Effect of shearing history on stress wave velocities of sands observed in triaxial compression tests

For the accurate design of geotechnical structures subjected to static and dynamic loadings, precise estimation of elastic wave velocities and hence, small strain stiffness of soil is essential. However, the interpretation of elastic wave velocities propagating in deformed/sheared soil has not been comprehensively explored. In this research, shear (Vs) and compression wave velocity (Vp) measurements have been undertaken on three kinds of uniformly graded sands during drained triaxial compression by keeping minor principal stress constant. Planar piezoelectric transducers have been used to overcome the limitations associated with the more commonly used bender elements, such as distortion of transducers during specimen shearing. This technical note reveals that the increase of major principal stress in the wave propagation direction has a more significant influence on Vp in comparison to Vs. The axial strain (ea) at which peak Vs is noted is comparable to the ea at which specimen dilation or phase transformation takes place. The Vs values show a substantial drop after phase transformation, although there is an increase in the mean stress level. However, Vp increases even after specimen dilation takes place, and it is the major principal stress that dictates its evolution during triaxial compression. Moreover, for a given material and initial stress level, elastic wave velocities of specimens prepared at different initial densities approach one another during shearing and later merge at a large ea.