The cone penetrometer test (CPT) is used extensively for the assessment of ground conditions for offshore wind farms due to its relative speed, accuracy and continuous profiling capability. For a recent offshore wind farm project in the North Sea, the CPT equipment and CPT procedure were innovatively modified to assess the residual strength and cyclic degradation properties of a highly overconsolidated clay in situ. The CPT equipment was modified to include a second sleeve, located behind the main sleeve, to assess the in situ shear strength degradation with a fixed shear strain. The CPT test procedure was modified to include a ‘cyclic CPT’ test, or ‘CPTc’ test, whereby the CPT was cycled up and down, to assess the in situ cyclic degradation of the soil at large cyclic strain amplitudes. The in situ tests gave consistent results in line with expectations for a highly overconsolidated North Sea clay. After comparing the results of the CPTc, the dual sleeve CPT and select relevant laboratory tests, a consistent response indicates the applicability of using these innovative in situ tests to assess the degradation of the highly overconsolidated clay.

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