Supply Chain - Reservoirs - Raw Water - Intake & Outlet Pipelines

Northumbrian Water are constructing a new 43 ML service reservoir at Springwell Village near Washington, Tyne and Wear, along with associated pipework and connections to the Derwent and Carr Hill mains. The project will supply water to the village and support the wider Wearside and South Tyneside areas, improving water storage and benefitting around 50,000 properties in South Tyneside, and supporting resilience for a further 200,000 properties in South Tyneside and Wearside. Mott MacDonald Bentley was appointed as principal designer and contractor for the £26m project which started on site in June 2023 and is due for completion by March 2025. Project details

The twin cell reservoir is 100m by 75m x 9m tall with a storage capacity of 43 ML. The construction of the reservoir is semi-precast, utilising precast wall panels with an in situ concrete base, in situ infills between the walls, precast columns and roof planks and an in situ screed placed over the top of them. The site is comprised of a sloping grassland field with a fall of 1 in 10 from north to south.

[caption id="attachment_15453" align="alignnone" width="1200"] Springwell Service Reservoir 3D model - Courtesy of MMB[/caption] Planning & environment

The new service reservoir and pipework was subject to planning and full statutory Environmental Impact Assessment. Environmental assessment and planning submission was undertaken by others during the definition design stage. Environmental constraints and any required mitigation were passed to MMB for consenting and implementation. This included the discharge of numerous planning conditions from local Councils, applications for: protected species licences (including great crested newt); prior consent under Section 61 of the Control of Pollution Act, and Scheduled Monument Consent from Historic England for tunnelling work under the Bowes Railway.

Landscaping upon completion of works will implement a non-statutory biodiversity net gain on the reservoir site through planting other neutral grassland using a locally sourced seed mix on the reservoir banks and surrounding land, the reinstatement/gapping up and planting of new species rich hedgerow, and the planting of new native trees within the site. A retention basin will be constructed within the site boundary to manage runoff.

Design strategy

Once appointed, MMB began to consult with the pre-construction designers at WSP and the client to further develop and detail design the structure and pipe network. MMB worked with FLI Precast at an early stage of the development to engage with them to design a semi-precast solution for the main structure, utilising lessons learned, and knowledge gained by both organisations from constructing multiple service reservoirs in this manner.

[caption id="attachment_15452" align="alignnone" width="1200"] Springwell Reservoir base construction - Courtesy of MMB[/caption]

From the beginning of their appointment, MMB has engaged with all stakeholders to develop solutions through collaborative working and to ensure that water quality standards are met throughout the construction process. MMB has particularly engaged with WSP and NWG, who have had an on-site presence throughout the works and have been able to identify opportunities within the design for cost and carbon effective design amendments, with shorter approval windows due to the collaborative nature of the design.

One opportunity highlighted by the design team was the proposal to remove the reservoir underdrainage from the scope. By studying available guidance, in particular the UK Water Industry Research Limited, Treated Water Storage Assets: Good Practice for Operation and Management, Version 2 report.

Underdrainage is more prevalent on structures that are not considered monolithic. Due to the construction sequencing and the type of jointing techniques considered, with no movement joints along with the required fall and depth of the drainage system due to project constraints, reservoir fall direction, the lack of presence of groundwater and subsequent uplift concerns, the underdrainage system would not have been beneficial to the structure. This was presented at an early stage to the Qualified Civil Engineer appointed to oversee the structural design and was deemed surplus to requirement and omitted.

The use of BIM360 as a shared working environment allowed the client, their consultants, MMB and their subcontractors to collaboratively work on drawings, showing real time changes to the design in 3D and allowing quicker design discussions, with the aim to shorten the design program and keep the site program on target. This worked particularly well when MMB re-designed the valve house layout and sizing using virtual reality.

[caption id="attachment_15456" align="alignnone" width="1200"] Springwell valve house - Courtesy of MMB[/caption]

It was noted on receipt of the outline design that the valve house was not sufficiently designed for operation and maintenance of the assets housed within it. Therefore, MMB had to increase the size of the building itself, change pipework arrangements to ensure adequate access to assets. To make this process as efficient as possible the 3D model was used in a VR headset to aid meetings and acceptances with the client. This was better than using 2D drawings as the team could all see in real space what the proposals were, and pick up any issues to be addressed.

