The site was part of Aylestone Road Gas Works from at least 1888, incorporating a gasholder, numerous buildings, railway tracks, tanks and an electricity substation at various times. Saffron Brook was adjacent to the south of the site.
Historic trial pits and boreholes inside the former gasholder had encountered Made Ground to 11.6m bgl, comprising brick and concrete rubble in a slightly sandy/silty clay matrix with localised ash, coal, coke, slag, copper wire, wood, polystyrene, brick and concrete cobbles, concrete boulders and black staining and gasworks/tar/hydrocarbon/burnt odours. Beneath was soft friable fine grained silty sandstone (Branscombe Mudstone Formation).
Intrusive investigation by Applied Geology in 2021 comprised cable percussive, driven continuous sample and rotary boreholes to depths of between 1.75m and 20.0m bgl and machine dug and hand dug trial pits up to 2.6m bgl.
Made Ground was encountered up to 10.0m deep inside the former gasholder and up to 5.0m deep outside, and comprised mixed granular and cohesive soils with brick, charcoal, concrete, quartzite and mudstone, locally with clinker, slag, granite, ash, chalk, flint, glass, plastic and asphalt and rare wood and wire fragments. Cobbles of brick, concrete and granite were also encountered, locally preventing drilling. Weak to strong hydrocarbon, creosote, coal-tar and gasworks-type odours were noted in the Made Ground across the site at variable depths. At the base of the Made Ground was a layer of puddle clay, which was locally stained black and had a strong hydrocarbon odour. Beneath was stiff, friable light grey locally silty clay with frequent mudstone lithorelicts (Branscombe Mudstone Formation), tending to a mudstone with depth. A slight hydrocarbon odour was locally noted in the weathered deposits. Groundwater was struck in the Made Ground between 1.5m and 9.00m bgl, standing at a higher depth inside the gasholder than outside. This suggested that the perched groundwater in the gasholder was not in continuity with the ‘true’ groundwater external to the gasholder, indicating the lining of the gas holder was still intact. Free product was not identified.
Elevated methane and carbon dioxide concentrations were recorded during monitoring, together with depleted oxygen and VOCs up to 67ppm.
Contamination testing of soil samples inside the gas holder recorded localised elevated concentrations of PAHs and VOCs, above published industrial/commercial end use screening values, indicated to be a mix of combustion products and coal derived materials, specifically in the range of coal tar and creosote. Whilst TPH concentrations did not exceed screening values, samples with the highest concentrations were suggestive of lube oil, substituted polyaromatic hydrocarbons and biodegraded diesel. There were no contamination exceedences from soil samples tested outside of the gasholder. Chrysotile, amosite and crocidolite were detected.
Contamination testing of groundwater samples from inside the gas holder recorded localised elevated concentrations of PAHs, sulphate, TPH and SVOCs. Outside of the gas holder, there were some marginal exceedences of PAHs, TPH, benzene and SVOCs during the first round of monitoring only.
There was considered to be a low risk to human health in areas external to the gas holder and low-moderate risk inside the gas holder. For Controlled Water receptors, there was considered to be a low risk external to the gas holder and a moderate risk inside the gas holder.
A piled foundation solution was recommended for the new office building, given the presence of deep Made Ground, extended into the Branscombe Mudstone Formation. Estimated bored pile capacities were provided. Since numerous obstructions were encountered during the ground investigation by Applied Geology, it was considered that either the proposed pile positions would need to be probed prior to construction or a bored (possibly cased) pile solution would need to be adopted. The piling method would need to avoid/mitigate any potential migration (cross-contamination) from the Made Ground to the natural strata, since it would involve penetrating the puddle clay, and would need prior agreement with the Regulators, including the production of a piling risk assessment.
A suspended floor slab was recommended, with CS2 gas protection measures. Sulphate resistant concrete was deemed necessary for new buried concrete.
Recommended remedial measures comprised a watching brief during development to check for the presence of previously unidentified shallow gross contamination, which may locally be present, localised removal of grossly impacted soils if encountered during the watching brief and the installation of a cover layer system in areas of soft landscaping. Remedial proposals would need to be discussed and agreed with Regulators and may involve further assessment (possibly including a DQRA) and the production of a Discovery Strategy and Verification Plan and subsequent Verification Report would be required.
