A record-breaking underground stormwater storage tank allowed precious green space to be retained for public use at a prestigious new homes development in Hertfordshire.
The largest SDS GEOlight® stormwater tank of its type in England, capable of collecting up to 8 million litres, is integral to the sustainable drainage infrastructure at the 2,200-home Stortford Fields development near Bishops Stortford and enabled much-needed sports facilities for the surrounding community.
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 8,000 m3 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.
Green Infrastructure
Protecting 58 hectares of green infrastructure, including ancient woodland and meadowland, and creating new public amenities were integral to plans for Stortford Fields. The development is divided in two by a natural valley and the ancient Hoggates Wood. Contractors Breheny Civil Engineering levelled a sloping area of land next to Hoggates Wood to form the new Hoggates Park. Located at the lowest point of the development 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
As part of a £1 million 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
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.
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 1200mm diameter surface water sewer which follows the line of the new spine road.
Construction Challenges
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. 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.”
As GEOlight® is lightweight, it is comparatively quick to install the SDS team was able to complete the GEOlight® installation over 8 weeks, working closely with Breheny Civil Engineering.
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.”
Located along the popular M11 corridor, Stortford Fields is being built by a consortium of Bovis Homes, Taylor Wimpey, Persimmon Homes and Kier Homes, beginning in August 2017 expected to be completed in 2026.