Due to constraints regarding planning consents, works on the project began in earnest in May 2023, whilst the detail design of the reservoir was still ongoing, leading to a truly ‘design and build’ approach on the scheme. This approach sharpened attention on the project’s critical path and detail design works proceeded with construction programme in mind.

Significant design works were conducted in relation to the geotechnical operation, which required approximately 55,000m3 of sandstone and superficial materials to be excavated. The excavated materials will be reprocessed at a local quarry to become the embankment backfills, reducing waste.

Geotechnical & earthworks

One of the initial challenges with the construction of the scheme was the variance in geotechnical properties of the founding material. Covering approximately 80% of the reservoir footprint is an area of sandstone, and the remaining 20% an area of superficial materials, comprising weathered sandstone and granular glacial till to the south side of the reservoir.

Due to this variance, there was a significant risk of differential settlement of the base, which was highlighted during the definition phase. Initial settlement calculations led to large differential settlements which would have created intolerable stresses within the structure, so a ground improvement solution was sought. The solution adopted was to use a large temporary berm of material, comprising of the excavated material for the reservoir, to surcharge the softer material to the south. This berm of material (measuring approximately 8m tall, generating greater bearing pressures than the final structure would when filled to maximum capacity) was constructed, along with monitoring points using settlement gauges within this material, to drive out the anticipated settlement.

[caption id="attachment_15459" align="alignnone" width="1200"] Springwell settlement bund - Courtesy of MMB[/caption]

The monitoring results were analysed showing that the settlement was achieved quickly, with a significant tail off from any additional settlement recorded. The trendline of the settlement indicated granular properties over more cohesive, which confirmed that rebound of the material was unlikely. The berm was removed and then an assessment of rebound of the material was made. Initial programmed time for this berm was significantly longer than eventually needed and due to the close monitoring of this settlement it resulted in some programmed time savings to the works.

Precast construction & value added

The use of the precast walls reduced the scale of wall reinforcement fixing reducing both a risk of the reinforcement cages moving during the temporary construction phases and the time duration of works at height for setting the reinforcement and pouring operations, making construction safer. The reduction in concrete pours reduced the number of deliveries to site and the number of concreting operatives at any one point.

Off-site production of the walls aided in finish control of the concrete and allowed for higher accuracy of the placement of pipe wall couplers and setting in submarine door frames with less likelihood of cracking around these openings.

Precast wall panels installation is quicker than traditional wall pour construction, which has aided the construction programme, however this has led to a significant number (231) of wall infill pours required. As these locations are a potential area for leakage, good curing and placing procedures were required. The infill pours are tall and thin (approximately 8.5m tall and nominally 500mm to 1m wide) and significant care was given for the placement using tremie pipes and rate of concrete pour, with considerable care on concrete delivery times taken. Bar spacing within the infill was designed with a view of construction procedures, ensuring adequate bar spacing for effective concrete compaction via vibratory pokers. [caption id="attachment_15455" align="alignnone" width="1200"] (left) Wall panels and (right) wall infills - Courtesy of MMB[/caption]

SMR Projects Ltd supplied a Xypex additive for the concrete mix, which reacts with water and forms crystalline structures closing small voids/cracks that may occur from various thermal effects. Controlled permeability formwork was used to densify the surface of the concrete by limiting evaporation of water from the concrete surface. This increases resistance to soft water attack which is prevalent in this area of England. An impermeable sheet was also used over the infill following striking of the formwork for the curing process to prevent the concrete from drying out too quickly, again to reduce early cracking of the concrete surface creating a higher quality product.

From an operational perspective, using submarine doors to access the reservoir cells through the valve house allowed MMB to design out a requirement for access directly through the reservoir roof, thus reducing the number of openings in the roof helping to prevent contamination ingress to the reservoir via additional roof hatches and reducing fall from height risks associated with such access.

A further saving was made during design by ensuring that the valve house was capable of housing the electrical and signal board components needed for monitoring of flows and operation of valves. This removed the need for an external electrical control building that was considered in the planning application.

Early planning & stakeholder engagement

The Springwell Project, located near a new housing estate close to the village, has led the team to focus on early engagement with local stakeholders to minimise disruption, including road closures, bus and footpath diversions, construction traffic, and the impact on local businesses, as well as noise and dust concerns. To ensure the community feels informed and involved, MMB have participated in meetings and workshops with residents to provide project updates, get local input and answer questions.

