Supply Chain - Stormwater Storage - Contractors

Offwell is a small village on the outskirts of Honiton just off the A35. The existing sewage treatment works was constructed in the 1980s and serves a population of around 350. It was typical of the era, with an old style Dortmund tank with percolating filter and humus tank off the back end. Final effluent from here gravitates out from the site to Offwell Brook. The driver for the scheme was to reduce the number of unconsented spills to the environment by providing a capacity of 30m3 storm storage (exceeding the EA’s required storage volume of 25m3). Project scope

At the scoping stage, Galliford Try worked closely with South West Water to establish the most cost-efficient solution when considering both operational expenditure (OPEX) and capital expenditure (CAPEX) to ensure South West Water achieved the best whole life costs. From this exercise it was established that the scope outlined below was to be included for construction on site:

One new part buried concrete rectangular stormwater storage tank with capacity of 30m3. New LCP for the control of duty/standby pump for return of effluent within the storm tank. Relocate the existing MCERTS final effluent flow meter and re-test. New pipelines between existing equipment and new storm tank. [caption id="attachment_12889" align="alignnone" width="1200"] Storm tank construction: (left) steelwork framework and (right) shuttering - Courtesy of Galliford Try[/caption] Stormwater storage tank

Being a part-buried tank, a significant excavation was undertaken with appropriate temporary works associated to the angle of repose. Once completed, a blinding layer was installed at an appropriate level for the steelwork to be built up from.

The steel for the base was installed, shuttered, and poured in two due to a sump for the install of return pumps. Wall steelwork was then installed up to 2.5m above ground and shutter pans used for the concrete pour. The overflow from this then spills to an existing chamber on site and then to Offwell Brook.

MEICA

To enable on-site efficiencies, the majority of the LCP and kiosk were fitted out off-site in Galliford Try’s local depot, with the LCP being constructed in Plymouth and the kiosk being fitted out with the LCP and services within the same workshop.

Once completed and tested, this was transported directly to site where Galliford Try’s electrical team connected the LCP into the site network for both power and communications. Once on site, all that remained was to install the already put together framework and brackets for the isolators and ultrasonic junction box and install the pre-wired flow meters back to the already mounted heads within the kiosk.

This in turn lead to betterment of the programme and benefited by undertaking the off-site build in a controlled environment.

Site access

Access to site was challenging. The site was only accessible for construction vehicles through one road, which was a narrow, single lane track with a steep drop off on one side and tall hedge banks on the other, and there were infrequent passing places for larger construction type vehicles. It was difficult for the site team to manage construction activities and getting plant and materials to site.

After the first week of mobilising to site, Galliford Try found that the road with the verge was starting to degrade and potentially become unsafe. This was reported back to the Highways Commission. While waiting for further consultation, several other routes were identified and the project team looked at each of these routes to identify which was best suited.  After the Highways Commission responded, it was finally decided that to ease further degradation of the road that a handling/staging area was to be found in a local farmer’s yard and smaller vehicles were to be used to transport plant and materials to site.  This had an effect on programme and cost but was more manageable and safer to undertake, but left Galliford Try with one inevitable issue; concrete deliveries for the storm tank.

Galliford Try’s Client Liaison Officer notified residents on the few occasions that the concrete had to be transported through the village. Deliveries were restricted in size and were escorted through the village to prevent damage to any of the dwellings. This was undertaken for the base pour and the wall pours; with multiple wagons on each of the days.

Confined site

The footprint of the working area was confined to the site boundaries, only providing 950m2 of space, of which only 180m2 was usable for construction activities. Alongside this, there were some enabling works that required undertaking first to enable access into the construction zone, this consisted of:

New final effluent flow meter chamber. Relocation of turbidity sensor. New sample chamber with weir plate. Offwell Stormwater Storage: Supply chain - key participants Client: South West Water Project delivery, civils & MEICA: Galliford Try Civil design: Eastwood & Partners Tank construction: D Wall Construction Services Pumps: Xylem Water Solutions Metalwork: GT Fabrications Flow meter: Siemens Pipework: Saint Gobain PAM UK Kiosk: GR PRO Precision Manufacturing Instrumentation: Pulsar Measurement Aggregates: Steve Wills Haulage [caption id="attachment_12892" align="alignnone" width="1200"] Regraded operational land - Courtesy of Galliford Try[/caption] Carbon savings & in-house capability

Local resources and materials were used where possible including subcontractors, which as a result provided the local economy with a financial boost during the construction phase.

