Partially saturated soils below the groundwater table usually exist because of decomposition of organics within the soil deposit or soil desaturation technologies for liquefaction mitigation. Therefore, the cyclic responses of partially saturated soils should be paid more attention to and it is necessary to establish a practical model to evaluate the liquefaction resistance improvement LRR20 (the ratio of liquefaction resistance of partially saturated soil to that of fully saturated soil) due to the decrease of saturation. Some researchers have tried to quantify the liquefaction resistance of partially saturated soil using the P-wave velocity Vp. However, whether the LRR20-Vp relationship is influenced by initial states such as relative density, structure or fabric of soil, and fines content, is not clear yet. For this purpose, an experimental programme comprising undrained cyclic triaxial and bender elements tests was performed on partially saturated soils under various conditions. Results show that the cyclic shear behaviors of partially saturated sands with different B-values are significantly different from those of fully saturated sands. For partially saturated sands, as excess pore water pressure (EPWP) accumulates to a certain proportion of initial effective confining pressure, it suddenly develops with a significant increase followed by the occurrence of more than 5 % of axial strain. Based on the test results including those from independent literature, a unique experimental LRR20-Vp relationship is proposed for different relative densities, sample preparation methods, initial effective confining pressures, and fines contents.
