Supply Chain - Site Preparation - Vegetation Management

Bourn Reservoir is an above-ground reservoir complex located approximately five miles outside Cambridge, England. Only one tank was still operational after the other two were removed from service due to natural degradation. A new tank was required to meet demand from large-scale developments in the supply area. A review was conducted to ascertain the total water volume required, revealing the need for an additional 8 ML of storage on top of the capacity of the remaining tank by the summer of 2022. Stonbury was contracted to demolish the disused circular prestressed tank in 2020, whilst the other remained in service, and in 2021, Stonbury commenced the construction of a new concrete tank next to the operational reservoir.

Scope of works

A summary of the works undertaken is as follows:

• Disconnection and demolition of the old Reservoir No 2.
• Ground investigation works and tree removal.
• Groundworks.
• Design of new concrete tank.
• Construction of new tank, drainage and connecting pipework.
• Installation of MEICA elements.

Site description

The Bourn Reservoir site is a small, spatially-constrained compound that originally contained Reservoirs 2 and 3. Land owned by the farmer adjacent to the site was utilised for the site compound which housed welfare units, a storage area, and turning space for vehicles and machinery. The land was returned after the programme was complete. [caption id="" align="alignnone" width="1200"] View of tank under construction with newly painted exterior walls - Courtesy of Stonbury[/caption]

Project team

The project was delivered for the client, Cambridge Water Company, by direct-delivery water sector specialist contractor, Stonbury, as principal contractor and principal designer, and Mason Clark Associates as their designer.

Bourn Reservoir: Supply chain - key participants

• Client: Cambridge Water Company
• Principal contractor: Stonbury
• Designer: Mason Clark Associates
• Subcontractor: GM UK Ltd
• MEICA installation: IWS Mechanical & Electrical
• Ecological appraisal: Middlemarch Environmental Ltd
• Concrete delivery: Hanson UK
• Concrete pump hire: Camfaud Concrete Pumps Ltd
• Crane hire: Emerson Cranes
• Handrail & staircase supplier: L&G Engineering Ltd
• Internal steps & ladders supplier: Chemglass
• Access doors & hatches: Technocover Ltd

Project challenges

The build possessed several unique challenges due to constraints on time and space. The planning application review period was extended for further environmental assessment as part of the Tree Retention Scheme, which introduced significant time constraints to the programme. Proactive action was taken to minimise delay, and a revised foundation plan was submitted accounting for the proximity of tree roots and including provision of a perimeter root barrier trench for additional protection. Planning permission was granted in October 2021, leaving under a year to complete the project by summer 2022. Innovative pre-construction planning was required to overcome the issues posed by the constrained size of the operational area. As the site footprint dictated a tank shape that was non-optimal for water flows, a computational fluid dynamics (CFD) analysis was completed to identify a ‘best practice’ design to reduce dead spots and protect water quality. As the new tank footprint was partially on virgin ground and partially on ground where the previous structure had stood, there was a risk of differential settlement. To mitigate this risk, a tailored ground investigation was conducted to obtain data for a predictive settlement analysis to inform the engineered fill design and ensure a robust outcome. [caption id="" align="alignnone" width="1200"] Tank under construction, showing precast concrete slabs being maneuvered into place by a crane - Courtesy of Stonbury[/caption]

Design

Mason Clark Associates was responsible for the civil and structural design of:

1. The reinforced concrete twin-compartment reservoir, excluding the detailed design of precast roof slab panels.
2. The reinforced concrete valve house.
3. The surface, underdrainage, and foul water drainage systems.
4. The external steel roof access staircase.

