Supply Chain - Asset Maintenance & Rehabilitation - Leak Sealing

King George V Reservoir (KGV) is an operational raw water storage reservoir located in the Lea Valley at Walthamstow and is a Site of Special Scientific Interest (SSSI) with restricted access. It is controlled by Thames Water’s Lea Valley/North London Abstraction Group based at Coppermills AWTW. Seepage through the clay core at the north-east of the reservoir was identified by a Willowstick survey in 2020. MWH Treatment was awarded a contract to address the seepage issue, with the works taking place 1.4km north of the main entrance from Lea Valley Road to the north of the tidal break on the east side of the reservoir. History of the site The reservoir was conceived as part of an overall plan for the Lea Valley and was laid before the Royal Commission of Water supply in 1893. At this time, the responsible authority was the East London Waterworks Company. Construction took place between 1908 and 1912, and the reservoir was opened by Their Majesties, King George V and Queen Mary in March 1913. At the official opening there was a ceremonial guard of honour review, and The King signed a Grant of Arms to the Metropolitan Water Board (owners and operators at the time). [caption id="" align="alignnone" width="1200"] Google Maps - Location of King George V Reservoir[/caption] Description of the current facility The reservoir was formed by the construction of a continuous embankment on the floodplain of the River Lea. An earth embankment divides the reservoir into two compartments that are connected by three large diameter culverts. The external grassed embankment supports a central puddle clay core. The reservoir has a maximum capacity of 12,492 ML at its top water level (TWL) of 20.18m AOD. It is normally operated on a ‘put and take’ basis which at typical through‐flows of 60 to 100 MLD gives a theoretical retention period between 100‐150 days. The crest level is 22.50m AOD with a length of approximately 7km. The reservoir is operated in conjunction with other local reservoirs to provide a continuous supply of raw water to Coppermills AWTW and Hornsey WTW. The embankment, wave wall and clay core were raised between 1938 and 1941 to restore the 1.5m design freeboard that had been lost due to gradual consolidation settlement. The reservoir was, however, kept 1.5m below original top water level, as a wartime precaution, until February 1945. At the higher TWL leakage occurred at several points, being attributed to dryness of the very high plasticity alluvial clays within the core; consequently, the current TWL was established. A 4.5m deep grout screen was installed along an 800m length of the southwestern embankment in 1983 in an attempt to address the leakage. Access to the reservoir is located at its southeast corner from the Lea Valley Road. This access is shared with a sailing club and activities club. Sheep graze the embankments as an environmentally sustainable way of managing grass growth. Scope of works The key scope of works for the project was the provision of a sheet-pile cut‐off wall. The wall was designed as 15m deep; based on the pile toe being driven into the London Clay a minimum of 0.5m, and 165m long; being installed between crest chainages 1,715m and 1,880m. A low vibration technique of pile driving was promoted to minimise potential for stability issues within the embankment during construction. The scope also included provision of access to the crest and provision of temporary access road, strengthening of existing site roads and protection to existing pipework and services. [caption id="" align="alignnone" width="1200"] Google Maps - location of site compound and works area[/caption] Technical description A survey was carried out late 2019 to early 2020 in the area around chainage 1,750m as this had been identified as an area of interest in a previous survey. Willowstick surveying is a non-intrusive geophysical method used to map and model the subsurface water flow paths. The procedure involves energising the study area with a signature electric current between paired electrodes. The surveying equipment consists of GPS system and an instrument known as a Willowstick which measures the magnetic field response of an electric current. As the electric current flows through the survey area the preferential flow paths are identified, mapped, and modelled. The information gathered can be used in making informed, guided, and cost-effective decisions concerning how to further, monitor, intercept and mitigate seepage. The main findings from the survey were: A weakness zone of preferential flow identified between chainage 1,723m and chainage 1875m. The flow path shows the characteristics of uniform sheet flow and did not indicate a concentrated leakage path, rather a 150m zone where water was passing evenly through the core. The preferential flow was identified at elevation 12m AOD through the foundation underneath the embankment. The preferential flow path strongly correlated to the location of the original stream channel. Its characteristics indicated that seepage was concentrating in flood plain sediments deposited by the original river channel meandering through and beneath the survey area before the dam was built. The survey report recommended remediation works in this area within a short timeframe to reduce any movement or damage to the reservoir embankment. King George V Reservoir: Supply chain - key participants Principal contractor/designer: MWH Treatment Designer: