Storming the tanks

Cheshire-based GHA Livigunn is working in association with contractor Volker Stevin to design three new storm tanks for United Utilities at Wigan Wastewater Treatment Works (WwTW).

The purpose of the project was to reduce the frequency of storm overflows to the nearby River Douglas in line with EU Bathing Water Legislation. The project was instigated by United Utilities to increase storm storage capacity from 45,000m3 to 110,000m3 via the construction of three new 133m x 62m x 4m tanks with a combined storage capacity of 65,000m3.

The design for the large tanks revolved around resistance to flotation. The design water level at Wigan had to be taken at a ground level that varied by over a metre across the tanks. The finished external walls are 5.0m high and 390mm thick, with thickness being kept to a minimum to reduce the requirements for thermal reinforcement and to achieve an overall reinforcement density of only 66kg/m3.

The storm tanks were reinforced with special mesh prefabricated into U-shaped cages that slot together on site with nominal lacer bars. The base slab design had to consider thermal movement and shrinkage and involved the adoption of 30m x 30m pours to reduce the number of joints and associated detailing problems.

Located in the DouglasValley, northwest of Wigan, the WwTW serves a total population of 200,000 and a catchment area of 95km2. Flows are received from 21 distinct drainage areas, each discharging to the WwTW via the Douglas Valley Trunk Sewer. The storm tanks are designed to meet a life requirement of 60 years.