Characterization of the Small-Strain Stiffness of Soils at an In-situ Liquefaction Test Site

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.