Supply Chain - Site Preparation - Piling

Starting on site in 2022, the conceived project was to construct a new fish passage around the Holme Sluice Gates on the River Trent, allowing bi-directional travel of all breeds of fish from salmon to coarse river fish. The existing gates were constructed in the 1950s and are currently operated by the Environment Agency to maintain navigable river levels within the Nottingham area.

This paper follows up on one published by Water Projects in 2022, and looks at the construction and commissioning elements of the project.

Background

When the Holme Sluice Gates structure was constructed, the River Trent was diverted from its original path around the Country Park through a newly excavated 80m wide cut channel. At the side of the sluice gates is the Holme Canal Lock with a further adjacent water structure of the Holme Pierrepont canoe slalom course; all of which create barriers for migratory fish back up the River Trent.

Project summary

As the sluice gates will remain in operation, the fish pass channel requires two bridges rated to 40t to be built to maintain access to that area. The channel is to be protected at the intake via a single floating boom and the water isolated from the channel by a combination of removable stop logs and an automated radial gate system.

[caption id="attachment_12348" align="alignnone" width="1200"] Aerial view of the completed fish pass - Courtesy of Jackson Civil Engineering Ltd[/caption]

The design of the fish pass has been provided by the Environment Agency’s designer, TEAM Van Oord (TVO), with Jackson Civil Engineering Ltd (JCE) undertaking design elements for the temporary works and the Mechanical, Electrical, Instrumentation, Control and Automation (MEICA) elements.

The fish pass design has a total flow rate of 4.4m³/s - 4.9m³/s with a varying water depth of 1.5m- 1.7m. Although the velocities within the vertical slots have a flow rate of 2m³/s, the pools created at each stanchion bay slows this to >1m³/s; allowing fish to rest as they travel through the pass.

Where we left off……

This paper follows on from an initial case study published by Water Projects in August 2022. In the time since, the piling works (which started in April 2022) were completed by Ivor King Piling, following a period of ground pre-auguring to ensure that the circa 10m AZ700-36 sheet piles could be driven with relative ease.

A total of 350 sheet pile pairs with a number of singular piles were driven into the ground to produce the working cofferdam.

Due to the inlet and outlet elements of the fish pass having to be installed within the river flow, it was imperative that the works hit the allowable in-river works period to avoid damage to spawning fish and suitable breeding grounds. This period ran from 15 June 2022 to 2 October 2022; these dates forming an X5 Contract Key Date. Work on the inlet/outlet sections started nine workdays after the period started, and all piling works were completed by the end of August 2022.

There were some significant challenges throughout this period, the depth and complexity of excavating safety behind the sheet wall to install tie piles at a depth of 4-5m requiring a significant portion of the river bank (50m³) to be removed stored and reinstalled and for a number of the tie bar being at a level below the surface of the river, meant a design change was needed to a welded beam solution on the backside of the piles.

[caption id="attachment_12352" align="alignnone" width="1200"] Modular wall shuttering in place for wall pour - Courtesy of Jackson Civil Engineering Ltd[/caption]

Channel design concept

In summary, the channel concept included a full height channel wall at the inlet and outlet, joined on both sides by a 2m high dwarf FRC sheet pile wall. Around the top of the sheet piles, a non-structural concrete pile capping beam (chainage 24m - 150m) was required, at which point the design changed to a structural capping beam.

This design was driven by the ground pressures seen on the sheet pile walls in that the channel floor would act as the only prop needed in the non-structural sections, however, due to the increasing depth of the channel towards the discharge end the top of the channel would also need to be permanently propped as well as using the base slab.

The start of earthworks & FRC

Originally the staging for construction was to temporarily prop the sheets at the top throughout the channel run, excavate down to design invert level, cast the base slab and the install the capping beams. This would require the need for significant soffit support systems and working at height.

Through detailed project discussions with the principal designers and supply chain, the decision was made to excavate only the area needed for the capping beam and use the ground formation to act as the soffit support there for removing a significant health and safety risk. To allow access throughout the construction period a section of capping beam was left out (5m of the 400m required) which would require a conventional support system for install.

Earth removal started in earnest in mid-August 2022 reducing the ground to install 26 (No.) SBS 400-185Hyd struts and 300m linear lengths of SSB wailing beams provided by Mabey Hire Ltd.

[caption id="attachment_12345" align="alignnone" width="1200"] Excavated fish pass awaiting FRC works - Courtesy of Jackson Civil Engineering Ltd[/caption]

Groundwater challenges

From the provided geotechnical report it was noted that groundwater was expected circa 2m below the ground surface and from engaging with a dewater drainage specialist it was possible that installing a dewatering system would require a significant number of well points discharging 25 l/s. The larger issue was that these wells would need to be installed through the capping beam (not aesthetically pleasing) or would be placed in the way of the bulk earthworks removal.

A number of trail pits were excavated to 4m to assess the realistic nature of the groundwater. Little groundwater was evidenced during these tests, therefore the project proceeded without the use of dewatering, with a mitigation measure that a local sump would be created to allow groundwater to be pumped our using a 6” end suction over pump, to aid in the drainage and working surface a 100mm thick layer of gravel (350 tonnes) was installed in the bottom of the channel to allow the water to flow beneath and work plant. The risk of significant groundwater did not materialise.

The installation of the temporary struts and excavation of bulk material were done in sections therefore allowing manoeuvrability of the excavator to reach between the piles. M2 Civils Ltd, the earthworks contractor, removed on average 24 road wagons of earth per day, totalling 1458 loads or 27,935 tonnes (13,760m³).

Holme Sluice Fish Pass: Supply chain - key participants

• Client designers: Team Van Oord
• Principal contractor: Jackson Civil Engineering Ltd
• Temporary works design: Waterman Group
• Mechanical design: KGAL Consulting Engineers Ltd
• FRC: STAM Construction Ltd
• Groundworks: M2 Civils Ltd
• Piling works: Ivor King Piling
• River works: Red7 Inshore Diving
• Ground support: Mabey Hire Ltd
• Electrical design & control equipment: IMAC Ltd
• Radial gate system & stop logs: Centregreat Engineering Ltd
• Eel tiles: Berry & Escott Engineering Services
• Fish monitors: FishTrack
• Precast concrete beams: Banagher Precast Concrete Ltd
• Tarmac & surfacing: CR McDonald Surfacing

[caption id="attachment_12347" align="alignnone" width="1200"] Non running fish pass mid-distance - Courtesy of Jackson Civil Engineering Ltd[/caption]

FRC

The FRC works was undertaken by STAM Construction Ltd and started in October 2023 with the installation of base slab using cut and bent reinforcement steel throughout the construction. This would then allow the high-level shoring props and wailing beams to be removed. Due to the nature of the design being effectively three straight runs of dwarf walls, the wall shuttering system (9m long by 2.5m high) was constructed on an adjustable wheeled cart, thus allowing the shutters to be installed, retracted, moved, and redeployed reducing the use of timber and removing significant lifting and manual handling risks.

To facilitate this the project teams worked on providing a solution to allow the shuttering system to fly past the starter bars required for the wall and floor stanchions. This was achieved by utilising a ‘Startabar’ product whereby the RC bars are encased under a steel cover which is then removed, and the bars bent out to the correct position after the wall concrete has been poured.

To construct the wall and floor stanchions, conventional shuttering systems were employed. In total 329 tons of steel reinforcement was used and 2,250m³ of concrete were pumped.

Bridges

The construction of the fish pass has created an island of the operational sluice gates requiring 24 hour. To service this asset, two permanent 40T rated road bridges were designed and installed. A major project challenge was to sequence the bridge construction start for after the bulk excavation had past the bridge locations and have them finished before the temporary access created across the sheet piles needed to be excavated out. This includes the bridge abutments, deck and on/off ramps. The bridge construction utilised MY3/MY3E precast concrete beams spanning 12.6m utilising seven beams per bridge and used scaffold soffit support system to all for cast in situ sidings and deck. The MY3 beams were planned and delivered in a ‘Just in Time’ strategy allowing a direct lift and install from the wagon to their installation location.

Construction started September 2022 and finished October 2022 including the installation of the vehicle restraint system, notably the bulk earth movement completed by removing the temporary access to the island in December 2022.

MEICA

The water passage through the pass is controlled by an automated radial gate system, using local river levels and area flow gauges. The gate opens when the flow is greater than 49m³ and the level is above 20.34m AOD; should either of these readings become false the gate will close.

The gate spans the full width of the channel with a radius of 3.5m and stands closed at a vertical height of 1.9m. Using a single actuator the gate (10 ton self-weight) is lifted and lowered via a steel cable winch system with a full extent of travel being circa 4m vertically.

The radial gate solution was chosen by the designers as the best suitable control mechanism due to the following benefits:

• Requiring less infrastructure than a vertical gate (no need for structures/lifting frame/gantry above the channel) all reducing the civil reinforcement design in those areas.
• Less energy to operate as loads are transferred through the pivots and the geometry of the gate means upwards water pressure will assist with lifting.
• Longevity of the seals as loads transferred through the pivots and not into guides and seals.
• No need for channels/slots in the walls/base of the channel which could become blocked with debris and would also affect hydraulics.

Even though the size of the fish pass requires significant volume of water to achieve a steady state, this is realised in approximately 90 seconds from the start of the gate opening even though the gate takes 8 minutes to fully clear the pass/river surface.

The isolation of the fish pass for maintenance purposes is undertaken by the installation of five 6.8m long stop logs. These are deployed using an 18m long lifting gantry with dual lifting hoist with a SWL of 2.5T and a hand cranked moveable storage cart.

While the pass is in operation there will be free bidirectional passage for aquatic life, however during non-operational periods eels are still likely to try and pass and with the water during non-operational period only filling the last third of the pass, there is the potential for the eels to become stranded in the pass. Therefore, the pass has been designed with eel chute compliant with the Eels Regulations 2009. The chute and subsequent rebates along the edges of the pass have been fitted with fixed plastic eel tiles therefore providing purchase to aid the eels in their journey. To assist the chute, which is connected to the radial gate and is lifted out of the water during gate operation, while the gate is closed there is a constant flow of water covering the eel tiles guided by the fall of the channel base and surface mounted fixed plastic lumber sections.

[caption id="attachment_12351" align="alignnone" width="1200"] Completed fish pass - Courtesy of Jackson Civil Engineering Ltd[/caption]

Pass/river interfaces

Extensive riverbank and river bend modifications were needed to ensure that the pass joined and exited the river and was engineered to prevent erosion. During the in-river works period of 2022 and detailed as a key date within the contract, specialist dive services were employed to dredge out the riverbank/bed and place 80-100T of scour protection amour using 5-300kg sections of rock.

This was all achieved using bank-side long reach excavators; therefore being more environmentally friendly, with greater efficiency than deployment of float pontoon platforms.

Due to the in-river works, stringent protection measures were placed on the project with regards to fish protection and silt migration. The project team deployed 100m of silt bubble curtains around the river work areas to reduce the any sit mitigation as well as undertaking on-site water quality monitoring for turbidity and dissolved oxygen both upstream and downstream, and within the works area. Data showed that the work area was a factor of 10 higher than the up/downstream areas, which remained within agreed permissible limits.

Further dive works included underwater cutting and removal of the cofferdam end sections to open the fish pass to the river. This solution was chosen as it reduced the risk of delays and damage to permanent structures during a vibration extraction method.

Monitoring fish usage

The fish pass is fitted with state of the art near-field communication hardware (passive induction transmitters) to facilitate the monitoring of fish passage. When the electronics are installed, the system will allow the client to understand how effective the pass is, the direction of fish travel and time spent within the pass.

[caption id="attachment_12350" align="alignnone" width="1200"] (left) Completed fish pass looking towards the intake structure and (right) fish pass in operation showing control radial gate - Courtesy of Jackson Civil Engineering Ltd[/caption]

Conclusion

The project completed on 14 May 2024 having run for 27 months. The following is a statement from Simon Ward, Fisheries Specialist with the Environment Agency.

“After two years in construction, and several years of planning and design, the Colwick (Holme Sluices) Fish Pass on the River Trent is now complete and ready for fish to use enabling them to reach their spawning and feeding grounds. This flagship project opens up the River Trent and its tributaries for migratory fish, including salmon, trout and eels, making more habitat accessible for fish. Whilst a complex task the construction ran smoothly throughout and the partnership of designer, constructor and EA Fisheries technical staff working together is an exemplar of what can be achieved for the environment.

“The fish pass is part of the Environment Agency’s work to improve fish passage across the country and provides a significant step in restoring the River Trent catchment to its former glory for salmon and other coarse and migratory fish. The project also includes an eel pass to help support the critically endangered European eel; and a public viewing platform above the water, with highly visual interpretation boards to inform and advise visitors about the local wildlife in and around the river, including the fish that are expected to use the pass.”

Witches Oak Water is a series of gravel pit lakes south-east of Derby, on the River Trent. The 80 ha site, part of a larger sand and gravel quarry, was acquired by Severn Trent more than 20 years ago to store water as part of a drought resilience project. Severn Trent constructed an intake, pumping station and pipeline to the nearby Church Wilne WTW. Water quality was poor and required more treatment to meet drinking water standards than was originally anticipated, so abstraction ceased and the site mothballed until a solution could be found. Severn Trent is now bringing Witches Oak Water back into operation by recommissioning the existing infrastructure and increasing raw water supplies to a maximum of 93 MLD. The additional water will improve Severn Trent’s resilience to hot and dry summers and secure water resources for future generations.

Solution

Severn Trent is keen to explore the potential of low-carbon, natural, biological water treatment processes in conjunction with conventional, chemical treatment to remove pollutants from the water abstracted from the River Trent. Historic data revealed that nitrate levels in the River Trent have occasionally exceeded the 50 Mg/l concentration value prescribed by the DWI, and regularly breached Seven Trent’s own, more stringent standard.

As a result, a natural, biological solution in the form of floating wetlands to provide preliminary water treatment is being combined with refurbishing the existing abstraction infrastructure at the lakes (called the Raw Project) to supply water to the new water treatment works being constructed in Church Wilne.

[caption id="attachment_14685" align="alignnone" width="1200"] Installing the floating wetlands - Courtesy of Matthew Nichol Photography[/caption]

Floating wetlands

This is the first time in the UK that wetlands have been used for operational water pre-treatment on this scale and in the ‘real world’, but theoretical studies show that, on average, these floating wetlands could remove in the region of 40-60% of both total nitrogen (ammonia, organic and oxidised nitrogen) and total suspended solids (particles >2 microns), helping to considerably reduce the amount of energy and chemicals used in the traditional water treatment process.

Floating wetlands were selected over conventional or static wetlands because Witches Oak floods regularly during the winter.

The pontoons can rise and fall with the water level, so are resilient to changes in load and seasonal fluctuations, whereas a static wetland would be submerged during a flood event, diminishing process benefits provided by the vegetation. This has been thoroughly tested in winter 2023/24 when the site experienced extensive 1 in 100-year flood events. The floating solution also typically requires less maintenance, including no watering or fertilisation.

The Biomatrix floating ecosystems selected for Witches Oak mimic elements of self-sustaining, naturally occurring riparian wetlands. Water quality is improved by the root mass, which provides a large surface area for the microbial communities needed to remove pollutants. Planted with several types of sedge, hard rush and flowering species, including purple loosestrife and meadow sweet, these islands provide biological treatment to reduce the turbidity and nitrates in the raw water.

[caption id="attachment_14682" align="alignnone" width="1200"] Floating wetlands (2023) - Courtesy of Matthew Nichol Photography[/caption]

In December 2022, Mott MacDonald Bentley initially focused on the construction and launch of three trial wetlands in Lake 2, as well as the removal of a redundant man-made wetland from the lake. Since then, a further 28 floating wetlands have been installed on the three northern most lakes during summer 2023.

The 31 floating treatment wetlands were positioned in the natural water flow path in Lakes 1, 2 and 3.

Work is also being undertaken to upgrade of the existing pumping station and river intake, along with the installation of a new fish recovery system and three new transfer pumps.

Undertakings

This is a true ‘One Mott MacDonald’ project with members of the consultancy teams having worked on the feasibility of the wetlands’ abilities to pre-treat water and then their outline design, before handing over to colleagues at Mott MacDonald Bentley through early contractor involvement and then design and build delivering the detailed design, pre-construction and construction activities.

Witches Oak RWPS: Supply chain - key participants

• Client: Severn Trent
• Principal designer & contractor: Mott MacDonald Bentley
• Outline design & specialist input: Mott MacDonald
• Floating wetlands: Biomatrix Water
• Civils installation: Aquatic Engineering Ltd
• Fish recovery: MEM Fisheries Consultants & Services Ltd
• Cofferdam: MGF Ltd
• Piling: Elite Sheet Piling Ltd
• Formwork: Doka UK Formwork Technologies Ltd
• Steelfixers: MTN Structures Ltd
• Electrical: Main Electrical Ltd
• Electrical: BGEN Ltd
• Dewatering: Selwood
• Dewatering: Alba Dewatering Services Ltd
• Pumps: Xylem Water Solutions
• Fish & eel screens: Intralox LLC
• Pipework supply: Freeflow Pipesystems
• Pipework Installation: Utiliqo Ltd
• Valves: AVK UK Ltd
• Penstocks: Waterfront Fluid Controls
• Kiosk: Morgan Marine Ltd

[caption id="attachment_14687" align="alignnone" width="1200"] Pumping station refurbishment - Courtesy of Sky Revolution[/caption]

Environmental measures

To minimise the environmental impact, temporary track mats were used for the base of the works’ compound and launch area at Lake 2, allowing the site to return to managed grassland when the mats were lifted.

A silt curtain was erected in the lake to prevent debris from the old wetland polluting the water. This was important because the wetland was colonised by invasive species, mostly Himalayan balsam on land and zebra mussels below water. The curtain also ensured that sediment disturbed by the removal work did not spread.

The design has also had to accommodate regular site flooding; in particular for the temporary works around the river intake where, as an example, the cofferdam was designed specifically to account for controlled and safe flooding of the excavation to manage the high river levels experienced on-site.

Programme

Launching and anchoring the 31 floating wetlands was the first phase of the Raw Project. Phase two is refurbishing the abstraction pumping station built in the 1990s to enable raw water to be supplied to Witches Oak WTW.

