Phase I and Phase II Ground Investigation – Pear Tree Street


The site located at 1 Pear Tree Street, Islington, EC1V was a proposed development comprising of new high density residential dwellings housed in an apartment block between 5-6 storeys high. The ground level space fronting on to Pear Tree Street was to be used for commercial/retail end use with the development also including a public square.

REC were previously commissioned to undertake a Phase I and Phase II ground investigation at the site. The intrusive works were undertaken in July 2011.

The site was a former Victorian gasworks which was demolished between approximately 1890 and 1914. Investigations completed by REC identified soils with concentrations of petroleum hydrocarbons, BTEX and PAH that exceeded the generic criterion based on a residential end use. It was proposed to undertake remediation of the site as a means of providing betterment of the soil and groundwater quality prior to development.

The objective of the remediation works was to implement measures in order to provide betterment of the soil and groundwater quality underlying the site and to reduce the long term risk to controlled waters. These measures included:

Surface Treatments

The entire site was ‘probed’ via the use of an excavator to check for the presence of subterranean structures. If during the course of removing structures, hydrocarbon-impacted soils were encountered, then soil arisings were treated via the addition of oxygen release compound (ORC) (calcium peroxide). The dry ORC powder was mixed into the soil via the use of the excavator and the soils replaced into the excavation. This approach effectively reduced the risk of cross-contamination of the relatively clean soils encountered at the surface. The dosage of ORC in for mixing with impacted soils was approximately 25kg to 50kg per 10m3 of soil.

Sub-surface Treatments

Chemical aerobic techniques were used to instigate a reduction in the concentrations of petroleum hydrocarbons, BTEX and PAH across the site.

Soils at selected locations across the site were injected with ORC on a nominal 3.0m grid. The grid was centred on those parts of the site where the concentrations of contaminants of concern exceeded the remediation target values.

The installation of ORC will accelerate naturally occurring in situ bioremediation of petroleum hydrocarbons and organic substances by indigenous microorganisms in the subsurface. The method provides a controlled-release, supplemental source of oxygen to the subsurface which is made available to aerobic, hydrocarbon-degrading bacteria.

The ORC was injected from the upper surface of the London Clay Formation to approximately 1.0m to 2.0m below reduced ground level and the injection point sealed with a bentonite plug.

REC were commissioned to prepare a Remediation Verification Report in order to provide documentary evidence that works to achieve proposed betterment of the Pear Tree Street site had been undertaken in accordance with the agreed Remediation Method Statement. This report was submitted to Islington Borough Council for their agreement.

The exposure pathway beneath the remaining site area has effectively been removed and the risk to human health from remnant organic substances within the soil is not considered significant. It is considered that the works undertaken at the site have been completed satisfactorily and will provide significant long term betterment of the soil and groundwater beneath the site. The site was successfully signed off.