Where local suppliers were not available, Galliford Try strived to use off-site build solutions where possible to minimise commuting. Contractors from Exeter were used for the construction of the storm tank, all concrete for the project also came from Exeter, and local suppliers were used for aggregates from local quarries.

In-house resources that were utilised included project management, electrical design, LCP design/build, process control, and commissioning. The in-house workforce undertook  the site establishment, civil engineering, landscaping, MEICA works, and finally the demobilising the site.

Stakeholder management

At pre-construction phase, all third-party stakeholders were identified and entered onto a management plan. Galliford Try’s Customer Liaison Officer contacted all stakeholders prior to commencement to provide them with the appropriate project contacts should they have questions, queries, or complaints and to establish their requirements and desires throughout the construction phase of the works. As part of the management plan all communications where complaints or agreements are made are formally recorded, resolved, and closed out.

Conclusion

Initial progress on site was good with setting up site and setting the project up for an efficient delivery until the road issues occurred.

Once this was managed and materials and plant were getting to site the remainder of the project ran through a smooth transition and was delivered by the regulation date of 31 March 2024.

Hawkchurch is a small village on the Devon/Dorset border, 3 miles north-east of Axminster and 4 miles north of the fishing town of Lyme Regis. The existing sewage treatment works was constructed in the 1980s and serves a population of around 500. It was typical of the era, with an old style Dortmund tank with percolating filter and humus tank off the back end. Final effluent is pumped from the site to an outfall into the Blackwater River. The project driver was to reduce the number of unconsented spills to the environment by providing a capacity of 45m3 storm storage and inlet works screening to remove solids and enhance the operation of the works. Project scope

At the scoping stage, Galliford Try worked closely with South West Water to establish the most cost-efficient solution when considering both operational expenditure (OPEX) and capital expenditure (CAPEX) to ensure South West Water achieved the best whole life costs. From this exercise it was established that the scope outlined below was to be included for construction on site:

A new above-ground concrete storm tank with capacity of 45m3 (exceeding the EA’s required storage volume of 40m3). New access platform to span the storm tank for washing down of the storm tank. New LCP for the control of duty/standby pump for return of effluent within the storm tank. One Jacopa wave screen to be installed on the overflow from the storm tank. One new HAIGH 590 series, 6mm 2D mechanically cleaned screen and 10mm 1D hand raked bypass screen from Haigh Engineering to be installed within existing inlet channel. The site

Access to site was challenging, as the site was only accessible via one narrow road with infrequent passing places for the larger vehicles required to get construction equipment and materials to site. Additionally, the footprint of the site was only 880m2 of which only 180m2 was usable for the construction activities.

[caption id="attachment_12435" align="alignnone" width="1200"] (left) Piling rig set up, (middle) piling reinforcement, and (right) piles cropped - Courtesy of Galliford Try[/caption] The stormwater storage tank

Prior to construction it was identified that there was contaminated water within the ground. After the initial sampling, Galliford Try requested further sampling to take place to identify if this was a one-off fuel spill or not. After the second survey, it was identified that contaminated water was still present and was an issue that required managing.

Galliford Try’s environmental team looked into the best practice options to manage this, and then engaged AGS Ground Solutions Ltd to undertake a Tier 2 survey of the site, as it was considered possible that a new fissure could be created during the piling stage which could expose the contaminated water to the environment.

Through further studies and multiple discussions around this topic, to mitigate the risk, Galliford Try utililised contaminated water handling plant and pumps from Siltbuster Ltd to manage flows. Along with this, Reconomy was engaged for the removal of any contaminated arisings.