Atkins Temporary works design: Noble Works Ltd UXO survey: 6 Alpha Associates Ground investigation, soil testing: Structural Soils GPR surveys: Technics Group Environmental surveys: Middlemarch Environmental Willowstick survey: Willowstick UK Noise & vibration monitors: RSK Acoustics Silent piling contractor: Sheet Piling (UK) Ltd Piling verticality survey: CMCS Ltd Civil works, labour/plant: Derryard Construction limited Electrical (site setup, gate works): SRS Electrical Ground maintenance: UK Landscapes Scaffolding/access: Clocktower Scaffolding General plant hire: GAP Group Ltd Welfare supply: Wernick Group Supply of Trakway Matting: Sunbelt Rentals Ductile iron pipework: Electrosteel Castings (UK) Ltd Valves: Glenfield Invicta CCTV, timelapse cameras: Wireless CCTV Ltd [caption id="" align="alignnone" width="1200"] Finished temporary road before track matting - Courtesy of MWH Treatment[/caption] Toe road and crane pad Atkins were instructed to advise on embankment loading constraints, construction methodology and the sheet pile solution. They developed the initially suggested remedial works to achieve a detailed solution which was then agreed with Thames Water. The existing track at the toe of the embankment was the only access route to the works area. The track had a loading capacity of 12t and the crest road a 5t weight limit; both limiting machinery and plant selection. A previous project adopted an 8t excavator and 6t tracked dumpers and Atkins used this information within their assessment to complete a slope stability analysis of the embankment. This confirmed that the use of an 8t excavator was acceptable for the works on the crest road. An existing 30” pre-stressed concrete (PSC) pipe main under the road at the toe of the embankment further constrained the size of vehicles to be used during construction and required enabling works. A disused section of pipe was recovered and tested to confirm dimensions and the material characteristics to be used in the loading analysis. Atkins confirmed that the pipe had sufficient capacity to support a maximum axle load of 12t. A temporary road was installed to minimise rutting and dynamic loading on the existing track. Trackway matting was installed from the sailing club ramp to the crane pad to disperse vehicular loading and mitigate impact on the pipe and vehicle speed was restricted to 5mph for all vehicle movements. An emergency plan was put in place to ensure the Thames Water Operations team were made aware of any incident and to cover the eventuality of a pipe burst. The temporary road was designed by Noble Works Ltd and installed by Derryard Construction Ltd over a period of 8 weeks. Over 1km of track matting was hired from Sunbelt Rentals and installed over a period of 2 weeks. Along with the temporary road, a crane pad was installed to facilitate the 200t crane used to lift the piling equipment and sheet piles from the toe to the crest of the embankment. The crane pad was designed by Noble Works Ltd and constructed in MOT Type 1 granular fill by Derryard Construction Ltd. A section of the adjacent aquifer recharge pipeline was dismantled to facilitate construction of the crane pad which also included a concrete protection slab above existing underground HV and telecommunications cables. [caption id="" align="alignnone" width="1200"] Finished installation of the track matting - Courtesy of MWH Treatment[/caption] Piling decision Following previous successful remediation works carried out at Sir William Girling, Staines South, and Island Barn Reservoirs, MWH Treatment selected Giken silent piling. This is a vibration free press-in method that allowed the pile installation to be virtually silent. This technique enables the piling plant to ‘walk’ over the tops of the driven piles. The initial press-in to install the dummy piles was carried out utilising a reaction platform. The Giken rig then used the previously installed piles to generate reaction to enable on-going pile installation. The sheet piling works (undertaken by Sheet Piling (UK) Ltd) commenced at chainage 1,880m which is the junction of the perimeter and the reservoir dividing embankments. A wider toe area had been identified as a turning and laydown area for the storage of equipment. It was also used for the reaction platform for the installation of the dummy piles. The first 100 piles were fed to the rig by the crane at the base of the embankment. Once the radius of this crane was exceeded, the ‘non-staging’ method was adopted utilising a clamp crane and pile runner to move the piles along the crest and to feed the piles to the Giken rig. A 15m long PU32 section pile was adopted being dependant on the chosen method of installation, length of pile and expected ground conditions. The pile clutches were lined with sealant to ensure low permeability through the wall joints. Press-in driving of the piles with the Giken rig was assessed as feasible with hard/difficult driving being a low risk when reaching the London Clay layer. Subject to the approval of the reservoir Qualified Civil Engineer (QCE) water jetting was an option to achieve required toe penetration. [caption id="" align="alignnone" width="1200"] Reaction stand setup excavation - Courtesy of MWH Treatment[/caption] Crest road works The Giken piling method minimised the access width requirement to work on the crest, however some excavation was still required. Using an 8T excavator the crest road was excavated to expose the clay core to allow the alignment of the