Supply Chain - Sewerage - Contractors

The village of Stratton, which is located just outside Bude, Cornwall, used to have its own sewage treatment works, but this was replaced by a sewage pumping station (SPS) many years ago when it was decided that Bude and the local villages would have all their sewage treated at a single, large treatment works at Helebridge. The Stratton SPS has its own storm tank and an overflow which discharges into a local stream and ultimately the River Neet, which in turn discharges into the local Bude Bathing Water Area. South West Water had a driver to ensure the Bude Bathing Water Area would meet the design standard of no more than an average of two significant (>50m³) spills per bathing season (as per Environment Agency definition).

The problem

In order to deliver the required bathing water quality driver, the Stratton SPS had to significantly reduce the volume and number of overflow discharges. The pumping station site itself is quite small and given the volume of stormwater which had to be held on site, it was deemed both impractical and not cost effective to try and store this liquid on site.

The solution

Galliford Try was employed by South West Water to find a solution to meet the bathing water driver. The most cost-effective option was to upgrade the existing Stratton SPS in order to increase flows to the Helebridge STW from 17 l/s to 75l/s, based upon a 2050 design horizon. This would negate the need for any additional stormwater storage at the pumping station location. The Helebridge STW falls under a different regulatory permit to the other sites around the Bude area as it has its own long sea outfall, therefore no further stormwater storage would be required at this site even though the project would significantly increase the Stratton pass forward rate. A new pipeline was required between Stratton SPS and Helebridge STE to accommodate the increase in pass forward flows. [caption id="" align="alignnone" width="1200"] Stratton SPS temporary works installation - Courtesy of Galliford Try[/caption]

The new pumping station

The existing Stratton SPS was designed and sized to only pass forward 17 l/s and therefore the existing structure could not accommodate the new pumps and associated requirements. During storm events, dry weather flows increased around ten-fold and this range was deemed too great to achieve efficiently with just a single pump, therefore three identical pumps were chosen which would operate as duty, assist and standby. As the existing building’s dry and wet well configuration did not lend itself to this arrangement, it was decided to construct a new wet well into which the three submersible pumps could be placed. The existing control building however did have sufficient space to house the new control panel. Due to the site size and arrangement, it was not possible to construct the new wet well chamber where flows entered the site, therefore a new chamber, along with a valve chamber and manifold were constructed on the opposite side of the existing pumping station building. A new pipe was installed through the existing dry well to enable incoming flows to the site to reach the new pumping station location. The existing SPS had to be maintained in full working order during the construction works so a suspended stainless steel channel was designed and installed in the existing wet well to bridge the wet well chamber and transfer incoming flows to the new pipe which passed through the dry well chamber. A gate valve installed on the new pipe was kept closed during construction and this allowed flows within the channel to overflow into the existing wet well chamber where it was passed forward by the existing pumps. The channel also allowed the site storm tank, screen and overflow to continue to operate unaffected by the construction works. Once flows were needed for the new SPS commissioning period, the valve could be opened and this would direct flows to the new station rather than the old one. The channel also provided permanent shuttering when it came to filling in the existing wet well chamber. The filled chamber was capped in concrete and benched to enable storm flows to pass up through the existing screen and into the storm tank, and once full, also allowed the site to overflow from its existing location. Ground conditions for the new 4m diameter, 3.6m deep pumping station were not ideal, so a designed sheet pile cofferdam was undertaken. Due to narrow access lanes and lack of available space on site, all equipment and precast rings were delivered to the sister project being undertaken at Marhamchurch, then loaded onto a tractor trailer and transported to the works. [caption id="" align="alignnone" width="1200"] Washout chamber construction - Courtesy of Galliford Try[/caption]

New rising main: route

The existing rising main route from Stratton SPS to Helebridge STW was not the most logical route for the new main to follow. It was clear that a more efficient route could be found which would reduce the overall elevation of the new main relative to the pumping station and also reduce the overall length. At pre-construction phase, various routes were identified using maps and other available data which were scrutinised by the Galliford Try design, construction and client estates team. Landowner acceptance of work being carried out on their land was as important as the routes buildability to South West Water as the aim was to work with the local community in delivering the overall improvement to the Bude Bathing Waters. Once a preferred route was chosen and agreed with key stakeholders, a comprehensive topographical and ecological survey was undertaken along the proposed 2.1km route, and this resulted in only a few minor tweaks to the final alignment.

