Property owners identified the sudden appearance of a circular subsidence feature in the garden area of an existing property, adjacent to the building which measured approximately 3m in diameter. The surface covering (paving slabs/subbase etc). had collapsed in to the feature.
We undertook a detailed geotechnical phase 1 desk study reviewing information that included historical mapping, a geological report, geological maps and memoirs, available online information associated with the local area which included information on surface mineral extraction (quarrying) and underground cave systems.
Our Desk Study identified that the site was located on the bedrock that comprised the Mercia Mudstone Marginal Facies, a formation that is known for the risks posed by solution features. Geological boundaries with the Avon Group (Limestones and Mudstones) and the Portishead Formation (Sandstones) were identified in close proximity.
A subsequent walkover of the site and immediate area, identified a number of other possible subsidence features, ponds and anomalous ground conditions that were suggestive of the wider impact of solution features in the local area. A limited ground investigation was undertaken and was aimed at only identifying the likely risk of locating an excavator adjacent to the feature to allow its excavation. Our exploratory site investigation comprised a combination of windowless sampling and dynamic probing to record ground conditions and insitu ground strengths.
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A ground model comprising limited Made Ground underlain by the weathered Mercia Mudstone Marginal Facies was identified. Variable in-situ ground strengths were recorded, including zones of very low strength soils in two locations to a maximum depth of 11m. This information suggested that whilst the surface expression of the feature measured approximately 3m, the likely zone of influence extends much further.
Our recommendations to the Client that placement of any plant to investigate the feature could not be recommended without further assessment of the investigation risks. At present, ESP continue to work with the Client to assess the site and allow the formulation of a remedial strategy.
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Get in touch with our team via our email, enquiries@earthsciencepartnership.com or phone number, 02920 813385 and we’ll be very happy to help with and discuss any geotechnical engineering project requirements you may have.
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