Located on the western shore of Lake 3, the pumping station is undergoing a complete refurbishment, including the installation of three new transfer pumps to deliver up to 96 Ml/d of pre-treated water; enough for a city the size of Derby.

[caption id="attachment_14690" align="alignnone" width="1200"] River intake - Courtesy of Sky Revolution[/caption]

The design layout for the station also includes a new water quality kiosk and a wash-water booster set to clean the fish screens.

The river intake in the north-east corner of Lake 1 is also more than 25 years old and is being modified. The design will help to protect both fish and the gravel lakes. Two new 2.5m x 4m concrete channels will be constructed to create a flow low enough to allow fish and eels to swim back to the river, while two large, actuated intake penstocks will be installed to protect the lakes if there is a river pollution incident.

The introduction of new recirculation pipework will enable water to be pumped back to the river from the pumping station when necessary. Under the proposals, river water quality will in future be monitored upstream of the intake as well as at the intake itself.

Barbel, bream, chub and roach are common fish species found in the River Trent and two new screens will be fitted to filter fish and debris out of the water drawn in by the pumps at Witches Oak Water. The fish recovery system will consist of an above-ground pipe connecting the pumping station intake culverts to Aston Brook, which runs alongside the river, and from where fish can eventually return to the Trent.

Summary

Witches Oak is one of seven projects that form Severn Trent’s Green Recovery Programme, worth more than £700m, which aims to develop low-carbon water resources, improve river water quality, replace redundant lead pipes, install smart metering and make urban areas more resilient.

The additional water supply from Witches Oak will help meet demand for water from a growing population and improve the resilience of supplies during hotter, drier summers.

[caption id="attachment_14683" align="alignnone" width="1200"] Installed wetlands - Courtesy of Sky Revolution[/caption] The 31 constructed wetlands use naturally occurring biological processes to contain and treat ammonia, suspended solids and trace metals. Coupled with installing ceramic membranes at Witches Oak WTW (to provide a filtration barrier, reduce turbidity and remove pathogens such as cryptosporidium), the project is a more carbon-friendly treatment solution.

By restoring natural functions to these gravel lake ecosystems, the project is promoting biodiversity recovery and contributing to enhanced ecosystem services. The wetlands provide amazing biodiversity benefits including:

1. Above the water, the planted vegetation is a haven for wildlife, including nesting birds.
2. Below the surface, the floating beds provide habitat for fish, eels and other aquatic species.
3. On the fringes of the lake, retained coppiced woodland from the construction activities has been used to create further wildlife habitat.

In addition, positive social outcomes and community engagement are key elements of Severn Trent Water’s 25-year water resources management plan. The delivery team presented to the local girl guide group to tell them about the work we’re doing, and a visitor engagement centre has been installed to showcase this exciting project to local school and community groups.

Seedy Mill WTW treats surface water from the Blithfield Reservoir. The existing supply to the works is gravity-fed up to a maximum of 85 MLD. Additional yield could be provided by utilising three in-line booster pumps located at Nethertown. In addition, there are two individual borehole contributions at Seedy Mill and Trent Valley, which can be introduced at various stages throughout the treatment process. The Water Resources Management Plan confirms maximum flow is 110 MLD (with or without the borehole yields), although the treatment capacity is 115-120 MLD dependent upon raw water quality. Flows through the existing plant are highly variable with minimum flows as low as 20 MLD. This tends to occur during Blithfield Reservoir conservation periods. Daily demand increases from 20 MLD to 110 MLD can be achieved with an average of between 50 and 110 MLD in six hours.

Existing works

The existing treatment works includes clarifiers with poly-aluminum chloride coagulant, polyelectrolyte, and sulphuric acid dosing. There are two clarifier streams, one with four accelators and one train with four Dissolved Air Flotation (DAF) streams. Chlorine is added seasonally to the clarified water before it enters several banks of rapid gravity filters. Next, the water passes through chlorine contact tanks, before being subjected to ultraviolet light irradiation, and then pumping to distribution. Filter-to-waste flows are decanted to the head of the DAF and are limited to 5% of the DAF flows. [caption id="" align="alignnone" width="1200"] (left) Piling works for the RGF building and (right) construction progress March 2022 (photo by Lobster Pictures) - Courtesy of Galliford Try[/caption]

Project driver

In 2019, following a Drinking Water Inspectorate (DWI) assessment of Seedy Mill WTW, South Staffs Water was required to review and revise and/or make operational the construction, installation and commissioning of an additional filtration stage or other agreed solution to reduce total trihalomethanes and other disinfection by-products.

Recommended solution

Two options for additional filtration were identified by South Staffs Water; Rapid Gravity Filters (RGF) and a ceramic solution. In May 2020, RGFs were chosen as the preferred option, with associated upgrades to the existing sludge treatment processes to deal with the additional expected dirty washwater. This solution satisfies the criteria laid down within the DWI notice, ensures the plant will consistently deliver all the required water quality parameters, and improves the capacity, performance and resilience of the sludge treatment system. Following Early Contractor Involvement (ECI) works, South Staffs Water issued Galliford Try with an NEC Option C contract for £21m for the detailed design and civil and MEICA construction of the new filtration stream and sludge treatment upgrades. These were required to be installed and commissioned before the Regulatory Date of 31 March 2023. [caption id="" align="alignnone" width="1200"] Clean water backwash tank, interstage pumping station and rapid gravity filter building - Courtesy of Galliford Try[/caption]

RGF treatment stream - project scope

• Ten new RGF cells with treatment capacity of 120 MLD.
• Common pipe gallery.
• 1000m diameter inlet and outlet process pipework.
• Interventions to connect new RGF process stream.
• New clean backwash tank and pumps.
• New Intermediate Pumping Station (IPS).
• Re-purpose the existing structural frame of the disused bottling plant to reduce the use of new materials.
• New MCC and systems integration.
• Transformer compound.
• Portal frame and cladding over RGF and MCC/blower building.
• Security enhancement works, including perimeter fence.

Sludge works - project scope

• New sludge holding tank.
• Two new Alfa Laval (Aldec 105) centrifuges to replace existing - duty/standby phased changeover.
• Duty/standby poly make-up and dosing systems.
• Sludge conveyors.
• Building extension to house new centrifuges.
• New MCC.
• Modifications to existing Opus SCADA system.
• Removal of existing plate press/centrifuges and existing MCC.
• The sludge feed of 12m³/hr @5% changed to 15m³/hr @3%.

[caption id="" align="alignnone" width="1202"] Interstage pumping station and rapid gravity filter building (from west) - Courtesy of Galliford Try[/caption]

HV works - project scope

• Replace existing 11kV switchgear.
• Provide an additional 1500kVa transformer with a civil compound and HV cabling to new 11kV switchboard.
• Modification to the generator synchronisation system to accommodate new switchgear.
• New 3.3kV switchgear to feed the new RGF transformers and the existing Block 4 transformers.
• New secure enclosure for the 3.3kV switchgear with a plinth, trenches and ducting for cabling.
• HV cabling to and from switchgear.

Supply chain engagement

The team employed a clear strategy - to utilise Galliford Try’s in-house capabilities where possible, including design, steelwork, lifting equipment, sample pump backboards, instrumentation backboards, point of application boxes and asset intelligence (SEMD). The team also used the company’s in-house civil and mechanical labour force whenever possible. The engagement and performance of the supply chain were fundamental to the project’s success. A number of key suppliers and subcontractors were engaged early and helped develop the design and constructability of key areas. Due to the confined space, the on-site reviews and pre-order minutes of meetings helped flush out access dates, laydown areas and points of interaction. Coupled with daily collaborative planning sessions, this approach helped to control the work areas on site and enabled better communication around supplier movements and working areas. [caption id="" align="alignnone" width="1200"] Interstage pumping station and RGF building from the north - Courtesy of Galliford Try[/caption] The Seedy Mill WTW project was faced with an extremely challenging regulatory target, by which point the new RGFs had to be in service. There were significant challenges with costs and programme, set against a backdrop of instability and unpredictability in supply chains and construction costs inflating at a much greater rate than CPI. However, Galliford Try and South Staffs Water’s proactive and collaborative approach to managing risk, delivered a project which was within the original target price, and the team met the regulatory date challenge with two weeks to spare. Galliford Try’s construction management was key to the delivery of the Seedy Mill WTW project, and their collaboration with South Staffs Water allowed them to meet and surmount a significant number of challenges by careful planning, risk mitigation and challenging the programme whilst always maintaining focus of health, safety and wellbeing of personnel and the customer.

Seedy Mill WTW: Supply chain - key participants

• Client: South Staffs Water
• Principal designer/contractor: Galliford Try
• Civil design checks: Eastwood & Partners
• Hydraulic modelling: Hydrotec Consultants
• RC works: STAM Construction Ltd
• Underground pipework/earthworks: Draper Construction
• Piling: Van Elle
• Shoring/temporary works: National Trench Safety UK
• Fabricated steelwork: GT Fabrications
• Structural steel & cladding: Structural Steel Limited
• Mechanical installation (RGF): Frankyln Yates Engineering
• Mechanical installation (dewatering): Watermark
• Electrical installation: Elsym Installations Ltd
• SCADA & systems integration: HBX Ltd
• RGF Leopold flooring: Xylem Water Solutions
• Filter media installation: Western Carbons Ltd
• Centrifuges: Alfa Laval
• Conveyors: CTM Systems Ltd
• Polymer systems: SNF UK Ltd
• Dosing rigs & instrument backboards: GT Plymouth
• MCC & RIO panels: BGEN
• Carbon steel pipework: Freeflow Pipesystems Ltd
• Ductile iron pipework: Saint Gobain PAM UK
• IPS & CWBT pumps: Sulzer Pumps
• Air blowers: Aerzen Machines
• Penstocks: GT Ham Baker Engineering
• Pressure instruments: Trafag Sensors & Controls
• Level & flow instrumentation: Siemens
• Level instrumentation: Hawker Electronics
• Flow instrumentation: ABB
• Valves: Affco Flow Control
• Valves: Cotswold Valves
• Security works: GT Asset Intelligence
• Ventilation: Air Technology Systems Ltd

[caption id="" align="alignnone" width="1200"] RGF pipe gallery - Courtesy of Galliford Try[/caption]

Construction - Filtration stage

The Seedy Mill project requirement was to provide an additional stage of filtration on site, primarily to reduce the turbidity in the water. The build was an off-line solution built on the south side of the existing treatment works. The scope included: Underground pipework Connections into the 1000mm diameter pipework downstream of the DAF were made with suitable valving to allow flows to be diverted to the new intermediate pumping station. Draper Construction undertook a significant number of underground pipework connections to connect the RGF to the existing process and also to the wastewater systems on the project. This required careful planning with all stakeholders and submissions and agreement of ‘authorised outages’ to reduce the output of the site while the connections were made. Intermediate Pumping Station (IPS) The IPS consisted of two compartments, which could be isolated from each other via penstocks to allow for future maintenance. Each compartment contained three 45 kw centrifugal pumps from Sulzer, each capable of operating at 30 MLD. Water is pumped directly up from the IPS into the bellmouth chamber, which contains baffling to regulate the flow into the RGF’s common inlet channel that allows the process water to enter the ten RGF cells via inlet actuated penstocks. [caption id="" align="alignnone" width="1200"] Interstage pumping station wet well - Courtesy of Galliford Try[/caption] Pipe gallery On the south wall of the RGF cells is a 71m long pipe gallery, which contains the backwash pipework system, the process outlet pipework system and the rinse pipework, used after the cell had a backwash but prior to the cell going into service. Rapid Gravity Filter cells The RGFs are housed in a cladded structure 71m long x 18m wide. There are ten cells built upon continuous flight auger piles, designed to take 10-15 MLD. The RGFs are concrete tanks, cast in situ using a mix of blending cement with fly ash to slow down the cure time and reduce thermal cracking and shrinkage. On the floor of each cell is a Xylem Water Solutions Leopold underdrain flooring system (rather than the traditional nozzle approach), which provides improved bubble coverage and reduced installation time. Above the Leopold flooring are layers of sand and anthracite media. Each cell has its own primary (ultrasonic) and secondary (level electrode) level measurement equipment as well as local control panels, turbidity monitor backboard, sample tap and differential pressure monitors. Access steelwork on top of the RGF cells allows access for maintenance of the associated equipment. [caption id="" align="alignnone" width="1200"] RGF air pattern test - Courtesy of Galliford Try[/caption] Clean Water Backwash Tank (CWBT) The new CWBT was built in two compartments, fed by treated RGF process water. There are two 75kw pumps (duty/standby) per compartment, each capable of pumping up to 330 l/s, which flows back up through and cleans the filters. This is in conjunction with the blowers adding air into the system at various parts of the wash. The dirty backwash water is discharged in the existing waste systems. Motor Control Centre (MCC) building This building houses the new MCC contained in the IPS and CWBT drives and the main Programmable Logic Controller (PLC), which controls the operation of the IPS, RGF and CWBT systems. It also houses the two (duty/standby) air blowers that are utilised for backwashing of the filters. Commissioning The new IPS/RGF/CWBT arrangement was commissioned as far as practical offline. After successful dry testing, wet testing was conducted utilising process flow in but running all flow that had been through the RGF to waste until satisfactory water quality laboratory results and Site Acceptance Tests were achieved. Three cells at a time were turned into supply, in order to stagger the impact of bringing RGF cells online into the works. Each stage had a proving period, after which the core team would meet to review the results and then decide ‘Go’ or ‘No-Go’ for the next bank of cells.

Construction - Dewatering stage

The existing dewatering system comprised of an existing building with centrifuges and ancillary equipment that was beyond its asset life. The new dewatering system comprised of a new sludge holding tank with mixers, centrifuge feed pumps, polymer make-up, day tank and dosing system, centrifuges capable of 15m³ @ 3% dry solids, conveyors and an MCC to control the system. [caption id="" align="alignnone" width="1200"] Alfa Laval centrifuges - Courtesy of Galliford Try[/caption] The new dewatering works had to be undertaken in the existing building around existing operational assets. A building extension was required to house the new Alfa Laval centrifuges and two screw conveyor systems from CTM Systems Ltd. However, the new polymer system and MCC were installed within the existing building, requiring careful planning to replace equipment in a phased manner with minimal disruption to the client’s process.

HV works

The HV works consisted of modifications to existing HV switchgear, new 3.3kV HV switchgear, the replacement of the 11kV mains incomer panel, new transformers and compounds, and associated cabling and transformers. GT utilised its HV SAP to work together with the South Staffs Water SAP on the isolations and phasing of the work to carry out these works with minimal disruption to the process. These works were also done in and around the new RGF, which required careful planning involving morning CP sessions with team coordination, exclusion zones and access areas. [caption id="" align="alignnone" width="1200"] Dewatering area - Courtesy of Galliford Try[/caption]

Time saving solutions

The Seedy Mill WTW project team achieved Plant In Use (PiU) more than nine days before the Regulatory Date of 31 March 2023. PiU is defined as all flow entering the site running through the new RGF filtration system, with capacity ranging from 60 MLD up to 120 MLD. At one stage, the PiU was forecast as being more than 37 days behind schedule but the team turned this around by running a number of programme workshops. Galliford Try and South Staffs Water collaborated and challenged each other on different construction and commissioning methods, resulting in a significant improvement on the PiU date. To have achieved PiU prior to the regulatory date is exceptional and is a credit to everyone involved. This achievement is a result of the high level of cooperation between the respective site operation teams, a collaboration which was essential for the delivery of the scheme.

Budleigh Salterton is a seaside town on the coast in east Devon, at the start of the World Heritage Jurassic Coast, with a population of just over 5,000 people. It has been described as a picturesque town with a two-mile pebble beach for the hardy swimmers braving the seas for a dip in the beautiful clear water. South West Water’s objective was to ensure that the bathing water quality at Budleigh Salterton does not deteriorate to below ‘excellent’ within the next 20 years. This was to be achieved by providing in-line tank storage in combination with the removal of 0.31ha of surface water from the combined system, in order to ensure that no greater than two significant (>50m³) spills per bathing water season was achieved by 31 March 2022.

Project background

The project comprised the installation of an in-line 190m³ stormwater tank constructed from two 83m lengths of 1200mm diameter precast concrete pipes laid adjacent to the existing sewer. The inlet to the storage tank is controlled by the level within the existing sewer. So when the flow increases within the sewer, the level rises and starts filling the tanks. The outlet flow has a weir fitted with a flow control Hydro-Brake® allowing 25 l/s to pass forward. The project was constructed within a grassed field which was within a flood risk area.

Temporary works piling

The excavation support system (the temporary works) consisted of a 90m long x 4m wide cofferdam within the flood risk area. Galliford Try constructed an engineered haul road which doubled as the piling platform from which specialist sub-contractor, Elite Sheet Piling Ltd, installed 310 (No.) GU13, 6m long sheet piles using a 35t excavator mounted, side griping MOVAX pile driver. Once the sheet piles were installed, the ground between the sheet piles was removed and taken off site. The sheet piles were designed to cut off any groundwater entering their excavation from the adjacent river. One of the major risks to the scheme was that as soon as Galliford Try excavated the large 90m long 4m wide trench it would be inundated with groundwater, making it impossible to safely install the precast concrete pipe sections. The cofferdam worked perfectly; cutting off all incoming groundwater. Once the ground trapped within the cofferdam was removed, the excavation remained dry during the pipe installation.

Drainage blanket

Following the excavation of the cofferdam, a drainage blanket was installed in the bottom of the trench. This comprised a 200mm thick layer of 50mm singled sized stone which was then covered over with a 1000 guage geotextile membrane from Terram; helping to prevent the intermixing of solid particles between the layers of soil and acting as a filter layer stopping any fine material getting into the drainage blanket layer. [caption id="" align="alignnone" width="1200"] (left) Piling installation by Elite Sheet Piling Ltd, and (right) drainage blanket installation - Courtesy of Galliford Try[/caption]

Pipe installation

With the drainage blanket complete, Galliford Try started the installation of the 62 (No.) 1200mm diameter precast concrete pipe sections, which were laid on 20mm pipe bedding materials and aligned using a pipe laser. The pipe laser was installed by the site engineer at the correct location with the exact fall from end to end of the 90m cofferdam. A 20t excavator was used to lift the pipe sections along the haul road and down into the cofferdam where the pipe laying gang would place them in accordance with the pipe laser. Once all the pipe sections were installed, the team backfilled the pipe sections with pipe bedding then, as dug soil back up to ground level.