New rising main: buildability

The construction team had the option of using either ductile iron or PE pipes as the preferred pipeline material choice. At the Stratton SPS end, the River Neet was classed as an EA major river, in the middle there was a 315mm medium pressure gas main, and at the other end of the rising main there was a horse gallop track; the pipeline had to cross all three of them. The gallop track owner and EA would not permit an open cut pipe laying solution, so directional drilling became the preferred installation method which also pleased Wales & West who operated the medium pressure gas main. As the pipelines were being installed in the summer months together with a few lane and hedge bank crossings, it was decided to directionally drill the entire rising main with the aims of reducing arisings and virgin material requirements and reducing time on site, providing both a cost and CO₂ reduction. A 315 PE main was therefore chosen as the pipeline material and a team was deployed ahead of the directional drilling team to butt weld the pipes together in 100m long runs. [caption id="" align="alignnone" width="1200"] Directional drilling remote field set up - Courtesy of Galliford Try[/caption]

New rising main: construction

In order to achieve the required depth under the River Neet, the rising main had to be drilled from the pumping station manifold location. A steep hill was present the other side of the river so this initial drive was in excess of 250m long. Given the restricted working area available at the pumping station location, a drilling fluids recycling plant was used utilised to reduce the amount of water and arisings movements required. At other locations along the rising main, local lagoons were created into which the drilling fluids could be settled then disposed of. Through conversation with local landowners, it was identified that most of the fields the main were to pass through were going to be re-seeded later in the year, and given the soil was dry due to the summer weather, the land owners permitted plant to travel directly on the existing pasture. The logic being that this would be less damaging to the soil structure than undertaking a full top soil strip along the easement, as the over compaction would be remedied when the field was reploughed just a few months later. The local rock formation was very shallow and in places had less than 100mm of top soil cover. The rock is known as Bude Metasandstone and is known to be extremely hard in places. The directional drilling rig managed to successfully drill below the river, gas main and gallop track, but in other places struggled to achieve the required line and level. In the end, the only option was to open cut around 400m of the main with the cut being over 5m deep in places to achieve the required level. Thankfully due to the competency of the rock, this could be battered back and open cut. [caption id="" align="alignnone" width="1200"] Directional drilling helped minimise impact on farmland - Courtesy of Galliford Try[/caption] Along the rising main route there were sensitive fibre optic cables to cross and additional control measures were required to safely locate and expose these through hand excavation with a full watching brief put in place when drilling the new main past these services. The A39 which is a major road in this area, also had to be crossed. The local council were not keen for the rising main to be drilled below this section of the road as it was constructed on an embankment, there was however an agricultural culvert nearby. Through conversations with the local council structures department, it was established that the concrete running surface in the culvert was not the structural floor but was actually built on top of a thin layer of Type 1 stone. The acceptable solution to both the council and land owner, who used the culvert, was to cut a trench through this concrete running surface down the middle of the culvert, dig out the stone and place the pipe in trench and backfill with structural concrete. This provided just enough protection to the new main and allow the existing culvert clearance to be maintained.

Stratton SPS to Helebridge STW Rising Main: Supply chain - key participants

• Client: South West Water
• Designer & contractor: Galliford Try
• Directional drilling: GMAC Utilities Ltd
• Temporary works/shoring: MGF Ltd
• Agricultural work & plant hire: AJ Heywood & Sons
• Precast concrete: Marshalls Civils & Drainage
• Pumps: Pump Supplies Ltd
• Generators: Addicott Electrics Ltd
• Instrumentation: Xylem Water Solutions
• Fencing: Topan Fencing

[caption id="" align="alignnone" width="1200"] Directional drilling recycling unit - Courtesy of Galliford Try[/caption]

Working with the local community and stakeholders

The Cornish countryside is known for very narrow, winding lanes with steep hedge banks. This presented the project delivery team with an issue of how to get lorry loads of materials delivered to the working locations. Luckily the sister project at Marhamchurch had good access from the main road for articulated vehicles and a large hardstanding laydown area. A local agricultural plant company based in Marhamchurch itself were engaged at the start of the Stratton project to assist with material logistics. As the hire company was only five minutes down the road, the team was able to call for assistance as/when required in 30 minute time slots. As the hire company also provided plant and assistance to the local farming community, strong relationships were already established, along with good knowledge of the local road network. This helped enormously in being able to traverse the network of Cornish lanes without upsetting any of the local stakeholders. Extensive letter drops were undertaken along key haulage routes for this project to ensure local residents understood why material movements were being undertaken in their area, and what benefit it would provide them and the local community. This helped enormously in keeping contacts with the dedicated project Client Liaison Officer to a minimum, with most contacts as a result of third party activities not associated with this project. The existing site was located at the end of a narrow dirt track with no room for construction vehicles or laydown. An agreement was made with the adjoining landowner to stone up and fence off an area which we could use as a construction compound, it would then be dressed and handed over to him upon completion of the works as a parking/storage area. This created a future asset for the owner and saved time, money and CO₂ emissions for the project. [caption id="" align="alignnone" width="1200"] Directional drilling below sensitive gallop track - Courtesy of Galliford Try[/caption] As previously mentioned, one of the landowners had a gallop track which the new rising main had to pass under. Part of the farmer’s business was specialising in race horse breading and training, and the gallop track was a highly engineered training surface where race horses could reach speeds of around 30 mph. The landowner was concerned that by open cutting the track, groundwater paths could change and result in a boggy area developing and this could cause significant injury to a jockey or race horse. It was agreed that directionally drilling below the track was an acceptable solution and whilst working in this area, additional controls were put in place as the gallop track remained operational through the construction work. On a few occasions, the site team got to hear and feel what it was like when race horses were running at top speed just a few meters away from where they were working, something which none of them will forget.