The new storm tank was built upon twelve piles due to the strata within this area a mixture of Charmouth mudstone formed some 190 million years ago and poorly stratified angular rock with clayey hillwash. To undertake the piling, a piling mat and temporary works were required, which allowed Aarsleff Ground Engineeing to mobilise to site and install the twelve CFA piles at 450mm nominal diameter.

Throughout the piling exercise, contaminated water was managed through the Siltbuster Ltd plant, and the piling activity was successfully completed with zero impact to the local water courses.

With the piling compete, steel for the base was installed and tied into the piling reinforcement, shuttered and poured in two sections due to the need for a sump to install the return pumps. Wall steelwork was then installed up to 3m above ground and shutter pans used for the concrete pour.

Overflows from the new tank will spill into an existing chamber on the site and to screen these flows further, Jacopa Ltd fitted a new 2D wave screen to screen all flows to 6mm within the storm tank.

[caption id="attachment_12436" align="alignnone" width="1200"] (left) Base shuttered and wall starters, and (right) steelwork for base slab - Courtesy of Galliford Try[/caption] Inlet works screening

One of South West Water’s aims is for everyone to feel confident about the water at their favourite beach, river, or lake. Galliford Try have installed a number of inlet works enhancement screens on multiple sites throughout Devon as part of SWW’s WaterFit Programme, and through this have gained useful experience in installing screens whilst managing flows through to the works.

Following discussions on how to manage flows at Hawkchurch STW, a new 590 Series screen from Haigh Engineering Ltd was installed into the inlet channel. Flows are screened to 6mm, with bypass flows screened to 10mm.

Hawkchurch Storm Tank: Supply chain - key participants Project delivery, civils & MEICA: Galliford Try Ground surveys: AGS Ground Solutions Ltd Civil design: Eastwood & Partners Contaminated water handling: Siltbuster Ltd Removal of contaminated arisings: Reconomy Piling: Aarsleff Ground Engineering Tank construction: D Wall Construction Services Steelwork: GT Fabrications Wave screen: Jacopa Inlet screens: Haigh Engineering Ltd Pumps: Xylem Water Solutions Pipework: Saint Gobain PAM UK Flow meter: Siemens Instrumentation: Pulsar Measurement Kiosk: GR PRO Precision Manufacturing [caption id="attachment_12431" align="alignnone" width="1200"] (left) Jacopa wave screen in the storm tank, (top right) Inlet screen installation in the channel, and (bottom right) inlet screen in action - Courtesy of Galliford Try[/caption] Carbon savings & in-house capability

Local resources and materials were used where possible including subcontractors, which as a result provided the local economy with a financial boost during the construction phase. Where local suppliers were not available, Galliford Try strived to use off-site build solutions where possible to minimise commuting.

In-house resources that were utilised included project management, electrical design, LCP design/build, process control, and commissioning. The in-house workforce undertook  the site establishment, civil engineering, landscaping, MEICA works, and finally the demobilising the site.

Stakeholder management

At pre-construction phase, all third-party stakeholders were identified and entered onto a management plan. Galliford Try’s Customer Liaison Officer contacted all stakeholders prior to commencement to provide them with the appropriate project contacts should they have questions, queries, or complaints and to establish their requirements and desires throughout the construction phase of the works. As part of the management plan all communications where complaints or agreements are made are formally recorded, resolved, and closed out.

Conclusion

Initial progress on site was good with setting up site and setting the project up for an efficient delivery until delays were encompassed by switching the piling rig out due to the environmental considerations. Once this was managed and piling underway, the remainder of the project ran through a smooth transition and was delivered by the regulation date of 31 March 2024.

