Undrained Dynamic Response of Partially Saturated Sands Tested in a DSS-C Device

Liquefaction-induced seismic vulnerability is still a major problem in geotechnical engineering applications. To address this issue, various liquefaction mitigation techniques have been proposed to reduce the excess pore water pressures and liquefaction-induced deformations. Recently, the induced partial saturation (IPS) method, by which the fully saturated condition in loose sands is altered to be partially saturated, was proposed. In the present research, undrained cyclic strain-controlled tests were performed on partially saturated sand specimens with degrees of saturation ($S$) of 70%–89% and relative densities ($D_r$) of 21%–43% in a dynamic simple shear device with confining pressure (DSS-C) at effective stresses and back pressures ranging from 50 to 200 kPa and under cyclic simple shear strain amplitudes ($\gamma$) of 0.1%–0.4%. Various specimen preparation techniques were explored, and the best specimen preparation method was determined to be wet pluviation of dry sand into a water-sodium percarbonate mixture to obtain loose, undeformed, partially saturated sand specimens at various degrees of saturation. The test results showed that as the degree of saturation decreases, the generation of excess pore water pressure ratios ($r_u$) also decreases significantly, especially under conditions of higher effective stresses. The $r_u$ values in partially saturated sand specimens decrease as the effective stress increases. However, $r_u$ increases as the backpressure increases. The back pressure has a significant effect on $r_u$ generation in partially saturated sands, unlike in fully saturated sand tests; the true liquefaction resistance of partially saturated sand samples should be investigated under the coupled effects of effective stress and back pressure, which is equivalent to the initial pore water pressure in the field. According to the test results, the beneficial effect of effective stress on liquefaction resistance is more prominent than the detrimental effect of back pressures; therefore, we have concluded that once IPS is implemented in the field, the degree of saturation can be reduced to lower values at shallower depths.