Meadow Lane Stormwater Tank: Supply chain - key participants

• Delivery contractor: Galliford Try
• Modelling: Arcadis
• Piling contractor: Elite Sheet Piling Ltd
• Hydro-Brake®: Hydro International
• Geotextile membrane: Terram
• Cofferdam shoring: MGF Ltd
• Precast concrete pipe sections: FP McCann
• Pump hire: SLD Pumps & Power

Other elements

At each end of the storm tank, Galliford Try installed tee sections of pipe creating a manifold, which connected the twin pipe together to form one large tank. Galliford Try then constructed six inspection chambers to provide access and to house the Hydro-Brake® flow control. This only allows a pass forward flow of 25 l/s at maximum design head; above these flow rates it weirs over a bypass to prevent the sewer backing up. Once the storm event is over, the stormwater within the tank is slowly released back into the existing sewer during periods of dry weather flow. [caption id="" align="alignnone" width="1200"] (top left) 1200mm precast concrete pipe sections, (bottom left) backfilling the stormwater tank, and (right) Hydro-Brake® from Hydro International in flow control chamber - Courtesy of Galliford Try[/caption]

Summary

By providing 190m³ of offline, gravity return storm storage in the field adjacent to Halse Hill, Budleigh Salterton, the spill performance at Meadow Road CSO has been improved in line with South West Water and Environment Agency targets. As the storage is to be provided offline, normal dry weather flow is to be maintained through existing sewer. The agreed storage tank volume, flow control device and overflow weir level were derived from the Arcadis modelling report and taken as the scheme output for the storage element. The final tank volume selection/spill performance of the promoted option was operationally reviewed by the SWW operations team, commissioning engineer and asset management.

The Thames Tideway Tunnel project is a 25 km (16 mile) combined sewer under construction under the tidal section (estuary) of the River Thames as it runs through London. It will significantly increase the capacity of London’s sewerage system, large sections of which were built in the 19th century, and be able to capture, store and convey almost all of the raw sewage and rainwater that currently overflows into the Thames Estuary. The Tideway tunnel will connect 34 of the capital’s most polluting combined sewer overflows (CSOs), transferring the foul water to the Lee Tunnel for onward delivery to Beckton Sewage Treatment Works. The completion of the Thames Tideway Tunnel scheme, currently anticipated for 2025, will significantly improve water quality, capturing more than 95% of the spills that currently enter the river to the benefit of London’s almost 9 million residents.

Location

As part of works on the Central section of the Thames Tideway Tunnel scheme, Barhale was contracted by the Ferrovial and Laing O’Rourke joint venture (FLO JV), the lead contractor for the central section of the project, to deliver the heavy civils and civils structures that would intercept the sewage overflows and transfer these to the main Tideway tunnel beneath the river Thames.

The Victoria Embankment site was located in the City of Westminster, on the north side of the river Thames adjacent to, and south-west of, the Hungerford Bridge and Golden Jubilee footbridges. The site was bounded to the south-east by the River Thames and the north-west by the A3211, beyond which are the Victoria Embankment Gardens.

[caption id="attachment_10008" align="alignnone" width="1200"] (left) Works within the cofferdam in the Thames and (right) works ongoing within the cofferdam - Courtesy of Barhale[/caption]

Scope of work

The works were delivered in stages to provide protection to existing assets and engineering control over the structure’s movement. One of the first elements of work undertaken at Victoria Embankment was the construction of a 50m long secant piled wall that would retain and minimise ground movement on the dual carriage way and the Circle and District tube lines.

The next stage of the works comprised the diversion of the existing gas mains and the completion of the cofferdam. Once these were completed, piling operations could start. In total, 117 piles were installed: 47 (No.) 600mm bearing piles for the CSO structure and 70 (No.) 1200mm piles for the new river wall. These were required to provide adequate bearing capacity for the future structures.

The groundwater levels along the Tideway Central route, including Victoria Embankment, did not allow safe excavation of the shafts and tunnels and required the project team’s attention. A temporary works assessment was carried out along the route and verified by using pump tests, which modelled the required pumping capacity to dewater the existing aquifers.

At Victoria Embankment, Barhale installed four 106m deep Chalk Wells, six 65m deep Thanet Sand Wells, and six 50m deep Lambeth Group Wells. This system was also accompanied by in-shaft and tunnel wells that have targeted local pockets of water.

The 15m diameter shaft was built using three techniques. For the first 14m, the shaft was sunk using the caisson method and precast segments. This method was chosen for this section due to the granular material that was reported in the existing boreholes.

[caption id="attachment_10006" align="alignnone" width="1200"] Secondary lining works - Courtesy of Barhale[/caption]

Once past this layer, London Clay was encountered, and the excavation method was changed to underpinning. This method was used for excavating down to 32m below ground level. Due to the limitations of precast segments, the lining technology was changed to sprayed steel fibre reinforced concrete with a thickness of 600mm. This method allowed for better control of shaft profiles and portals for future works.

At formation level, a 2m deep reinforced concrete base slab was cast. The construction of this base slab required approximately 80 tonnes of steel and over 450m³ of high GGBS (ground granulated blast-furnace slag) concrete.

From this level a series of temporary works were implemented to allow the short connection tunnel to be excavated and supported using SCL (sprayed concrete lining). The connection tunnel is 18m long and varied in shape. During the first 14m, the tunnel internal diameter (ID) is 3.6m and in the last 4.5m the tunnel transitions to an ID of 4.9m.

The completion of the SCL works allowed for the site to transition to the full civils stage and start the works on the shaft and tunnel secondary lining and internal structures. Above ground level works that were released by the achievement of this milestone comprised the construction of new chambers that will deal with the intercepted flow and the new river wall that encompasses these structures and protects them from possible ship impact.

For these works at Victoria Embankment, Barhale used approximately 1400T of rebar and over 7000m³ of low carbon emission concrete, making this one of the bigger schemes that has been delivered during the Thames Tideway Tunnel project.

Tideway Central: Victoria Embankment: Supply Chain - key participants

• Main designer & contractor: FLO JV
  • Ferrovial
  • Laing O’Rourke
• Delivery contractor: Barhale
• RC piling: Expanded Piling
• Dewatering: WJ Groundwater
• Civils works: Flatley Construction
• Sawcutting/drilling: EDD (Euro Diamond Drilling) Ltd
• Waterproofing: Renesco
• Temporary electrical set-up: Select Site Solutions
• Concrete supplier: CEMEX UK
• Concrete supplier: Hanson UK
• Additives for SCL works: Normet
• Waterproofing: Sika
• Steel supplier: Express Reinforcements Ltd
• Steel supplier: HY-TEN Ltd
• Plant supplier: Select Cranes
• Plant supplier: Flannery Plant Hire

Challenging tunnel construction

The short connection tunnel at Victoria Embankment was design to be excavated and supported using the sprayed concrete lining method. This allowed for the short connection tunnel to wrap around and support the main tunnel during the break-out and final connection operation.

To put this in stages, the Victoria Embankment team was required to construct the tunnel using SCL and line the asset using cast in situ reinforced concrete and reduce its diameter to a constant 3m ID. The main tunnel team would be required to build the main tunnel using a segmental lining and cast in situ a reinforced liner, missing only the break-out area towards the Barhale connection tunnel. Once the segments were broken out, the two independent concrete structures would be joined together using cast in situ reinforced concrete.

The ground condition investigations for the connection tunnel identified a band of sand in the middle of the tunnel excavation, with a thickness of 1m and the presence of water.

On the assumption of the layer thickness and presence of water, a temporary design was progressed to support the excavation while spraying concrete. The designer prescribed the use of a pipe arch canopy support system for the narrower, 3.6m ID section of tunnel, with piles providing support for the remaining 4.9m ID section. The dewatering specialist prescribed the use of shaft wells and in-tunnel wells to allow efficient dewatering of the layer.

[caption id="attachment_10007" align="alignnone" width="1200"] (left) Commencement of shaft works, and (right) diversion works within Victoria Embankment -Courtesy of Barhale[/caption]

During the initial 3 to 4m of tunnel excavation, the face log showed an increase of the sand layer band and larger volumes of water than expected.

The sand layer covered approximately half of the excavation face at this point. As works progressed through the next 4m, additional wells continued to be drilled into the tunnel to target the water presence and the band of sand kept extending. Nine meters in, and the sand layer presence ranged from midway up to, and 3m above, the tunnel crown.

The encountered sand layer was also very fine which made spraying concrete challenging. As the concrete was sprayed, the sand bond was so small that the sprayed concrete was pilling off in small increments, making the excavation bigger. To prevent this occurrence, fast setting grout was sprayed as an initial layer and only then sprayed concrete.

This amendment to the plan did not solve the issue of the water presence and problems wet sand causes to excavations, especially tunnels.

An additional amendment to the plan had to be made. The team set out to space proof and conducted a feasibility study to understand if banker bars could be deployed. The temporary works design was then independently checked by a second design consulting company and approved by client FLO JV.

Barhale safely implemented the temporary works support, which meant installing eight advances of banker bars and spraying the primary concrete lining against them.

Works completion

The complex civils works on Victoria Embankment CSO Project to intercept the combined sewer overflows and to divert them into the Thames Tideway Tunnel presented challenging and unexpected ground conditions. Demonstrating agility and innovation, the Barhale team was able to deliver an excellent solution which both met the project timeline and provided best value for money.

Lower Woodburn Wastewater Pumping Station (WwPS) serves the Greenisland Drainage Catchment and is located close to the town of Carrickfergus on the foreshore of Belfast Lough. The storm outfall from the WwPS flows directly into the lough. Although recognised that discharges have minimal impact on the Belfast Lough Shellfish Water Protected Area (SWPA), there is localised impact and a need to protect and preserve amenity on the accessible shoreline. Upgrading of the WwPS was needed to comply with the Environmental Regulator’s (NIEA) consent requirements.

Living with Water Programme

As part of Belfast’s strategic ‘Living with Water Programme’ (LwWP), NI Water has invested £1.8m to upgrade Lower Woodburn WwPS by increasing the volume of storm emergency overflow storage and installing a static screen. The solution also facilitates proposed housing (315 units) and hospitality/recreational developments that discharge directly to the WwPS and delivers storm separation of 17.8 hectares of impermeable ground. The project delivers against the LwWP goals:

1. PROTECT against Flooding.
2. ENHANCE the Environment.
3. GROW the Economy.

[caption id="attachment_12584" align="alignnone" width="1200"] Lower Woodburn WwPS site pre-construction - Courtesy of DAWSON-WAM Ltd[/caption]

Project delivery

Delivery partners for this project were appointed through existing NI Water frameworks. Tetra Tech was appointed to provide professional services, (including project management, cost management and site supervision) and to act as NEC Project Manager. The Murphy/ DAWSON-WAM Ltd JV (MDW) was appointed as main contractor.

Building on earlier work done to investigate catchment-wide solutions for Carrickfergus as part of the LwWP, MDW was engaged to develop the design and build solution under an Early Contractor Involvement (ECI) NEC contract. The design was reviewed and approved in collaboration with NI Water, and budgetary approval was granted for construction works under an NEC-ECC Main Option A contract in summer 2023.

Delivery was a collaborative effort between client, consultant and contractor teams to improve water quality and environmental conditions in the receiving Belfast Lough. The works comprised addition of surface water storage in a new concrete cofferdam constructed alongside, and connected to, the existing pumping station wet well. The constrained nature of the site lead to the need for access to neighboring lands from which to operate construction plant and machinery.

The scope was further developed during construction to improve the overall solution performance by installing a non-return valve on the storage overflow to enhance separation between storage and the marine environment and to improve the design life of the pre-existing pumping station.

Change was addressed using the provisions of the NEC contract and was managed within the overall budget for construction works. The new storage tank was integrated from an ICA perspective by linking to NI Water’s telemetry system for effective monitoring of storage levels and overflow incidents.

Throughout construction, regular site inspections were undertaken by Tetra Tech’s Site Supervisor and Health & Safety Manager to affect joint inspections with NI Water’s and MDW’s H&S Managers. This made sure of a combined approach to H&S management; issues were addressed face to face in real time and resulted in an excellent H&S performance throughout, with no accidents or incidents.

[caption id="attachment_12583" align="alignnone" width="1200"] 3D models of new storm storage tank - Courtesy of RPS[/caption]

Early contractor involvement (ECI)

The ECI process proved its worth as extensive design and build challenges were overcome and a buildable solution was developed that satisfied the construction brief and costs criteria. This led to Murphy/DAWSON-WAM Ltd being appointed as principal contractor for the design and build of Lower Woodburn WwPS upgrade in August 2023. Contract works involved the design, supply, installation, testing and commissioning of an additional 317m³ of additional gravity storm storage on the existing site.

Certain elements of the project proved extremely challenging, none less than the design and construction of the additional storage tank on an existing site footprint that was extremely compact. Construction was further restricted as access to the site was from the busy A2 road, (the main road corridor from Belfast to Carrickfergus), immediately to the east of the site was a live petrol station with underground fuel tanks, on the west was an existing domestic occupied property and the access to the property and to the local strand, straddled the site on the east and south side. This was further complicated by the fact the existing pumping station had to be kept operational during the entire construction process.

The extensive ECI (which included site investigations, topographical surveys, network modeling, liaison with key stakeholders and utility providers, design development and costs approval), was completed post contract award. MDW appointed RPS as their civil design partner and through collaboration with NI Water and Tetra Tech, overcame the buildability challenges of this compact site.

The ground investigation report undertaken on the footprint of the only available space on the site identified glacial till overlying bedrock which varied from 6 to 8m below ground level. Early tank storage design calculations indicated that a tank depth of approximately 6.1m deep below ground level would maximise the footprint available within NI Water’s site, and enabled a gravity return into the existing pumping station.

[caption id="attachment_12588" align="alignnone" width="1200"] (left) CFA cased auger secant piling cofferdam installation and (right) excavation of storm tank exposing secant piles - Courtesy of DAWSON-WAM Ltd[/caption]

Temporary works

Several temporary works solutions were considered to facilitate the tank construction including the use of a steel sheet pile cofferdam. This solution was omitted for several reasons but predominately because of the vibration risk to the surrounding properties and being unable to achieve sufficient embedment into the mudstone bedrock.

For the solution, DAWSON-WAM Ltd developed a secant piled cofferdam design which deployed a hard/hard continuous flight auger with reinforcement cages in every male pile (second pile). Utilising a double auger drive motor, it was capable of coring into the mudstone while maintaining pile verticality.

Working with piling design partner, Cassidy Geotechnical Ltd, a 450mm diameter secant piled cofferdam with 9m long piles was developed as the most viable solution. This was further valued engineered by incorporating the piles as part of the permanent works and introducing a single face, in situ, internal reinforced concrete lining to form the internal wall of the tank. Through design analysis, it was identified in the temporary condition the secant piled cofferdam would require propping during excavation, but this was omitted by collaboration between the piling designer and contractor by introducing a reinforced capping beam that was poured prior to excavation which reduced the deflection enough during excavation to eliminate the requirement of the temporary prop. The capping beam would also provide support the permanent in-site reinforced roof structure.

Pre-construction structural surveys were undertaken on all the surrounding properties and vibration monitors were installed at two locations adjacent to the site during piling operations. The secant method proved a success as the piles were installed to the designed length without triggering any vibration limits and when excavated provided a watertight cofferdam.

[caption id="attachment_12586" align="alignnone" width="1200"] Aerial view of site during storm tank reinforced concrete base slab construction - Courtesy of DAWSON-WAM Ltd[/caption]

Lower Woodburn WwPS: Supply chain - key participants

• Main contractor: Murphy/DAWSON-WAM Ltd JV
• Project management & professional services: Tetra Tech
• Civils design: RPS
• Secant piling design: Cassidy Geotechnical Ltd
• Civils contractor: DAWSON-WAM Ltd
• MEICA: J Murphy & Sons
• Shuttering contractor: Form Builders
• Mixer: Xylem Water Solutions
• Static screen: Eliquo Hydrok Ltd
• Washwater booster pumpset: Dutypoint Ltd
• Combisafe fall protection: Honeywell

Construction

Due to the storage requirements of the tank, and given the small site footprint, there was not adequate space on the site to undertake the construction works safely. Therefore, a detailed construction phasing plan was developed by the contractor at the ECI stage which incorporated keeping the existing pumping station in service during construction. This plan identified that by temporarily incorporating the existing access lane (leading to the beach and neighbouring property) as part of the site, construction could be undertaken. An agreement was reached with the owners of the house - who had just recently purchased the property – to delay their relocation to the dwelling until works had been completed, and the council agreed to temporarily close access to the strand to aid construction. [caption id="attachment_12587" align="alignnone" width="1200"] Single sided reinforced concrete wall pours against secant piles to form internal tank walls - Courtesy of DAWSON-WAM Ltd[/caption]

Consultations were carried out through NI Water’s internal communications team and members of the project management team, and included public drop-in meetings and approximately 100 letter drops.

Due to site constraints, the establishment of an off-site construction compound also facilitated just in time delivery of the construction materials to the site, which maximized the small amount of on-site space and reduced site congestion.

MEICA installation included a single Xylem Water Solutions mixer to prevent silt build up in the tank, an Eliquo Hydrok 6mm screen was fitted on the existing 315mm emergency overflow pipe from the pumping station and a new 6 bar washwater booster pumpset was commissioned on site for maintenance of the screen.

Site finishes included a new perimeter fence, EMS area and a wildflower mix was used to finish final surfaces to enhance biodiversity.

Works programme

Construction works commenced on site in August 2023 and were completed on programme and within budget by late April 2024.

The collaborative approach of the project team effectively delivered a solution to improve the water quality of Belfast Lough, supporting NI Water’s agenda to Protect, Enhance and Grow through the delivery of the Living with Water Programme.

St Augustines Road is a busy thoroughfare in Chesterfield linking the town to Matlock and the south-eastern edge of the Peak District. Lying within the Yorkshire Water (YW) waste catchment area, the road is steep in its nature with gradients often exceeding 7%. Due to the local hydraulic catchment and topography, during heavy rainfall events flooding occurred at the road’s intersection with Derby Road (A6). The flooding lifted manhole covers from their frames which caused significant damage to the highway. It also caused internal and external damage to the surrounding properties. To combat this, mitigation was carried out by Yorkshire Water and Chesterfield Borough Council (CBC), including the implementation of mitigation measures such as non-return air bricks and flood gates, however, this had limited success. CBC also commissioned the construction of a detention basin to the rear of the flooding properties; but again with limited success.

