Measurements and modeling of surface waves in drilled shafts in rock

Surface wave measurements were made in horizontal drilled shafts in rock salt to characterize the stiffness of the rock around the shafts. The Spectral-Analysis-of-Surface-Waves (SASW) method was used to determine dispersion curves of surface wave velocity versus wavelength. Dispersion curves were measured for surface waves propagating axially and circumferentially in the shafts. Surface wave velocities determined from axial testing increased slightly with increasing wavelength due to the cylindrical geometry of the shafts. On the other hand, surface wave velocities determined from circumferential testing exhibited a completely different type of geometry-induced dispersion. In both instances, finite-element forward modeling of the experimental dispersion curves revealed the presence of a thin, slightly softer disturbed rock zone (DRZ) around the shafts. This phenomenon has been previously confirmed by crosshole and other seismic measurements and is generally associated with relaxation of the individual salt crystals after confining stress is relieved by excavation.