Gaskell Street CSO & Detention Tank (2026)

The site of the Gaskell Street CSO & Detention Tank Project adjacent to Sankey Brook - Courtesy of Shone’s Drones

Located in St Helens in the north-west of England, this project was undertaken to address a longstanding issue associated with a Combined Sewer Overflow (CSO) at Gaskell Street. Under storm conditions, the existing outfall regularly discharged stormwater into Sankey Brook, posing both environmental and regulatory challenges. The scheme was therefore driven by the requirements of the Environment Agency’s Storm Overflow Discharge Reduction Plan and delivered as part of United Utilities’ AMP8 Better Rivers programme, which seeks to significantly reduce the frequency and impact of storm overflows across the network.

Project drivers

The Gaskell Street CSO & Detention Tank Project was developed to address several key environmental improvement drivers:

1. EnvAct_IMP2: Reducing spill frequency to no more than an average of 10 rainfall events per year.
2. EnvAct_IMP4: Ensuring no local adverse ecological impact.
3. WINEP screening driver: 6mm solids separation up to the 1 in 5-year design storm critical flow rate with 2050 climate change allowance.

Casagrande C6 XP2 rig used by Van Elle for grouting of fissures & voids – Courtesy of Ward & Burke

Stakeholder collaboration

With a challenging regulatory deadline of 31 March 2026 driving an accelerated 11‑month delivery programme, the scheme was undertaken within a complex risk environment.

Ground conditions presented significant uncertainty, including a suspected former brickworks, areas of historic landfill, and legacy coal mine workings. These constraints required a proactive and risk-informed approach throughout design and construction. Against this backdrop, close and continuous collaboration with the project delivery team, St Helens Borough Council, local stakeholders, key suppliers, and the Environment Agency was critical to successful delivery. This integrated approach enabled risks to be effectively managed, solutions to be aligned with stakeholder expectations, and project momentum to be maintained despite the challenging conditions.

The chosen solution

To address these requirements, the preferred solution brought together several complementary civil, mechanical and hydraulic interventions designed to improve screening performance, provide additional storm storage capacity and upgrade the existing overflow infrastructure.

3D Revit models: (left) key structural elements of the scheme and (right) section through combined CSO & detention tank – Courtesy of Ward & Burke

The principal elements of the scheme comprised:

1. A new CSO chamber and 700m³ underground detention tank constructed within Sunshine Park.
2. A replacement 6mm screen sized for significant peak design flows of 1,481 l/s (1 in 5-year event) and 1,739 l/s (1 in 30-year event).
3. A new overflow sewer and outfall headwall to replace undersized existing assets.

Project details

At the heart of the scheme was the 12.5 m internal diameter, 14.6m external diameter, 10m deep in situ reinforced concrete (RC) sunk caisson shaft which provides the required 700m³ of storm storage volume. Flows are screened upon entry into the detention tank via a new CSO channel housing an Eliquo Hydrok 180° MecMex removable screen, 3m in length, sized to deliver 6mm solids separation for a design flow rate of up to 1500 l/s.

liquo Hydrok MecMex screen installed on removable guide rails within CSO channel – Courtesy of Ward & Burke

During storm conditions, screened flows are discharged to Sankey Brook via the new overflow sewer and a precast outfall headwall from Tracey Concrete constructed on the riverbank. The headwall incorporates a flap valve from Althon Ltd to prevent high river levels from backing up into the sewer network during storm events, while integrated energy dissipation and erosion protection measures were included to manage discharge velocities and protect the bed and banks of the watercourse from scour.

Following the end of a storm event, the screened effluent retained within the detention tank is returned to the foul network by a duty-standby return pump set operating at a controlled rate of 20 l/s, allowing the stored volume to be released gradually without overloading downstream assets. To maintain long-term operational performance and minimise the risk of sediment accumulation within the tank, a low-head mixer pump fitted with hydro-ejector venturi nozzles was provided to periodically mobilise and clean deposited solids.

The wider scope of works also included the construction of a series of new manholes and connecting sewers to integrate the upgraded assets with the existing network, together with dedicated instrumentation chambers housing flow switches and associated monitoring equipment.