Hydraulic study

As a result of the mitigation’s limited success, a hydraulic study was commissioned by Yorkshire Water. This confirmed the root cause of the flooding was hydraulic overloading of both the foul and surface water sewers. A notional solution was developed by Yorkshire Water which included the upsizing of these sewers, with some form of temporary storage for the foul system to retain flows in a rainfall event. Later iterations of the hydraulic model revealed that improvements to the surface water network would in fact also reduce the frequency of flooding on the foul water network due to interactions between the two systems during storm conditions. In light of this information, in May 2021 Yorkshire Water secured funding to implement a series of significant improvements on St Augustines Road.

Preferred solution

The preferred solution included the construction of a new outfall into the River Rother and the transfer of all flows from the surface water sewer in Derby Road. This would provide a much greater flooding resilience in the existing system. The existing 225mm surface water sewer was to be upsized to Ø750mm along its 320m length. It would then be diverted via a tunnel underneath Derby Road into the wetland area immediately adjacent into the River Rother. Here, a new outfall would be constructed to discharge the surface water sewer into the river network. [caption id="" align="alignnone" width="1200"] (left) The preferred option and (right) flooding on St Augustines Road - Courtesy of Galliford Try[/caption]

Constraints and engineering challenges

The preferred solution progressed to detailed design with several technical challenges to be overcome. Several of these challenges were exacerbated by the need to construct the outfall into the River Rother outside of the salmonoid spawning season. An Environment Agency (EA) bespoke Flood Risk Assessment Permit, (FRAP) was required to construct the outfall and all in-channel activities were required to be undertaken before 1 October 2021 in order to mitigate the impact on any spawning salmon. A significant flow rate of up to 0.89m³/s would be discharged into the river during storm events and river scour was a concern. In order to combat the scour on the riverbed and riverbanks, there was a requirement to reduce the velocity of the flow before it interacted with the River Rother, causing excessive turbulence. To achieve this, box culverts were selected due their unique cross section and significant channel width. This slowed velocities down to near 1m/s, which is widely regarded to have negligible effects on turbulence and thus reducing scour. The headwall was also designed to allow flows to enter the River Rother at a 45° angle, as per EA Standard Rules SR2015 guidelines, further reducing the risk of scour on the opposite riverbank and riverbed.

Outfall design

This design however did not come without complications, such as the need to carry out the structural design, build and installation of a bespoke outfall within a few months. BSP Consulting was commissioned to carry out the structural design of the outfall which was to be precast, reducing the need for in situ concrete handling adjacent to the sensitive receptor. Off-site build (OSB) also reduced the construction programme and allowed the headwall to be manufactured concurrently within the determination period of the FRAP. [caption id="" align="alignnone" width="1200"] Outfall under construction - Courtesy of Galliford Try[/caption] Poor ground and underfoot conditions adjacent to the River Rother proved too risky and problematic for traditional mobile crane lifts, so all lifts of the precast sections were to be undertaken with a tracked excavator. This further constrained the design and restricted the maximum mass of each section to 13 tonnes. Consequently, in its final iteration the precast headwall was designed in two sections and was combined with an in situ toe and infill slab/wall which completed the structural design.

Design

During the design stages, extensive ground investigation was carried out. This identified water bearing sand and gravel strata adjacent to the river which transitioned to mudstones and clays towards St Augustines Road. It was this change in strata which posed a significant challenge for the no-dig section of the work beneath Derby Road. This section of work was critical as it linked the outfall to the upsizing work on St Augustines Road. Discussions were held with Active Tunnelling Ltd who proposed the use of the guided auger bore technique to install the sewer in this transitional strata. Although initially designed as a 750mm diameter sewer, this was reduced to 600mm which was the maximum diameter which could be accommodated by the tunnelling contractors. Hydraulic modelling was undertaken by RES Environmental to confirm the viability of this proposal and in order to compensate for this localised throttling, the pipeline had to be upsized locally to 900mm immediately upstream of the tunnel. The alignment of the tunnel was severely limited due to proximity of existing buildings on Derby Road. BSP Consulting was again called into action as pre-condition surveys and settlement calculations were commissioned in order to understand the impact of the tunnelling works on these at-risk properties. [caption id="" align="alignnone" width="1200"] (left) Installation of drive pit on St Augustines Road, (middle) guided auger bore rig, and (right) the transition between 600mm and culverts - Courtesy of Galliford Try[/caption] Settlement calculations were undertaken using the “Guide to best practice for the installation of pipe jacks and microtunnels technique” and a predicted settlement of <3mm was deemed to be acceptable. The pre-condition surveys highlighted only minor cosmetic issues and were used as reference to the state of repair of the properties prior to construction work commencing. The majority of the civil design was undertaken in house. This element included a design coordination role combining and processing the information received from the consultants and stakeholders. An example of the numerous stakeholders which were consulted is that of the Coal Authority. Initial searches indicated the presence of historical coal workings. More detail was sought and records indicated the presence of an adit within 20m of the proposed pipeline. A coal permit was sought from the Coal Authority and the design progressed whilst this was processed.

St Augustines Road (Chesterfield) Flood Alleviation: Supply chain: key participants

• Principal contractor & civils design: Galliford Try
• Temporary works & headwall design: BSP Consulting
• Hydraulic modelling: RES Environmental Ltd
• Ecological consultants: Middlemarch Environmental
• Traffic management: Amberon Traffic Management
• GRP & utility mapping: Cloud Conversion
• Ground investigation: Grange Geo Consulting Ltd
• Pipelaying & outfall construction: Arthur Civil Engineering
• Trenchless installation: Active Tunnelling Ltd
• Cofferdam design & supply: National Trench Safety UK
• ‘EndSafe’ safety barrier: Groundforce Shorco
• Cofferdam installation: Ivor King Piling
• Dewatering: Alba De-Watering Services Ltd
• Precast concrete: Bonna Sabla
• CCTV, CIPP & cleansing: CPS Drainage Solutions
• Handrailing & grille design: Galliford Try (Fabrications)
• Silt curtain: Murlac Land and Marine Environmental Solutions
• Pumping: Selwood Ltd
• Ridgistorm pipes: Polypipe Civils
• Pipe & flap valve supplier: Keyline Civils Specialists
• Landscaping & site clearance: Landscape Maintenance Services Ltd
• Reinstatement of traffic signals & street furniture: Swarco
• Aggregates & muck away: JC Balls & Sons
• Brickwork: Derwent Joinery & Construction
• Gates & fencing: SFC (Midlands) Limited
• Surfacing: BDS (Yorkshire) Ltd

[caption id="" align="alignnone" width="1200"] Service detection - Courtesy of Galliford Try[/caption]

Installation

The Environment Agency granted the FRAP on 13 August 2021, and the detailed design was finalised and submitted to Yorkshire Water’s technical approval committee on 16 August 2021. Work began on site a few days later, leaving just six weeks to construct the headwall and cease all in-channel activities as per the conditions of the FRAP. Work commenced immediately on the enabling works, with particular attention paid to the temporary works. Haul road To provide drainage to the waterlogged land adjacent to the River Rother, a land drainage scheme was commissioned, designed and installed by Lincolnshire Drainage. This provided a more suitable foundation on which to install the haul road and working laydown areas. The haul road was designed by BSP Consulting and was of substantial construction given the underlying strata, at 700mm thickness with geotextile layers to improve longevity. Ground dewatering After the installation of the haul road, wellpoint dewatering was installed around the outfall area and along the proposed culvert alignment. The dewatering was installed by Alba De-Watering Services Ltd at 1.5m centres. However, due to the underlying mudstone, each well had to be pre-augered and was then installed using the more traditional spout method to ensure that a reasonable well depth was achieved to adequately dewater the working area. A silt curtain was supplied and installed by Murlac Land and Marine Environmental Solutions prior to the channel works to minimise silt and sediment damage downstream from the works. It proved to be very effective during the cofferdam installation. [caption id="" align="alignnone" width="1200"] Cofferdam and silt curtain - Courtesy of Galliford Try[/caption] Cofferdam A cofferdam was required to retain the River Rother and this was designed and supplied by National Trench Safety UK and installed by the primary subcontractor Arthur Civil Engineering. The cofferdam was unique in design in that it utilised Larsson piles in a trapezium shape (plan) to minimise over-dig yet still accommodate the headwall and first culvert. The cofferdam was installed by Ivor King Piling using a 35t excavator with a Movax side grip pile driver attachment. The cofferdam was effective but 4” end suction pumps supplied by Selwood Ltd were still required to maintain the cofferdam and keep water levels low. Headwall installation Once the cofferdam was watertight, the headwall toe was cast in situ, after which the precast headwall arrived in two sections as planned. It was lifted into position by Arthur Civil Engineering using a 50t tracked excavator. An administrative issue led to the need to negotiate a 2-week FRAP extension with the EA and the headwall was completed and the cofferdam removed by 14 October in accordance with the conditions of the revised FRAP. Pipeline works Construction works on the remainder of the project continued in March 2022. A temporary one-way system was implemented to provide a safe working area to install the remainder of the pipeline with open cut works commencing on 14 March. The open cut works progressed steadily with over 100 services safely crossed. The later spring and summer months provided excellent conditions for online sewer relay with drought-like conditions, culminating in hosepipe bans for the majority of the UK. [caption id="" align="alignnone" width="1200"] Open cut pipelaying - Courtesy of Galliford Try[/caption] Prior to the tunnelling works commencing, concern was raised over the proximity of the tunnel as it crossed under a 225mm foul water sewer in Derby Road. With a clearance of just 300mm, the existing foul sewer was inspected by CPS Drainage Solutions using CCTV and a precautionary 5m cured-in-place patch liner was installed to strengthen the sewer prior to the guided auger bore operation. No-dig works commenced in early June and progressed well without incident and were completed towards the end of the summer. No settlement was detected during the installation and no visual deformations were reported from the nearby structures. Open cut works continued late into the summer with flooding benefits delivered by 5 October. Significant rainfall events have since occurred and the feedback from residents previously affected by the flooding has been positive.

Off-site build

Off-site build was utilised heavily without which it would not have possible to abide by the conditions set in the FRAP and achieve the tight programme when constructing the outfall into the River Rother. The headwall and culvert manhole cover slabs were precast by Galliford Try off site and the hand railing and grille were fabricated by Galliford Try (Fabrications). The flap-valve was fabricated off-site by Keyline Civils Specialists. [caption id="" align="alignnone" width="1200"] 900mm Ridgistorm pipe from Polypipe Civils ready for installation - Courtesy of Galliford Try[/caption]

Public engagement & community impact

Throughout the project customer engagement was a priority for Galliford Try and Yorkshire Water. Although COVID-19 curtailed some of the engagement meetings which were planned at the start of the project, regular communication with residents of St Augustines Road and surrounding area took place. A significant quantity of the communication was made to the local residents Facebook group, which provided timely and accurate updates on progress. Traditional letters were also posted as back up and for those who didn’t have access to Yorkshire Water’s online updates. Towards the end of the project volunteers from Galliford Try and Yorkshire Water spent a day helping out at the local food bank on St Augustines Road and made a donation of £500 to assist with their ongoing charitable cause.

Project success

The project is considered huge a success for Galliford Try and is the largest single project completed on the Infrastructure Framework for Yorkshire Water within the AMP7 period.

West Hallington Reservoir is a Northumbrian Water Group (NWG) asset situated 1km to the northeast of the village of Colwell in the Tynedale district of Northumberland. The reservoir was constructed between 1884 and 1890 for Newcastle & Gateshead Water Company for the purpose of municipal water supply. The reservoir was built adjacent to the earlier East Hallington Reservoir and operates as a non-impounding structure. It has a maximum depth of 12.1m, a capacity of 3.3Mm³ and a surface area of 50 hectares at the Full Supply Level (FSL) of 155.1m AOD.

Scope of works

An inspection carried out under Section 10 of the Reservoirs Act 1975, identified that the reservoir had insufficient drawdown capability, and thus needed to be increased such that the reservoir level can be drawn down by 4.8m in 8 days. Mott MacDonald Bentley (MMB) developed a design solution which utilises two 700mm diameter siphons on the west embankment to operate at a combined flow rate of 3.2m³/s. The new siphon pipeline, alongside a temporary pump arrangement and the existing scour pipes, will meet the required drawdown capacity. The siphon flows will be conveyed across a stretch of adjacent field which NWG has purchased. The flows will then either percolate to ground or gravitate to the Coal Burn and eventually to the River North Tyne.

Detailed design and engineering challenges

Hydraulic analysis MMB has designed and constructed over 25 siphons for various water companies in the UK. The siphon arrangement was analysed using an in-house design spreadsheet, and demonstrated that the siphon would break when the water level dropped 3m below top water level which was deemed acceptable. The siphon location was moved to the west embankment due to concerns about stability and leakage through the south embankment (the original proposed location of the siphon) and because it was easier to access this area of the site. [caption id="" align="alignnone" width="1200"] Summary chart from the reservoir drawdown hydraulic analysis - Courtesy of MMB[/caption] Pipes and thrust restraint Hoops stresses within the DN700 mild steel siphon pipes were checked and a DWI Regulation 31 coating specified. The stainless steel pipe supports were analysed for bending and shear and concrete thrust blocks were designed for the system test pressures. The pipe wall thickness was reduced from 10mm to 8mm as an efficiency. The pipework arrangement drawing was uploaded into the federated model in BIM360 to check for clashes and when installed on site was found to fit perfectly. Ground investigation and geotechnical analysis Following the original construction of West Hallington Reservoir (between 1884 and 1890) the Resident Engineer Charles Henzell wrote a report to the Institution of Civil Engineers describing the works and detailing a number of challenges which included:

• Areas of soft book leaf marl overlaying harder boulder clays on the route of the embankment needed to be excavated.
• Embankments sinking or slipping and the toe of the slope reinforced with concrete walls, sheet piles and additional stepped embankments.
• Springs in the vicinity of the works.

[caption id="" align="alignnone" width="1200"] (left) Cable percussive borehole rig on the crest of the embankment and (right) scaffold decked area required for rotary bore works on the slope - Courtesy of MMB[/caption] A desk study and intrusive in situ ground investigation (cable percussive and rotary bored boreholes, standard penetration tests (SPTs), piezo installation, lab testing) were carried out to classify and determine the thickness of the superficial and solid geology along with groundwater levels, identify any contaminated or aggressive ground and determine the engineering properties of the native soils. Slope stability and ground bearing pressure checks were carried out to ensure large plant would not compromise the stability of the embankments. The clay core and embankment material of the Pennine-type dam was found to be very silty and exhibit poor engineering qualities and so was disposed of, and a source of good quality puddle clay and Series 600-2C material was found locally at a brick factory in Birtley, Gateshead. This was installed in compliance with the test and inspection plan (T&IP) to a good standard. Thankfully the three springs in the west embankment mentioned in the Henzell report were not encountered. [caption id="" align="alignnone" width="1200"] Extreme weather conditions during the construction stages: (left) winter 2022 and (right) flooding from Storm Babet (October 2023) - Courtesy of MMB[/caption] Temporary works A borehole at the toe of the embankment on the dry side indicated the presence of water-bearing sands and gravels and so interlocking sheet piles were specified that punched through the sands and gravels into the underlying clays and so cut off groundwater flows into the excavation. A Portadam supplied and installed by OnSite Central was employed to provide a dry area to construct the intake bay. The Portadam was erected in the wet by operatives wearing buoyancy suits, and then pumped out and an excellent seal was achieved. The Portadam also allowed plant access around the pipework whilst the stone pitching was reinstated. Flood risk assessment A flood risk assessment (FRA) was carried out to support the planning permission and, as due diligence, to ensure MMB would not flood a property by accident. The assessment looked at the extent of flooding during operation of the siphon along with fluvial, surface water, groundwater and finally reservoir flood risk due to an uncontrolled release (which of course the siphon is designed to help prevent). The FRA concluded that:

The maximum modelled flood extent from a drawdown of Hallington Reservoir is in agreement with the flow path identified by the Environment Agency in the long-term flood risk map and no communities are shown to be at risk due to the proposed drawdown. As periodic testing of the reservoir drawdown will occur during dry periods, it is not considered that a drawdown of West Hallington Reservoir at this location would increase flood risk elsewhere.

Synchro 4D MMB chose SYNCHRO 4D from Bentley Systems during the latter stages of the design process. SYNCHRO 4D is software that enables MSP/P6Programme to be linked with a Revit model and to run a construction simulation. [caption id="" align="alignnone" width="1200"] A screen dump from the SYNCHRO 4D simulation - Courtesy of MMB[/caption] The simulation highlighted a number of pinch points when materials must be delivered or when water levels must be lowered or raised and helped drive efficiencies. SYNCHRO 4D was also useful for explaining the construction process to the client, to site operatives and to visitors. Solar power The mechanical equipment associated with the siphon is housed within a high security kiosk. Due to the remote site location, the cost of a permanent power supply for the kiosk building services was prohibitive. A solar powered solution has been installed utilising roof mounted PV cells, DC/AC converter and battery storage. This sustainable solution provides sufficient energy for lighting and heating within the kiosk with negligible running costs.

West Hallington Reservoir: Supply chain - key participants

• Construction delivery partners: Mott MacDonald Bentley
• Feasibility study & ARPE: Stantec UK
• Ecological surveys: OS Ecology Ltd
• Temporary water barrier (Portadam): OnSite Central Ltd
• Large diameter pipes: Powerrun Pipe-Mech Ltd
• Electrical/instrumentation installation: Intelect UK
• Large diameter valves: AVK UK Ltd
• Large diameter valves: Glenfield Invicta
• Valve actuators: Varley Hydraulics Ltd
• Stonemasons: Stuart Marley Ltd
• Steelwork fabrication: AJ Engineering & Construction Services Ltd
• Steel fixing: D&N Reinforcements Ltd
• Kiosks: Technocover Ltd
• Bespoke steel access cover: EJ
• Solar panels: Green Leaf Engineering

[caption id="attachment_9915" align="alignnone" width="1200"] Solar panel display - Courtesy of MMB[/caption]

Construction: Problems, successes and lessons learned?

