Joseph Gallagher Ltd

Finham Storm Tank

Engineering Agility Delivers Improved River Water Quality for Coventry

By: Joseph Gallagher Ltd

Published: April 3, 2026

Joseph Gallagher Ltd constructed storm tank and supporting infrastructure at Finham STW, forming a key component of Severn Trent’s Green Recovery programme

Project Overview

Finham Sewage Treatment Works (STW) serves approximately 500,000 people across Coventry, processing around 115ML of wastewater each day. As part of the UK Government’s 2022 Green Recovery programme, Severn Trent committed to substantial investment in water quality, sustainable growth, and nature‑based solutions.

The Finham Storm Tank Project is a key deliverable within the Bathers River Challenge. It provides a new 4,800m³ stormwater tank designed to significantly cut storm overflow events into the River Avon and deliver measurable improvements in river water quality.

Alongside the core STW upgrade, JGL delivered extensive civil engineering and construction works, including a 5,960m³ reinforced‑concrete stormwater tank, major piling operations, structural concrete works, and the installation of associated pipework and access infrastructure.

Project Background

The scheme was designed to work in conjunction with the storm tank at Unicorn Lane CSO, increasing stormwater storage capacity across the catchment. These combined assets allow storm flows to be held safely and redirected to treatment once operational conditions return to normal – reducing spills and improving compliance.

The early phases progressed efficiently through strong collaboration between design, operations, and delivery teams. However, unexpected ground conditions discovered during excavation required the redesign of the original tank solution. The Integrated Project Team (IPT) approach enabled rapid decisions, allowing JGL and partners to maintain progress and reach operational readiness ahead of the 31 March 2025 regulatory deadline.

Joseph Gallagher Ltd delivered storm tank structure at Finham STW, including RC walls, slab works and access infrastructure

Challenges

The initial concept proposed a 15-20m deep circular shaft supported by diaphragm walls. During excavation, the team encountered previously unknown sub‑artesian groundwater pressures that posed risks of water ingress and soil instability. This discovery stopped works immediately and triggered a full redesign.

Key challenges included:

Managing groundwater inflow and mitigating ground instability
Reducing excavation depth to avoid triggering further groundwater movement
Re‑engineering piling, slab, and reinforcement design
Protecting the programme despite significant technical change
Maintaining safe, coordinated working across multiple contractors and work fronts

Through collaborative design sprints, the team—supported by JGL’s temporary works and engineering expertise—adopted a shallower rectangular tank supported by rotary bored piles (650–750mm diameter, providing up to 1000kN tension resistance). This reduced groundwater risk while preserving structural performance. Reinforcement optimisation further lowered steel volumes, reducing carbon impact and cost.

Storm tank structure at Finham STW

Services Provided

JGL delivered major construction and engineering services, including:

Installation of 77 rotary bored piles with casing and subsequent pile cropping.
Construction of a 1,800m², 400mm‑thick reinforced‑concrete slab with power‑float finish.
Casting of 4‑metre‑high tapered RC walls with Sika Supercast movement joints.
Installation of three new culvert runs.
Cutting and removal of existing reinforced‑concrete wall sections.
Installation of 400mm ductile iron pipework and 160mm SDR17 PE pipework (approximately 140m open‑cut).
Use of trench boxes, trench sheets and service‑crossing protections.
Backfilling and compaction around the tank structure.
Construction of footpaths, kerbs, bollards, grasscrete, and a new tarmac access road.
Installation of hot‑dip‑galvanised handrailing around the tank.

Temporary and permanent works delivered or supported by JGL included:

Temporary formwork for tapered RC walls (DOKA).
Internal JGL temporary-works design, including crane platforms and concrete‑cutting assessments.
Ground Force trench support for rising main installation.
Permanent piling design and structural detailing.
Supply of crawler cranes, telescopic cranes, access equipment, ventilation, lifting gear, and storage.

Environmental and Community Impact

The new storm tank dramatically reduces storm overflow discharges into the River Avon, enhancing water quality and supporting compliance with the Water Framework Directive. Environmental improvement initiatives included Biodiversity Net Gain assessments, bird‑box installations, and a tree‑replanting programme to restore habitats impacted by works.

Community engagement was strengthened through newsletters, construction updates, video content, and a dedicated Bathing Rivers virtual room, helping residents understand the project’s role in improving river health.

Legacy

Despite major engineering challenges and a significant mid‑project redesign, the 4,800m³ storm tank was delivered to a high standard and within the regulatory timeline. The wider Finham programme demonstrates how adaptive engineering, collaborative working, and proactive problem‑solving – supported by JGL’s civil and temporary works expertise – can deliver robust wastewater infrastructure that benefits both the environment and the community.

The project leaves a lasting legacy of improved river quality, strengthened ecological conditions, and a model for integrated delivery that can be replicated across future UK water‑sector schemes.