Supply Chain - Site Preparation - Cofferdams

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.

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:

Above the water, the planted vegetation is a haven for wildlife, including nesting birds. Below the surface, the floating beds provide habitat for fish, eels and other aquatic species. 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.

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 The Piling People 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 The Piling People 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.

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:

Inlet works. Primary settlement tanks (PSTs). Activated sludge plant (ASPs). Final settlement tanks (FSTs). 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.

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.

Scope of works National Grid, working with North Lincolnshire Council and East Riding of Yorkshire Council, developed a plan to construct a tunnel under the River Humber to replace the existing pipeline. The three-year project will provide National Grid with a 5km long, 3.65m diameter tunnel bored 30m under the river bed. A single 1200mm concrete-weight coated steel pipe will be inserted into the tunnel in a single string, making it the longest gas pipeline insertion of this kind in the world. Issues and solutions The River Humber pipeline is an important pipeline - connecting an import location for gas at Easington, on the East Yorkshire coast, to the national network – delivering gas supplies to millions of households throughout the UK. The original pipeline, Feeder 9, is buried in a cut-and-cover trench under the river bed where tidal patterns have eroded the river bed uncovering the pipeline. Remedial work was undertaken to provide a short-term solution while a long-term solution was developed. Working with the joint venture consisting of Skanska, PORR Bau GmbH and A.Hak, MGF’s in-house design engineers developed and manufactured a bespoke temporary works propping solution for the tunnel boring machine (TBM) launch pit excavation. The contractor required the design to allow for the following key factors: Use of a load monitoring system. Design of bespoke connection details to sheet piles or capping beams where steel walers were not used. Design of bespoke 'headwall' beam to eliminate knee bracing at the end of the tunnel portal to increase working space. Design of prop connections to withstand 10 tonne accidental load. The excavation was 209m long by 8m at its widest point, ramping from ground level to 11m depth. The deepest section, including the portal wall, was constructed with secant piles with PU32 sheet piles forming the remaining walls. The propping solution consisted of 406UC hydraulic braces with 2500kN 400 series struts. To accommodate the TBM installation the longer, high-level props were twinned with spans of up to 11m between pairs of props. The headwall beam was primarily required for ease of excavation as knee braces are notoriously hard to navigate around with an excavator. This ensured that the temporary works was sufficiently robust whilst maintaining an accessible working space; a technical aspect which separated us from the competition and was completed to Eurocodes. Additionally, there were concerns that accidental impact of the struts by plant could cause failure of the system, particularly during the TBM lift, as there was only 200mm space between the paired props. To mitigate this risk MGF designed bespoke connections to the capping beam, wailing beams and sheet piles to withstand an impact of 10-tonnes. MGF used its in-house load monitoring system which actively measured the load and temperature of each individual prop, waler axial loads and bending and axil load in the headwall beam. The data collected during the monitoring regime will have a significant impact on creating efficiencies in the design of hydraulic strutting and support of narrow closed trenches going forward. Stephen Barker, Major Projects Engineer said:

"This project was particularly challenging due to the multiple levels of framing, high loads and strict robustness criteria. As the design was required to survive the loss or damage of a prop and withstand a 10-tonne impact load more props were required, however we had to balance this with ensuring that the contractor had sufficient space to excavate efficiently. The design of the headwall beam contributed significantly to this as it removed the requirement of having knee bracing at the end of the excavation, opening space and making bulk excavation easier."

The project will take approximately two years to complete and top level of the temporary works will stay in place during this time. In an adjoining field, the pipework is being welded in top strings in readiness for insertion. The tunnelling is currently underway with a slurry TBM.   For more information: MGF Ltd | +44 (0)1942 402700 | www.mgf.co.uk    

