Record-breaking stormwater tank installed

The largest underground stormwater tank of its type in England has been installed as part of the sustainable drainage infrastructure for a new Hertfordshire housing development, enabling the provision of much-needed sports facilities for the local community.

By installing an SDS GEOlight® tank, capable of collecting up to eight million litres of rainwater below ground, green space could be retained for use as a full-size football pitch, part of newly-created public sports facilities serving the 2,200-home Stortford Fields development.

Located along the popular M11 corridor, the new development is being built about 1km to the North West of Bishop’s Stortford and is part of a wider District Plan to create 18,400 new homes across East Hertfordshire by 2033. Stortford Fields will be built in two phases by a consortium of Bovis Homes, Taylor Wimpey, Persimmon Homes and Kier Homes, beginning in August 2017 and expected to be completed in 2026. Breheny Civil Engineering are responsible for delivering all infrastructure works to prepare the site including drainage, services and building of a new spine road through the development.

Green Infrastructure

Protecting 58 hectares of green infrastructure (around 45% of the site area), including ancient woodland and meadowland, and creating new public amenities were integral to plans for Stortford Fields from the outset. The SDS GEOlight® tank forms part of a Sustainable Drainage Systems (SuDS) infrastructure that incorporates a mixture of above- and below-ground components to manage water quality and quantity across the site, including detention ponds, swales and roadside filter drains.

The GEOlight® tank’s 8,000cu m capacity is calculated to capture and retain surface water running off the impermeable surfaces created by the first phase of the development, even during peak rainfall. Known as the Western Neighbourhood, phase one includes new roads, 857 homes, a new primary and secondary school, and local shops.

The Stortford Fields development is divided in two by a natural valley and the ancient Hoggates Wood. Breheny Civil Engineering levelled a sloping area of land adjacent to Hoggates Wood to form the new Hoggates Park. Located at the lowest point of the development and positioned between the Western and Eastern neighbourhoods, it was the ideal place to locate the GEOlight® tank. Rather than creating above-ground attenuation in a pond or detention basin, Hoggates Park could then be protected for recreational use and still provide green open space.

As part of a £1M investment for Hoggates Park, new sports facilities will be developed in close consultation with local residents, including a proposed sports pavilion, full-size football pitch, five-a-side pitch, and children’s play area. The sports facilities will also be used by the new schools.

Site constraints

Among the challenges for achieving efficient surface water management for the site were a clay soil that left little opportunity for infiltration, an undulating topology, together with a pre-existing 132Kv underground electricity cable supplying Stanstead Airport. A deep surface water pipe network was required to avoid the cable and achieve the necessary gravity falls. Surface water is piped to the GEOlight® tank via a 1,200mm diameter surface water sewer which follows the line of the new spine road.

Planning requirements stipulated that the surface water drained from the Western Neighbourhood is discharged into an existing agricultural drainage ditch that runs along the Eastern perimeter and links, after about 1.5km, into the Bourne Brook, a tributary of the River Stour. A vortex flow control on the tank’s outfall regulates the discharge flow rate to a maximum of 53.7 litres per second.

Construction Challenges

SDS began by working with the contractors to optimise the design of the tank to minimise materials and construction costs. Manufactured from 100% recycled PVC, GEOlight® has a unique honeycomb structure that gives it high structural strength while enabling a 95% void rate. So, the tank design could be enhanced to deliver the maximum volume of water in the minimum space.

Said Chris Mitchell, Site Manager for Breheny Civil Engineering: “SDS helped us to reduce materials, excavation time and costs by recommending the use of lighter weight storage crates, while minimising the depth of the sub-base layer above the tank. The solution still met the necessary loadbearing requirements and satisfied Sport England’s specifications for the football pitch.”

Installing such a large tank presented some logistical challenges. However, as GEOlight® is lightweight, it is comparatively quick to install – a 1,000cu m storage tank can be completed in a week. At Stortford Fields, the SDS team was able to complete the installation over eight weeks, working closely with Breheny Civil Engineering to coordinate the schedule of deliveries needed to bring the crates to site in stages, minimising storage onsite and working around the needs of other suppliers.

Co-ordination

Chris Mitchell continued: “The big advantage of using SDS was that they took over the complete installation of the tank with only minimal input from ourselves. We only needed to dig the hole and prepare the ground for them to begin the installation. This saved us from providing equipment, such as forklifts, and manpower to unload deliveries or move storage crates around the site.

“Despite the huge scale of the installation, it proceeded without a hitch, and the SDS team on site communicated with us very well to request groundworks support as and when it was needed. The GEOlight® tank was covered in a sealed geotextile membrane that the SDS team welded in stages. We were able to promptly backfill each section to avoid any damage to the membrane as the works proceeded.”

SDS also worked with Breheny Civil Engineering to realise the low maintenance advantages of a central 1,200mm perforated-pipe distribution system for the GEOlight® tank. By surrounding the pipe with a fill of rolled and washed stones, the system is self-cleansing and silt build-up in the tank will be minimal. As a result, less jetting will be required, and ongoing inspection and maintenance reduced to a minimum.