sheet pile wall with the centre of the core. The base width of the trench was specified at 2m to allow the Giken rig to operate. The top of the core was covered with plastic sheeting to prevent desiccation and protect the clay core from being damaged. Following the completion of the sheet piling works, the trench and crest was reinstated. Site installation of the piles Sheet Piling (UK) Ltd commenced work on site at the end of October 2022, with a planned 10-12 week site duration. The 200T attendant crane was stripped of all non-essential weight in order to maintain the 12T per axle load weight restriction. Once in position on the crane pad a smaller 45T service crane off loaded the initial crane ballast and commenced rigging; once part rigged the remaining ballast was self-rigged. The reaction stand was lifted into position by the 200T crane to the 6m x 6m hardstanding area finished 600mm below the top pile design level. The Giken rig was then positioned onto the reaction stand followed by counterweights. The first pile was then driven utilising the weight of the rig and counterweight as reaction. After the first pile were installed, it became a reaction pile for subsequent piles to be installed. When the rig fully sat on the dummy piles, the reaction stand and counterweights were removed. Once a pile was sufficiently driven and stable the rig releases its clamps from the reaction piles and travels forwards. This ‘self-moving’ system eliminates the need for support by a crane during the piling operation. Following installation of the first 100 piles a clamp crane was utilised as the piling works had exceeded the working radius of the 200T crane. After the clamp crane had travelled along the piles as far as possible inside the 200T crane working radius a track, formed of a temporary pile welded along the completed wall, allowed a handling train to deliver the piles to the clamp crane. Pile verticality was critical as the piles must not exit the puddle clay before penetrating the underlying London clay. During piling verticality was monitored via spirit level. Once completed Sheet Piling (UK) Ltd employed CMCS Ltd to undertake a verticality survey. This included the first five 5 piles and then 10% thereafter, resulting in a total of 32 tests. CMCS Ltd utilised the lancing tubes on the piles that were welded prior to installation. The data reported the tilt of the pile and to what extent, whether leaning towards or away from the reservoir. [caption id="" align="alignnone" width="1200"] Train moving a pile to the clamp crane - Courtesy of MWH Treatment[/caption] Success of piling solution After the first week of initial set up and pressing of the 3 dummy piles, Sheet Piling (UK) Ltd installed 275 piles over the next 6 weeks. The site team completed the work in advance of programme despite difficult weather conditions. The crest of the embankment was very exposed; strong cross winds resulted in several days when piling was prevented, but the team were able to boost productivity to recover lost time. The Sheet Piling (UK) Ltd team de-mobilised mid-December; this was during the weekend of a severe snowstorm with over 6 inches falling over the Sunday night. The MWH Treatment team and subcontractors mobilised additional resources to clear the site of snow, including the 1km toe road, enabling several lorries and the 200T crane to safely leave site. Site demobilisation took place earlier than planned realising 3-weeks’ efficiency on the track matting hire period. After the Christmas shut down, the trench on the crest road was reinstated and an embankment survey was carried out by Derryard Construction to confirm the final ground levels were within tolerance. A post-installation Willowstick survey was conducted which reported that the previous flow path had been cut-of. The sheet pile wall appears to be working as designed by cutting-off the preferential flow of electric current through and beneath the northern half of the sheet pile wall; interpreted as cutting off the seepage flow. The post completion survey confirmed that the remediation works had been successful. Biodiversity The King George V Reservoir is a major wintering ground for wildfowl and wetland birds, including nationally important species. A total of 85 wetland species have been recorded and the reservoir also forms a moult refuge for a large population of wildfowl during the late summer months. Over the course of the project environmental consultants Middlemarch visited site monthly to monitor bird activity and to ensure site works were not affecting wildlife. The site compound took up the space of the sailing club/water sports club car park so MWH Treatment constructed a temporary car park providing sailing club patrons with continued safe parking. On project completion it was agreed with Thames Water that the car park should become a permanent overflow car park. A wildflower meadow is being planted to compensate for the biodiversity loss of the car park. Conclusion MWH Treatment have delivered a very successful project overcoming access, logistic, weather and environmental challenges. The sheet piling cut-off wall achieved the desired project outcome: mitigation of water seepage. The project was delivered early with an excellent safety and environmental performance. Thames Water additionally gained an upgraded toe road and improved protection for critical services. The project also fostered and maintained excellent relationships with neighbours at the sailing and activity clubs.

