A Unified Model of Cyclic Shear–Volume Coupling and Excess Pore Water Pressure Generation for Sandy Soils under Various Cyclic Loading Patterns

Accurate prediction of excess pore water pressure (EPWP) generation in saturated sandy soils remains one of the most challenging issues in sandy site responses to strong earthquakes and extreme marine environments. This paper presents experimental results of undrained and drained multidirectional cyclic hollow cylinder (MCHC) tests on saturated coral sandy soils under various cyclic loadings. The results show that threshold generalized shear strain ${\gamma }_{\mathrm{ga},\mathrm{th}}$, below which EPWP and volumetric strain can be neglected, is an inherent property depending only on the soil type and initial state. Furthermore, there exists a virtually unique form of relationships between the generalized shear strain amplitude (${\gamma }_{\mathrm{ga}}$) and the cumulative dissipated energy per unit volume of soil ($W_{\mathrm{c}}$) at different relative density ($D_{\mathrm{r}}$), irrespective of drainage conditions and cyclic loading conditions. These findings highlight the fundamental mechanism for cyclic deformation behavior and the uniqueness of correlations among $r_{u\mathrm{p}}$ (peak EPWP ratio), ${\epsilon }_{\mathrm{vp}}$ (peak volumetric strain), and ${\gamma }_{\mathrm{ga}}$ of saturated sandy soil at the similar $D_{\mathrm{r}}$, regardless of cyclic loading conditions. Based on these findings, a novel unified model of ${\gamma }_{\mathrm{ga}}$-based cyclic shear–volume coupling and EPWP generation is established, which is independent of cyclic loading conditions over a wide loading frequency range. Then the applicability of the proposed model is validated by the experimental data of the same tested coral sandy soil and siliceous Ottawa sand, as well as the data of siliceous fine sands in previous work. It is found that the proposed model surpasses the existing strain- and stress-based models in accurately predicting EPWP generation under complex cyclic loadings, which can offer new insights into the mechanisms of the EPWP generation in saturated sandy soils.