A geotechnical site investigation by surface waves

Near surface geotechnical site investigations are generally performed using invasive mechanical techniques. Even though these techniques provide fairly accurate information, the time and costs involved increase exponentially with the size and depth of the investigation. Recent advances in non-invasive in situ geophysical techniques have led the drive to pursue the potential of combining both mechanical and geophysical techniques. It is apparent that geophysical techniques cannot offer the same level of accuracy as mechanical techniques, but they provide a more time and cost effective approach. A limitation of non-invasive geophysical techniques is the necessity of providing a constraint to prevent the misinterpretation of measured data. Hence, a combination of both mechanical (to constrain) and geophysical techniques, benefits from the advantages while limiting the disadvantages of both. However, in order to reduce the necessity of mechanical testing to provide constraining information, the paper proposes a novel approach by combining the passive Horizontal to Vertical Spectral Ratio (HVSR) technique and an array based active surface wave technique. This proposed technique relies on the notion of the phase velocity dispersion curve, array based technique providing constraining information to the HVSR technique. Results so far suggest that this two-step procedure of combining these surface wave techniques (HVSRplus), allows for very reasonable estimation of the near surface shear wave velocity (Vs) profile, in the absence of constraining information from mechanically measured data. Of course, care must be taken when not calibrating or verifying against reliable mechanical data. Results produced by this technique have been verified against invasive mechanical (boreholes, SPT) data.