Programme

Construction of the £1.75m rising main project started in June 2021 with project completion in July 2022.

Marhamchurch is a small village around 2.4km south of Bude and has a population of just over 800. Sewage from this village is pumped to a nearby large sewage treatment works which processes the waste from Bude and the surrounding area. The existing Marhamchurch Sewage Pumping Station (SPS) was built around 50 years ago and was not designed to hold the volume of stormwater nor pass forward the volume of water which is now required to achieve the bathing water quality driver. Marhamchurch SPS does have an emergency overflow which connects to the River Neet and it is this river which discharges into the Bude bathing water area. South West Water had a driver to ensure a design standard of no more than an average of two significant (>50m³) spills per bathing season (as per the Environment Agency definition).

Solution

Galliford Try was appointed by South West Water to find a solution to meet the bathing water driver. A two-fold approach was undertaken; namely to increase the pumping station’s pass forward flows from the current 8 l/s to 18 l/s, and to provide a new pumping station which would contain 100m³ of on-site stormwater storage and thus reduce the likelihood of having to utilise the pumping station’s emergency overflow. The increased pass forward flow rate and on-site storage capacity was based on a 2050 design horizon. The existing Marhamchurch SPS sent flows to Helebridge STW via a now mothballed sewage treatment works at Helscott, which had been turned into another sewage pumping station. Not only was this a very convoluted route for the flows to take as they were pumped in the opposite direction to the Helebridge STW, but the existing pipe diameter and condition did not lend itself to accommodate the increase in flows. The solution was to therefore lay a new rising main from the Marhamchurch SPS and connect into the existing Helscott Rising Main heading to Helebridge, thereby removing the ‘dog leg’ section of existing pipework. [caption id="" align="alignnone" width="1200"] (top left) Existing site prior to improvement works, (top right) site clearance, (bottom left) construction of temporary footpath and haul road to shaft and (bottom right) shaft cutting edge construction - Courtesy of Galliford Try[/caption]

The new pumping station

The existing Marhamchurch SPS site is on a very small parcel of land which could only be accessed down a narrow un-metalled lane, with low hanging trees. It would not have been possible to send construction plant down this lane, however the adjoining landowner (farmer) was more than happy to allow access to his fields to undertake the improvement works to the pumping station. With easy access now available from the A39 (main road) a shaft solution was adopted for the pumping station storage solution. A 6.4m diameter shaft would just fit on the existing South West Water plot of land adjacent to the existing control building which, at just under 9m working depth, would provide the additional storage volume required for this project. The shaft would house new pumps and with a vehicle rated cover slab installed, parking would still be available for the South West Water maintenance teams. The shaft was located on the opposite side of the site to the incoming flows and existing wet well. The existing pump dry well was positioned below ground between this existing wet well and the new shaft, so the solution was to core holes through the dry well and pipe the incoming flows through and into the new shaft. This meant that the limited wet well capacity could also be used for stormwater storage and by raising the existing emergency overflow screen within this existing wet well chamber, the overall shaft depth could be reduced to just under 9m depth as previously mentioned and thus saving money and reducing construction related CO₂ emissions, compared to a solution which did not utilise the existing asset storage capacity.

Marhamchurch SPS: Supply chain - key participants

• Client: South West Water
• Project design & delivery: Galliford Try
• Shaft construction: Active Tunnelling Ltd
• Horizontal directional drilling (HDD): Gmac Utilities Ltd
• HDD equipment: Ditch Witch UK
• Temporary works/shoring: MGF Ltd
• Pumping: Pump Supplies Ltd
• Control panel: Galliford Try
• Screens with lifting system: Eliquo Hydrok Ltd
• PE pipe: Jewson Civils Frazer
• Pumps: Xylem Water Solutions
• Precast concrete: FP McCann
• Slurry recycling plant: Gmac Utilities Ltd
• Shaft steelwork: Galliford Try Fabrications
• Bulk materials: Steve Wills Haulage

