The lateral pressure coefficient at rest of expansive soils in landfill at various vertical stresses and moisture contents

When expansive soils in the original location are artificially transferred to landfill in different seasons, and subject to engineering activities afterwards, the corresponding deformation and stability of retaining structures become unpredictable. This necessitates the determination of lateral pressure coefficient at rest (k(o) value) for expansive soils in landfill. Considering compaction, excavation of expansive soils, as well as construction of landfill in different seasons, series of stepwise loading and unloading consolidation tests at various moisture contents were carried out in this work to explore the evolution characteristics of k(o) value and assess the dependence of k(o) value on vertical stress and moisture content. Besides, scanning electron microscope (SEM) was used to track the change in microstructural features with vertical stresses. The results indicated that the k(o) value of expansive soil shows a pronounced nonlinearity and is inextricably linked with vertical stress and moisture content, based on which a prediction formula to estimate the variation in k(o) value with vertical stress during loading stage was proposed; there is a significant exponential increase in k(o) value with overconsolidation ratio (OCR) during unloading stage, and OCR dominates the release of horizontal stress of expansive soil; SEM results revealed that with an increase in vertical stress, the anisotropy of expansive soil microstructure increases dramatically, causing a significant directional readjustment, which is macroscopically manifested as an initially rapid increase in k(o) value; but when vertical stress increases to a critical value, the anisotropy of microstructure increases marginally, indicating a stable orientation occurring in the soil microstructure, which causes the k(o) value to maintain at a relatively stable value.