Offshore shear wave velocity measurements for the assessment of soil sampling quality
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Shear wave velocity Vs is a critical soil parameter for several geotechnical and geophysical engineering applications including seismic site response analysis, liquefaction risk assessment and design of shallow and deep foundations. Moreover, the comparison of shear wave velocity between laboratory and in situ measurements has become a standard acceptance criterion for the assessment of sampling quality. Offshore in situ shear wave velocity testing is considerably more challenging than onshore, due to the difficulties in the correct deployment of the instrumentation as well as of the wave source, in the absence of direct visibility of the ground level below water. This paper describes the methodology employed for offshore shear wave velocities (Vs) measurements in the harbour of Barcelona. Medusa SDMT tests were performed in sea depths ranging between 15-17 m from a jackup and employing a drill rig to penetrate the probe down to 40 m below the seafloor. The paper includes examples of recorded S-wave seismograms, analyses Vs repeatability for same depth measurements and displays Vs profiles with depth. In the same test locations, carefully prepared specimens of undisturbed samples were tested with bender elements in triaxial cells. The obtained lab shear wave velocities, under equivalent at rest stress conditions, were compared with the in situ values of the Medusa SDMT tests to assess sample quality.