Intake bay The intake bay was located along the Small Burn which originally ran across the middle of the reservoir and was diverted into the adjacent Coal Burn in the 1880s via a DN375 clay gravity drain. The ground around the intake bay was found to be poor and was excavated down to competent sub-grade and reinstated with 340 tonnes of gabion stone and 60 tonnes of Type 1 capping material. As this area is normally under a 5m depth of water it was inaccessible during the ground investigation stage. Pressure testing As the DN700 pipes are fabricated from coated mild steel, a Type 1 pressure test (water loss method) was required. All bolts had been tightened using Hytorc© equipment and the torque readings recorded in the T&IP, but achieving a pass proved difficult due to sunny weather. A temperature rise on the above -ground pipework led to a pressure loss of more than 0.2 bar in the water filled pipe voiding the test on a number of occasions. Eventually cloud cover enabled a valid test; one of the only times a site team has prayed for the sun to go in! [caption id="" align="alignnone" width="1200"] Siphon intake bay constructed within a Portadam barrier supplied and installed by OnSite Central - Courtesy of MMB[/caption] Stone pitching Reinstatement of the original stone pitching (to resist wave action) took seven weeks longer than planned; mainly down to availability of stone masons. Productivity increased as the works progressed and the three man gang went from replacing 3 to 5m² per day. The stone pitching was reinstated to a good standard, but it was felt that the detailing around one of the pipe supports would be built differently next time. The 2,500m perimeter of the reservoir had been faced with stone pitching down to original ground level; installation of which in the 1880s without an excavator and selector grab must have been a job and a half! Ecology Prior to the construction works, NWG employed OS Ecology to undertake trapping for white clawed crayfish, a native species that is in decline due to the introduced North American signal crayfish. No white clawed crayfish were found within West Hallington Reservoir but three were trapped within the adjacent East Hallington Reservoir. A pied wagtail nest was spotted by eagle-eyed site workers next to the engine of an excavator. The excavator was withdrawn from service until the eggs had hatched and the fledglings left the nest. [caption id="" align="alignnone" width="1200"] Reinstatement of the stone pitching - Courtesy of MMB[/caption]

Testing, commissioning & conclusions

The siphon is designed to operate above a water level of 152.5m AOD and to self prime above a level of 154.1m AOD (TWL = 155.1m AOD) and so the All Reservoirs Panel Engineer (ARPE) requested that during the testing and commissioning stage, MMB should demonstrate the following:

• That the siphon can be primed between the water levels 152.5m AOD and 154.1m AOD.
• That the siphon can self-prime above a water level of 154.1m AOD.
• That the Full Supply Level of 155.1m AOD can be achieved.

The water level rate was restricted to 300mm/week and any seepage through the clay core will be monitored in future via a flow measurement chamber. During operation of the siphon, receiving watercourses and culverts were monitored at a number of locations to ensure scour or carriageway flooding were recorded and the test halted if required. The works were completed within budget (final NEC3 contract value £3.3m, £60k gain pot) and on time. The original compliance date of 31 September 2023 was extended to incorporate obtaining the Section 166 agreement and a new date of 30 June 2024 was agreed with the ARPE. [caption id="" align="alignnone" width="1200"] DN700 siphon pipework during installation - Courtesy of MMB[/caption]

Leaving a legacy

Northumbrian Water is very keen on creating community projects and “leaving a legacy”. MMB widened the access from the main road and provided three passing places as the access route is used by both residents and members of the fishing club.

Looking ahead

Northumbrian Water are investigating another 10 reservoir sites that potentially require siphons, and so Mott MacDonald Bentley hope to use the good practice and experience learned at West Hallington Reservoir on other schemes in the future. A Northumbrian Water Engineering Briefing Note for siphon design and construction is also underway, which captures all the learning and good practise and will sit alongside the newly published CIRIA 813 Siphons in Dams: Design, Installation, Operation, Management and Testing.

Keadby Terminal Assisted Outfall (TAO) is a strategically important asset, protecting homes, businesses and agricultural land from flooding in the Isle of Axholme. The TAO was originally constructed in 1939 and required upgrading and modernisation to ensure the continued operation of the pumping station as resilience and reliability had been lost due to age, obsolescence and general wear and tear, resulting in the need for a major investment. Keadby TAO is the outfall for the River Torne within the Isle of Axholme to the tidal River Trent. It originally comprised a gravity sluice which maintained water levels within the Isle of Axholme and discharged fluvial flow to the tidal Trent when tide levels permitted. High flows were pumped via six pumps, providing approximately 32m³/s discharge capacity when required.

Finding a viable solution

Previous business cases had failed to find a viable solution and detailed design work had developed plans for the project to completely renew the existing facility. However, multi-disciplinary teams from the Environment Agency, Arcadis, WSP and Galliford Try Binnies JV (GBJV) worked collaboratively to deliver value engineering achieving savings greater than £12m by re-using existing assets.

The new solution made a switch to fish-friendly electric pumps, avoiding the long-term management of fish and eel screening, and recommended refurbishing rather than replacing the station which would deliver environmental benefits, operational savings and a 90% carbon saving.

[caption id="attachment_14188" align="alignnone" width="1200"] Outfall construction (November 2021) - Courtesy of GBJV Ltd[/caption]

Project scope

The upgrade included:

1. A comprehensive heavy electrical system replacement.
2. Replacement of all pipework
3. Replacement of the entire control system.
4. The installation of six bespoke vertical lift pumps providing 19.2m³/s discharge capacity against a hydrostatic head of 3.7m when required.
5. A complete redesign of the outfall structure to provide three individual discharge cells which will allow safe access to the twelve (1.8m wide) gravity/pumped culverts and twelve 1.8m tidal flaps for inspection and maintenance purposes.

The project team assisted in the development of the strategic case for investment alongside an update to the Isle of Axholme Flood Risk Management Strategy, and worked closely with the strategy asset management and delivery group ensuring that stakeholder engagement was central to the delivery of the project.

Undertakings

GB JV Ltd, a joint venture between Binnies UK Ltd and Galliford Try, has been the sole engineering design and build provider for the Outline Business Case (OBC), Final Business Case (FBC), and design/construction stages. This ‘cradle to grave’ approach has provided many benefits including the complex construction sequence document (for the whole scheme) created during the FBC which considered severe site constraints and proved an invaluable guide during the design and build phase.

[caption id="attachment_14184" align="alignnone" width="1200"] Outfall construction constraints - Courtesy of GBJV Ltd[/caption]

Keadby Terminal Assisted Outfall Pumping Station: Supply chain - key participants

• Designer & delivery contractor: GB JV Ltd
  • Binnies UK Ltd
  • Galliford Try
• Client NEC project management: Turner & Townsend
• Client technical advisors: KGAL Consulting Engineers Ltd
• Client cost consultant: Arcadis
• Electrical enabling works: Fairfield Control Systems
• Sheet piling: Keltbray Ltd
• Mechanical installation: Alpha Plus Ltd
• Outfall construction: CPC Civils Ltd
• Electrical installation: Centrica
• MCC/PLC/HMI/SCADA & systems integration: Blackburn Starling Ltd
• Penstocks & flap valves: Glenfield Invicta
• Large knife gate valves: Ebro Valves Ltd
• Large knife gate valves: Stafsjö Valves
• Pumps: Pentair Fairbanks Nijhuis
• Stop logs: Aquatic Engineering Ltd
• HV switchgear & transformer: Midland Power Networks
• Standby generator: YorPower Ltd
• GRP kiosks: Industrial GRP Ltd

Constructability challenges

The design involved constructability challenges as the Environment Agency operational team could only allow the release of two pumps at any one time to ensure that the operational plant was never compromised in terms of readiness for flood resilience. It also involved innovative fish pass solutions as site constraints prevented the use of traditional fish ladders and passes.

Binnies’ design team provided a solution which allowed the retention of the existing structure while completely refurbishing the installation. This was achieved by replacing the existing ‘carbon-hungry’ diesel-driven centrifugal flow pumps with electrically driven permanent magnet motors and vertically mounted axial flow pumps within the existing building envelope.

[caption id="attachment_14186" align="alignnone" width="1200"] Aerial view of final pour - Courtesy of GBJV Ltd[/caption]

Given the age of the buildings on site, the initial concept design was for the complete demolition of the existing reinforced concrete structure to make space for a new installation. A solutions-focused team looked at plans differently and included extensive early supplier engagement with pump suppliers that crucially unlocked the re-use of the existing structure; a cornerstone of the significant cost and carbon savings. A complete refurbishment rather than the replacement of the existing delivered effective resilience to future climate change.

Fish and eel friendly pumps

The existing facility presented a barrier for fish and eel migration and The England and Wales Eels Regulations (2009) required the installation to be upgraded to allow migration of elvers past the facility.

Accordingly, the fish-friendly pumps allow fish to pass downstream safely when pumping and an innovative ‘Elver Mode’ was added into the software controlling the sluicing operation allowing elver access during their migration period and subsequently safe passage into the Three Rivers Catchment and the wider Isle of Axholme, with 224km of main river available to fish and eel passage.

Pentair Fairbanks Nijhuis, the pump supplier, developed a fish-friendly impeller design that ensures fish are not struck on entering the pump, by incorporating vanes to guide them unharmed through the system. The impeller design in conjunction with the variable speed drives ensures optimal efficiency at all times. Low running speeds also improve passage for fish and reduce noise and vibration.

[caption id="attachment_14183" align="alignnone" width="1200"] Pump hall - Courtesy of GBJV Ltd[/caption]

Operational access

Binnies’ design has also improved operational access to the culverts and flap valves which previously posed significant health and safety risks. Previously, some areas were accessible only by divers and other areas have never been accessed since the TAO was commissioned in 1939 as it was considered too dangerous.

The improved operational safe design has minimised ongoing maintenance difficulties.

Outcomes

Through collaboration with partners and the supply chain the team delivered the OBC and FBC stages to time and budget winning the Best Business Case of the Year at the Environment Agency’s ‘Flood and Coast Excellence Awards’ (2019). GB JV Ltd attained partial sectional completion of the flood defence asset at the end of April 2021 (four pumps operational and available for beneficial use), resulting in the Environment Agency attaining 80% of its stated outcome measures; the reduction in flood risk to 2484 of the total 3105 properties to be delivered by this project.

The remaining 621 properties were realised in November 2021 upon successful integration of the two remaining pumps. This was achieved despite COVID-19 disrupting construction work and storms in 2019 and in 2020 when the Environment Agency needed to operate the diesel pumps to reduce flood risk to the catchment, preventing construction work for approximately three months.

The former pumping station had six large and dangerously loud diesel engines (over 98 dBA when measured outside with the doors closed) and emitted large amounts of exhaust fumes. It was not safe to be inside the building with the pumps operating (even with ear defenders). The building is within 100m of residential properties so pumping activities disturbed the local community.

The new pumps are much quieter with the standby generator designed to be less than 60dBA at 1m. As the pumps can run 24 hours a day for several weeks at a time, the local community have greatly benefited from the removal of noise and air pollution.

[caption id="attachment_14187" align="alignnone" width="1200"] Outfall structure including fenders - Courtesy of GBJV Ltd[/caption]

The site is immediately adjacent to the Port of Keadby on the tidal River Trent. Close management was required with both PD Ports Ltd and ABP who had equipment within the working area, so the team worked with the port to respond to and work around vessel movements and port operations. A new river fender structure was designed and installed to reduce the risk of damage to the outfall structure from berthing vessels.

In conclusion, the carbon footprint was reduced by 90% throughout the project’s lifecycle using modern efficient fish-friendly electric pumps negating the need for a 3mm eel screen and reduced ongoing lifetime operational expenditure by the use of the existing structure.

When COVID-19 restrictions prevented foreign travel to the pump supplier in Holland, the team implemented remote attendance to witness the programme-critical Factory Acceptance Testing, allowing us to maintain programme. This also enabled a wider team to witness the testing and provided a learning opportunity for team members that would not otherwise have been able to attend. The approach continued with Design Workshops that avoided the need for the Design Team to physically attend site, without compromising the quality of work. The reduced travel also contributed to carbon reduction.

Awards

The project has won a number of awards including:

Environment Agency ‘Project Excellence Awards 2019

1. Winner Best Business Case of the Year at the Environment Agency’s ‘Flood and Coast Excellence Awards’.
2. Highly Commended Asset Management Project of the Year at the Environment Agency’s ‘Flood & Coast Excellence Awards’.

Institution of Civil Engineers, East Midlands Branch

1. Large Project Award 2023.
2. Sustainability Award 2023.

Dawlish is a traditional seaside town on the south coast of Devon, approximately 12 miles south of Exeter and is a popular holiday and day trip destination. South West Water (SWW) and Galliford Try’s objective was to ensure that the bathing water quality did not deteriorate within the next 20 years by ensuring that there are no more than two significant (>50m³) spills from the Brook Street CSO per bathing season by 31 March 2022. The proposed solution was to be achieved by providing 30m³ of offline, pumped return stormwater storage in the Manor House council office car park on Brook Street to improve spill performance at Meadow Road CSO in line with SWW/Environment Agency targets, in combination with the removal of surface water from the combined system; being delivered as a separate element of the overall project by another delivery partner.

Description

As part of the overall SWW objective to achieve less than two significant spills from Brook Street CSO per bathing water season, this project supplemented the scope from the infra delivery partner.

• Provision of 30m³ of unscreened, stormwater storage upstream of the Brook Street CSO, by the installation of a 4m diameter x 3m deep tank with duty pump return.
• Provide a flow control device (Hydro-Brake^®) to limit PFF to 10 l/s with an unrestricted bypass arrangement within an overflow chamber.
• Once the tank is full, additional flow will bypass the Hydro-Brake^® via a high-level weir.
• The agreed storage tank volume, flow control device and overflow weir levels have been derived from the Stantec UK modelling report and taken as the scheme output for the storage element. The final tank volume selection/spill performance of the promoted option has been selected at SWW IPC and has been operationally reviewed by SWW operations team, commissioning engineer and asset management.

[caption id="" align="alignnone" width="1200"] (left) Flow control chamber showing overflow weir and Hydro-Brake^® and (right) looking down into the 30m³ stormwater tank showing the float controls, pump, pump guides and sump - Courtesy of Galliford Try[/caption]

Project scope

• Site mobilisation and temporary works, to include road closure of Brook Street and suitable traffic management/diversion.
• Construct 4m diameter x 3m deep (internal) PCC shaft in Brook House car park.
• Construct 30m of connecting pipework and associated manholes to connect new tank to existing sewerage network.
• Construct PCC overflow chamber to include Hydro-Brake^®, spill weir and bypass weir.
• Install new single 5 l/s return pump and associated control cabinet, cabling, ducting, instrumentation and return rising main.
• Site reinstatement.

In-house capability

Galliford Try self-delivered this project, designing all civils and MEICA elements, as well as being the principal contractor and lead designer on the scheme. Galliford Try managed the site, direct labour, installation and commissioning, all within an incredibly tight programme (made tighter by severe storms).

Brook Street CSO: Supply chain - key participants

• Design & construction: Galliford Try
• Modelling: Stantec UK
• Shoring/temporary works: MGF Ltd
• Hydro-Brake^®: Hydro International (UK) Ltd
• Pumps: Xylem Water Solutions
• Access covers: Technocover Ltd
• Traffic Management: Amberon Ltd
• Fabrications: Thorne Fabrication Limited
• Electrical items: Edmundson Electrical Ltd
• Diamond drilling works: 24-7 Diamond Drilling & Sawing Services Ltd
• Civils items: Keyline Civils Specialist
• Plant hire: Gap Group Ltd
• Aggregates: Hanson Aggregates
• Construction Products: Jewson
• Plant hire: Plantforce Rentals Ltd
• Crane hire: Spence Crane Hire Ltd
• Tarmac surfacing: Devon Tarmasters

Planning & collaboration

The site footprint consisted of a small section of The Manor House carpark (which belongs to the Dawlish Council offices), a 20m x 20m area in which Galliford Try managed to contain their welfare facilities and site office, working area and laydown of plant, equipment and materials, a 5.5m diameter, 5m deep shaft and two flow controlling combined sewer inspection chambers with a pumped return, MCC cabinet, two weirs and a Hydro-Brake^®. Galliford Try also had to construct two inlet and outlet inspection chambers within a very tight street with multiple existing services. They accomplished this using road closure notices and daily communication with the residents of the local area. During the works, Galliford Try had to excavate the shaft using a 14t wheeled excavator, an MGF TWD of sheets and frames, 5m away from a listed building. SWW estates team carried out a structural survey on the building before they started, and no evidence of any movement was detected associated with the works. Galliford Try mucked away 225t of spoil from the shaft while working closely with another SWW contractor during the construction phase to reduce their footprint further, they arranged joint use of their muck away process meaning reduced footprint of their working area and around half the amount of muck wagons going through Dawlish town centre. Galliford Try also required a 40t mobile crane to lift in the large pre-cast concrete shaft section which required an intricate lift plan and careful logistics. To connect the stormwater storage tank and flow control inspection chambers into the existing network Galliford Try had to accurately survey the existing sewer and optimise the inlet point inspection chamber missing two water mains, a gas main and a surface water drain. To return the flows to the existing sewer they had to precisely know at which point their tank would spill to contain the required volume of storage. The existing sewers in Brook Street were incredibly old and fragile, they had to delicately expose the existing sewer and break in without cracking the upstream and downstream section of pipework. Galliford Try managed to install the chambers using the minimum amount of new pipework. [caption id="" align="alignnone" width="1200"] (left) All weather kiosk constructed off-site, containing the alarms, telemetry, pump return and mains in-comer and (right) access cover within the Manor House Car Park into the 30m³ stormwater tank all completed day before handover - Courtesy of Galliford Try[/caption]

Off-site build

• All four new inspection chambers were concreted from PCC sections and all sections of the shaft were also PCC, reducing installation time and costs.
• The kiosk was constructed off-site by Galliford Try and installed by their panel shop and then commissioned three days before the commissioning, date reducing storage and on-site presence.

Challenges & risks

Galliford Try used a vacuum excavator (the fastest and most economical method of excavation) to dig around existing services. Benefits include: reducing reinstatement costs, eliminating the hazard of utility strikes, and reducing the amount of manpower/equipment required. The construction challenges:

• Small footprint/working area.
• Existing services in road.
• Managing access for stakeholders.
• Temporary works - excavating deep shaft adjacent to existing buildings.