Shaft construction

Active Tunnelling Ltd was engaged at the pre-construction phase for the shaft construction works which included full design and installation of the shaft and the corresponding cover slab. To ensure ease of construction, a haul road was cut through the adjoining field from the existing gate access to the works area. The field was steeply sloped and therefore the haul road had to be cut into the slope to follow the natural ground contour to provide safe access for road going vehicles such as concrete lorries, articulated delivery lorries and cranes. [caption id="" align="alignnone" width="1200"] (top left) Shaft collar constructed ready for sinking, (top right) shaft excavation with 27t excavator, (bottom left) first ring constructed ready for casting concrete jacking collar and (bottom right) shaft sinking in relation to public footpath - Courtesy of Galliford Try[/caption] Ground conditions as identified in the borehole logs indicated a few meters of made ground and sub soil overlaying the Bude Metasandstone formation. The Bude formation had a reputation of being very hard and competent in places so Active Tunnelling Ltd opted to sink the shaft as a caisson with the option of underpinning if it could no longer be pushed down. Thankfully at this location the Bude formation did not prove too much for the excavators used to sink the shaft and it was successfully sunk as initially planned. The arisings were removed from the shaft using an excavator mounted grab, before being loaded onto a tractor and trailer and tipped for drying further down in the field located adjacent to the pumping station. One of the most challenging activities associated with the shaft was construction of a crane pad for the roof slab installation. A 180t mobile crane was needed to offload and install the various precast concrete panels which made up the shaft roof, however as the field was sloping steeply near the working area, a significant cutting around 3.5m deep had to be made in order to create an area level enough for the crane to set up on. Groundwater also became an issue at the cut edge into the hill and a drainage system had to be installed to divert the groundwater away from the crane pad to comply with the temporary works requirements on the design.

The new rising main

From the new shaft pumping station the rising main route was quite obvious. It would pass through two fields on the east side of the A39 before passing below the A39 and the River Neet, then connect into the existing rising main from Helscott to Helebridge, located on the western bank of the River Neet. The River Neet is classed as a major river and the connection point was on a designated flood plain so confirmation was obtained at pre-construction phase with the Environment Agency that they had no objection to the proposed solution, construction method and that South West Water’s statutory undertaker rights were applicable. Similar permissions were obtained from Cornwall Council for installation of the rising main below the A39. [caption id="" align="alignnone" width="1200"] Directional drilling with slurry recycling operation - Courtesy of Galliford Try[/caption] The river and A39 were not to only obstacles to overcome in this local area, there were sensitive fibre optic cables and also a medium pressure gas main. The chosen solution to overcome these obstacles was to directionally drill into the Bude Metasandstone and pass below everything at varying depths agreed with each stakeholder. Due to the location of the directional drill and also working on a flood plain next to a tourist ‘hotspot’, it was decided to minimise the construction site footprint west of the A39 as much as possible. To achieve this a slurry recycling plant was used to process drilling arisings with the processed arisings loaded straight into site dumpers and taken off to the tip. The rising main itself was 180PE pipe which was butt fusion welded in advance of the drilling operation. The initial plan was to directionally drill the entire 486m of rising main however the Bude Metasandstone is a very strong rock and in places was less than 600mm below the field surface. The Gmac Utilities Ltd directional drilling team successfully drilled the road and river crossing at depth however the remaining section of rising main which was located at the interface between the sub-soil and the Metasandstone was almost impossible to control the drill line and level. It was therefore decided that traditional open cut excavation methods would be used to complete this section of the rising main.

Environment

This project was ultimately driven by the need to improve the environment through the improvement of the local bathing water quality. Prior to works commencing a thorough environmental, ecological and habitat survey was undertaken to ensure the project would have minimal impact on the surrounding area. [caption id="" align="alignnone" width="1200"] (left) Shaft drop pipe and pump sump with baffle plate and (right) shaft fit-out - Courtesy of Galliford Try[/caption] At pre-construction stage it initially made sense to locate the new pumping station on the western side of the existing building next to the existing wet well however this would have meant the felling of a very large and mature Ash tree (showing no signs of Ash die-back) and would have also resulted in around 2m of the new structure being visible above ground, due to its location on the downward side of the boat lift incline. Both the felling of a mature tree and the visual impact of a structure at this location was deem too significant to ignore and by moving the new pumping station location to the east of the existing building, constructability improve and therefore cost reduced. An existing hedge bank and a few immature trees had to be felled in order to allow access to the site to undertake construction works. A replacement Cornish hedge bank was constructed to replace what was lost at the end of the project and replanted with native hedge and tree species with the aim of improving what was removed once the young plants had established.

Stakeholders

From pre-construction phase through to completion, keeping key stakeholders informed of the works was identified as critical for the successful delivery of the project. A dedicated Galliford Try Client Liaison Officer was allocated to the project and the local village was informed of works through information signs, letter drops and information posted in the local village shop. The existing pumping station was located on a popular locals and tourist footpath which followed the old Bude canal system, with its location positioned half way up an old canal boat lift. A public information board was positioned at this location and was aimed at providing some basic information to tourists who were walking past the site. The local landowners were personally kept informed of progress by the site team and a great working relationship was established which resulted in successful reinstatement and handing back of the farmland following project completion. [caption id="" align="alignnone" width="1200"] Shaft crane pad construction - Courtesy of Galliford Try[/caption] Through this close relationship, one of the landowners asked that a manhole cover be removed following abandonment of the soon to be redundant rising main and the site team suggested that they could fill in and re-profile a small depression left following the removal of an overhead power cable pylon. A few little gestures such as these went a long way with the landowner and were not underestimated by the site team.

