Pore Pressure Buildup in Resonant Column Tests

Resonant column tests were performed on fully saturated and dry hollow cylindrical and fully saturated solid cylindrical specimens of Monterey No. 0 sand of 60% relative density subjected to 96 kPa confining pressure. The hollow specimens were tested by torsional excitation and the solid specimens by longitudinal excitation. It was observed that the threshold shear strain am plitude below which no pore water pressure buildup occurs is approximately $1.5\times {10}^{-3}$ for specimens which were not previously shaken. Between the approximate shear strain amplitudes of $1.5\times {10}^{-3}$ and $8.5\times {10}^{-3}\%$ there was a cyclic strain hardening effect which increased the threshold strain amplitude for specimens which were previously shaken, and caused the pore water pres sure of virgin specimens subjected to cyclic strain to stabilize at a predictable level (liquefaction did not occur). All specimens, whether previously shaken or not, liquefied at the threshold strain of $1.2\times {10}^{-2}\%$ which was previously identified to be the shear strain below which gross particle displacement is frictionally blocked. The effect of previous shaking on damping ratios and shear moduli was also investigated.