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Geotechnical investigation at a disused quarry using CPT

Geotechnical Support Site assessments Feasibility support

EPS was requested to assist a client to fulfil the pre-planning requirements for a new residential housing development at the site of a former quarry. The investigation was designed to evaluate risks to future residents whilst also providing geotechnical information for appropriate foundation design at a site with potentially highly variable ground conditions.

  • Initial desk study work established the site to have been used as an ironstone quarry which was later backfilled. Records indicated that the quarry was potentially up to 30m deep and had been excavated in long strips with the waste arisings from the quarry process used as the initial backfill, likely resulting in a fill with variable settlement characteristics.
  • Previous reports suggested that a significant amount of settlement had already occurred, approaching 1m in places and so the key objective of the EPS intrusive investigation was to address the depth of the cohesive capping layer and the potential remaining settlement of the fill.
  • In order to investigate the prevailing ground conditions and assess the characteristics of the near surface soils, 11 window sampler boreholes were drilled at the site to depths of around 5m and Electronic cone penetration testing (CPT) was performed to profile the characteristics of the deeper fill. This latter method provided information from 10 tests to approximately 12m depth with constant electronic in-situ data to provide a strength / depth profile for the whole distance of the test, which helped to identify inconsistencies in the fill.
  • The ground conditions were found to consist of medium to high strength clays (engineered backfill) from ground level to beyond the maximum depth of each position. Extensive sampling and chemical analysis of shallow reworked soils was undertaken with all materials identified to be suitable for use within a residential setting and therefore it was not necessary to make any recommendations for remedial measures. A programme of gas monitoring was subsequently undertaken, on a precautionary basis, which successfully demonstrated a low risk, negligible gas regime.
  • On the basis of EPS’ investigation, the Client has been able take into consideration the variability in the ground conditions and refine their proposed development scheme as required prior to submitting a full planning application.