The Exmouth Offshore Project is a key element of the wider Exmouth programme of works to deliver no more than 10 significant (>50m³) spills per annum from the aggregation of Maer Road Sewage Pumping Station (SPS) Combined Sewer Overflow (CSO) and Phear Park SPS CSO. The output from the programme will reduce spill frequencies for better environmental conditions as a long-term solution with a positive impact on the bathing waters. The works were undertaken in Sandy Bay Devon Cliff Holiday Park in Exmouth and was delivered around Sandy Bay’s winter maintenance window, compressing 15,000 working hours within just 13 weeks. Overrunning the programme would have directly impacted Sandy Bay’s tourist season, with significant revenue loss (approximately £2m/day). The works were located only 4 meters from a Blue Flag beach.

Introduction & background

The Exmouth Offshore Project required the installation of an 804m long, 900mm diameter, gravity long sea outfall, to achieve an increased flow rate of 991 litres per second.

The works took place in Sandy Bay Devon Cliff Holiday Park (Exmouth), one of the UK’s largest caravan sites, with over 2200 units and 10,000 visitors per week in the summer season and were only permitted during the Sandy Bay winter maintenance period.

South West Water contracted Galliford Try as principal designer and contractor for the project.

[caption id="attachment_17044" align="alignnone" width="1200"] (left) Pipe string along coastal path and (right) Pushing pipe lower car park and compound - Courtesy of South West Water[/caption]

Construction

To meet this challenging programme, specialist horizontal directional drilling (HDD) contractor Peter McCormack & Sons mobilised one of the largest and most powerful drilling rigs ever used in the UK, to complete the largest forward reaming (48”) operation in the country.

The works required 18 months of planning and the programme challenge included expediting the specialist reamers, 1.2m in diameter manufactured in America and imported to precise programme requirements. The methodology and accurate planning minimised programme risk by minimising the use of marine spread.

This relied on forward reaming (pushing the pipe from land), which is an unconventional method for the length and diameter of pipe of this scale, but reduced the reliance on marine support in winter sea conditions reducing weather risk.

Early marine trenching support was mobilised to clear boulders at the exit point at seabed over two days. Survey marine vessel continued throughout the project to confirm HDD exit point.

818m of HDPE pipe was welded on land, stretching in a single length at strategically high ground to slide the pipe. This allowed to fully pressure test the pipe to give complete confidence in the weld quality. The project team considered both the safety of the public accessing the coastal footpath adjacent to the works and maintenance operations within the holiday park with accurate interface management. In addition to this, the project team worked closely with landowners to agree a route for the welded pipeline to be positioned, in readiness for the pipe push with regular communications with landowners.

[caption id="attachment_17047" align="alignnone" width="1200"] (top left) 500t drill rig compound and entry point and (bottom) pipe pushing operation at sunrise - Courtesy of South West Water[/caption]

The works naturally attracted attention, and allowed the team the opportunity to explain the work that were undertaking and the long-term benefits South West Water outfall would provide to wider Exmouth storm overflow project.

The project compressed 15,000 working hours within 13 weeks which equates to 10 months of normal working hours. This could only be achieved by running a continuous 24x7 drilling operation. Despite the intensive nature of this challenge, the team completed the HDD works ahead of programme, on budget with zero harm to the team, zero pollution incident, and minimal impact to the public. This pipe is a wider part of Exmouth stream of works including storm overflows to achieve long term goal.

Exmouth Outfall: Supply chain - key participants

• Client: South West Water
• Principal contractor: Galliford Try
• Design: Pell Frischmann
• Design of outfall route & drilling specialist/sub-contractor: Peter McCormack & Sons Ltd
• Topographic, GPR & boreholes: AGS Ground Solutions
• Unexploded ordnance survey report: 1st Line Defence
• 900MM PE pipe supplier: Egeplast international GmbH (Germany)
• HDD 500t kit & drilling tools: American Augers (USA)
• Pipe pusher: VanLeewen Trenchless Technology (Netherlands)
• Welding plant supplier: Ritmo remedy 1000FA (Italy)
• Concrete: Aggregate Industries
• Welding plant operator: Pipe Force Martin Woodcock
• Fiberglass systems: Pipex px
• Crane hire: Spence Crane Hire Ltd
• Plant hire: Plantforce Rentals Ltd
• Plant: Sunbelt Rentals Ltd
• Aggregate: Steve Wills Haulage

[caption id="attachment_17043" align="alignnone" width="1200"] 500 T drilling rig - Courtesy of Galliford Try[/caption]

Teamwork and Inclusion

Working collaboratively towards a common agenda, multiple partners were involved including the pipe supplier, drillers, pipe pushers, pipe welders, crane operators, mud pump operators, water tank suppliers, fuel suppliers etc.

