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Castle Mill, Oxford

The client:Longcross Construction Ltd
Services:In-situ stabilisation, excavation, geotechnical testing.

The Challenge:

The 1.5 hectare site was previously occupied by railway sidings and was redeveloped with the construction of eight student accommodation blocks each between six and eight storeys. The buildings were to be constructed using piled foundation but due to the risk of vibration during piling works, Cognition was asked to consider alternative options to piled foundations. Cognition carried out trials to demonstrate that  ground stabilisation would provide geotechnically  suitable subgrade material on which to construct conventional raft foundations.

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The Approach

Cognition was contracted by Longcross Construction to undertake soil stabilisation works at the Castle Mill site in Oxford. Cement stabilisation was carried out to 3.5m below ground level extending 1m below the water table. This approach eliminated the risk of vibration and improved the natural ground to allow for raft foundations. The key elements of the works included;

  • Site investigations to gather geotechnical information and samples for the stabilisation mix design for the made ground and soft natural clay;
  • CFA rig used to drill boreholes to a maximum depth of 5m bgl;
  • Strata arisings sampled for laboratory analysis;
  • Standard Penetration Tests (SPT) carried out at intervals of 1.5m, 2.5m, 3.5m and 4.5m bgl and N values were recorded between 10 and 16 at 4.5m bgl;
  • In situ geotechnical testing, including natural moisture content, plasticity and CBR ;
  • Excavation of made ground and soft natural clay;
  • Auguring of the ground to locate the sand/gravel stratum (approximately 4.0m bgl);
  • Combination of sump pumping and well pointing to dewater the base of the soft natural clay above the sand/gravel;
  • In-situ stabilisation using an excavator mounted allu bucket;
  • Geotechnical testing to ensure the compaction specification for the stabilisation was met;
  • Core cutter samples taken for nuclear density testing to compare with the compaction results and compressive strength for bearing pressure;
  • Stabilisation below all building plots.

The Outcome

Cognition completed the soil stabilisation works and delivered an alternative approach to piling work which eliminated the risk of vibration, allowing all work to be completed safely. Student accommodation buildings were constructed successfully without complex alternative foundations.

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