The client stipulated that each cell would require the following features:

• Opposed high-level inlet and a low-level outlet, connected to the existing network.
• Automated control valves.
• Combined metered washout/overflow pipe.
• Overflows with sufficient capacity to take design inflow whilst maintaining sufficient freeboard to the soffit of the roof.
• Washouts, sized suitably to enable the drain down of each cell over a 24 to 48 hour period without causing flooding to surrounding watercourses.
• Sufficient ventilation to ensure that each cell is not over-pressurised during filling or a vacuum created during drawdown.
• Access doors and hatches to meet LCPB 4 rating and SEMD requirements.
• All necessary ductwork for telemetry and security cabling.
• Mains alterations to accommodate the new reservoir.
• Arrangements for taking compliance samples.

The proposed tank was to have an approximate footprint of 850m² and operate with a top water level of 79.2mAOD (ground level is approximately 71.4mAOD). The inlet was to be connected to the pumping main, whilst the outlet connected to the same main as Bourn Reservoir 3 so that they would operate hydraulically together. The placement of inlet and outlet valves, baffle walls and weir boxes were designed in accordance with results from the CFD analysis. [caption id="" align="alignnone" width="1200"] Demolition of the disused circular tank prior to construction - Courtesy of Stonbury[/caption] Addressing the drive to build assets with lower embodied carbon and to reduce the length of the programme, the design incorporated the use of precast concrete roof slabs in addition to conventional in-situ pour concrete. This was preferred to a fully precast solution due to the risk of perennial ingress issues at precast joints. This would help to reduce embodied carbon by approximately 10% compared with a traditional build and make the 48-week programme possible. The structural elements of the reservoir and pipework were to have a minimum design life of 50+ years, and 20+ years for all other elements. All structural and geotechnical design was in accordance with the relevant section of the Eurocodes, and all concrete elements were designed with a water-tightness classification of 1. All work was designed to be carried out in accordance with the latest CESWI, WIMES and Cambridge Water Company general engineering specifications and to meet DWI requirements for potable water storage units and the current SEMD guidance.

Environmental management

A Project Site Environmental Assessment sought to register and mitigate site-specific environmental impacts such as noise, land and watercourse pollution, waste materials and damage to local ecology. Identified environmental risks were controlled through measures detailed in Risk Assessments and Method Statements. A preliminary ecological appraisal was undertaken during the pre-construction phase to determine the presence of dedicated nature conservation sites and protected species in proximity to the site. The site was not within or near an SSSI. No notable plant species were identified but some notable animal species were. Recommendations were provided based on the ecological mitigation hierarchy to avoid, mitigate, and compensate during and after the programme. [caption id="" align="alignnone" width="1200"] Construction workers building reinforcement cage for walls - Courtesy of Stonbury[/caption]

Health & Safety

A detailed H&S file was prepared for the client. During construction, special site-specific health and safety considerations were planned, managed and monitored using a variety of means, including:

• Site-suitable PPE.
• Approved RAMS.
• Vibration, noise and CoSHH assessments.
• Daily Point of Work Risk Assessments (PoWRAs).
• Suitable barriers and safety signage.
• Obstructions or trip hazards protocol.
• Agreed working hours and strict adherence to quiet periods.
• Asbestos protocol.

Demolition

In 2020, existing services into Reservoir 2 were disconnected and removed in preparation for tank demolition. Due to the spatial constraints of the site, demolition was carefully planned with an appropriate methodology to mitigate risk to the live operational tank. An excavator was employed to demolish the roof, with an attached claw to cut the concrete walls into small vertical sections. All broken material was pushed into the middle of the tank during demolition to protect the adjacent asset. Once demolished, material was removed from site. [caption id="" align="alignnone" width="1200"] The site for the new reservoir after demolition of the previous tank - Courtesy of Stonbury[/caption]

Site preparation

A site compound was constructed for offices and welfare facilities. This area also had space for limited parking, temporary materials storage and a turning area for construction vehicles. Heras fencing was erected on the boundary of the compound area and construction warning notices were posted at the site entrance gates. To prepare the area for the reservoir, the locations of services were pre-determined using a combination of existing service drawings, site knowledge and CAT scanning using a signal generator followed by hand-excavated trial holes.