Scope of works The purpose of the job was to design and supply temporary and permanent support for a hydro power house and turbine chamber (plus associated inlet channel and tailrace) constructed at the side of an existing weir on the banks of the River Derwent in the centre of Derby. Issues & solutions The design incorporated a single level of modular 406UC Hydraulic Brace system in conjunction with Larssen piles up to 10m in length, toed in mudstone, covering a perimeter of over 150m. MGF's design allowed early removal of the brace by using a permanent concrete base slab as a low propped cantilever. The design had to take into consideration the complicated construction sequence; and the location of a large crane during installation and removal of the river piles. During the project Balfour Beatty and MGF collaborated to develop the Stairsafe system (the UK's first temporary steel stair system designed specifically for excavations). The system proved a great success in providing rapid and safe access into excavations leading to greater site productivity. The system is now being used throughout the UK on excavations up to 6.5m in depth. [caption id="" align="alignnone" width="1200"] Courtesy of MGF Ltd[/caption] Project details Sector: Energy & Waste Client: Derby County Council Contractor: Balfour Beatty Consulting engineer: Atkins MGF Depot: Midlands - Rugeley Product hired: 406UC Hydraulic Brace, 10M Larssen Piles The verdict From early in the tender process MGF have been heavily involved with Balfour Beatty to develop an innovative, cost effective and highly complicated cofferdam design; which helped make Balfour Beatty's tender attractive to the client, Derby City Council. Michael Rummens, Balfour Beatty Site Agent, commented:

"MGF were brought in at tender stage to assist in the value engineering of the cofferdam and support frames. Upon contract award, MGF's technical team were extremely proactive in producing a workable design in line with the projects requirements. The professional, amenable and expedient attitude continued throughout the process from design, technical support, plant hire and delivery and site assistance."

    For more information: MGF Ltd | +44 (0)1942 402700 | www.mgf.co.uk    

Scope of works Excavation works began in 2017 at the Dancing Hill Reservoir site as part of a £6 million project to upgrade the Water Supply GRID for Bridgwater and the surrounding areas. Major upgrades are required due to large-scale developments in the area. Issues & solutions The design was particularly challenging due to the complicated nature of the ground surrounding the reservoir tank. A sizeable berm was built on the ground profile around Cofferdam 4 to ensure a safe and stable excavation. This exaction also had to accept other loadings from Cofferdams 1, 2 and 3. Multiple 203UC tank brace frames were used due to the strict lift plan limit of 1 tonne. Larger or heavier brace systems could risk the collapse of existing foundations. Vibration pile driving was not permitted due to the detrimental effect it could have on the high-pressure water mains that supplies customers. There was also a HV cable running through the site that had to be avoided. The exact location of other services created additional challenges and ongoing surveys had to be monitored throughout the duration of works. As of 2017 there are four connected cofferdams in place allowing safe working space to combine new pipeworks to the main reservoir tank. Multiple back-to-back 203UC tank braces with special alignment of bracing has been key in the design, combined with KKD 600 sheets to secure the trench. Several Lightweight DavitSafe systems allow safe access into the 10m deep cofferdams. Sector: Water & Utilities Client: Wessex Water Location: Bridgewater, Somerset Scheme: Water Supply GRID Upgrade MGF depot: Bristol Products hired: Multiple 203UC brace, KKD 600 sheets, lightweight davit [caption id="" align="alignnone" width="1200"] Dancing Hill - Courtesy of MGF Ltd[/caption] For more information: MGF Ltd | +44 (0)1942 402700 | www.mgf.co.uk    