Lower Woodburn Wastewater Pumping Station (WwPS) serves the Greenisland Drainage Catchment and is located close to the town of Carrickfergus on the foreshore of Belfast Lough. The storm outfall from the WwPS flows directly into the lough. Although recognised that discharges have minimal impact on the Belfast Lough Shellfish Water Protected Area (SWPA), there is localised impact and a need to protect and preserve amenity on the accessible shoreline. Upgrading of the WwPS was needed to comply with the Environmental Regulator’s (NIEA) consent requirements. Living with Water Programme

As part of Belfast’s strategic ‘Living with Water Programme’ (LwWP), NI Water has invested £1.8m to upgrade Lower Woodburn WwPS by increasing the volume of storm emergency overflow storage and installing a static screen. The solution also facilitates proposed housing (315 units) and hospitality/recreational developments that discharge directly to the WwPS and delivers storm separation of 17.8 hectares of impermeable ground. The project delivers against the LwWP goals:

PROTECT against Flooding. ENHANCE the Environment. GROW the Economy. [caption id="attachment_12584" align="alignnone" width="1200"] Lower Woodburn WwPS site pre-construction - Courtesy of DAWSON-WAM Ltd[/caption] Project delivery

Delivery partners for this project were appointed through existing NI Water frameworks. Tetra Tech was appointed to provide professional services, (including project management, cost management and site supervision) and to act as NEC Project Manager. The Murphy/ DAWSON-WAM Ltd JV (MDW) was appointed as main contractor.

Building on earlier work done to investigate catchment-wide solutions for Carrickfergus as part of the LwWP, MDW was engaged to develop the design and build solution under an Early Contractor Involvement (ECI) NEC contract. The design was reviewed and approved in collaboration with NI Water, and budgetary approval was granted for construction works under an NEC-ECC Main Option A contract in summer 2023.

Delivery was a collaborative effort between client, consultant and contractor teams to improve water quality and environmental conditions in the receiving Belfast Lough. The works comprised addition of surface water storage in a new concrete cofferdam constructed alongside, and connected to, the existing pumping station wet well. The constrained nature of the site lead to the need for access to neighboring lands from which to operate construction plant and machinery.

The scope was further developed during construction to improve the overall solution performance by installing a non-return valve on the storage overflow to enhance separation between storage and the marine environment and to improve the design life of the pre-existing pumping station.

Change was addressed using the provisions of the NEC contract and was managed within the overall budget for construction works. The new storage tank was integrated from an ICA perspective by linking to NI Water’s telemetry system for effective monitoring of storage levels and overflow incidents.

Throughout construction, regular site inspections were undertaken by Tetra Tech’s Site Supervisor and Health & Safety Manager to affect joint inspections with NI Water’s and MDW’s H&S Managers. This made sure of a combined approach to H&S management; issues were addressed face to face in real time and resulted in an excellent H&S performance throughout, with no accidents or incidents.

[caption id="attachment_12583" align="alignnone" width="1200"] 3D models of new storm storage tank - Courtesy of RPS[/caption] Early contractor involvement (ECI)

The ECI process proved its worth as extensive design and build challenges were overcome and a buildable solution was developed that satisfied the construction brief and costs criteria. This led to Murphy/DAWSON-WAM Ltd being appointed as principal contractor for the design and build of Lower Woodburn WwPS upgrade in August 2023. Contract works involved the design, supply, installation, testing and commissioning of an additional 317m3 of additional gravity storm storage on the existing site.

Certain elements of the project proved extremely challenging, none less than the design and construction of the additional storage tank on an existing site footprint that was extremely compact. Construction was further restricted as access to the site was from the busy A2 road, (the main road corridor from Belfast to Carrickfergus), immediately to the east of the site was a live petrol station with underground fuel tanks, on the west was an existing domestic occupied property and the access to the property and to the local strand, straddled the site on the east and south side. This was further complicated by the fact the existing pumping station had to be kept operational during the entire construction process.

The extensive ECI (which included site investigations, topographical surveys, network modeling, liaison with key stakeholders and utility providers, design development and costs approval), was completed post contract award. MDW appointed RPS as their civil design partner and through collaboration with NI Water and Tetra Tech, overcame the buildability challenges of this compact site.

The ground investigation report undertaken on the footprint of the only available space on the site identified glacial till overlying bedrock which varied from 6 to 8m below ground level. Early tank storage design calculations indicated that a tank depth of approximately 6.1m deep below ground level would maximise the footprint available within NI Water’s site, and enabled a gravity return into the existing pumping station.