Usk is a large raised reservoir formed by an earth embankment dam, which completed constructed in 1955 with an approximate capacity of 12,268,000m3. The dam is 480m in length, with a maximum height of 31m, and supplies raw water to Bryngwyn Water Treatment Works and as well as providing compensation water to the River Usk, which is classified as a Special Area of Conservation (SAC) and a Site of Special Scientific Interest (SSSI). The project focuses on the replacement of the reservoir draw-off pipework within the dam’s outlet tunnel. Project drivers Welsh Water appointed Mott MacDonald Bentley (MMB), who form part of the Capital Delivery Alliance, to design and construct the replacement of both historic 18-inch scour mains in the tunnel to improve emergency drawdown capacity and to provide facility for enhanced releases to the River Usk. The works will improve drawdown capacity and as such form part of the works required to address a statutory measure in the interest of safety (MITIOS) under the ambit of the Reservoirs Act 1975. Works to provide ‘enhanced releases’ allow a range of discharges from the reservoir, with the aim of providing benefits to the River Usk and its habitats, which is in line with the European Habitats Directive. Furthermore, the works provide enhanced releases to contribute to the United Nation’s Sustainable Development Goal (SDG) 6. In particular, it contributes to achieve the target of 6.6: ‘By 2020, the aim is to protect and restore water-related ecosystems, including mountains, forests, wetlands, rivers, aquifers and lakes’. Optioneering The historic pipework and tunnel characteristics caused a variety of constraints on the new system that needed to be considered when finalising the desired pipework arrangement. The project aimed to safely maximise the potential drawdown capacity whilst working within these constraints, some of which are described below. The historic arrangement did not allow for easy inspection or maintenance. At the upstream end of the tunnel, the historic 18-inch pipes pass through a concrete plug, which cannot be replaced without a full drawdown of the reservoir. Emptying the reservoir was not feasible due to Usk Reservoir supplying large volumes of raw water for supply and compensation purposes. As a result, the historic 18-inch pipe section formed a constraint on the design and construction of the permanent works. [caption id="" align="alignnone" width="1200"] Pipework and valve arrangement within the valve chamber – Courtesy of MMB[/caption] The tunnel cross-section is a 2.4m diameter horseshoe shape. Access to the upstream end of the dry side of the tunnel is necessary to operate, inspect and maintain the new pipework and valve arrangement. The pipework arrangement and diameter needed to consider the ability to remove sections of pipework if required at a later date. A long list of options were developed, however the chosen solution was to replace the twin 18-inch pipework with a single larger diameter pipe, offset to one side of the tunnel. This option maximised the outflow from the reservoir and maximised space for access, inspection and maintenance. To merge the two 18-inch pipe sections from the tunnel plug, an asymmetric manifold was designed to combine the flow, with guard and duty valves upstream of this. Enhanced releases are provided by two flow control valves installed offline to the new drawdown pipework, that could be remotely operated using a telemetry system located in a new control kiosk. Other options considered during the optioneering included: Increase diameter of one 18-inch diameter pipe downstream of plug, with the other 18-inch diameter pipe to discharge into tunnel downstream of plug. Larger single pipe solutions, up to 1200mm diameter. Twin 600mm diameter pipes. Moving valve house upstream 50m to immediately upstream of a bend in the tunnel. Design considerations Another consideration in the pipework design, was to limit the flow velocity through the twin 18-inch sections of pipework and valves. If the system was operated for a prolonged duration with excessive velocities, there would be a risk of causing damage to the system through cavitation and excessive wear. [caption id="" align="alignnone" width="1200"] (left) Gate valve transportation and (right) bulkhead valves with concrete encapsulation - Courtesy of MMB[/caption] Following the feasibility stage, the pipework system was further optimised to improve hydraulic efficiency whilst maintaining velocities to a suitable level. The manifold was optimised to achieve balanced flows between both legs, to prevent significantly higher velocities within a single leg. The results of the optimisations enabled the diameter of the larger pipework to be reduced from the initial proposal at the feasibility stage, leading to a reduction in material costs and embodied carbon by approximate 10%. The diameter of the larger pipework was 800mm, and the flow rate measured in the commissioning phase was approximately 5m3/s. Concrete tunnel investigation As some cracking had been observed to the tunnel soffit, a condition assessment of the concrete tunnel was completed. Intrusive investigations were undertaken to obtain cores to determine the cause and depth of cracking, mechanical properties, and petrographical assessment