OVERVIEW

The city’s expanded retail provision supports the role of the city centre as a designated regional centre. Westgate is one of the most environmentally friendly shopping centres in the UK, with stringent sustainability targets including a full long-term embodied carbon analysis, a UK first for a retail centre.

SDS worked closely with Waterman Group who were commissioned to provide multidisciplinary services covering structural, civil engineering, environmental and flood risk services.

Work commenced on site in February 2015 including construction of a 1,000 space, two level car parking facility with a combined footprint of 45,000m².

WHAT WE DID

As part of the sustainability strategy, no pumping is allowed on site, requiring part of the enabling works to include a 2m² culvert to divert captured rainwater via a major system of attenuation tanks into Castle Mill Stream.

With the capacity to store up to 950m³ of water, the SDS GEOlight® geocellular attenuation tanks have been suspended above the basement car park and beneath the John Lewis store. The tanks located beneath the footpath that adjoins the bus link road are ultra-high strength GEOlight® 800. In order to meet demands for their longevity, the tanks are required to have a minimum design life of 60 years.

The tanked basement and service yard has been designed to take 600mm depth of flood water. The second level basement car park has the potential for use as a flood water storage area, should this ever be required. Furthermore, attenuation ensures the surface water discharged will be no greater than the pre-existing situation and will cater for the effects of climate change.

RESULTS

The inherent flexibility of GEOlight® systems meant that SDS alone was able to meet the client’s brief.

As the scheme is founded on Oxford clay, ground conditions and the extensive basement precluded the potential for infiltration basins, wet ponds, swales and constructed wetlands, filter drains, perforated pipes and filter strips. The most viable solutions were identified as living roofs, rainwater harvesting and attenuation tanks. As the site was located next to a river, the tanking system was required to prevent lateral water movement.

The principle of sustainability is fundamental to the development of Westgate Shopping Centre and the adherence to this principle provided the surface water drainage requirements with a number of difficulties to overcome. In close co-operation with SDS, a unique solution was established, comprising a combination of attenuation tanks, suspended above the basement car park and sandwiched between down stand beams and ground floor slab.