Structural Drainage

Reservoir Base Uplift, Deckdrain - Rushmoor Sewage Treatment Works, Telford, UK

Client Severn Trent Water

JN Bentley

Consultant Mott Macdonald

Deckdrain Drainage Geocomposite



  • High flow drainage core eliminates stone, saving on material costs
  • Easy to lay and joint
  • Acts as a shutter and key for the concrete

Project Description

Serving Telford from its rural setting, Rushmoor STW is approximately a mile west of the town and forms part of the Shropshire Area Sludge Strategy, a strategic centre for sludge digestion, de-watering and cake storage.

The project is part of Severn Trent Water’s e5 (efficiency 5) Major Projects Programme, an alliance formed between four framework contractors and Severn Trent Water to deliver 11 major wastewater, non-infrastructure projects. Collaborative working and innovation have been at the core of the delivery; maximising efficiencies to drive results for both the client and alliance partners alike, facilitating the One Supply Chain (OSC) ideology and driving the success of Severn Trent Water’s AMP5 Capital Programme. Between 2008 and 2026, the Rushmoor STW catchment (population equivalent) is expected to grow by nearly 100%. If Rushmoor STW is to effectively treat the flows and loads derived from the expected growth, the site’s existing primary settlement tanks (PSTs) would need refurbishment and additional secondary treatment capacity provided.

The Challenge

The area around the site has a high water table. This provides a particular challenge to the stability of below ground water storage structures that include treatment operations, with constantly changing water levels within the tanks. When empty, the tanks suffer considerable uplift pressures. A simple solution is to create thick, heavy and costly sealed concrete base slabs. In addition, a 500mm thick under-slab stone drainage layer is required. Encouraged by the e5 programme theme of innovation, a novel solution was proposed.

The Solution

The ABG Deckdrain 700S/NW8 drainage geocomposite was proposed to perform the dual function of uplift pressure relief and under-slab drainage. The virtually impermeable cuspated core of Deckdrain is supplied thermally bonded to a filtration geotextile to prevent soil fines from blocking the drainage path. Deckdrain drainage geocomposite is placed facing down and an initial screed layer is poured into the cuspates. The cuspates are filled with the screed, bonding it to the underside of the slab and enhancing the compressive strength. Deckdrain’s open core structure far exceeds the drainage capacity of a 500mm stone layer. Removal of the stone provided significant savings in excavation, and the import and placement of costly granular materials. Non-return valves were placed at intervals across the floor slab to relieve groundwater pressure when the tanks are empty, thus avoiding uplift pressures and allowing the minimum practical concrete floor slab thickness.

The ABG Service

ABG provided calculations to show equivalency of flow to stone drainage layers and gave installation guidance on site.

Contact ABG today to discuss your structural drainage project and to discover how our technical team can work with you to design a solution to meet your requirements.


Additional information