To support the operation of the site’s mechanical and electrical installations, a new motor control centre panel from Lloyd Morris Electrical Ltd was installed within a purpose-built kiosk, providing a secure and accessible location from which the screening, pumping and ancillary systems could be powered, monitored and controlled. The kiosk also accommodated a new telemetry panel, enabling operational data and alarms to be communicated directly to United Utilities’ control systems and supporting effective remote monitoring and site management.

Electrical installation within MCC kiosk – Courtesy of Ward & Burke

Supply chain: Designers, consultants & contractors

• Principal contractor: Ward & Burke
• Outfall FRAP application: JBA Consulting
• Ecological consulting & surveys:
  • The Environment Partnership
  • Tyrer Ecological
  • Bowland Ecology
• Vegetation clearance: Treefellers Ltd
• Invasive species management: Budget Knotweed Solutions
• Testing services: Socotec
• Road safety audit: Six:TEN Highways & Traffic
• Utility surveys & CCTV: Lanes Group
• Excavation shoring: MGF Ltd
• Ground treatment & permeation grouting: Van Elle
• Electrical installation: LEC Electrical
• Groundworks & enabling works: JR Hind Ltd
• Reinforcement drawings & schedules: Paul Benham Ltd
• Concrete cutting services: Access Drilling Services Ltd

Caisson shaft construction

Construction of the underground detention tank presented a number of significant geotechnical challenges, with the ground investigation identifying weak mudstone bedrock, a high phreatic groundwater level and potential voids beneath the working footprint in a historical coal mining area. To address these challenges, Van Elle was engaged to undertake initial ground treatment, using injection grouting to stabilise and fill the potential voids identified in the ground investigation prior to the main shaft works commencing.

Excavation and sinking of the 12.5m internal diameter caisson shaft to formation level – Courtesy of Ward & Burke

A sunk in situ caisson method was then selected as the preferred construction approach, providing a robust means of mitigating the risks associated with groundwater ingress, ground instability within the weak mudstone, and flotation forces generated by the high groundwater table.

As the caisson was progressively excavated and sunk, bentonite slurry was injected through ports cast into the in situ concrete shaft wall to lubricate the concrete-to-ground interface, reducing skin friction and enabling controlled sinking of the structure. The bentonite was batched on site using a bespoke mixing and storage tank, designed and manufactured in-house by Ward & Burke. The caisson was successfully sunk to formation level in just 4-weeks, to form the structure of the new underground detention tank.

Supply chain: Suppliers of process plant & equipment

• Control panels: Lloyd Morris Electrical Ltd
• Kiosks: Quinshield Ltd
• Screen supply & install: Eliquo Hydrok Ltd
• Precast CSO channel & roof slab: Shay Murtagh Precast
• Precast manhole rings, sewer pipes & headwall: Tracey Concrete
• Concrete washout tanks: Kelly Tanks Ltd
• Pipework supply: Electrosteel Castings (UK) Ltd
• Coated steel dropper pipes: PMJ Mechanical Ltd
• Submersible return & mixer pumps: Xylem Water Solutions
• Flap valve supply: Althon Ltd
• Davit socket & lifting arm: T Allen Engineering Services Ltd
• Lifting equipment: Rope & Sling Specialists (RSS)
• Access covers: Technocover Ltd
• Geotextile supply: Tailor Engineering
• Instrumentation: Siemens
• Fabricated steel components: C&D Engineering
• GRP ladders, platforms & handrails: Chemglass
• Reinforcing steel supply: Dominic Lydons Steel
• Concrete pumps: Camfaud Concrete Pumps Ltd
• Concrete supply: Cemex
• Welfare buildings: Nixon Hire
• Site security services: Direct Security Ltd
• Muck removal & aggregate supply: Envirovue | J Phillips Grab & Tipper Hire Ltd | JD Haulage & Aggregates
• Plant hire: GAP Group
• Mobile crane services: Cork Crane Hire | AMF Lifting

Gaskell Street site following the installation of the RC precast channel – Courtesy of Shone’s Drones

Innovation & efficiencies

Integrated structure

A key innovation on this project was the decision to combine what would typically be two separate underground RC structures – a rectangular CSO chamber and a circular detention tank shaft – into a single integrated structure. The early-stage design adopted the conventional arrangement, with the CSO chamber and detention tank developed as standalone elements, each requiring its own excavation, formwork and temporary works elements.

