Derelict gasworks transformed for landmark housing
A combination of traditional and state-of-the-art techniques has been employed at the former Orwell Gas Works to regenerate a derelict site for housing development
Investigation & remediation design
Card Geotechnics has been awarded a £500,000 contract by Persimmon Homes for the design and management of the remediation clean-up at the former Orwell Gas Terminal. The contract includes the investigation, risk assessment and remediation design and on-site contract supervision.
The consultancy initially undertook a comprehensive review of local and national archives to obtain as much information as possible relating to the historical development of the site. A ground investigation strategy was then formulated to quantify the degree, nature and extent of contamination present and to provide geotechnical information for the structural design. To these ends a staged approach to the site assessment was adopted that encompassed a range of investigation techniques. These included trial pitting, cable percussive and rotary cored boreholes and monitoring wells, Dutch cone penetration testing and a ground probing radar survey.
Ground conditions were found to comprise a considerable thickness of made ground over a thin layer of alluvium, which were underlain by River Terrace Gravels over the Upper Chalk. Adjacent to the dock, a glacial buried channel was identified that extended to a depth of over 20 metres into the chalk and was infilled with dense gravels and beds of very stiff clay.
A major programme of chemical testing of samples retrieved from the site showed that widespread contamination was present within the shallow deposits. This included hydrocarbons and heavy metals, phenols, tars and cyanide compounds. In addition significant PAH contamination was found in the groundwater within the Terrace Gravel deposits.
On the basis of the investigation findings, hydrogeological modelling and a quantitative risk assessment were undertaken to establish a set of site specific target levels for site remediation. In parallel with this a remediation strategy was formulated to achieve the required targets.
The proximity of the site to the Ipswich Wet Dock and the River Orwell, coupled with its situation immediately above the main chalk aquifer, have had to be considered in the formulation of an appropriate remedial strategy as have the safety of future users of the site and construction workers.
Remediation works commenced in June 2002 under the supervision of Card Geotechnics. The approach that was adopted was designed to optimise the reuse of site won material and to minimise the need for disposal off site.
To prevent any short-term impact on water quality within the wet dock, a mastic-sealed sheet pile wall was installed along the dock boundary of the site prior to the start of any bulk excavation.
Extensive former foundations were encountered at shallow depth and these have been broken out, screened and crushed for reuse as aggregate. Excavated material has been assessed during excavation and subdivided into grossly contaminated material requiring disposal to landfill and material that can be treated on site for re-use. Soil treatment is being undertaken using ex-situ bioremediation with the creation of a number of large biopiles on the site.
Excavation is being undertaken in phased areas, with a bulk dig to a depth of 2 metres below existing ground level and then localised hotspot removal to a further 1m below this level. The excavations are then backfilled using crushed concrete below the water table and compacted treated materials to finished remediation level.
Treatment of groundwater is being undertaken by a combination of in situ and ex-situ methods. The water within the shallow deposits (made ground and alluvium) is being treated using an on site treatment plant and is then disposed of to foul sewer. Impacted water within the Terrace Gravel is being undertaken using an ozone-enhanced air-sparging system.
A strict regime of environmental monitoring is being undertaken to ensure that the site operations have minimal impact on the surrounding area. All site works are being validated by Card Geotechnics to demonstrate to the Environment Agency and the Environmental Health Officer that works are being undertaken in a correct manner. It is hoped that occupation of the first units at the site will commence in the spring/summer of 2003.