Completion & carbon savings

Galliford Try completed this work following on from a previous project to remove the surface water run-off from the local catchment; greatly reducing the combined sewer flow rate and reduced the time the storm tank would fill, further reducing the risk of a spill during the bathing water season. The overall scheme (including the surface water separation) would at some point become carbon negative, due to reducing the combined sewer flow to the local STW and not paying the energy bill to treat the additional storm flow now going straight into the Brook. This reduces the amount of flow being stored and pumped back into the combined sewer on time. [caption id="" align="alignnone" width="1200"] Completed car park area day before handback - Courtesy of Galliford Try[/caption]

Community

Whilst road closures were in place, Galliford Try helped residents by assisting with taking their bins out to the new temporary location, as well as helping the older residents bring their shopping in.

Guildford Borough Council (GBC) are progressing with the development of approximately 41 hectares of brownfield land located along the west bank of the River Wey as it runs through central Guildford and north through Slyfield Industrial Estate. Guildford Sewage Treatment Works (STW) lies within this land and requires relocation in order to enable the development to proceed. This relocation forms a critical part of the Slyfield Area Regeneration Project, and will allow the delivery of up to 1,550 new homes, partially on land occupied by the existing STW, to enable local housing needs to be met.

The new sewage treatment works will be built to the north of Slyfield Industrial Estate on land that has been used for quarrying, and in turn used as a landfill site, and is currently largely unused. In addition to the construction of a new works, relocation of the works to this location will require the construction of a new transfer tunnel to divert existing incoming flows to the new site, and the construction of a new outfall to the River Wey. The existing STW must also be decommissioned to enable use by the Weyside Urban Village (WUV) Development.

Design flows

The new Guildford STW is designed for a population equivalent (PE) of 125,300, with room to expand the site in the future.

Design criteria	Flow rate
Dry weather flow (DWF)	322 l/s
Average flow	368 l/s
Flow to full treatment (FtFT)	746 l/s
IPS peak capacity	2,250 l/s

Location of new works

The site was formerly quarried for sand and gravel during the 1960s and 1970s, with the resultant voids filled with waste materials progressively moving from west to east (i.e., towards the River Wey), which continued until the late 1980s. There is no record relating to the exact nature of the fill materials. The site was understood to have taken household and commercial wastes, but not special or difficult wastes. Following this, the site has remained vacant and undeveloped. [caption id="" align="alignnone" width="1200"] (left) Location of new sewage treatment works and tunnel route and (right) Slyfield Regeneration Project area - Courtesy of Adams Hendry[/caption]

Transfer tunnel

The STW transfer tunnel will consist of a 1.4km long, 1500mm internal diameter, 1780mm outer diameter tunnel, made of precast concrete pipe, which will be installed by pipejacking (microtunnelling) method. Six shafts are required to facilitate its construction and will serve as temporary launch and reception shafts. Network modelling by Arcadis verified the designed flow velocities remain self-cleansing and provide for the flow rates required. A geotechnical interpretive report has been prepared in review of the ground conditions for the scheme as whole, and to ascertain suitability for shaft sinking and tunnelling. The principal geological stratification for the site consists of Made Ground overlain principally with River Terrace Deposits, with sections of overlapping Alluvium, overlaying the solid geology of firm to very stiff clay of the London Clay Formation (LCF). The selected method of microtunnelling was chosen as it allows long pipeline sections to built, and its suitability to the ground conditions within LCF at Guildford, while limiting the exposure to personnel to underground works, against traditional methods. The microtunnelling is operated from a central control cabin at ground level, and only limited personnel access into the tunnel is required to service the microtunnel boring machine during construction. A ground movement assessment has been prepared and modelled using XDisp software by Oasys. The analysis constitutive model is based on the O’Riley & New method. The method predicts ground movements resulting from ground loss which produces strain of the soil within the influence zone, and projects the settlement in a gaussian distribution curve.

Ground improvement & remediation

The project team has consolidated decades of site investigation data into a unified format and 3D model. The process involves first combining data within OpenGround software and applying scripted translation to a common description - for example having all London Clay called ‘London Clay’, rather than ‘LC’ or ‘Clay’. [caption id="" align="alignnone" width="1200"] RIC 1 and 2 pass areas - Courtesy of BAM Enpure Ltd JV[/caption] This combined dataset is then used in Leapfrog 3D geological modelling software, which applies intelligent interpolations, and highlights anomalies that can be quickly assessed and removed. The resultant 3D model has then been used to enhance communication in the engineering AutoDesk BIM 360 federated models, and in the proprietary Arcadis SmartAtlas GIS viewer. Rapid Impact Compaction (RIC) is a method of imparting energy to the ground using a repeated but relatively short drop-weight, where a base plate (or “puck”) is in constant contact with the ground. RIC is preferred over a larger drop-weight technique due to its more progressive and less disturbing impact on the ground it is being used on. The method is also faster and uses smaller scale setups. A challenge for the RIC method was whether the energy it imparts is sufficient to densify the ground, versus other higher energy drop-weight methods. A trial was deemed necessary to demonstrate the suitability of this method. RIC is proposed to be applied over the majority of the site with an expected densification of landfill down to approximately 4.5m. Areas where assets will not be supported on driven full depth piles will undertake a second pass of the RIC method, reducing the long-term settlement of the landfill material. The predicted reduced surface will be levelled as required by the infill of depressions with imported stone or other suitable material. Improvement of the existing ground by reinforcement through installation of Vertical Rigid Inclusions (VRI) method across the site. Supporting of level sensitive and heavier elements will be on precast driven piles founded below the landfill material. The combination of methods will be designed to achieve Thames Water’s settlement performance values over 40 years, and remain within the allowable values of 50mm and 100mm. This will reduce the need for full depth driven piling which would otherwise be required. Reduction of infiltration to the landfill materials by ensuring impermeable surfaces drain to the treatment works and are released at the outfall away from the site. Gas and leachate well monitoring is provided along the perimeter of the site to monitor any impact and provide leachate removal points if required. [caption id="" align="alignnone" width="1200"] (left) Rapid impact compaction rig and (right) pile under test - Courtesy of BAM Enpure Ltd JV[/caption]

Guildford STW Relocation Project: Supply chain - key participants

• Client: Thames Water
• Principal contractor & designer: BAM Enpure Ltd JV
• Client designer: Jacobs
• Civil designer: Arcadis
• Planning consultant: Adams Hendry Consulting Ltd
• Process & M&E: Enpure Ltd
• CFD analysis: The Fluid Group
• Physical modelling: Hydrotec Consultants Ltd
• Tunnel design: Joseph Gallagher Ltd
• Temporary works/shoring: MGF Ltd
• Precast structures: A-Consult Ltd
• Precast structures: FLI Precast Solutions
• Rapid impact compaction: Cofra Ltd
• Piling: Aarsleff Ground Engineering Ltd
• XDisp software: Oasys
• Controlled modulus columns: Vibro Menard Ltd
• Inlet works and PST odour covers: Power Plastics Ltd
• Inlet pumps: Grundfos
• Imported sludge Strainpress: Huber Technology
• Escalator screens: Longwood Engineering Company Ltd
• Grit removal plant: Jacopa
• PST scrapers: Colloide Engineering
• FST scrapers: Purac AB
• Odour control plant: Odour Services International Ltd
• FBDA grids and blowers: Xylem Water Solutions
• Sludge thickening & dewatering: Kent Stainless Ltd
• Cake Silo: Saxlund International
• Cloth pile filters: Eliquo Hydrok Ltd
• Polymer batching & dosing: Richard Alan Engineering
• Ferric plant: Colloide Engineering
• Tank logging system (WASP): JR Pridham Services Ltd
• Glass lined steel tanks: Balmoral Tanks Ltd
• Sludge tank mixing: Landia UK Ltd
• Pumps, mixers & flocculators: NOV
• Welfare building: M-AR Limited

Guildford STW - the new sewage treatment works

Inlet pumping station The new STW is being designed to maintain inflow from the external catchment ranging from dry weather to 1 in 100-year storm (plus climate change allowance) with future-proofing beyond the 2041 design horizon. [caption id="" align="alignnone" width="1200"] (left) inlet pumping station physical model and (right) inlet pump testing physical model - Courtesy of Hydrotec Consultants Ltd[/caption] The pump design has been developed to fulfil the project brief load requirements (future peak IPS capacity 2585l/s) and some flexibility either side of the requirements is available due to the standard capacity equipment design offered from the supply chain. The proposed arrangement now consists of a wet well 12.5m diameter, approximately 20m deep. A common inlet will be provided to dissipate any entrained air, and two selectable pumped sections with double isolation will be provided by wall mounted penstocks on full height walls. The pumping well will be constructed in two halves, hydraulically connected in normal operational mode however each half will be able to be isolated (using motorised penstocks) in accordance with specification requirements surrounding double isolation, to allow person entry into half of the wet well at a time. Each half of the wet well will have three pumps installed giving a capability to provide full flow to treatment from two pumps with a standby permanently installed. The pumping station was first modelled by computational fluid dynamics (CFD) to demonstrate the assumed flow patterns that will occur. The pumping station was then physically modelled to comply with the specification and process design requirements particularly with respect to distribution, mixing, settlement, turbulence, dispersion, and freeboard for the full range of flows that each structure is expected to receive. [caption id="" align="alignnone" width="1200"] Storm overflow physical models - Courtesy of Hydrotec Consultants Ltd[/caption] Inlet works The inlet works is being designed to receive all pumped flows from the inlet pumping station (including future peak IPS capacity 2585 l/s), inlet works drainage and screening liquor return flows. Flows will enter the inlet works and then separate across the four escalator screens inclined at an angle of 45°. Each screen will be provided with a local proprietary control panel. Each panel will incorporate a differential ultrasonic level control. An emergency bypass will be provided upstream of the screens to accommodate all four screens blinding at the same time. Three 1200mm wide duty screens will be required to screen flows from low flow up to 2585 l/s, screening 6mm in two directions. The inlet works and grit treatment plant will be covered with odour covers which will be connected to the inlet works and PST odour plant. The inlet works has been classified as a Zone 2 Area under the odour covers and all equipment beneath the odour covers will be ATEX rated. Storm tanks Storm flows from the inlet work storm separation chamber will flow into two storm tanks via a 1400mm above-ground pipe. Within the 1400mm pipe between the storm separation chamber and the storm tanks there will be a partial pipe MCERTs certified flow meter to measure the total flow into the storm tanks. During normal operation the blind tank will fill first, then the spill tank will fill. In the event that both tanks are full to capacity (and still in storm flows) the spill tank will overflow via a weir and the storm tank will discharge to river. [caption id="" align="alignnone" width="1200"] Elevated inlet works - Courtesy of BAM Enpure Ltd JV[/caption] The storm tanks contents will automatically return to the inlet pumping station via 700mm gravity pipe when the flow being pumped to the inlet works drops below a predetermined set point. The return flows will be controlled to approximately 0.5 DWF via an automated valve controlled by an electromagnetic flow meter located in the gravity main, returning to the common inlet well of the inlet pumping station. Storm tank cleaning will be a pumped system with the two pumps located external to the storm tanks. Storm tank cleaning will be initiated during the latter part of the storm tanks draining back to the inlet pumping station and will not operate when the tanks are filling. The pump discharge pipe work will include connections for flushing with washwater. Ferric storage & dosing Two 45m³ ferric storage tanks will be located in a concrete bund with the dosing skid packages and control system located within an adjacent kiosk. The bund will be capable of containing 110% of the one of the storage tanks plus 10% allowance for rainwater. A drive-in bunded area for chemical deliveries will be provided and an underground chemical interceptor will contain any local spillages, complete with a Castell interlocked 3-way valve to allow the bunded drainage area either to drain into the interceptor (during a chemical delivery) or to drain into the site drainage (normal operation). Chemical dosing points will be provided upstream of the PSTs, on the common outlet channel of the ASP, and upstream of the tertiary treatment plant. The dosing points will utilise sparge systems to ensure adequate distribution of chemicals into the channels. Odour control plant The odour plant is a 2-stage system. Stage 2 filters will discharge into an 18-metre-high odour stack. The plant has been designed to achieve a minimum removal efficiency of 98% of H2S and 99% removal of odour, based upon a total system duty of 8432 m³/hr (of which 4813 m³/hr is from the inlet works and 3619 m³/hr from the primary settlement tanks). Service water A potable water main supplies a service water break tank via a ballcock (float valve). The service water break tank has a working volume of approximately 30m³ which gives a nominal retention of approximately three hours at average throughput. Service water booster pumps draw water from the tank and transfer it to the service water main network where it feeds the polymer make-up system for all thickeners and dewatering belt presses. Primary settlement distribution chamber The distribution chamber has been CFD modelled to calculate the flow split to each downstream primary settlement tank from first principles, and assess whether the proposed design would deliver an equal flow split (+/-5%) for seven future flow scenarios. This required the inlet bellmouth to be turned down to improve flow splitting. [caption id="" align="alignnone" width="1200"] PST distribution chamber CFD models: (left) with four PSTs in service and (right) with six PSTs in service - Courtesy of The Fluid Group[/caption] Primary settlement Primary settlement consists of four 47.15m long by 10m wide tanks, with a depth ranging between 6.24m at the hopper end and 5.3m at the outlet length (equating to 1 in 50 floor slope). Each PST tank has three sludge hoppers at the inlet end. The outlet of the PST discharges into a common 1000mm wide outlet channel. From this outlet channel the 1000mm outlet pipe from the drop box combines with the 700mm RAS lines into an above-ground 1200mm pipe. Retractable odour covers will be provided across the PST tanks to allow for future inspection, cleaning and maintenance. The odour covers will be connected via ducting to the inlet works/primary settlement tank odour plant. All mechanical equipment installed below the covers will be constructed from 316 stainless steel or high performance plastics. The primary settlement tanks have been classified as a Zone 2 Area, and all electrical equipment beneath the odour covers will be ATEX rated. The design of the PST Tanks is based upon a maximum inflow of 3022m³/hr to all four in-service tanks, and a peak flow to each tank of 1007m³/hr with one PST out of service. In normal operation all four PST tanks will be in service. Within each PST tank a chain and flight scraper system will be provided which will utilise high-tech non-metallic materials as the main materials of construction. Scum from the primary tanks will gravitate via a rotating surface skimmer pipe to the above-ground primary settlement scum tank. The primary scum return pumps are progressive cavity pumps, operating in a duty standby configuration. The pumps are fixed speed pumps operating between a high and low level in the scum tank. The PST sludge hoppers will desludge based upon a SCADA-adjustable interval and duration. The primary desludge pumps are progressive cavity pumps, operating in a duty/standby configuration. The pumps are variable speed pumps operating to maintain a SCADA-adjustable flow set point. Once the effluent has flowed through each PST it will enter into a common 1000mm wide outlet channel. From this outlet channel flows transfer to the ASP distribution chamber via a drop box and 1000mm above-ground pipe. [caption id="" align="alignnone" width="1200"] ASP overview - Courtesy of BAM Enpure Ltd JV[/caption] Activated sludge plant The ASP plant will have a distribution chamber using the same general arrangement as the primary settlement distribution chamber. The activated sludge treatment tank consists of four ASP lanes. Each ASP lane has a selector tank followed by an anoxic zone and then the three aerobic zones. The main aeration lane will be fitted with aeration pipework and fine bubble diffusers. The aeration vessel is 80m long and 6.1m wide with a water depth of 7.05m. The total instantaneous maximum air demand of the ASP is 10,438 m³/h. The first third of the aerated fraction of each lane will contain 50% of the aeration capacity, the middle third 30%, and the final third 20%. Each aeration lane will have three passes, arranged as channels of 6.15m width. The air for the diffuser grids will be supplied from three ASP air blowers operating on a duty/assist/standby basis. The aeration system will be controlled by an ammonia based real time control (RTC) system utilising a feed forward ammonia load input and a feedback ammonia concentration. Once the effluent has flowed through each ASP lane it enters into a common 1000mm wide outlet channel. From this outlet channel flows transfer to the FST distribution chamber via a drop box and 1200mm above-ground pipe. [caption id="" align="alignnone" width="1200"] Anoxic and aeration zones flows - Courtesy of BAM Enpure Ltd JV[/caption] Final settlement tanks The FST plant will have a distribution chamber using the same general arrangement as the primary settlement distribution chamber. The four FSTs will consist of 32m diameter flat bottomed circular tanks with a side wall depth of 6.15m. The FST will separate solids from the flow via settlement, producing a clarified effluent which overflows a peripheral weir into a launder channel, before gravitating to the tertiary treatment distribution chamber via a 1000mm below-ground pipe. The combined clarified outlet flow from the four tanks will be monitored by a common flow meter, as well as suspended solids and phosphorus monitoring instrumentation, all provided on the common line to tertiary treatment. The design of the FSTs is based upon a maximum inflow to each tank of 1741m³/hr with one FST tank out of service. In normal operation all four FST tanks will be in service. Each tank will have a 700mm inlet pipe, which will flow into a central diffuser drum before entering the FST tank. Each tank features a 600mm outlet pipe fed via a peripheral launder channel, as well as a 400mm RAS gravity outlet pipe. Each of the four tanks features a full bridge rotating scraper system which will run continuously in normal operation. [caption id="" align="alignnone" width="1200"] 32m final settlement tanks & sludge return (RAS pumping station) - Courtesy of BAM Enpure Ltd JV[/caption] Sludge return (RAS/SAS pumping station) Sludge from each pair of FSTs will enter one of two RAS/SAS desludge chambers. The two chambers will be located side by side and will share a set of three duty/duty/standby RAS transfer pumps, which will recycle the settled sludge from both RAS/SAS desludge chambers to the activated sludge plant distribution chamber feed line. The variable speed centrifugal RAS transfer pumps will be capable of pumping 2200m³/hr at a maximum head of 8.9mhd. The pumps will operate between high and low level in the duty RAS/SAS desludging chamber, with both chambers provisioned. A pair of duty/standby SAS pumps is provided, which will facilitate wastage of surplus sludge, with each pump drawing from a single RAS/SAS desludge chamber. The fixed speed centrifugal pumps will transfer sludge to the SAS buffer tank. Control of the SAS pumps will be carried out either on a timer or based on the level in the SAS buffer tank, with the common discharge line featuring flow monitoring. Tertiary treatment Clarified effluent flow from the FSTs will pass to the tertiary ferric mixing and flocculation (post-precipitation) stage, where the option will exist to dose ferric sulphate solution into the flow in order to encourage flocculation of remaining solids. This stage will be split into two parallel streams, with flow from the FSTs initially entering via a single bellmouth inlet into a common inlet channel, before passing via twin overflow weirs into two rapid mix chambers, and subsequently overflowing into two flocculation chambers. The design of the rapid mixer chambers will ensure complete mixing (>95% CoV) of chemical in less than 4 seconds. Flow from the flocculation chambers will weir into a common outlet channel leading to a flow distribution chamber feeding the tertiary treatment stage. The distribution chamber has been CFD modelled to calculate the flow split to each downstream cloth pile filter from first principles, and assess whether the proposed design would deliver an equal flow split (+/-5%) for seven future flow scenarios. The tertiary treatment stage will comprise six cloth pile filters, which will operate with all filters in service - however the peak flow will be able to be treated with only five filters in service. The filters will be sized to accommodate a peak flow of 840 l/s, producing a final effluent of 20 mg/l TSS (95%ile) and 0.2mg/l TP (average). Bypass pipework is to be provided adjacent to the cloth pile filter structure, by which clarified effluent from the FSTs may bypass the tertiary ferric dosing/flocculation and cloth pile filter stage. Each of the six cloth pile filters will feature a central rotor drum onto which filter discs are attached. The rotor and discs will be fully submerged within the tank, with the discs supporting the physical cloth filter media. Flow entering the filters will flow from outside the discs through the cloth into the discs’ interior, with solids collecting on the external surface of the cloth pile, creating a headloss. As this headloss increases, the water level in the tank will rise and at a pre-set water level (as detected by an ultrasonic level sensor) a cleaning cycle will be initiated. [caption id="" align="alignnone" width="1200"] Tertiary treatment flocculation chambers: Eliquo Hydrok Mecana cloth pile filters - Courtesy of BAM Enpure Ltd JV[/caption] Filtered final effluent (FFE) from the inside of the filter disc and rotor drum assembly will pass forward through the unit via a low-level aperture to an outlet chamber, from which flow will weir into the common final effluent outfall chamber. An emergency overflow weir is to be provided, allowing unfiltered effluent from the filter tank (external to the discs) to weir over into the outlet chamber. Backwash liquors from the backwash pumps will exit the filters via a common manifold, draining by gravity to the grit plant liquor PS. Eight backwash pumps will be provided per machine, giving a total of 48 pumps across all six machines. The pumps will be submersible units placed within the tank, with each pump being dedicated to three filter discs. The backwash flow rate is rated for 10 l/s (subject to the installed pipework design/headloss). Each of the six filter tanks will see some solids settlement occurring (and potential accumulation of other debris), which will create a build-up of sludge in the bottom of the tank. This sludge will be periodically removed by a set of two duty/standby sludge pumps per machine (i.e. 12 sludge pumps installed in total). The discharge from each sludge pump will be connected to the common backwash manifold, leading to the grit plant liquor pumping station. Final effluent discharge Flows leaving the cloth pile filters will pass out from the main filter chamber via a low level aperture into a small outlet chamber, from which flows will weir over into the common outfall channel. The final effluent (FE) will flow under gravity through an FE flow measurement flume, with an ultrasonic instrument for measurement of final effluent flows discharged to outfall. The FE outfall channel will feature a pipe leading to the FE washwater system, with a further line branching off this pipe servings as the suction line for FE Meteorburst Analyser. On exiting the FE outfall channel, flows will enter a dedicated FE outfall main, which will drain by gravity, discharging to the river north of the STW site. [caption id="" align="alignnone" width="1200"] New Guildford STW: 3D model night-time overview - Courtesy of BAM Enpure Ltd JV[/caption]