The approach to community engagement has also included regular drop-in sessions, quarterly information events, newsletters, and site tours to keep residents up to date throughout the project. These initiatives have helped build a strong, positive relationship with the community, reflecting MMB’s commitment to minimising disruption and improving the local environment.

There has been significant emphasis on supporting education, particularly in STEM. By partnering with Newcastle College, we’ve organised site visits and provided two students with work experience, while local primary schools have been engaged through activities such as site safety aiming to spark interest in engineering and construction. Beyond education, MMB have collaborated with local organisations and the project’s subcontractors have also contributed to the community by installing a reading area at Springwell Village Primary School, demonstrating how MMB have harnessed resources and partnerships to deliver meaningful improvements.

[caption id="attachment_15451" align="alignnone" width="1200"] Springwell Reservoir under construction - Courtesy of MMB[/caption] Springwell Reservoir: Supply chain - key participants Client: Northumbrian Water Main designer & contractor: MMB Client’s designer: WSP Earthworks: Thompsons of Prudhoe Excavations: Cheetham Hill Construction Ltd Soils material testing: Dunelm Geotechnical & Environment FRC: Cidon Group Precast design & install: FLI Precast Solutions Ltd Precast roof panels design & supply: Stresslite Precast Xypex concrete waterproofing chemical additives: SMR Projects Ltd (UK Xypex Distributor) Pipeline cleaning & dechlorination: Pipeline Testing Consultants Ltd Tunnelling works: Active Tunnelling Ltd Directional drilling: Ken Rodney Construction Underpressure connections: Power Technologies Services Ltd Connections to NWL network: EJ Peak Technology Pipe welding: Serimax Ltd Pipework & fittings: UTS Engineering Ltd Pipework & fittings: Electrosteel Castings (UK) Ltd Pipework & fittings: George Green (Keighley) Ltd Flow controls: AFFCO Flow Control (UK) Ltd Pipe brackets: Deritend Group Ltd Metalwork: JHT Fabrications Fencing: Middleton Forge Ltd Conclusions

The reservoir is part of a broader package of works to improve the resilience of Northumbrian Water’s clean water network and the reservoir aspect of the scheme is approximately 70% complete at the time of writing (August 2024).

[caption id="attachment_15454" align="alignnone" width="1200"] Springwell Reservoir columns and roof - Courtesy of MMB[/caption]

Through MMB’s commitment to protecting the environment and social engagement the project has limited disruption where possible and aimed to build a stronger connection with the local community.

Direct engagement with the client during the enabling works and beyond has expedited the programme of works, allowing longer lead items to be procured earlier and aid in achieving this challenging programme.

The use of a large berm of excavated material during the enabling works for the scheme has induced settlement within the ground prior to the structure installation, thus reducing stresses on the structure and allowing a more efficient design to be made.

Engaging with the precast concrete panel designer at early stages of the construction programme allowed construction efficiencies to be achieved with respect to structural arrangements and thickness, reducing embedded carbon for the structure. Through the team’s collective experiences of building modern service reservoirs, MMB have endeavoured to create a structure which should be less prone to leakage. Further works and testing are ongoing to realise this so far successful project.

