Strong ground motions from earthquakes can result in the reduction of the shear modulus and wave propagation characteristics of soils, and can produce liquefaction and cyclic softening during shaking and for some time following the seismic event. While the ground motions produced from controlled blasting are different than those in an earthquake, the consequences of the ground motions with respect to changes in stiffness and strength are similar. Shear wave velocity provides a means to quantify the potential for change in the seismic response of soils and predict liquefaction potential of the soil. The small-strain shear wave velocity of soils indicates their stiffness, and this parameter can vary with both stress and soil fabric. Shear wave velocity can be measured using downhole, crosshole, and surface wave techniques, all of which were assessed in this work.
This study provides a comprehensive comparison of in-situ shear wave velocity measurement before and after a series of controlled blasting liquefaction tests at the Port of Portland, in Portland, Oregon. The objectives of this work are to compare shear wave velocity profiles, in-situ anisotropy, and the change in shear stiffness of the soil before and after blast-induced liquefaction. Shear wave velocities in two different soil layers at this test site were measured in this study: a deposit of medium stiff silt and a deep deposit of medium dense poorly graded sand. Crosshole, downhole, and surface wave tests were conducted before and after blast-induced liquefaction. The downhole results showed that velocity increased with depth in the sand and silt layers, and downhole velocities from the different trials generally varied by less than 10 percent. While conclusive findings on fabric-induced anisotropy were not obtained from this study, less invasive techniques provided reliable one-dimensional velocity profiles for the layers of interest at the test site.
In general, the three techniques showed similar results prior to blasting. A significant reduction in the shear wave velocity was observed in the downhole tests after blast-induced liquefaction and softening. These results can be used to guide inferences regarding the loss of stiffness following a seismic event.