A mutually inclusive team worked. Collaborative teamwork environment with no risk-taking policy with a culture of everyone working on site and delivery people have same respect with different roles and responsibilities.

Cultural Behaviour Safety videos were shared on regular basis to remind site team safety, culture, and respect aspect. Site team was well trained and fully skilled to follow safety rules religiously. No one was either bigger or lower than anyone which was demonstrated by actions through a mutually inclusive respectful culture. Shift change and intermittent toolbox talks were carried out. Several graduates and other interested people were taken to site for their learning. One of the UK’s Water Plcs were visited the site for their learning.

[caption id="attachment_17046" align="alignnone" width="1200"] (left) Drill pipe and pilot drill bit and (right) outfall pipe, roller and guide - Courtesy of Galliford Try[/caption]

Health & Safety considerations

Accurate and realistic RAMS were produced. Uninterrupted CCTV monitoring was placed. Each shift had a team of 25 highly skilled and well-trained site team with zero risk taking policy. Specific H&S steps:

• Strict one-way system with separate entry and exit points for plant and construction vehicles.
• Separate entry and exist of South West Water Operators to make existing pumping station in safe operation.
• Detailed interface management plan of each operational activity including regular on site maintenance of drilling rigs.
• An hourly check of whole plant, Noise barriers and vibration control equipment for public safety and wellbeing with fully secured fence - large visible metal sign boards and multiple laminated signs in and around the Sandy Bay for public awareness.
• On duty several field marshals to navigate foot path users. Same drilling team was kept. Compressed programme 15,000 working hours within 13 weeks. Injury free 15,000 working hours into 13 weeks. No near miss. No pollution incident.

Noise barriers and vibration control equipment was installed for the safety and benefits of local community and footpath users. This was checked daily by the site team and the environmental manager to ensure there were zero environmental and H&S incidents.

Protecting the Blue Flag beach

To protect the Blue Flag beach, the entire working area was surrounded by flood barrier to prevent drilling fluids from escaping the site to avoid any risk of pollution of sea. All drilling fluids were recycled by using a recycling centrifuge system. This sustainable innovation allowed the drilling arisings to be fully dewatered, allowing transportation of the inert material without the risk of silt on the road network, causing unnecessary nuisance to the travelling public. This initiative minimised water use and mitigated 364 lorry movements through Exmouth, saving 92T eCO₂.

[caption id="attachment_17042" align="alignnone" width="1200"] 900mm diameter pipe welder - Courtesy of Galliford Try[/caption]

Conclusion

Despite the intensive nature of this challenge, by running a continuous horizontal directional drilling operation over a 24/7 operating pattern, the works were completed on budget and ahead of the programme. There were no time delays, no environmental impacts to the blue flag beach (just 4m from the site), and any impacts to the Holiday Park were minimal.

South West Water have showcased this project through the media and LinkedIn focusing on the engineering challenges the team faced and promoting both construction and the wider environmental benefits of this scheme.

The team have produced a presentation given by Galliford Try’s team to over 250 employees nationally to share the best practice and learnings from this project across their entire business.

South West Water are extremely pleased with progress of these works to meet their commitments with the Environment Agency.  Relationships with the Holiday Park management are extremely good, and the owners are grateful for the efforts the team and SWW have made to complete these works with minimal disruption to their business.

The WINEP Durham Transfers Project was designed to improve the quality of the Blackdene Burn, a watercourse located in the north Durham area. During AMP7, sewage treatment works (STW) sites within Northumbrian Water Group (NWG) ownership were identified for improvements in phosphorus concentrations within final effluent discharge to the environment. The scope of this project is to address the improvements at both Plawsworth STW and Pity Me STW, which currently discharge to the Blackdene Burn.

Background

Esh-Stantec was appointed by NWG to progress the proposed solution; a phased approach whereby, during AMP7, final effluent from both STWs will be transferred to a discharge point on the River Wear via transfer pumping stations. The transfer of flows will be facilitated via new rising mains and gravity sewerage which will connect immediately upstream of the existing outfall from Brasside STW. In line with the phased approach, planned upgrades to Brasside STW in AMP8 will increase its treatment capacity and allow for the eventual abandonment of Plawsworth and Pity Me STWs along with chemical dosing to meet planned phosphorus limits discharging to the River Wear.