Llyn Bran is located in Denbighshire, north-east Wales and was a natural lake that was raised in 1899 by a concrete dam. The reservoir was used to supply water to the North Wales Hospital in Denbigh. The hospital closed in 1995 and Llyn Bran has not been used for drinking water since. As part of a separate discontinuance scheme at Llyn Anafon, Llyn Bran was identified as a ‘compensatory site’ for the loss of habitat at Anafon. One element of turning Bran into a compensation site was the removal of the dam, as well as other ecological enhancements. Through careful restoration work, Llyn Bran will eventually become a Special Area of Conservation. The reason for Llyn Bran discontinuance Llyn Bran Reservoir is required as a ‘compensatory site’ for the loss of habitat area that will be caused by discontinuing the dam at Llyn Anafon Reservoir, as required by the Habitats Directive. Llyn Anafon Reservoir was originally a natural lake which was raised by 1.4m in 1931 through the construction of an embankment dam. The use of this site for local water supply ended in 2002. A 2016 S10 inspection of Llyn Anafon resulted in a MITIOS (Measures in the Interests of Safety) to “develop a long-term solution to the poor and deteriorating condition of the dam”. In January 2019, it was agreed with relevant regulators that to provide a long-term solution, the reservoir would need to be decommissioned, with the dam structure removed and the lake lowered by 1.4m. [caption id="" align="alignnone" width="1200"] Llyn Bran before discontinuance - Courtesy of Welsh Water[/caption] Llyn Anafon is nationally and internationally protected for specific conservation features. This means prior to any project taking place, a full Habitats Regulation Assessment (HRA) had to be undertaken. The outcome of this assessment was that it was not possible to avoid adverse impacts to protected features and as such, the only route to proceed with the project was under the “Imperative Reasons of Overriding Public Interest (IROPI)” (Article 6.4 of the Habitats Directive) on the grounds of public safety. The Anafon Dam Discontinuance Scheme is the first IROPI case of its kind in the UK and has recently been approved by Welsh Government. To satisfy the IROPI case, Welsh Water was required to secure ‘compensatory measures’ for the loss of habitat at Anafon. Using a set of criteria agreed with National Resources Wales (NRW), Llyn Bran was identified as the most suitable site to provide the necessary compensation. A key element of the work was to ‘restore and maintain hydrological functioning’ i.e., removal of the dam to create a natural lake and restore connectivity to the downstream watercourse. Although the primary reason for discontinuing Llyn Bran was for compensation, it also had the added benefit of removing a dam which was no longer needed for operational purposes. [caption id="" align="alignnone" width="1200"] Llyn Bran dam before discontinuance - Courtesy of Welsh Water[/caption] Scope of works Create temporary construction access using bog mats. Draw down the reservoir by means of siphon pipes and pumps. Remove silt from the area immediately upstream of the dam. Demolish the entire dam and remove arisings for disposal off-site. Restore the watercourse channel downstream of the retained natural lake, incorporating silt management and retention measures. Create new water vole habitat along the shore areas of the retained natural lake using coir matting. Demolish outbuildings and associated structures with all building waste removed for disposal off-site. Remove exposed sections of pipework. Buried pipework to be sealed and filled by grouting. Install new roadside fencing along the north end of the reservoir. Install rip-rap protection around the upstream end of the B4501 culvert. [caption id="" align="alignnone" width="1200"] Ground conditions - Courtesy of Welsh Water[/caption] As with all discontinuance schemes the various surveys, permits, consents and negotiations in advance of the physical works were particularly complex, taking more than two years to complete. An overview of the consents and constraints required is below: Impoundment Licence and SSSI consent from Natural Resources Wales (NRW). Permitted Development Certificate from Denbighshire CC. Ordinary Watercourse Consent from Denbighshire CC. Protected species licence required for trapping and temporarily removing water voles (this was needed before permitted development could be granted). Annex 1 Habitats present around lake margins. Reservoir drawdown rate limited to 120 l/s. HRA level 1 & 2 required. Peat bog deposits along route of temporary access track. Bat surveys required prior to demolition of valve house & boat house. Challenges & constraints Site access With no formal access track to the dam, and the surrounding land being designated as Annex 1 Habitat due to the presence of peat, access was one of the main challenges. With 140 peat depths probes taken along the route of the proposed access track, and working closely with NRW, a methodology was developed for installing over 280m of bogmats to the dam without the need to excavate and remove the top layer of peat. This minimised the potential damage to the peat and made site reinstatement far easier. [caption id="" align="alignnone" width="1200"] (left) Bog mat access track and (right) amphibious excavator used to load barge with coir matting - Courtesy of Welsh Water[/caption] Peat The extent of peat encountered on site was far greater than expected once construction began. This resulted in an entire change in the contractors methodology for the coir matting. It was initially planned to use Bog Master excavator and tracked dumper to access the