Demolition and Remediation for LIDL

Fallon were contracted by Lidl as Principal Contractor to provide a fixed price combined demolition, remediation and enabling works package at the site of a former gasworks and council depot in North Wales. Approximately half of the site was open storage land, whilst the remaining half was formed from depot buildings and external hardstanding.

The site investigation reports had identified area of open storage had been previously remediated to achieve environmental objectives set by the Environment Agency but had not undergone geotechnical remediation for the proposed store. Consequently, a number of large relict foundations from the former gasworks remained in-situ. The site investigation and risk assessment had also identified that shallow groundwater with the superficial deposits beneath the council depot had been contaminated with hydrocarbons by a leaky interceptor.

Excavation of interceptor

The Remediation Strategy required all relict foundations were removed beneath the proposed store footprint so that there were no obstructions to future piling operations. Elsewhere relict foundations were removed to a minimum of 1m depth to enable proposed car park and drainage construction. The interceptor had to be excavated and removed and the zone of downgradient impacted groundwater treated in-situ to reduce potential risks of contamination to an off-site surface water receptor.

Implementation and Verification of the Remediation Strategy and works were subject to pre-commencement planning condition which also required a period of three months post-remediation monitoring of groundwater.

Injection of ORC in grid of wells

In order to achieve the considerable programme constraint, the works commenced with the removal of the interceptor and drilling of a grid of injection wells in the zone of groundwater contamination. The groundwater was then treated in-situ using an Oxygen Release Compound (ORC) to promote long-term degradation of hydrocarbon compounds. The ORC was mixed on-site and injected under pressure to create flow through water bearing sand lenses within the superficial deposits. Following injection, the groundwater was monitored to measure the degradation of hydrocarbon compounds over a period of three months.

During the three monitoring period, Fallon demolished the depot buildings and excavated all made ground to a minimum depth of 1m to remove relict foundations. Beneath the proposed store footprint, all relict foundations were excavated to their full depth and this included a former gas-holder that remained intact. The circular gasholder wall had formed a barrier was withholding shallow groundwater within the superficial deposits and its removal resulting the flow of hydrocarbon impacted groundwater from outside the wall into the excavation. The excavation was backfilled with free draining gravel and six wells were inserted to enable passive product skimmers to be installed to remove any free product that may have accrued from the in-flow of groundwater to the excavation.

Breaking out of gas holder wall

All paving, slab, brick and relict foundations were crushed to produce a 6F2 fill material that was placed across the site to form a remediation finished level in preparation for piling and construction.

A Verification Report was compiled on completion of the project to: record the works described above; present groundwater monitoring data; soil test data; and analyses of the fill materials produced during the works.

Product Skimming in backfilled gasholder excavation