The designed solution allows for projected growth across the Plawsworth and Pity Me catchments (including the completion of the significant Sniperley development) with an AMP8 solution transferring all raw flows from Plawsworth and Pity Me to the upgraded Brasside STW for treatment, with final effluent from Brasside STW ultimately discharging to the River Wear.

The objective of this appointment is to provide the assets required to enable to the transfer of flows from Pity Me and Plawsworth STWs in the AMP7 and AMP8 scenarios. It is anticipated that the project will be completed in December 2024.

[caption id="attachment_18551" align="alignnone" width="1200"] Piling operations by Francis & Lewis International Ltd, for the construction of the pipe bridge substructure - Courtesy of Esh-Stantec[/caption]

WINEP Durham Transfer: Supply chain - key participants

• Client: Northumbrian Water
• Principal designer & contractor: Esh-Stantec
  • Esh Group
  • Stantec UK
• Structural consultant: James Christopher Consulting Ltd
• M&E, including supply & installation of generators, MCCs & pumps to wet wells: Retroflo Ltd
• Directional drilling of 3km of rising main & installation of all AV chambers: Ken Rodney Construction Ltd
• Auger boring under the East Coast Mainline, coordinating with Network Rail, & supply/installation of track monitoring: Terra Solutions Ltd
• Design, manufacture, & installation of pipe bridge: Francis & Lewis International Ltd
• Design & sinking of segmental shafts: Active Tunnelling Ltd
• Temporary works/shoring: MGF Ltd

New transfer stations

The first stage of the scheme involves the transfer of fully treated, final effluent to the River Wear, meaning that Plawsworth and Pity Me STWs remain fully operational throughout the construction of all assets which will become live prior to the upgrade of Brasside STW.

Plawsworth Transfer Station: The new transfer assets at Plawsworth STW were carefully designed and sequenced around the limited working room in the site and the requirement to maintain treatment operations during construction. The Plawsworth Transfer Pumping Station comprises:

• A 4m diameter, 6m deep wet well shaft (sunk as a caisson).
• MCC and associated kiosk.
• Generator and associated kiosk.
• Above-ground valve arrangement.
• Flow meter chamber and all associated interconnecting pipework and ducting.

In AMP7, final effluent flows are intercepted and directed into the new wet well before being pumped to the River Wear. Due to the transfer of final effluent, a reduced emergency storage volume has been provided within the new wet well.

In AMP8, following upgrades to Brasside STW, raw flows will be intercepted upstream of the inlet works and diverted to the new SPS for transfer to Brasside via a new 250mm rising main. As part of the conversion from a treatment works to a terminal pumping station, the existing humus tank will be re-purposed to provide additional emergency storage in line with EA guidelines for the transfer of raw sewage flows and minimise emergency overflow spills to the environment.

In addition to emergency storage, a permanent standby generator has been incorporated into the design which will automatically cut in if there is a loss of power to the site.

The pumping station was designed as a three pump station (duty/assist/standby) to transfer ultimate AMP8 flows up to 32 l/s, however, in the AMP7 scenario,  the pumping station will operate as a duty/standby/standby station, with the individual pumps sized to transfer 24 l/s of final effluent to maintain a minimum velocity in the rising main.

Pity Me Transfer Station: The main works at Pity Me STW comprises the following:

• A 6m diameter, 9m deep wet well shaft (sunk as a caisson) with built-in emergency storage volume sized for ultimate raw flows.
• MCC and associated kiosk.
• Generator and associated kiosk.
• Valve chamber.
• Flow meter chamber and all associated interconnecting pipework and ducting.

Similar to Plawsworth, the pumping station was designed as a three pump station to transfer 34 l/s in AMP7, with a duty/standby/standby arrangement, and up to 61 l/s in AMP8, after conversion to a duty/assist/standby arrangement. In both the final effluent and raw flow scenarios, pumped flows will exit the works via a new 280mm rising main.

[caption id="attachment_18550" align="alignnone" width="1200"] Active Tunnelling constructing the new wet well shaft at Pity Me STW
Courtesy of Esh-Stantec[/caption]

Transfer rising mains

Approximately 2.1km of 250mm PE rising main and associated air valve/washout chambers has been installed from Plawsworth STW. The rising main pipework was installed predominantly via a trenchless technique, namely horizontal directional drilling (HDD), primarily due to lack of available access from the adjacent A167 highway to facilitate traditional open cut construction, as well as unfavourable ground conditions.

A new 280mm  rising main was installed over an 850m length from Pity Me STW. Due to a tight working corridor situated between a garden centre and the Blackdene Burn, as well as the requirement to cross beneath the A167, a highly trafficked highway, the rising mains was also installed via HDD. This use of a trenchless installation technique minimised disruption to local business and removed the requirement for traffic management on a busy commuter route.

eDNA samples taken from ponds adjacent to Pity Me in the feasibility stage confirmed the presents of great crested newts. In lieu of protracted traditional population surveys and subsequent licenced activities to mitigation including fencing and capture to clear the working area, a District Level License (DLL) was secured from Natural England and works were completed under an approved method statement with supervision from a consultant ecologist.