western shore of the reservoir, however it quickly became clear this was not practical. Thanks to the contractor being proactive, the methodology was quickly changed to using an amphibious excavator and barge, to transport the coir matting across the reservoir. Water voles Water voles were identified on site during the early site survey stages, meaning a licence was required to temporarily remove the voles during construction. Six water voles were captured by specialists and were transferred to a facility in Kent, where they were looked after while the work took place. A condition of the licence was that there must be 5-days with no water voles captured before it could be concluded that all voles were removed from site. Rather unluckily, there were two occasions where a vole was captured at the end of the fifth day, meaning the process took far longer than expected and the licence had to be extended. The voles are due to return home to Llyn Bran this summer. [caption id="" align="alignnone" width="1200"] Water voles were captured and transferred to a facility in Kent for the duration of the works - Courtesy of Welsh Water[/caption] Compensation flow A condition of the NRW impoundment licence was for a continuous compensation flow of at least 20 l/s for the duration of the works. This meant pumping to the watercourse, which had to be continually monitored. It was hoped to use a cloud-based telematics tool that makes it easy to monitor the pumps off-site but unfortunately, due to lack of signal, this did not work meaning William Hughes Civil Engineering needed to attend site over the weekends to check on the compensation flows. The summer of 2022 proved to be exceptionally dry, which crated a challenge in meeting compensation flows whist also not allowing the remaining lake to become too low. Flood risk The existing reservoir provided significant flood attenuation meaning the discontinuance increased the flood risk to a downstream highway culvert. Initially, there were concerns from the highways department that the culvert would need to be upsized, which would have added significant costs to the project. Following a detailed flood risk and stability assessment, Welsh Water were able to prove that the increased flood risk would not adversely affect the culvert and only minimal slope protection work around the culvert was required. [caption id="" align="alignnone" width="1200"] Amphibious excavator forming the new channel connecting to the downstream watercourse - Courtesy of Welsh Water[/caption] Llyn Bran Reservoir Discontinuance Project Construction timeline: 16 May 2022 - 4 November 2022 Project cost: £1.7m Drivers: Dam safety | Habitats Directive | Reducing costs | Ecological compensation measures Supply chain - key participants Principal Contractor: William Hughes Civil Engineering Design Partner: Stillwater Associates Environmental Consultants: Ricardo Energy & Environment Ecology/water vole licensing: Wildwood Ecology Ltd Silt management: Salix River and Wetland Services Amphibious excavator hire: Land and Water Plant Opportunities & Savings Forestry culvert At the design stages there were concerns about the condition of a nearby downstream forestry culvert and its ability to accommodate the reservoir outflows during emptying. Expertise within Welsh Water was utilised by requesting a CCTV survey of the culvert via the wastewater team. The team provided a prompt and efficient service and enabled us to confirm the capacity and condition of the culvert. [caption id="" align="alignnone" width="1200"] Cores being drilled through concrete dam to allow any extreme storm flows to pass through - Courtesy of Welsh Water[/caption] Dam safety mitigation During detailed design, the Dam Safety Team suggested coring holes though the concrete dam once the reservoir was drawn down to the required level. This would mitigate the risk of the reservoir refilling during severe weather, before the dam was fully demolished. Fortunately, on this occasion the weather was favourable and the cores were not needed. This approach will be considered for any future concrete dam discontinuances. Waste removal It was agreed with NRW that the stone from the haul road and compound could be transferred to a nearby quarry, just down the road from site. This was done under a waste transfer notice, saving costs. The arisings from the dam were removed off site. The dam was demolished far easier and quicker than expected, making the removal of concrete straightforward. [caption id="" align="alignnone" width="1200"] Dam demolition in progress - Courtesy of Welsh Water[/caption] Coir matting Due to the silt being drier than anticipated, there was spare rolls of coir matting left over. Rather than this being wasted, it was moved to nearby Alwen WTW where it is being stored before being used for silt management on the upcoming Cilcain Discontinuance Scheme which is due to start this spring. The future for Llyn Bran The discontinuance will give significant biodiversity opportunities in relation to the marginal areas around the retained natural lake and with the restored connectivity to the watercourse. The retained lake will also be used as a receptor site for aquatic plants translocated from Llyn Anafon. Long term ecological monitoring of the lake is required to ensure the compensatory criteria are met. This includes water quality and macrophyte monitoring as well as botanical surveys to check the natural regeneration of exposed shore habitats. A condition of the water vole licence is that activity is monitored until 2034. Through careful management and monitoring of the site, it is hoped that Llyn Bran will become recognised as a Special Area of Conservation.