[caption id="attachment_12588" align="alignnone" width="1200"] (left) CFA cased auger secant piling cofferdam installation and (right) excavation of storm tank exposing secant piles - Courtesy of DAWSON-WAM Ltd[/caption] Temporary works

Several temporary works solutions were considered to facilitate the tank construction including the use of a steel sheet pile cofferdam. This solution was omitted for several reasons but predominately because of the vibration risk to the surrounding properties and being unable to achieve sufficient embedment into the mudstone bedrock.

For the solution, DAWSON-WAM Ltd developed a secant piled cofferdam design which deployed a hard/hard continuous flight auger with reinforcement cages in every male pile (second pile). Utilising a double auger drive motor, it was capable of coring into the mudstone while maintaining pile verticality.

Working with piling design partner, Cassidy Geotechnical Ltd, a 450mm diameter secant piled cofferdam with 9m long piles was developed as the most viable solution. This was further valued engineered by incorporating the piles as part of the permanent works and introducing a single face, in situ, internal reinforced concrete lining to form the internal wall of the tank. Through design analysis, it was identified in the temporary condition the secant piled cofferdam would require propping during excavation, but this was omitted by collaboration between the piling designer and contractor by introducing a reinforced capping beam that was poured prior to excavation which reduced the deflection enough during excavation to eliminate the requirement of the temporary prop. The capping beam would also provide support the permanent in-site reinforced roof structure.

Pre-construction structural surveys were undertaken on all the surrounding properties and vibration monitors were installed at two locations adjacent to the site during piling operations. The secant method proved a success as the piles were installed to the designed length without triggering any vibration limits and when excavated provided a watertight cofferdam.

[caption id="attachment_12586" align="alignnone" width="1200"] Aerial view of site during storm tank reinforced concrete base slab construction - Courtesy of DAWSON-WAM Ltd[/caption] Lower Woodburn WwPS: Supply chain - key participants Main contractor: Murphy/DAWSON-WAM Ltd JV Project management & professional services: Tetra Tech Civils design: RPS Secant piling design: Cassidy Geotechnical Ltd Civils contractor: DAWSON-WAM Ltd MEICA: J Murphy & Sons Shuttering contractor: Form Builders Mixer: Xylem Water Solutions Static screen: Eliquo Hydrok Ltd Washwater booster pumpset: Dutypoint Ltd Combisafe fall protection: Honeywell Construction Due to the storage requirements of the tank, and given the small site footprint, there was not adequate space on the site to undertake the construction works safely. Therefore, a detailed construction phasing plan was developed by the contractor at the ECI stage which incorporated keeping the existing pumping station in service during construction. This plan identified that by temporarily incorporating the existing access lane (leading to the beach and neighbouring property) as part of the site, construction could be undertaken. An agreement was reached with the owners of the house - who had just recently purchased the property – to delay their relocation to the dwelling until works had been completed, and the council agreed to temporarily close access to the strand to aid construction. [caption id="attachment_12587" align="alignnone" width="1200"] Single sided reinforced concrete wall pours against secant piles to form internal tank walls - Courtesy of DAWSON-WAM Ltd[/caption]

Consultations were carried out through NI Water’s internal communications team and members of the project management team, and included public drop-in meetings and approximately 100 letter drops.

Due to site constraints, the establishment of an off-site construction compound also facilitated just in time delivery of the construction materials to the site, which maximized the small amount of on-site space and reduced site congestion.

MEICA installation included a single Xylem Water Solutions mixer to prevent silt build up in the tank, an Eliquo Hydrok 6mm screen was fitted on the existing 315mm emergency overflow pipe from the pumping station and a new 6 bar washwater booster pumpset was commissioned on site for maintenance of the screen.

Site finishes included a new perimeter fence, EMS area and a wildflower mix was used to finish final surfaces to enhance biodiversity.

Works programme

Construction works commenced on site in August 2023 and were completed on programme and within budget by late April 2024.

The collaborative approach of the project team effectively delivered a solution to improve the water quality of Belfast Lough, supporting NI Water’s agenda to Protect, Enhance and Grow through the delivery of the Living with Water Programme.