Guildford STW - Sludge Treatment Plant

Sludge thickening equipment Primary sludge from the PSTs will be pumped via the primary sludge pumps to the primary sludge buffer tank, which will have a volume of 120m³ and will be sized to provide 4-hours of intermediate storage prior to thickening. A set of two duty/standby primary sludge thickener feed pumps will be provided to transfer sludge (at up to 7.0%DS) from the primary sludge buffer tank to the primary sludge gravity belt thickeners. Two primary sludge gravity belt thickeners (GBTs) will operate in a duty/standby configuration in normal operation, although the streams will be able to run as duty/assist if required. Each machine will be able to handle up to 51m³/h (771 kgDS/h) of primary sludge, at up to 7%DS achieving a solids capture rate of 95%. Surplus activated sludge (SAS) from the FSTs will be pumped via the SAS pumps to the surplus activated sludge (SAS) buffer tank, which will have a volume of 67m³ and has been sized to provide 4-hours of intermediate storage prior to thickening. A set of two duty/standby SAS thickener feed pumps will be provided, which will transfer sludge (at up to 2.0% DS) from the SAS buffer tank to the SAS gravity belt thickeners. Two SAS gravity belt thickeners (GBTs) will operate in a duty/standby configuration in normal operation but will be able to run as duty/assist if required. Each machine is sized to handle up to 55m³/h (332kgDS/h) of SAS, at up to 2%DS, achieving a solids capture rate of 95%. Sludge balancing/blending tanks Thickened primary sludge, surplus activated sludge, and screened sludge imports will be pumped to two thickened sludge blending tanks. The tanks will have a combined volume of 2150m³ (1075m³ each), and will be sized to provide a combined total of 4-days retention time prior to dewatering of the blended sludge. The tanks will operate in a duty/standby configuration, with either tank being available for selection as the duty unit for either filling or emptying. [caption id="" align="alignnone" width="1200"] Thickened sludge blending tanks - Courtesy of BAM Enpure Ltd JV[/caption] Each tank will feature separate inlet connections for the three types of sludge being received, with actuated valves and crossover lines allowing all sludges to be directed to either of the two tanks. The tanks will also feature outlet actuated valves, which will allow either tank to be isolated from the sludge belt press feed pumps downstream, thereby ensuring the pumps only draw from a single tank at any given time. A set of three duty/duty/standby sludge belt press feed pumps will be provided, which will transfer blended thickened and imported sludge from the thickened sludge blending tanks to the sludge belt presses. The pumps will be variable speed progressive cavity units, with a peak capacity of 30m³/h (i.e. 15m³/h per pump) at a maximum head of 48.5m, based on sludge of up to 7.0%DS. Dewatered cake leaving each belt press will enter a discharge cake hopper leading to the suction end of a sludge cake pump. Three pumps will be provided, with a single pump dedicated to a single machine; each pump will operate in conjunction with the associated belt press upstream. The pumps will be variable speed progressive cavity units, with a peak capacity of 7.4m³/h of sludge (up to 30%DS). The pumps will transfer the cake via individual discharge lines to the sludge cake silos, with manual discharge crossover facilities allowing sludge from the pumps to be directed to either of the two silos as required. Cake silo storage & discharge Dewatered sludge cake will be pumped to two truck-loading cake silos, with a combined storage capacity of 600m³, sized to provide 7-days of storage at average cake production. Each silo will feature a discharge screw conveyor (with a sliding frame ‘anti-bridging’ device) which will discharge the cake through a hydraulically operated slide gate valve. Odour control plant The odour plant will be a 2-stage system. Stage 2 filters will discharge into an 18-metre-high odour stack. The plant has been designed to achieve a minimum removal efficiency of 98% of H2S and 99% removal of odour, based upon a total system duty of 6629m³/hr, extracted from the various tanks and thickening/dewatering machines in the sludge plant area.

New STW outfall

Outfall route The outfall route from the new works site location was assessed thoroughly in the ITN 2 stage of the project’s life and from exiting towards the middle of the works site, proceeds north towards the River Wey. The route deviates initially to minimise any impact on significant trees that exist within the floodplain, and any impact on the band of vegetation around the perimeter of the site. Final effluent & stormwater pipeline The final effluent pipeline will be a 750mm diameter pipe. This will be situated above ground from the final effluent channel to the edge of the site, and will be supported by piles when within the treatment works site. The pipeline then proceeds below ground through the embankment (to be reinstated using clay stanks) and under the adjacent ditch via an inverted siphon arrangement. A jetting/access point will be provided just before the pipeline dives below ground, giving the facility to clean this arrangement and flush from a more direct point than the pipes exit from the final effluent channel. Air relief may also be provided at this point, subject to detailed design. The storm overflow pipeline will be a 1050mm diameter pipe. This will be situated below ground from the storm tanks to a sampling manhole at the edge of the site; supported by piles when within the treatment works site. The pipeline will then proceed from the manhole through the embankment (to be reinstated using clay stanks) and under the adjacent ditch via an inverted siphon arrangement. The sampling manhole will provide an ability to clean this arrangement. [caption id="" align="alignnone" width="1200"] (left) Outfall route plan - Courtesy of Bam Enpure Ltd JV, and (right) outfall low flow scenario CFD model - Courtesy of Arcadis[/caption] Outfall structure The outfall structure will consist of a reinforced concrete headwall, set back from the river channel, to be founded in the river terrace deposits and flared to direct flows downstream and minimise the impact of upstream debris from collecting. The headwall will have duckbill valves on the discharging pipes, and erosion control products utilised in the surrounding area to tie the headwall into the natural riverbank and bed. The erosion mattress employed following the headwall structure will provide a secondary point at which turbulent flows can dissipate energy without eroding the natural bed of the river. Where the headwall ties into the natural riverbank, the hard/soft interaction between headwall and natural soil will be reduced by the installation of Rock Rolls erosion control and willow stakes at the base, and the installation of a natural hydro-seeded CocoBN blanket or equivalent (to be seeded with local seed varieties) which will retain the slope shape until vegetation establishes and provides an effective natural erosion control. This will also provide a gradual slope to access down to the river.

Programme

The project obtained planning permission in October 2022, with construction works starting on site in July 2023. Detailed design of the civil, mechanical and electrical works are due for completion by December 2023. Construction of the works will be complete by August 2025, with commissioning due to commence in from April 2025 through to December 2025. Flows from the existing works will be diverted to the new works from December 2025 (10% flow) in a 7-stage commissioning process with 100% flows diverted to the new STW by March 2026. Following 100% flow diversion the existing STW will be de-commissioned ready for handover to Guildford Borough Council.

The Thames Tideway Tunnel Project is a 25 km (16 mi) combined sewer under construction under the tidal section of the River Thames as it runs through London. It will significantly increase the capacity of London’s sewerage system (large sections of which were built in the 19th century), and be able to capture, store and convey almost all the raw sewage and rainwater that currently overflows into the Thames Estuary. The Thames Tideway Tunnel will connect 34 of the capital’s most polluting combined sewer overflows (CSOs), transferring the foul water to the Lee Tunnel for onward delivery to Beckton STW. The completion of the Thames Tideway Tunnel scheme, currently anticipated for 2025, will significantly improve water quality capturing more than 95% of the spills that currently enter the river to the benefit of London’s almost 9 million residents.

Project summary and location

As part of works on the west section of the Thames Tideway Tunnel scheme, Barhale was contracted by the BAM Nuttall Morgan Sindall Balfour Beatty (BMB) joint venture, to construct a series of complex underground structures that would intercept the sewage overflows from the West Putney Storm Relief Sewer for transfer to the main Tideway tunnel beneath the River Thames. The site was located in Richmond, on the south side of the River Thames between Hammersmith Bridge and Putney Bridge, and adjacent to Barn Elms Sports Centre.

Scope of work

The scope of works included the construction of a 35m deep x 6m internal diameter drop shaft and a 215m long x 3m outside diameter connection tunnel. Surface level works include 6m deep reinforced concrete (RC) interception chambers, an RC culvert and an RC MEICA building, comprising an electrical intake room, telemetry room and control room for system alarms and automated actuated valves installed by the lead contractor. Barhale also built an 800m long, 4m wide access road, and installed additional sections to address Environment Agency and local authority requirements relating to existing flood defences and protected trees in the proximity of the works. The drop shaft’s construction required initial and primary spray concrete lining (SCL), with a secondary cast-in-place concrete lining to encase a new 26m long x 90mm diameter stainless steel vortex drop tube pipe. The shaft was constructed using steel fibre reinforced concrete, applied by an Oruga robotic spray machine as the excavation progressed in 1.2m increments. [caption id="" align="alignnone" width="1200"] (left) Excavation at the shaft base and (right) 2.5m ID tunnel excavator machine (TEM) being lowered into drive shaft - Courtesy of Barhale[/caption]

Detailed mapping and innovative control

Unlike most other Tideway sites, Barn Elms is comparatively small and the connection tunnel drive through the London Clay Formation had to be delivered from a relatively narrow 6m diameter shaft. More than two months were spent preparing a plan and mapping out the tunnel drive route to ensure the team did not encounter any unexpected problems during the drive. The requirement was that the tunnel excavator machine (TEM), which needed to be compatible with 3m diameter jacking pipes, would not have been recoverable from a standard reception shaft as the connection to the main tunnel was right under the River Thames. To address this challenge, Barhale collaborated closely with Tunnel Engineering Services (UK) Ltd to design and construct a TEM whose internal parts could be dismantled, removed and re-used after the project’s completion. The new TEM was brought to site without any engineering guiding systems so the team also needed to address the question of control. Drawing on Barhale’s extensive mining experience, traditional and digital equipment was installed on the machine to control the steer, roll and the pitching degree. Digital pressure gauges were also fitted to ensure that the limit on the jacking pressure was not exceeded. The TEM was steered by four pairs of hydraulic cylinders located between the hood and the machine body. Using information available on the geometry of the TEM and the jacking pipes, plus knowledge of the maximum allowable joint deflection of the jacking pipes, it was calculated that the steering cylinder should not be opened more than 20mm when all other cylinders are fully retracted. [caption id="" align="alignnone" width="1200"] (left) Barn Elms shaft under construction and (right) the 2.5m ID tunnel excavator machine (TEM) being lowered into drive shaft - Courtesy of Barhale[/caption] The TEM roll represents the machine’s rotation about its central axis and there was a continuous tendency to clockwise or anti-clockwise spin. To overcome the issue, inclinometers and laser targets were installed within the machine and used for base readings for roll calculation. Installing the bespoke guidance equipment, and in-detail mapping of every stage of the drive, ensured that the TEM was able to operate effectively and carry out the pipe jack successfully. A further challenge was the safe handling and maintenance of the 14 tonne, 3m diameter precast concrete jacking pipes. To address the issue, Barhale purchased a 20 tonne SWL welding roller to be used as a base stand so that the pipes could be rotated 360°. This solution enabled the team to inspect each pipe thoroughly without the need for access equipment and working at height. It also brought a huge benefit in the preparation of the inter-jack stations. Using this system, the inter-jack hydraulic cylinders could be installed around the circumference of the jacking pipes by the fitters from standing position. Overall, the use of the welding rollers saved the Barhale team time, eliminated the risk of working at heights, and reduced the number of lifting operations. This idea was nominated for a Tideway RightWay award.

Barn Elms CSO Connection Tunnel: Supply chain - key participants

• Main designer & contractor: BMB JV
• Delivery contractor: Barhale
• Reinforced concrete works/formwork: Flatley Construction
• Secondary lining shuttering: EFCO UK
• Spray concrete lining: Shotcrete Services Ltd
• Steel fibre reinforced concrete: CEMEX UK
• Tunnel boring machine: Tunnel Engineering Services (UK) Ltd
• Precast jacking pipe segments: Macrete (Ireland) Ltd
• Stainless steel vortex generator: BMB JV
• Stainless steel vortex drop tube pipe: BMB JV
• Precast concrete cover slabs/planks: Barhale
• Ventilation/odour control: Specialist Plant Associates
• Demolition robot: Brokk Global
• Robotic concrete spray machine: MEYCO Oruga

[caption id="" align="alignnone" width="1200"] Shaft cover slab under construction - Courtesy of Barhale[/caption]

Challenging tunnel construction

The tunnel was constructed using a pipe jacking technique in which the TEM was attached to a muck conveyor. The pipes were jacked continuously from the drive shaft as the excavation of the tunnel face progressed. This was a relatively large pipe and the team encountered a significant challenge about 30m into the pipe jack when small cracks were identified in nine of the eleven pipes that had been installed. The defective sections needed to be replaced. The two un-cracked pipe sections were at the rear of the pipe jack and had not entered the ground so were relatively easily removed, however, it remained necessary to remove the other nine sections. This would only be possible by breaking out the pipe sections. A recovery strategy was developed which was centred on the fabrication of a bespoke shield which could be jacked along the line of the existing cracked pipes. A special hood was devised which, as the shield moved forward, would pass above the damaged sections providing support to the tunnel roof. It would protect the workforce and equipment and allow work on the removal of the pipe sections to begin. A further consideration was that damage to the original concrete lead pipe meant it could no longer be used. It would have taken too long to recast a new concrete section so the steel shield was also designed so that it could be jacked in and used as a replacement. To start the recovery, each of the individual damaged concrete pipe sections was circumferentially and longitudinally cut into small sections. The breaking out was carried out by a Brokk demolition robot which moved down the existing pipework ahead of the shield. [caption id="" align="alignnone" width="1200"] (left) The probe drill in use, forward probing of the tunnel face before full excavation as part of the WPP and (right) temporary support from within the sacrificial outer skin of the TEM once the tunnel drive was completed - Courtesy of Barhale[/caption] Once each concrete section had been broken out, the Barhale team could move in and pull the concrete from the invert back to the pit bottom for removal. Positioning the Brokk in advance of the shield meant that the removal of the material was not impeded. Proceeding section by section, it took two and a half weeks to reach the TEM. Removal of the damaged lead pipe revealed that, under the pressure of sitting stationary in the ground for five months while the recovery strategy was devised and implemented, the tail skin of the machine had deformed more than anticipated. By restarting and moving the TEM followed by the recovery shield forward into virgin ground, the Barhale team was able to relieve the pressure on the tail skin. This permitted the removal of the hood and the docking of the steel lead pipe after which the drive could be restarted. Replacing the damaged reinforced concrete pipes created quite a challenge, but with the support of the project team, a pioneering solution to change the cracked pipes was developed. The recovery shield that Barhale developed not only enabled the damaged tunnel to be replaced but also enabled access to the TEM face where repairs and enhancements had to be conducted. Additionally, the purpose made tunnel shield was designed to allow complete dismantling upon completion of the tunnel drive, allowing the final connection to the main tunnel.