Ward & Burke and United Utilities devised an alternative solution, in which a CSO channel structure was integrated within the detention tank and designed to be supported on rebates cast directly into the detention tank shaft concrete walls.

This integrated approach delivered a significant reduction in overall structural footprint within the park, enabling the existing football pitch to be retained in full for continued use by local residents following the works. It also eliminated the need for a second deep excavation in the challenging ground conditions, removing the associated geotechnical, groundwater and programme risks. The combined structure delivered considerable savings in construction programme and cost, alongside a meaningful reduction in embodied carbon and the overall volumes of concrete and reinforcing steel required, delivering better outcomes for the community, the client and the environment.

CSO channel

An additional efficiency involved the CSO channel itself being fabricated and installed as a single precast unit rather than being cast in situ within the shaft. Manufacturing the channel off-site ensured tighter dimensional tolerances and a higher likelihood of a higher quality finish; eliminating the need for installing complex formwork and falsework at height within the caisson.

(left) CSO channel structure being lifted into position and (right) the precast RC CSO channel and coated steel dropper pipes installation – Courtesy of Ward & Burke

Manufactured by Shay Murtagh Precast, the 62-tonne unit was installed in a single lift onto the rebates cast into the shaft wall, significantly reducing installation time on site and removing a sequence of high-risk activities from the confined working environment when compared with the alternative in situ construction method.

Outfall headwall Installation

Constructing a new outfall headwall directly within Sankey Brook required regular engagement with the EA and JBA Consulting to develop the ecological, technical and third-party materials necessary to support the Flood Risk Activity Permit application.

Several ecological surveys were undertaken across the affected watercourse and surrounding habitats, including assessments for water voles, otters, bats and nesting birds. Precautionary working method statements were developed in response to the survey outputs and briefed to all site personnel prior to works commencing.

A geomorphology assessment was carried out to understand existing channel behaviour, sediment transport and erosion characteristics at the discharge location, which informed the erosion protection design including a baffle wall and downstream riprap to dissipate discharge energy and protect the bed and banks against scour. A bespoke construction methodology was also developed to minimise working within the footprint of the river and to protect water quality throughout the headwall installation.

New outfall headwall installation on bank of Sankey Brook – Courtesy of Ward & Burke

Social value & community benefits

Social value and community engagement were built into the project from the outset. A tailored traffic management plan was developed with St Helens Council and local residents to coordinate construction traffic and access arrangements, minimising disruption to the surrounding community.

To support biodiversity, the team produced a tree-planting plan and ecological assessment that identified opportunities to increase planting within the park. The project also delivered a programme of volunteer-led community activities, including litter-picking and river-cleaning events in Sunshine Park, helping to improve the wider environmental health of Sankey Brook during the works.

In addition, a resident and stakeholder liaison event gave the community an opportunity to meet the project team, learn about the purpose and progress of the works, and raise questions or concerns directly. Together, these measures helped create a lasting positive impact for the people of St Helens.

Conclusion

The Gaskell Street CSO & Detention Tank scheme demonstrates how innovative engineering, close collaboration and a strong focus on environmental outcomes can deliver meaningful improvements to local river health. By combining the CSO chamber and detention tank into a single integrated structure, and by engaging early and regularly with the Environment Agency, St Helens Borough Council and the local community, Ward & Burke and United Utilities delivered this complex project on time and ahead of the 31 March 2026 regulatory deadline.

The scheme will significantly reduce the frequency and volume of wastewater spills at Gaskell Street CSO, improve water quality in Sankey Brook, and support the wider aims of United Utilities’ AMP8 Better Rivers programme: cleaner watercourses, enhanced public spaces and lasting community benefit for the people of St Helens.