Base construction

The site for the new tank was excavated, removing spoil and saving a small stockpile for backfilling. Old balancing pipework was removed, and the existing pipes were capped so that they could be reconnected upon project completion. Underdrains were installed. Fill-to-base was completed in accordance with the design informed by the predictive settlement analysis. The area was levelled, filled with type 1 material to the specified formation level and rolled. A 50mm concrete blinding base was poured from a dumper and levelled by hand. Concrete square bars were laid by hand to support the reinforcement steel placed directly on the blinding slab and tied, ensuring correct laps in accordance with the design. Timber formwork was levelled using a laser level placed on each end. Once this was braced, the floor was completed in four separate concrete pours. The sump was poured at the same time using a concrete pump and vibrated constantly as the concrete entered the reinforcement. [caption id="" align="alignnone" width="1200"] Formwork panels - (left) outside view during construction of the upper wall, and (right) inside view during construction of walls and columns – Courtesy of Stonbury[/caption]

Wall construction

A panel system with film-faced plywood was built for the wall and column formwork according to drawings. Completed shutters were lifted into place using an excavator for the 3m high lower walls and a tower crane for the 7.8m high upper walls. Reinforcement cages were fabricated on the ground and lifted into position with a tower crane to minimise working at height. Inner shutters were held in place with push-pull props drilled and fixed to the base slab. Outer shutters were fixed using Dywidag bolts and tie bars on the outer edge. Timber stop ends were fixed to the ends of the shutters. The concrete was poured in sections and vibrated as filling proceeded, employing both external formwork vibrators and 10m long internal vibrators to achieve the required concrete finish. Shutters were lifted, cleaned and re-positioned for each 5m wide pour section. Once all sections were full, laser levels were used to check levels and tops were floated off.

Pipework

At this stage, trenches were excavated for overflow and outlet pipework. Outlet pipework was installed first and connected to the existing valves. A flow meter was installed in line and a chamber was built around it. Pipes were capped prior to installation, chlorinated and bagged off to prevent contamination. Once installed, pipes were pressure-tested and the trenches were filled with stone and backfilled in layers. The flow meter chamber was excavated using a 5-tonne excavator and concreted with formwork and vibration. A Technocover lid was drilled and fixed to the cover slab and a galvanised ladder was affixed to gain access to the flow meter. Ducting was installed for the control panels and mains power. Once the main walls were complete, internal access and pipework were installed. [caption id="" align="alignnone" width="1200"] Construction workers guiding precast concrete roof slabs into place - Courtesy of Stonbury[/caption]

Roof construction

To construct the roof, 30 steel beams were lifted onto the reinforcement cage using a tower crane with two operatives in scissor lifts to guide them. The tower crane was then used to manoeuver 86 precast concrete panels (150mm) to create the roof slab. The panels were attached to the crane with a four-legged chain and lifted into position, guided by operatives in scissor lifts. Once all the panels had been placed on the roof, the joints were in-filled with concrete ready to take a 150mm structural concrete screed and Lytag levelling screed, laid with a fall of 1:80 front to back to allow water to run off from the roof. The outer walls were washed, spray-primed and painted with anti-carbonation green camouflage paint to protect the concrete from weathering and reduce visual impact from the road. A waterproof liquid seamless coating system was applied to the tank roof with anti-slip walkways. Finally, a permanent exterior access staircase and perimeter handrailing were installed.

Leak tests

The reservoir was filled with water and underwent a seven-day drop test to ensure there were no leaks. Once successfully passed, the tank was drained for a final inspection, clean and chlorination. Outside, attenuation drainage was installed and security fencing was erected. MEICA elements were installed by specialist contractor IWS Mechanical & Electrical. Their scope included a full ICA upgrade of the site, reservoir level monitoring, electronic security and electrical distribution for the site. [caption id="" align="alignnone" width="1200"] (left) Construction worker using the newly installed exterior staircase, and (right) the completed tank undergoing an integrity inspection - Courtesy of Stonbury[/caption]

Post-construction

Earlier CFD modelling and ground investigations had indicated the requirement to fill both cells immediately upon commissioning and keep them simultaneously in use for the first six months of operation to ensure ground was consolidated following the build. As a consequence, the inspection of the internal dividing wall could not be completed during the asset handover. To provide assurance to the client, Stonbury arranged to undertake a further inspection after six months, which confirmed integrity.