Located near Maerdy in South Wales, Lluest Wen Reservoir is a Category A Large Raised Reservoir as defined by the Reservoirs Act 1975. The reservoir is impounded by a 200m long, 25m high earth embankment dam across the Rhondda Fach River and currently impounds 1.07Mm3 of water. Construction of the reservoir was originally completed in 1898. Lluest Wen Reservoir is owned and operated by Welsh Water and has historically been used with a smaller, downstream reservoir, Castell Nos for water supply. Investigation and scope Following an inspection of the reservoir in February 2018 under Section 10 (3) of the Reservoirs Act 1975, the Inspecting Engineer made a series of recommendations for measures to be taken in the interests of safety (MITIOS). These recommendations included that flows through the spillway chute be modelled under the 10,000 year and probable maximum flood (PMF) conditions, that the performance of the chute arrangement be evaluated in safely conveying the associated flows to aid scoping remediation works and to action any works as necessary to preserve dam safety. The improvement works were statutory and enforceable by the regulating authority Natural Resources Wales. Mott MacDonald Bentley (MMB) was commissioned to undertake the design and construction of the works under Welsh Water’s Capital Delivery Alliance framework. [caption id="" align="alignnone" width="1200"] (left) Base concrete pour and (right) the upper spillway - Courtesy of MMB[/caption] Lluest Wen Reservoir Spillway Remedial Works: Supply chain - key participants Principal designer & principal contractor: MMB Flow modelling: Mott MacDonald Ground investigation: Strata Geotechnics Geophysical surveys: Terradat FRC works: Concrete Structures & Floors Ltd Metalwork: Industrial Pipework Services Ltd Diving works: Edwards Diving Services Ltd Manholes: Marshalls CPM Drainage pipework: Polypipe Civils and Green Urbanisation Concrete cutting & drilling: Holemasters Concrete supply: Hanson UK Scaffolding: Mii Engineering Site buildings: Wernick Hire Flow modelling and spillway design Prior to the scheme commencing, the existing flood study was reviewed and updated to reflect current industry guidance. A computational fluid dynamic (CFD) model was developed to simulate conveyance of the PMF and 10,000 year conditions. The PMF scenario anticipated flows approaching 100m3/s, with peak velocities approaching 20m/s discharging through the spillway. Outputs from the model indicated that the existing spillway was unable to contain flows arising during each flood scenario therefore representing a risk to dam safety through erosion of the earth fill embankment. An Options Report was prepared to identify possible solutions to remedy the shortfall in spillway capacity. The report considered the following options for remediation: Construct a new spillway off-line along a straight alignment. Rebuilding the spillway along the existing alignment. [caption id="" align="alignnone" width="1200"] Armourstone and lower spillway - Courtesy of MMB[/caption] Preferred option The preferred option was to re-build the spillway along its existing alignment. This would involve re-lining the upper section of the spillway which had previously been re-built in concrete during the 1960s. Lining would take the form of a 500mm thick reinforced concrete base connected to the underlying structure via a system of dowels. A similar approach would be taken for the downstream section of the spillway following demolition and removal of the historic masonry structure. Through this section, the new spillway base was doweled to a layer of mass concrete provided for global stability. New spillway walls were constructed on the hydraulic left and right hand sides capable of containing the PMF flows having utilised the CFD model to rationalise the wall heights as far as practicable. Additionally, a central training wall was provided through upper and lower sections of the spillway to evenly distribute flows across the width of the spillway and reduce water superelevation at the two changes in direction. The CFD model was also used to extract hydrodynamic pressures which allowed optimisation of the reinforcement within the walls and bases. Global stability checks for bearing, sliding, overturning and seismic were also undertaken to verify the suitability of the design. The new spillway construction would be approximately 200m in length and 15m at its widest point. Additionally, new drainage networks would be provided to the back of walls to control groundwater levels and underneath the spillway base to protect the formation and prevent undermining of the spillway base. At the downstream end, armour stone was provided for scour protection. Temporary works A 90-tonne crawler crane was used to provide full coverage of the spillway footprint. In order to access the embankment side of the spillway, it was necessary to construct a number of culverts, embankment and roadway at the downstream end of the spillway to cross the receiving watercourse. The upstream face of the bridge was concreted to prevent erosion and migration of fill into the downstream watercourse. [caption id="" align="alignnone" width="1200"] Finished spillway - Courtesy of MMB[/caption] Design considerations The gradient at the lower section of spillway approaches a fall of 1 in 2 which presented significant challenges both in terms of pouring and finishing the concrete bases as well as the flow velocities generated within the spillway. Base pours lengths were reduced to 3m and the bays were installed in a hit and miss sequence through this section in order to prevent slumping of the concrete prior to curing. High flow velocities necessitated that the joints be protected with steel plates to prevent scouring of the joint sealant and significant hydrodynamic pressures developing on the underside of the downstream base. Particular care had to be taken to ensure the absence of abrupt deviations across the joints which could invoke cavitation and ultimately lead to concrete degradation. The water level in the reservoir is known for rising quickly during periods of extended or heavy rainfall due to its location. A temporary drawdown of 3m below top water level was initially agreed with Welsh Water and a robust management plan was implemented to prevent the spillway from operating during the construction phase. Completion The scheme was mobilised to site in October 2020 beginning with improvements to the access track to the reservoir. Concrete works were scheduled to commence in late March 2021. Following a dry summer, concrete works were completed by January 2022 with landscaping and other finishing works completed by June 2022 whereupon the Qualified Civil Engineer issued the 10(6) certificate ahead of the required October 2022 statutory deadline.