In the Norfolk seaside town of Great Yarmouth stands Grist Mill. Dating back to the late Victorian era, the building is of local historical significance and stands directly alongside the western bank of the River Yare in the Cobholm Island area of Great Yarmouth. The existing quay wall and apron had deteriorated to a quality where it provided little, if any protection to the foundations of the timber framed Grist Mill, so the services of Ivor King were engaged to install a new sheet pile flood defence around the building. The works were part of wider £42.5 million Environment Agency Tidal Defence Initiative in the area to protect against the threat of East Coast flooding in the Norfolk seaside town. We carried out a virtually silent and vibration-free installation into hard ground to form a three-sided cofferdam waterside to the building, ensuring its future longevity. The Challenge The mill, located directly next to the River Yare, three miles up from where it flows into the North Sea, is set in a residential area, with neighbouring properties in close proximity. In addition, 23.5 metre ‘Z’ section steel sheet piles would need to be delivered into site and passed over the three-storey building in pairs for installation into hard ground conditions. The Solution Giving consideration to the impact of works on the local neighbourhood, along with the fragile nature of mill’s foundations and its environmentally sensitive riverside setting, our Giken Supercrush solution was selected as the only truly viable way of meeting all challenges. Supercrush is an extension of the Giken silent and vibration-free installation method, whereby sheet piles can be pressed into the ground without causing noise or vibration, whilst also overcoming hard ground conditions by simultaneously augering the ground during the installation process. Equipped to deliver Delivering the 23.5m long (more than twice the length of a London bus) sheet piles into site in pairs was facilitated by Ivor King’s in-house transport team. The route in (the last part being particularly challenging – narrow and twisty) was meticulously planned and deliveries were carried out using our special extendable rear-steer trailer, specifically designed for transporting long loads into sites with limited access. Two 300 tonne Liebherr Crawler Cranes were used for the lifting operations. One to manoeuvre the collosal sheet piles over the three-storey building in pairs and into the chuck of the Silent Piler. Another to steady the Supercrush auger (necessary due to the hard ground conditions), itself standing some 30 metres tall and weighing circa 30 tonnes, during the piling process. Using reaction force, each pair of ArcelorMittal AZ40-700 sheet piles, up to 23.5m in length, were installed by one of Ivor King’s mighty Giken ‘Double-Crush’ Silent Pilers, which clamps onto previously installed sheet piles and is capable of ‘walking’ along the piles to travel forward.

Vibration-free installation of cofferdam - Courtesy of Ivor King - The Piling People

Respecting the environment Further to our environmentally friendly installation technique, compressed air was used to create a bubble curtain, (sometimes known as a pneumatic barrier) to obstruct silt caused by augering from escaping into the flow of the river. Following sheet pile installation, wailing and capping beams, plus anchor ties were added to increase the integrity of the structure, ready to accept a concrete slab to complete the upgrade to the mill’s foundations. Client: JN Bentley on behalf of Environment Agency Project cost: c. £1.7m Completion date: October 2022  Project period: 19 weeks Plant utilised: Giken SCZ675 'Double Crush' Silent Piler 300T Liebherr LR1300 Lattice Boom Crawler Cranes Broshuis Rear Power-Steer Triple Extender Trailer The specialist silent and vibration-free piling technique provided by Ivor King and their Giken Supercrush helped us deliver our objective of better protecting the people and property of Great Yarmouth from flood risk whilst overcoming the challenges of working in a very residential area, within meters of a locally significant building and on the edge of the environmentally sensitive site of Breydon Water. Thanks to Ivor King for their support of JBA Bentley in delivering this project. Tom Stanley, Environment Agency Project Manager

J Murphy Group awarded Aarsleff the contract to install a temporary sheet piled cofferdam on the River Severn, as part of the £300 million Birmingham Resilience Project. The scheme is one of Severn Trent’s largest ever infrastructure projects, specifically to develop an alternative water supply for Birmingham to complement the Elan Valley Aqueduct (EVA). The Birmingham Resilience Project will create a new abstraction point and pumping station on the River Severn, at Lickhill Quarry near Stourport, to which water will then be pumped along a new 25km pipeline, through pipes with a 1.0m diameter, to the water treatment works in Birmingham. Aarsleff’s sheet piling works allowed a dry working area for J Murphy to construct the secant piling to the intake structure. Sheet piles will then be subsequently removed and the entire structure submerged. Water is to be taken from the River Severn which will provide a new source for the aqueduct, while maintenance takes place on the original EVA. When the resilience plan is in operation, the city of Birmingham will be drawing water from at least four sources, rather than just one. Early contractor engagement between J Murphy and Aarsleff identified significant technical deficiencies in the early engineering proposals for the temporary works cofferdam. Aarsleff’s proposed design was safer, quicker and cheaper with significantly less risk to the environment, their clients program and operatives. Aarsleff installed the bespoke temporary sheet piled cofferdam which comprised 154 (No.) AZ26-700 steel sheet piles, with an integral enhanced toe support system comprising of 53 (No.) H-Section piles. These were required due to the very shallow underlying bedrock, which only allowed for a limited sheet pile penetration. Access was provided by a temporary access works platform allowing safe working over water. Aarsleff employed a high frequency vibratory hammer and an impact hammer to drive its piles (using panel driving techniques) and employed a 110-tonne capacity mobile crane operating at a maximum 25m radius to unload and handle piles and hammers. [caption id="" align="alignnone" width="1200"] Courtesy of Aarsleff Ground Engineering Ltd[/caption] Senior technical estimator Ashley Carter said:

“This was a technically challenging scheme to be involved with requiring a lateral thinking approach to resolving the challenging problems. The prestigious project demonstrates the high level of experience and expertise Aarsleff’s sheet piling department can offer to its clients”.

Aside from delivering the project on time and within budget, Aarsleff were also able to reduce risk and environmental damage. Risk reduction through application of practical construction methodology, and Reduction in Environmental impact, (as the original scheme involved drilling underwater and spoil flushed directly into the watercourse). Steel sheet piles will also be recovered for re-use making this element of the scheme carbon neutral. Aarsleff took the technical challenge and applied sense and reason to develop a practical, constructible and engineered solution, which benefitted all parties for the aforementioned Environmental, Commercial and Safety aspects. Meeting a tight construction programme at the front end of a long complex scheme gives Aarsleff great satisfaction. Scope of works 154 (No.) AZ26 steel sheet piles (section modulus 2600cm3/m of max length 8.0m) Integral enhanced toe support system comprising of 53 (No.) 254/254/132 S355 JO H-Section piles Prefabricated box section connection detailing at 1.26m centre to centre Construction period 21st July 2017 - 13th Sep 2017   For more information: Aarsleff Ground Engineering: +44 (0)1636 611140 | www.aarsleff.co.uk

   

The project required the contractor to excavate within 25m of Beaumaris Castle – a UNESCO World Heritage site and one of the best examples of 13th and 14th century architecture in Europe attracting over 100,000 visitors last year. Faced with the challenge of installing 600m of 1.5m diameter polypipe in 6m sections between existing manholes, John Kelly Construction spent time researching the products available on the market, before finding that MGF’s High Clearance Trench Boxes were ideal for the job. [caption id="" align="alignnone" width="1200"] Courtesy of MGF Ltd[/caption] The 7m x 2.9m wide High Clearance Trench Boxes coupled with Top Boxes saved time and money to the contractor when compared to sheets and frames. The strength of the box allows suitability and capability of going to 5m depth and having high clearance struts on both ends of the box allows access at both sides – ideal for installing larger diameter pipes. [caption id="" align="alignnone" width="1200"] Courtesy of MGF Ltd[/caption] Paul Cowen, Technical Sales for MGF commented:

“Beaumaris’ most recent floods in 2017 was the catalyst that something needed to be done regarding the surface water drainage system. MGF became involved to supply the excavation support equipment to alleviate the threat of flooding to protect the natural beauty and tourism trade in the area”.

MGF’s stock logistics team confirmed availability of the kit at our Dartford depot and coordinated transfer to the Astley depot before delivery to site with three 40ft articulated vehicles and two 15t wagons to ensure the order was met. [caption id="" align="alignnone" width="1200"] Courtesy of MGF Ltd[/caption] John Kelly, Managing Director of John Kelly Construction, stated:

"I was very impressed with MGF as a company as they were very quick to respond to our queries, provided designs and all equipment ordered arrived on site with no delays."

Sector: Water & Utilities Client: Isle of Anglesey County Council Contractor: John Kelly Construction MGF depot: North West - Astley Products hired: High clearance trench box, top boxes & titan manhole box   For more information: MGF Ltd | +44 (0)1942 402700 | www.mgf.co.uk