Postcyclic Reconsolidation Strains in Low-Plastic Fraser River Silt due to Dissipation of Excess Pore-Water Pressures

The postcyclic reconsolidation response of low-plastic Fraser River silt was examined using laboratory direct simple shear testing. Specimens of undisturbed and reconstituted natural low-plastic Fraser River silt and reconstituted quartz powder, initially subjected to constant-volume cyclic loading under different cyclic stress ratios (CSRs) and then reconsolidated to their initial effective stresses $\left({\sigma }_{\text{vo}}^{\prime }\right)$ , were specifically investigated. The volumetric strains during postcyclic reconsolidation $\left({\epsilon }_{\text{v-ps}}\right)$ were noted to generally increase with the maximum cyclic excess pore-water pressure $\left(\Delta u_{\max }\right)$ and maximum cyclic shear strain experienced by the specimens during cyclic loading. The values of ${\epsilon }_{\text{v-ps}}$ and maximum cyclic excess pore-water pressure ratio $\left(r_{u\text{-}\max }\right)$ were observed to form a coherent relationship regardless of overconsolidation effects, particle fabric, and initial (precyclic) void ratio of the soil. The specimens with high $r_{u\text{-}\max }$ suffered significantly higher postcyclic reconsolidation strains; ${\epsilon }_{\text{v-ps}}$ ranging between 1.5 and 5% were noted when $r_{u\text{-}\max }>0.8$ . The observed ${\epsilon }_{\text{v-ps}}$ versus $r_{u\text{-}\max }$ relationship, when used in combination with the observed dependence of cyclic excess pore-water pressure on CSR and number of load cycles, seems to provide a reasonable approach to estimate postcyclic reconsolidation strains of low-plastic silt.