Secondary lining

The pipe jack was followed by a cast-in-place secondary liner to the pipe jack tunnel. Barhale worked with suppliers to design and procure an automated tunnel liner system that significantly improved the quality of the tunnel secondary liner. [caption id="" align="alignnone" width="1200"] Secondary lining shutter in place within the sacrificial TEM outer skin at the junction of the main Tideway central tunnel - Courtesy of Barhale[/caption] This consisted of a specialist automated tunnel formwork system that would ensure the tunnel secondary lining was constructed efficiently by pouring each section in one pass. This improved quality by reducing the number of construction joints compared to more traditional methods. As the equipment is fully automated, the hydraulic stop ends ensure a robust seal when pumping concrete. A mechanical travel system enabled quick set-up for each subsequent pour with minimal resources and it removed the need to manually strike and reconstruct formwork for each section of the lining.

Designed for life

The client required 120-year design life for the precast concrete jacking pipes. To achieve this, Barhale’s engineers worked closely with the client’s designer, materials engineers and precast experts to develop a detailed quality assurance system that included concrete mix trials and quality audits at our supplier’s facility. In addition, Barhale implemented a stringent quality control regime that covered the pre- and post-pouring of the pipes, curing, handling and transportation of the pipes. Carrying out the works required excellent stakeholder management throughout. Heavy plant deliveries were required to fall within stringent Tideway traffic management plans. Thorough planning and consultation with the supply chain, client representatives and third party stakeholders were also key for excellent HSE performance. Barhale supported BMB with meetings and workshops with asset protection teams, Thames Water, the Environmental Agency, the local authority and local council groups. [caption id="" align="alignnone" width="1200"] Shaft cover slab under construction - Courtesy of Barhale[/caption] Peter Leyton, Construction Director for the BAM Nuttall Morgan Sindall Balfour Beatty JV, praised the Barhale team for its hard work and effort in getting the TEM to the main tunnel connection:

“The resolution in the team, to come from where we were earlier and then to work through all the issues that were put in the way, is a huge testament. Also, the control of the movement in the main tunnel has been excellent and a great reflection on the efforts put in by all of the team.”

Guildford town and the surrounding residential areas are currently served by an existing sewage treatment works (STW) located adjacent to the River Wey, just north of the town centre. The land which this occupies has been identified as part of the Slyfield Area Regeneration Project (SARP), which includes the new Weyside Urban Village (WUV). This mixed-use regeneration project will include circa 1,500 new homes, new community facilities and 6,500m² of commercial space to improve the local area. Thames Water Utilities Ltd (TWUL) is working in partnership with Guildford Borough Council (GBC) to move the STW to a new location, which is not suitable for housing.

The location chosen is on the north-east extent of the existing Slyfield Industrial Estate and is on an area previously used as a landfill site. In August 2023 Water Projects published a case study which provided insight into the extensive pre-construction design for civil and process engineering elements of the project. This article delves into the construction of the project and provides an update on the significant progress made to date on site.

Site historical use

The site for the new STW is approximately 70,000m² and was previously used as a landfill site. It had originally been used as sand and gravel quarries and then subsequently used as landfill from the 1960s to the 1980s.

The landfill material is generally domestic and light industrial. It consists of black bag waste, timber, plastics, fabrics but contains fragments of asbestos and other materials.

The site was capped off with a granular layer in the early 1980s and has laid dormant ever since. The landfill depth varies across the site from between 2m and 6m thick. This then overlies an alluvium layer, sands and gravels and London Clay. The depth of the London Clay is approximately 10m below existing ground level.

[caption id="attachment_16131" align="alignnone" width="1200"] A screenshot of the 3D model used for design and construction - Courtesy of Bam Nuttall Ltd[/caption]

Civil engineering: Basis of the design

Typically, major structures within a sewage treatment works are built below or partially below ground level to maximise gravitational energy. The new Guildford STW differs in that the strategy is to deliberately design and build above ground as far as practicable.

This is to minimise the disturbance to, and removal of the existing landfill thereby reducing the impact on the environment and the cost of disposal.

Undertakings

After a period of Early Contractor Involvement, Thames Water awarded a contract to BAM-Enpure Joint Venture (BEJV) in 2021 to design, build and commission this significant new works and once operational, to de-commission the existing works.

BEJV is a non-integrated joint venture, formed specifically to combine the excellence of experienced civil engineering contractor Bam Nuttall Ltd and the process engineering/MEICA specialism of Enpure Ltd to deliver a turnkey solution for Thames Water. The workscope split is as follows:

Bam Nuttall Ltd: Civil engineering workscope

• Principal contractor
• Civil engineering design
• Ground stabilisation & piling
• Structures
• Underground pipework
• Tunnels, shafts and connections
• Buildings
• Outfall pipeline & structure
• Roads, fencing & general civil engineering works

[caption id="attachment_16143" align="alignnone" width="1200"] Construction of the high level inlet works structure - Courtesy of Bam Nuttall Ltd[/caption]

Enpure Ltd: Process engineering/MEICA workscope

• Principal designer
• MEICA design
• Process design
• Mechanical works
• Electrical works
• Control systems
• Commissioning

Decommissioning of the existing STW will also be undertaken by BEJV, in addition to working collaboratively with TWUL operational and networks teams to integrate existing TWUL operational assets into the new sewage treatment works.

Site trials

BEJV undertook a series of site trials in 2021/2022. The purpose of this was to investigate the proposed ground improvement techniques further in order to validate proposals and inform detailed design. There are a combination of techniques which interface with each other in a complex manner.

The trials also gave important information about potential effects to both leachate levels and ground gas within the existing landfill to ensure there would be no impact on environment, ecology and human health.

Four trial areas were chosen, to provide a variety of conditions and in each of these areas a set of boreholes was established at increasing offsets away from the trial areas. These boreholes contained monitors for both leachate levels and ground gas volumes and were monitored 24/7. In addition, noise and vibration monitoring was also undertaken. [caption id="attachment_16140" align="alignnone" width="1200"] (left) Rapid impact compaction underway and (right) a selection of site plant in use - Courtesy of Bam Nuttall Ltd[/caption]

The three techniques tested, and subsequently used within the works were as follows.

1. Rapid impact compaction (RIC): RIC was undertaken to remove the risk of long-term settlement to the landfill. Whilst the main structures would be supported on piles, this was to remove the risk of long term ground settlement underneath and around the structures. RIC is a method of imparting energy to the ground using a repeated but relatively short drop-weight, where a base plate is in constant contact with the ground. It is undertaken on a pre-determined grid pattern. At Guildford a 9T hammer was chosen, fitted to a 35T base machine and working on a 2.5m x 2.5m grid. Each ‘pass’ of the technique used 40 blows at each point.

Within the trial areas the technique was tested with 1, 2 or 3 passes (ie, 40, 80 or 120 blows per location). Cone penetrometer tests were used to measure the extent of deep level ground compaction before and after the trials. The continuous monitoring from the boreholes demonstrated that at each ‘thump’ of the ground there was no discernible change in leachate levels or ground gas volumes, and that there was no dispersal of leachate happening throughout the landfill.

From the trials it was determined that all areas of the site that are being built on should receive either 1 or 2 passes of RIC for the permanent works.

[caption id="attachment_16135" align="alignnone" width="1200"] (left) Driving precast concrete piles and (right) vertical rigid inclusion installation - Courtesy of Bam Nuttall Ltd[/caption] 2. Precast driven piling: The main structures and pipelines are founded on precast driven piles. These are end-bearing into the London Clay, approximately 10m below ground level. Precast driven piles were chosen as the solution as the operation does not bring any landfill to the surface. Various size piles were driven to various depths and tested with both dynamic and static tests. Borehole monitoring showed that there was no discernible change to leachate and ground gas levels and that there was no dispersal of leachate happening throughout the landfill.

3. Vertical rigid inclusions (VRI): VRIs are mass concrete piles which are used for lightly-loaded structures and areas such as roads and hardstandings. They are a displacement pile, with the surrounding soil pushed sideways by an auger. As the auger is withdrawn the resulting void is filled with concrete up to ground level. They therefore work by increasing the density of the ground. A granular layer is then installed above to remove any ‘hard spots’. Again, this technique does not bring landfill material to the surface. It was trialled in two areas and found to be suitable for use, without causing any effects to leachate levels or ground gases. It is therefore being used as part of the permanent works over the whole site wherever there are not piled structures.

The trials of all three techniques, together with the subsequent site testing proved that the design principles were correct, and gave information on how to refine the solutions further. It also demonstrated that the techniques would not cause any adverse environmental effects.

[caption id="attachment_16142" align="alignnone" width="1200"] (foreground) Ferric mixing and flocculation chambers and (background) concrete bases for sludge tanks - Courtesy of Bam Nuttall Ltd[/caption]

Guildford STW: Civils works supply chain - key participants

• Main contractor: Bam Nuttall Ltd
• Planning consultant: Adams Hendry Consulting Ltd
• Civil engineering design: Arcadis Group Ltd
• Boreholes & testing: Geo-Environmental Services Ltd
• Asbestos monitoring: Socotec Ltd
• Rapid impact compaction: Cofra Ltd (Boskalis Group)
• Vertical rigid inclusions: Foundation Piling Ltd
• Concrete piling: Aarsleff Ground Engineering Ltd
• Temporary works specialist: Mabey Hire Ltd
• Temporary works specialist: MGF Ltd
• Tunnels & shafts: Joseph Gallagher Ltd
• In situ concrete works: Kelly Formwork Ltd
• Precast concrete works: DJ Civils Ltd
• Precast concrete manufacturer: FLI Precast Solutions
• Precast concrete manufacturer: A-Consult Ltd
• Soil processing: Remediation Waste Management Services
• Concrete supplier: Heidelberg Materials
• Gas protection: PAGeo Contracting Ltd
• Crane supply: Delden Cranes

Enabling works

Enabling works took place during Spring 2023 and consisted of site clearance, setting up the site offices and welfare and establishing fencing. The whole site was then covered with a 100mm thick layer of imported granular material to provide further protection against the risk of asbestos within the existing capping layer.

A series of 20 further boreholes were installed around the perimeter of the site to enable daily monitoring of both leachate and ground gas within the landfill and to ensure that none was being mobilised by our works. Mobile monitoring stations for noise, dust, vibration and asbestos were all set up, which are monitored continuously throughout the works. Over half a million data points have already been captured.

[caption id="attachment_16138" align="alignnone" width="1200"] Piles and blinding concrete for the four final settlement tanks and (left) temporary ground support for pipelaying into the centre of the tanks - Courtesy of Bam Nuttall Ltd[/caption]

Guildford STW: MEICA works supply chain - key participants

• Main designer/contractor: Enpure Ltd
• CFD analysis: The Fluid Group
• Cake silo: Saxlund International
• PST scrapers & ferric dosing plant: Colloide
• Inlet works & PST odour covers: Power Plastics Ltd
• Grit remover plant: Jacopa
• Escalator screens: Longwood Engineering
• Imported sludge Strainpress: Huber Technology
• Sludge thickening & dewatering: Kent Stainless Ltd
• Odour control plant: Odour Services International Ltd
• FBDA grids & blowers: Xylem Water Solutions
• Cloth pile filters: Eliquo Hydrok Ltd
• Storm tank cleaning system: Sulzer Pumps Wastewater Ltd
• Tank logging system (WASP): JR Pridham Services Ltd
• Glass lined steel tanks: Balmoral Tanks Ltd
• Sludge tank mixing: Landia UK Ltd

Groundworks and foundations

The permanent works started in earnest in Summer 2023 and in August, rapid impact compaction was conducted over the site, with a combination of one or two passes, as informed by the results of the previous site trials. In total the ground was hit 360,000 times! Precast piles were then driven for the major structures and pipelines. These were all 250mm square piles and varied in length from 16m to 20m. A total number of 3,422 piles have been driven using three piling rigs.

Vertical rigid inclusion installation is currently underway, with a total of 1,700 out of 9,000 already completed. The works are sequenced to avoid clashes with other work areas, on what is a congested site. Following installation, each work area is left for the piles to cure for the following seven days, thus effectively sterilising large areas of the site at a time. This therefore requires detailed collaborative planning on all work fronts to ensure that the site can remain operational.

[caption id="attachment_16136" align="alignnone" width="1200"] Activated sludge plant precast walls erected, prior to pouring base slab and concrete stitches - Courtesy of Bam Nuttall Ltd[/caption]

Concrete structures

Of the 59 concrete structures, some are simple reinforced bases, however ‘the big 5’ are significant structures:

1. Inlet works.
2. Primary settlement tanks (PSTs).
3. Activated sludge plant (ASPs).
4. Final settlement tanks (FSTs).
5. Tertiary treatment plant (TTP).

Due to the repetitive nature of their construction, BEJV chose to construct the PST and the ASP using precast concrete techniques, which are then stitched together on site. The four FSTs and two storm tanks are also being constructed using precast panels which are then post-tensioned.

The inlet works and TTP are complex, bespoke structures with very little repetition and so are more suited to traditional in situ concrete construction. The inlet works is a particularly complex structure, and is the tallest structure on site at 13m above ground level and being formed of five separate elements.

As of August 2024, the inlet works concrete structure is half complete and the TTP structure underway. The PST precast walls are mostly in place and stitched together, with roof beams now being installed. Works have also started on the ASP walls.

Pipework to the FSTs is underway, which will then allow the construction of the bases to follow on. To date (August 2024), 4,000m³ out 14,000m³ of concrete has been poured, and 205 out of 454 precast wall elements installed.

[caption id="attachment_16141" align="alignnone" width="1200"] (foreground) PST precast walls with concrete stitches and bases and (background) ASP precast walls - Courtesy of Bam Nuttall Ltd[/caption]

Transfer tunnel

The flows to the existing STW enter at the southern end of the works. A new transfer tunnel has therefore been constructed to intercept this and transport the flows to the new STW. This tunnel is 1.4km long, 1.5m internal diameter and flows by gravity. It is formed at an invert depth of 10.6m to 15.7m below existing ground level, entirely within the London Clay.

To construct the tunnel a series of six shafts were sunk. The largest of these is 12.5m diameter and forms the inlet shaft in the new STW. The raw sewage will then be pumped from this to the inlet works, from where it will then flow by gravity through the new STW. The remaining five shafts were used to launch or receive the tunnel boring machine (TBM) and pipe-jack, as well as being used for auxiliary connections to sewers in the existing network which don’t get picked up by the main connection.

The tunnel was constructed using the pipe-jacked method using a tunnel boring machine. This was named ‘Daphne’ in a competition by Bisley Church of England School. Daphne Jackson was the first female physics professor in the UK and taught and researched in Guildford. The tunnel consists of four straight sections, one curved section and one short hand-dug section to connect to the existing network.

Current progress is that all of the shafts have been sunk, the main tunnel is complete and fit out the shafts has commenced.

[caption id="attachment_16139" align="alignnone" width="1200"] Safdar Ali, Andrew Dawes and Izhar Ahmad from the BAM team at the completion of the tunnel - Courtesy of Bam Nuttall Ltd[/caption]

Outlet

The outlet for the new STW consists of a 750mm diameter pipe from the TTP and a 1050mm diameter pipe which acts as a storm overflow during extreme weather events. These pipes are laid across a floodplain to the River Wey, a distance of approximately 330m before discharging via a bespoke headwall into the River Wey. This has been designed in conjunction with Environment Agency to minimise erosion in what is classified as a salmon river.

Due to Environment Agency and Local Authority permitting requirements as well as practical requirements of working on a floodplain with underlying running sand, these works are practically restricted to a short duration in the summer. This is being undertaken during Summer 2024 and the works are on target for completion within this period.

Below-ground pipework

The majority of underground assets in the new works are being constructed within the existing landfill. This brings plenty of challenges in both design and construction. Assets must be designed to withstand both differential settlement and potential degradation from existing contaminants in the land fill material.

A variety of ductile iron, carbon steel, and concrete pipes are being used with various protection methods. The work is being self- delivered by the BAM workforce and the installation of below-ground pipework is underway with a focus on where it interfaces with the new structures. This requires extensive temporary works solutions to provide a safe working environment for the workforce within the band of landfill material.

Steel sheet piles are being used to provide temporary ground support and excavated landfill material is being sorted in a material handling area on site. This is being done to reduce the amount of hazardous material being sent off site, halving the amount of excavation and disposal required using traditional open cut methods.

[caption id="attachment_16133" align="alignnone" width="1200"] Threading buried process pipework between new structures requires extensive temporary works - Courtesy of Bam Nuttall Ltd[/caption]

Integration of existing assets

As well as the main connection taking flows form the existing works to the new, there are three other existing assets which are to be integrated into the new system, at various points along the length of the new tunnel. This involves extensive research and planning in an open and collaborative approach with TWUL networks teams.

Bowers Farm Main was successfully diverted in November 2023, and has been constructed such that the final connection and commissioning to the new system will be a more straightforward operation. Planning works are currently underway on both Stokes Road syphons and Abbotswood syphons in order to construct similar scenarios for future connection and commissioning.

The main connection to the existing Guildford Main at the head of the new scheme entails connecting into an existing 10m deep concrete and brick built chamber, which carries live flows 24/7. The site team have undertaken several successful drawdowns in conjunction with TWUL to ascertain the dimensions and condition of the chamber in order to plan the final breakthrough works in great detail. This design work is currently underway and the new tunnel hand drive has been stopped a few metres short of the final breakthrough and shored up, prior to the final connection to the existing network.

[caption id="attachment_16164" align="alignnone" width="1200"] (left) Concrete bases for the tertiary treatment plant, (top right) cross-sectional view through the primary settlement tank, showing the piles and gas dispersal layer beneath the base, and (bottom right) precast wall panels are joined with an in situ concrete stitch - Courtesy of Bam Nuttall Ltd[/caption]

Future works

Construction works of the ‘Big Build’ are continuing at pace and the BEJV Team are now focusing on ensuring a safe, effective and efficient integration of civils and MEICA elements. This will start in late 2024 and continue throughout 2025.

Following successful completion of commissioning of the new sewage treatment works in 2026, the existing works will be de-commissioned making way for the next stage of the Weyside Urban Village, providing homes and leaving a true legacy for local people and businesses.