Influence of Particle Morphology on Hysteretic K0 of Granular Soils

In many applications of geotechnical analysis and design, correct determination of the coefficient of earth pressure at rest, K0, is essential. The K0 in soils is typically estimated using empirical equations that depend on the soil’s friction angle and stress history in terms of the overconsolidation ratio (OCR). However, this estimation ignores any effects of particle morphology in granular soils, potentially leading to unrealistically large hysteresis upon loading–unloading cycles. Existing experimental and numerical studies only reported K0 of granular soils with limited ranges of particle morphology during loading. This study conducted a comprehensive experimental campaign to systematically study K0 of soils with controlled morphology along loading and unloading paths. An oedometer equipped with flexible tactile sensors to precisely measure horizontal and vertical stresses during loading–unloading cycles was developed to obtain K0 in five granular soils with a wide range of particle shapes but similar particle size and initial density. Results showed that K0 during loading and unloading were strongly associated with particle morphology. Soils with a lower particle regularity (REG, i.e., average of roundness and sphericity) or overall regularity (OR, i.e., average of aspect ratio, convexity, and sphericity) mobilized more friction angles during loading, returning smaller K0 values. They also hold most of the locked-in stresses developed upon loading at different unloading stages, causing greater hysteresis. When employing REG and OR, equations were obtained for K0 during loading that are identical to Jaky’s equation, even though these overall descriptors have different definitions and ranges of value. New empirical equations that incorporate REG and OR into the K0–OCR relationship were developed to markedly improve the estimation of hysteretic K0 after validating against high-quality laboratory data of 10 sand types from the literature.