Conclusion

The project was completed according to programme without delays or health and safety incidents. The new tank was connected to the network in October 2022 and will increase water security within Cambridgeshire for decades to come.

The Strelley to Redhill Pipeline Project involved constructing and installing a large steel pipeline to connect the Strelley and Redhill Reservoirs. This pipeline will significantly boost the flow rate between the reservoirs, allowing up to 35 million litres of water to be delivered each day. This strategic upgrade would ensure a robust and reliable water supply for Nottinghamshire and surrounding areas; particularly in anticipation of future AMP8 developments. The route for the new pipeline, located 6km northwest of Nottingham City Centre, traversed a diverse landscape that included agricultural fields, residential areas, railways, highways, car parks, business parks and existing Severn Trent land and infrastructure.

Avove was contracted by Severn Trent to construct the 16km major infrastructure project. To facilitate the pipeline’s construction, Avove’s Vegetation Management teams worked closely with operational and construction teams to clear hedgerows, vegetation and woodlands along the designated route. Throughout the process, environmental sustainability was at the forefront of operations, safeguarding local ecosystems and fostering positive relationships within the community. This collaborative, ecologically responsible approach resulted in successful outcomes for all stakeholders, including Severn Trent, Avove, local wildlife, local residents and the broader community. [caption id="attachment_18185" align="alignnone" width="1200"] The Strelley to Redhill site compound showing the TBM being lowered into the shaft and the pipes ready to be laid - Courtesy of Avove[/caption]

The challenge

The Strelley to Redhill pipeline project presented several significant challenges, which required the Avove Vegetation Management team to conduct land surveys to identify potential issues with the proposed route. The following specific challenges were identified at this phase:

• Complex route intersection: The proposed pipeline route needed to intersect major UK infrastructure, including a Network Rail line and the A610. Navigating these intersections required careful, comprehensive planning and coordination to avoid disruption in the local area and to ensure the safety of both the team and residents.
• Design adaptations & approvals: Land surveys revealed potential risks associated with the original design, necessitating the quick development of alternative design solutions and diversions. These solutions required thorough testing for feasibility and swift approval to avoid delays.
• Community engagement & representation: Ensuring that local residents were adequately represented and consulted during the vegetation management works was crucial. The team needed to address community concerns and ensure that the project proceeded with minimal disruption to the daily activities of local residents.
• Environmental & ecological risks: The clearance of vegetation across agricultural sites introduced ecological risks, such as the presence of invasive Japanese knotweed that presented a significant health and safety risk due to its ability to cause damage to infrastructure; including sewers, drains and water mains. In addition, the work needed to be completed before the onset of nesting season, to prevent disruption to local wildlife. The team had to develop a delivery strategy that mitigated these risks, whilst maintaining the project’s timeline.
• Preservation of heritage sites & vegetation: The project needed to include the preservation of local heritage sites, such as Bestwood Country Park and the protection of mature wildlife, including a 150-year-old oak tree. The team faced the challenge of balancing the project’s infrastructure goals with the preservation of these valuable natural and cultural assets.

[caption id="attachment_18762" align="alignnone" width="1200"] Courtesy of Avove[/caption] To overcome these challenges, the Vegetation Management team worked alongside other Avove teams involved with the project to develop solutions that reflected best practices. Being able to leverage the existing expertise of colleagues ensured that bespoke solutions could be developed, representing simplicity, innovation and cost efficiency for Severn Trent.

