Cyclic Response and Reconsolidation Volumetric Strain of Sand under Repeated Cyclic Shear Loading Events

Many regions of the world have witnessed repeated earthquakes that have caused repeated liquefaction of soil. Therefore, the effect of repeated earthquakes on the dynamic behavior of soil is an important aspect that must be considered. In this study, a series of cyclic shear loading events and consolidation phases was repeated for loose sand to investigate the cyclic stress–strain response and postcyclic reconsolidation volumetric strain that are usually induced by repeated earthquakes. After an initial consolidation, samples were subjected to repeated cyclic shear loading events and subsequent reconsolidations in a cyclic direct simple shear (CDSS) device with various cyclic stress ratios ($CSRs$) under undrained conditions for nonliquefied and liquefied stages. Based on the test results, the number of cyclic shear events ($N_{ev}$), built-up excess pore water pressure ratio ($r_u$), $CSR$, and maximum double amplitude shear strain (${\gamma }_{\max }^{DA}$) were shown to be the factors influencing the behavior of samples. For the samples that were not subjected to any prior liquefaction stage, the reliquefaction resistance continuously increased as $N_{ev}$ increased. For those subjected to repeated liquefaction, the samples presented cyclic liquefaction accompanied by high flow shear strain in the first event. The reliquefaction resistance drastically decreased in the second cyclic shear loading event as a result of the critical anisotropy induced by the first cyclic shear loading event when the contraction tendency prevailed. The samples showed cyclic mobility behavior from the fourth event onward, corresponding to the predominance of the dilation tendency. The obtained ${\gamma }_{\max }^{DA}$ and ${\epsilon }_{rc}^V$ increased as $CSR$ and/or $r_u$ increased and decreased as $N_{ev}$ increased. The relative density ($D_r$) was not a major factor affecting the reliquefaction resistance; however, it might have promoted a lower induced anisotropy level, lower induced ${\gamma }_{\max }^{DA}$, lower ${\epsilon }_{rc}^V$, and the change in behavior of samples from cyclic liquefaction (contraction tendency) to cyclic mobility (dilation tendency). A higher contraction tendency in samples resulted in a higher ${\gamma }_{\max }^{DA}$ and ${\epsilon }_{rc}^V$. The shear strain induced in the immediate-past cyclic shear loading event was found to be the main factor affecting ${\epsilon }_{rc}^V$. From the ${\gamma }_{\max }^{DA}-{\epsilon }_{rc}^V$ relationship, two separate zones were observed, namely (1) noninduced flow shear strain; and (2) induced flow shear strain, regardless of $CSR$ and $N_{ev}$. Additionally, a relationship between ${\gamma }_{\max }^{DA}$ and ${\epsilon }_{rc}^V$ was proposed for practical applications.