Construction Only Award
Client: SEEDA/HCA and Portsmouth City Council
Designer: CampbellReith Consulting Engineers
Contractor: Tamdown Group and Merebrook Consulting
The site was located on the Tipner Peninsula of Portsmouth to the east of the M275 and comprised four parcels of land as follows:
1. Parcel I: a triangular parcel of land to the north east of Tipner lane and part of a former landfill.
Land in Parcels I-III was impacted by asbestos, localised hydrocarbon contamination and Japanese Knotweed.
4. Parcel IV: a larger northern part known as PD Fuels and a smaller southern section known as the Pavilion site which together occupies 1.7 hectares. Historically the PD Fuels site was used for a range of industrial activities, including a form brick pit, chemical (tar) works, and more recently a timber yard and the PD Fuels coal depot.
The widespread hydrocarbon contamination in Parcel IV constituted a hazard to groundwater and surface water. The degree of contamination was a major constraint on how the land could be developed.
Each Parcel was remediated as follows:
1. Bulk asbestos was removed from the formation surface and a multifaceted barrier installed. Knotweed was removed from affected areas and site won soils prior to reinstatement as part of an engineered cap. A geotextile was installed prior to the placement of at least 250 mm of site won 6F2 crushed concrete.
2. Excavations were advanced to between 250 and 300 mm below original ground level. A geotextile was installed prior to the placement of at least 250 mm of site won 6F2 crushed concrete.
3. Excavations were advanced to between 250 and 600 mm below original ground level. Site won subsoils were screened/picked for knotweed prior to being placed on a root barrier as part of an engineered cap. The cap was completed with a geotextile and at least 250 mm of site won 6F2 crushed concrete.
4. The cohesive properties of the soil in Parcel IV necessitated a number of changes to the initial remedial strategy. As soil washing was not practicable given the cohesive soils encountered, stabilisation with cement was carried out following bioremediation to asymptote condition. The effect of stabilisation has been to reduce both the effective porosity and hydraulic conductivity of treated soils. Parcel IV capping comprised a minimum of 300mm of site-won 6F2 crushed capping, 400-450mm of imported clean 6F1/6F5 Graded recycled glass followed by 150mm of imported 6F2 crushed concrete The recycled crushed glass was placed on a woven geotextile to prevent fines accumulation within the underlying crushed concrete.
As well as the main remediation, works also included demolition of existing structures, vegetation clearance and habitat relocation for existing wildlife. The remediation contract was part of a wider regeneration scheme around the Tipner area that will see new housing created, a Park and Ride scheme and a new junction to the M275.