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How an off-site underground tank improved the environment and made the record books

It’s the largest tank of its kind in the UK, holds 16,500m3 of storm sewage and has already given a significant boost to the local environment. WSP's Rob Sharpe looks at a perfect example of how civil engineering projects using precast elements can satisfy the client, facilitate our lives…and break records.

 

WSP designed a huge underground detention tank at Ash Lane, Widnes to enable United Utilities to store the increasingly larger volumes of discharge that are caused by higher rainfall and denser urban development. The result has been immediate: lower volume, less frequent discharges from the Ash Lane combined sewer that have greatly improved the quality of the local watercourse and environment in nearby Ditton. And by reducing the sewer’s peak flow, the tank has brought lower capital and operating costs at two other nearby combined sewer overflow (CSO) locations.

United Utilities’ requirement was for a design that was simple, safe, quick to build and resulted in less disruption to customers. So it came up with an outline design developed with drainage and water management solutions provider Kijlstra.

The rectangular tank’s modular precast construction is a first for both the UK and United Utilities. It also features the first UK use of the innovative flushing system that holds back a reserve of water which is subsequently released under gravity and is used to sluice down the main tank. Measuring 135m x 42.1m x 3m high, the tank has lower operational costs due to a reduced pumping head. And compared to conventional construction, the tank delivers an estimated £1m of capital cost savings.

The highly efficient Design for Manufacture and Assembly (DfMA) construction method has resulted in:

  • A lowest whole-life cost solution for the Client;
  • A relatively shorter construction programme, and
  • Reduced impact on local residents due to fewer traffic movements around the site, plus less noise, dust and vibration.

Concealed by 3m of earth, the tank has an insitu base with a 1:200 fall, but its external walls and roof are made from 300mm-thick precast concrete panels. An additional 200mm-thick layer of insitu concrete was applied over the roof slab.

But the greenfield site threw up numerous challenges. First was the risk of silt pollution, so the site team came up with a way to treat surface water run-off prior to discharge. Second, there was no way heavy vehicles could access the site. We had to construct a temporary kilometre-long access road using 12,700 tonnes of aggregates. These were removed for recycling when the project was completed and, along with excavation spoil being reused on site, gave the project an impressive 99.98% waste diversion from landfill. Furthermore, no net loss of biodiversity was experienced thanks to the team’s efficient environmental management.

We enjoyed a further boost thanks to United Utilities’ procurement through an alliance contract that allowed early involvement of the designer and contractor. This allowed the team to formulate the optimal solution at an early stage, allowing construction to start in good time.

Client service is one of our key differentiators; this was evidenced at Ash Lane by meeting all the Client’s and regulator’s requirements, and achieving zero accidents.

Collaboration with our joint venture partners (United Utilities, MWH and KMI+ (Kier, Murphy, Interserve and Mouchel JV)) was a critical part of the project’s delivery.

Rob Sharpe is Principal Engineer with WSP.