The approach

The Avove Vegetation Management team, working in collaboration with in-house design and ecological functions, successfully delivered the work according to Severn Trent’s requirements, providing cost-effective design amendments that rectified the challenging planned route.

• Design diversions & alternative solutions: Avove quickly identified operational risks and suggested route diversions to avoid potential hazards. For example, they a small diversion was designed to avoid uprooting Japanese knotweed, an invasive species that grows rapidly and requires costly chemical treatment to successfully eradicate it. This solution not only mitigated the risk this plant caused to the project, but also provided a cost efficiency of approximately £250,000 for Severn Trent.
• Protection of vegetation: The team took special care to protect local heritage sites and mature trees. For instance, they implemented excavation techniques that preserved the root systems of protected lime trees near a local church. Excavation was undertaken using vacuum techniques to avoid damaging the root plate. The team also designed a route diversion to protect a 150-year-old oak tree near Bestwood Country Park. In areas where excavation was necessary to the project, such as outside of Bestwood Country Park, near to a local housing estate, the team engaged with the community to develop replanting initiatives that ensured the greenspace would be restored to its former state.
• Community engagement: Avove prioritised community engagement throughout the project. The team held replanting forums and consultations to ensure that the local community needs were met and that their concerns were addressed. This approach helped to build positive relationships with residents and reinforced both our and Severn Trent’s commitment to social responsibility.

The Solution

Avove successfully delivered this vegetation management project in line with Severn Trent’s requirements, maintaining a significant portion of the original design whilst ensuring that all vegetation clearance was completed within an 8-week period. This rapid execution was crucial for facilitating the construction of the pipeline and ensuring that the pipeline project, as a whole, remained on schedule. In addition to the efficient delivery of this project, we collaborated with Severn Trent to ensure that that the execution of the original design didn’t present unexpected commercial complications by suggesting alternative routes, leveraging the expertise of our in-house functions to provide a one-stop shop solution to risks that would have otherwise been complicated to manage. [caption id="attachment_18761" align="alignnone" width="1200"] Courtesy of Avove[/caption]

Benefits

The successful execution of this stage of the Strelley-Redhill Pipeline Project, delivered numerous benefits for Severn Trent, the local community and the environment. These included:

• Cost savings & efficiency: The design amendments, such as the diversion around Japanese Knotweed, resulted in substantial cost savings for Severn Trent and ensured the project would be completed within the designated timeframe.
• Proactive community engagement: The team built trust and fostered positive relationships within the community, which alleviated concerns and ensured comprehensive communication.
• Environmental responsibility: The commitment to environmental responsibility ensured that the project had minimal impact on local ecosystems, safeguarded mature trees and involved replanting initiatives, which preserved the area’s natural heritage.
• Enhanced water supply infrastructure: The new pipeline significantly increases the flow rate between the Strelley and Redhill reservoir, ensuring a reliable water supply.

Innovations

• Efficient route diversions: By leveraging the expertise of their integrated functions, we developed and implemented route diversions that minimised the environmental impact and avoided costly complications.
• Multi-disciplinary approach: The synergy between the Avove Vegetation Management, Design, Ecological and Customer Service teams offered a holistic approach to project management, which ensured that potential risks were managed effectively and the project’s design and execution were optimised for simplicity, innovation and cost-efficiency.

The Results

The Strelley to Redhill Pipeline Project stands as a successful example of Avove's ability to deliver complex infrastructure projects, whilst balancing environmental, community and client needs. The project was completed on time, within budget and to the highest standards of quality and safety. The results of this project also demonstrated the value of a multi-disciplinary approach to project management, which integrated Avove's expertise across vegetation management, design and environmental sustainability. In conclusion, the project’s success highlights commitment to innovation, environmental responsibility and community engagement and reinforces Avove's reputation as a leader in delivering large-scale infrastructure solutions that provide lasting benefits for clients, communities and the environment.