Influence of Strain History on Postliquefaction Deformation Characteristics of Sands

An experimental study aimed at improving the understanding of the role of maximum cyclic strain during earthquake loading on the subsequent response of liquefied sands is presented. The behavior of two sands, one generally strain-hardening and the other strain-softening over a range of stress and density states during virgin monotonic loading, is presented to demonstrate that regardless of whether a sand is contractive or dilative in preliquefaction loading, its response following liquefaction is dilative. It is shown that postliquefaction stress-strain behavior can be characterized by four distinct phases. The influence of relative density, confining stress, and the level of maximum shear strain during earthquake loading prior to liquefaction (preliquefaction strain, ${\mathrm{\gamma }}_{\text{preliq}}$) on the response of liquefied sand varies during each of these phases. The first phase of deformation, essentially at zero stiffness, is highly dependent on the level of preliquefaction strain, but is not significantly affected by the density. The final strength of the material in postliquefaction loading (liquefied strength, $S_r$), on the other hand, is highly dependent on the density and loading mode. No clear relationship exists between liquefied strength and preliquefaction strain, but a somewhat weak inverse correlation is noted between strength ratio normalized by initial confining stress and preliquefaction strain. Normalized strength ratio of the liquefied sand varies over a wide range depending on the density and strain history. It varied between $\text{∼}0.04$ and 0.7 in pluviated strain-softening sands, but values as high as two were measured in strain-hardening dense sands. In general, the normalized strength ratio increases with increasing density, but it decreases somewhat with increasing stress level and increasing preliquefaction strain.