Donegan Civil Engineering

New CSO and Storm Detention Tank – St Helens

Principal Contractor delivery of a new CSO and 700m³ storm detention tank on United Utilities' £10m Better Rivers investment in St Helens

By: Valentine Donegan

Published: August 6, 2026

New CSO and Storm Detention Tank - St Helens

Client: United Utilities

Value: Approx. £4,200,000

Project Description / Main Operations

V J Donegan & Co Ltd delivered the civil and MEICA works for a new combined sewer overflow (CSO) and storm detention tank at Carr Mill, St Helens. The project was undertaken as part of United Utilities’ £10m Better Rivers investment in St Helens, reducing the frequency of storm overflow spills and improving water quality in Black Brook.

Acting as Principal Contractor, V J Donegan & Co Ltd delivered the full civils and MEICA package from initial site establishment through to commissioning, handover and full site reinstatement. The works centred on a 17.5m diameter shaft forming the new 700m³ storm detention tank, a new reinforced concrete CSO chamber with screening, and the associated below-ground network connecting the new asset into the existing system.

The works were delivered on a constrained urban site between a live highway and neighbouring residential properties, requiring careful planning of access, lifting operations and the construction sequence to maintain safe working within a confined footprint while keeping the existing CSO operational until the new asset was ready to take over.

Scope and method:

Excavation of a 14.0m deep, 17.5m diameter shaft to provide the new 700m³ storm detention tank, including temporary works and a reinforced concrete base, internal benching, pipework, access ladder and platforms, and installation of a precast concrete cover slab.
Construction of a new reinforced concrete CSO chamber housing a Longwood Stormguard 450 screen, within an 8m deep pre-driven sheet piled cofferdam.
Installation of associated deep manholes and connecting sewers, diameters ranging from 1200mm to 450mm.
Twin rising mains with return pumping to relieve flows back into the network.
Installation of new control and DNO kiosks with all associated ducting and drawpits.
Decommissioning of the existing CSO chamber and redundant pipework.
New permeable access road off Hawes Avenue, with footpath, fencing reinstatement and landscaping.
Testing, commissioning and handover to the client.

Outcome / Client benefits

To ensure high standards of safety and quality, V J Donegan & Co Ltd worked closely with specialist MEICA supplier Water Engineering Services Ltd, who delivered the screen, pumps, rising mains and control kiosks and completed dry and wet commissioning of the new asset. Regular engagement with United Utilities throughout ensured smooth progress and timely completion.

Now operational, the new CSO and detention tank reduce the frequency of storm overflow spills from the Carr Mill Road CSO and intercept solids before any discharge reaches Black Brook, improving water quality in the watercourse. The project demonstrates the company’s capability to deliver complex water infrastructure on constrained sites in support of United Utilities’ storm overflow reduction commitments