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Case Studies - Brownfield, Waste & Geotechnical

Slope Stability and Contamination Investigation, South Wales, UK

Client: McCarthy & Stone
Location: Tonypandy, South Wales Coalfields, UK
Date: 2008
Project: Slope Stability Investigation & Recommendations; Contaminated Land Investigation of site in Tonypandy.

Abricon were commisioned to carried out a site investigation, including slope stability, of the Renault Garage site in Tonypandy, Wales, on behalf of the client, a housing developer, as part of a pre-aquisition study.

The site's former use as colliery and current use as a garagewas recognised as being potentially contaminative. In addition, the site was situated alongside a steep 12m slope towards the river Rhondda, and plans obtained of the proposed sheltered housing development showed that several structures were to be situated directly atop the crest of the slope.

Abricon recommended that a slope stability investigation and intrusive contamination investigation be undertaken to inform on these potential points of concern.

Contamination

Several boreholes were drilled and wells installed to monitor groundwater and check for the presence of contaminants. Although PAH's, some metals and hydrocarbons were present in near surface soils, these were generally both of low concentration and were sited under areas of proposed hardstanding, thereby removing pathways to potential receptors.

Slope Stability investigation

The slope stability investigation reported that the site is variable, sloping downwards and to the north at an average angle of between 35 and 40 degrees, but is locally both more and less steep.

The investigation, involving the construction of rotary open hole and cored boreholes and hand held sampler boreholes across the site, revealed that the majority of the site was covered by a layer of colliery spoil/ made ground, overlying alluvial deposits containing pockets of peat, and underlain by weathered sandstone. Groundwater is present at the site with the water table dipping towards the river valley.

The investigation proved that the site soils are highly heterogeneous. In conjunction with the variable slope angles observed, it was concluded that the site did not therefore lend itself to quantitative analysis.

The existing boundary slope gradient is consistent with the angle of repose shown by colliery spoil excavated during the site investigation, indicating that the slope may already be at the natural limit of its stability - an investigation of the local area revealed that a significant landslide had occurred in similar materials, downstream on the opposite riverbank slope.

It is therefore considered that stability at the slope is near unity (i.e. a Factor of Safety of 1), and that slope stabilisation works are necessary to facilitate development in the vicinity of the slope’s crest. Planned driven piled foundations would also not be possible due to the presence of the boulder clay underlying the site.

Soil re-engineering, with the use of soil anchors and erosion control in areas requiring only marginal re-inforcement was recommended for the site, however it was also reported that the anticipated cost of such remedial works across the site would range between £609 000 and £791 000, depending on choice of method and area of treatment.

Given the costs involved, it was concluded that the site would be uneconomical to develop and the Client was advised to withdraw their bid for the land.