The 280mm and 250mm rising mains from Pity Me and Plawsworth connect to a common chamber located at the high point of the new network, where they will discharge flows to a new DN400 gravity sewer.

[caption id="attachment_18547" align="alignnone" width="1200"] Construction of new valve chamber and associated pipework at Pity Me STW - Courtesy of Esh-Stantec[/caption]

New gravity sewerage

2.4km of new DN400 gravity sewerage, with associated manhole chambers, was constructed to receive the pumped flows from both Pity Me and Plawsworth STWs. Careful consideration was given to the design of the new sewerage to ensure capacity (including future growth) and self-cleansing requirements are met in all flow scenarios, including when Pity Me and Plawsworth transfer pumps operate either in isolation or in parallel.

The construction of the new gravity sewer poses some complex crossings and challenging ground conditions. The crossing of Chester Low Road, another busy road for both commuters and customers of a popular local retail park, The Arnison Centre, was originally planned for open cut excavation. However, subsequent on-site observations with regard to topography, a number of high-risk services including high voltage electric cables and a uPVC water main, and running sands within nearby open cut trenches led to the conclusion that it would not be possible to safely support the excavation during construction. Therefore, the road crossing was completed via guided auger bore. Not only did this mitigate the disruption and health and safety risks associated with damaging services, but it also minimised disruption to local businesses and residents as the road remained operational throughout the process.

East Coast Main Line crossing

Further downstream, the new gravity line is also set to cross beneath Network Rail’s East Coast Main Line. The proposal is to install a 610mm sacrificial steel sleeve via a guided auger bore beneath the embankment, once complete the 400mm NB gravity pipe will be sleeved and the anulus grouted. Extensive liaison is ongoing with Network Rail in the lead up to the under track crossing (UTX) to gain the necessary approvals and fully understand the constraints which must be met during construction. These constraints include but are not limited to 24/7 working for the duration of the tunnelling works, installation of track monitoring and an approved contractor on standby in case there is a requirement for emergency track repair.

[caption id="attachment_18549" align="alignnone" width="1200"] Aerial view of working area at Brasside STW - Courtesy of Esh-Stantec[/caption]

Brasside pipe bridge

Approximately 800m upstream of the Brasside STW outfall connection, the sewer crosses a watercourse located within a steep ravine and dense woodland at the existing Brasside Sewage Pumping Station (SPS).

At the optioneering stage of the project, it was proposed that this crossing would take place via an inverted siphon arrangement. However, subsequent hydraulic modelling deemed that, due to topography constraints, it would not be feasible to achieve sufficient capacity while maintaining self-cleansing velocities in both the AMP7 and AMP8 flow scenarios without connecting to the existing outfall further downstream. This would have required construction works within ancient woodland, imposing the requirement for Environmental Impact Assessment (EIA) and potentially preventing the obtention of a Lawful Development Certificate.

Therefore, to mitigate the risk of repeat maintenance issues and potential asset failure, a pipe bridge was progressed as the preferred option having discounted a further interstage pumping station at the low point due to a lack of space within the existing Brasside SPS site and the associated operational carbon and expenditure.

The route of the pipe bridge was designed to mitigate environmental impact, by including bends within the bridge structure and above ground pipework to take advantage of existing operational land and avoid clearance of significant trees within a conservation area. The interface between the bridge structure and the above ground ductile iron pipework has been carefully modelled and iteratively designed to assess the impact of bridge movement and deflection on the pipe joints and select joints which provide appropriate tolerances for these movements.

[caption id="attachment_18552" align="alignnone" width="1200"] Construction of pipe bridge structure by Francis & Lewis International Ltd - Courtesy of Esh-Stantec[/caption]

Conclusion

The design of the WINEP Durham Transfers Scheme serves to meet regulatory deadlines by achieving a sustainable multi-stage solution which meets the needs of the current and future catchment population.

At the time of writing (July 2024), construction is progressing well, and all rising main pipework and transfer station substructures have been installed. The gravity sewer is predominantly complete with the two key crossings remaining, pending final Network Rail approval to complete the rail crossing and final structural design of the pipe bridge to install the above-ground pipework adjacent to Brasside SPS.

The current construction programme shows that flows will be turned in December 2024, transferring final effluent to the River Wear in the interim and, in AMP8, transferring raw flows to Brasside STW for treatment. This will achieve the removal of treated effluent from the Blackdene Burn, in turn significantly improving water quality while meeting WINEP obligations. Furthermore, the completion of the scheme in AMP8 will serve to rationalise the NWG sewage network in the area and improve the resilience of the network by serving catchment growth and improving treatment capacity.