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    

The challenge The reservoir had an estimated 180 meters of cracks that needed repair, as well as the main element of the concrete that required coating to waterproof and add durability to the concrete structure. The reservoir was taken out of commission but it needed to be returned to service as soon as possible. To do this, we looked at the order of the processes to ensure the tight time frames could be met. The Xypex Solution In accordance with the Reg. 31 IFU (Instructions For Use) all cracks were broken out to a minimum depth of 37mm and a width of 25mm. The walls were prepared by removing the laitance and the whole area was saturated. To stem the ingress of water, every crack that showed moisture was sealed with Xypex Patch’n Plug which sets in seconds stemming the flow. To meet the required time frame, the Xypex Concentrate coating work was then undertaken to allow it to air cure for the specified time. Once applied, the crystalline technology takes time to migrate deep into the concrete and become fixed within the concrete matrix as a permanent solution. When the curing is complete the coating could be removed if required, in this instance it remained. During the curing process of the coating, the crack repairs were then filled using Xypex Concentrate as a Dry Pack and then finally topped off with a thin layer of Xypex Patch’n Plug to enable the fast return to service. [caption id="" align="alignnone" width="1200"] The Xypex solution; Concentrate and Patch'n Plug - Courtesy of SMR Projects Ltd - Xypex UK Distributors[/caption] The results The allowed time frame meant undertaking the processes out of the normal sequence, but this allowed the repairs to be to be delivered on time. The coating was undertaken to increase the durability but also to pick up any minor unidentified cracks. The larger cracks were repaired and sealed using the standard Xypex repair methodology, however ultimately over 300 metres of crack repairs were completed.   For more information: SMR Projects Ltd - XYPEX UK Distributors: +44 (0)1295 279747 | www.smrxypex.co.uk

 

The challenge The South East Water contact tank has a constant output of 18.1 million litres per day, it contains water which is treated with Sodium Tripolyphosphate and sodium hypochlorite before entering the distribution network. The structure is of a ‘certain’ age and has a number of active leaks that have progressively worsened over time. As the tank is in constant use, it cannot be taken off-line and so a solution was required which could be undertaken against heavy flow of water with the tank still in use. The Xypex solution Xypex not only likes water, but the repair method requires a wet environment therefore an active leaking tank is not a problem. The cracks were chased out in accordance with the DWI Reg. 31 approved Instructions for Use Document i.e. a minimum of 37mm depth and 25mm wide, following the line of the crack. Either side of the crack was cleaned and the laitance removed to expose the capillary and pore network. [caption id="" align="alignnone" width="1200"] The Xypex solution; Concentrate and Patch'n Plug - Courtesy of SMR Projects Ltd - Xypex UK Distributors[/caption] A bleed pipe was installed and secured with Xypex Patch’n Plug at the lowest point of the crack where water flow was at its greatest. Then the base of the crack was plugged using Xypex Patch’n Plug to stop the flow of water. This was then left to see if there were any additional issues that manifested themselves over night. Once the flow was proven to have stopped, the repair was finished using Xypex Concentrate both as a dry pack and a slurry in line with the IFU and Xypex’s standard repair methodology. The results The Xypex methodology using Xypex Patch’n Plug stopped the flow of water instantly and allowed the repair to be undertaken. The crystalline chemistry within the Xypex Concentrate will migrate deep in to the concrete matrix around the repair, driving water out and increasing the life and durability of the tank in those areas. [caption id="" align="alignnone" width="1200"] Courtesy of SMR Projects Ltd - Xypex UK Distributors[/caption] As the original repairs were undertaken, additional pressure was transferred to other cracks that were visible, but not leaking, and these were all repaired using the same method. In all, only 2 materials were used on site and no specialist methods were used. SMR Projects Ltd attended and were on hand for technical help throughout the installation.   For more information: SMR Projects Ltd - XYPEX UK Distributors: +44 (0)1295 279747 | www.smrxypex.co.uk