of the concrete and the section thickness. Several crack monitoring locations were monitored over a period of 21 months to determine ongoing and potential future movement. The results suggested that limited future movement will take place. Some of the intrusive investigations also showed evidence of alkali silica reaction (ASR). Following a suite of expansion testing, the results showed that future effects of ASR would be limited and unlikely to cause significant deterioration. To help mitigate the risk of deterioration due to water ingress into the cracks, a proactive repair to the cracks was undertaken, and long-term monitoring of the cracks was established. Usk Reservoir: Supply chain - key participants Principal Designer: MMB Principal Contractor: MMB FRC works: Doohan Civil Engineering Electrical & control: Bridges Electrical Engineers Ltd Diving works: Edwards Diving Services Metalwork: D Hughes Welding & Fabrication HPPE siphon pipe: Aliaxis UK - GPS PE Pipesystems Pipework supplier: UTS Engineering Ltd Submersible pumps: Selwood Valves: Glenfield Invicta Flow meters: Siemens Concrete repairs: RAM Services Concrete cutting & drilling: Gnat UK Ltd Concrete supply: Hanson UK [caption id="" align="alignnone" width="1200"] Siphon and over pumping intakes - Courtesy of MMB[/caption] Key temporary works To maintain mandatory water compensation from the reservoir to the River Usk once the historic pipework removal had commenced, a twin siphon system was installed from the reservoir basin to the spillway’s stilling basin. Each siphon consisted of 280m long, 400mm OD HDPE welded pipework, with ductile iron fittings. The siphon was designed to operate down to -4m below top water level, which was typical for Usk Reservoir during the summer as the water level fluctuates. It used a pump to prime the siphon, and the pump could also be used in the event the siphon loses prime to discharge the minimum compensation requirement. The siphon was found to be reliable, and throughout the construction phase, saved approximately 260,000 litres of diesel, and 700 tonnes of CO2, when compared to over-pumping. Construction phase and commissioning Scope of work: Temporary siphon for compensation releases. De-commission of existing Portis supply main (including diving works). Demolition/re-construction of the valve house. Construction of new control kiosk and compound. Removal of three existing pipes within the tunnel, each at 180m in length. Installation of new electrically actuated guard and duty isolation valves, whilst working under single isolation. De-commissioning of the existing manual guard valves. Installation of the new 450mm>800mm pipework inclusive of manifold. Installation of valve house control valves/access metalwork. Security provisions. [caption id="" align="alignnone" width="1200"] Valve chamber and tailbay - Courtesy of MMB[/caption] The historic valve house was demolished and replaced with a new buried valve chamber, and the tailbay was relined with a new reinforced concrete structure. This was due to the visual condition of the historic structures, to create a larger area for the new valve and access arrangements, and to facilitate the installation of the new pipework and valves. The first task within the tunnel was to remove the three existing pipes. During the feasibility surveys it was found that the existing pipework and fittings contained asbestos, MMB commissioned a specialist asbestos contractor to remove the pipework adopting a pipe wrapping containment and use of a wire saw cutting technique. Once the pipework was removed all existing concrete pipe supports were also removed and the existing tunnel invert made good. Once all of the existing pipework had been removed up to the existing guard valves (at the tunnel bulk head), works to remove the existing duty valves and replace with new continued under client Gold Command control due to the single isolation nature of the works. The works then progressed onto installing the new twin 450mm to 800mm diameter carbon steel pipework in tandem with installing a new level concrete tunnel floor. Due to the limited space within the tunnel and lack of lifting facility MMB used a small pick and carry crane which was able to transport the new pipework into the tunnel for installation. The tunnel was 180m in length with two 45° bends, this presented some construction challenges due to the limited flexibility in the flanged system. The new valve house arrangement was then installed and incorporated the compensation, enhanced releases, and scour valves complete with extension spindle and pedestal arrangements for operation via electrical actuation on the new raised flooring arrangement. [caption id="" align="alignnone" width="1200"] (left) Scour pipework arrangement at the bulkhead and (right) scour pipework & thrust block - Courtesy of MMB[/caption] Programme The scheme was mobilised on site in July 2020 which presented additional unprecedented safety challenges due to the global COVID-19 pandemic. The site adopted stringent social distancing working on site and use of respiratory PPE when working in close proximity within the confines of the tunnel. The works were undertaken under the supervision of the QCE, and the pipework and valve system were successfully wet commissioned in April 2022.