Suction-Induced Hardening Effects on the Shear Modulus of Unsaturated Silt

Abstract

The small-strain shear modulus Gmax is a key material parameter in modeling the behavior of soils subjected to dynamic loading. Recent experimental results indicate that seasonal weather interaction with near-surface soils causes Gmax to change by up to an order of magnitude in some climates, with a hysteretic response upon drying and wetting. The increase in Gmax during drying and the stiffer response during subsequent wetting have been postulated to be due to plastic hardening during drying. To further understand this behavior, a series of isotropic compression tests were performed on compacted silt specimens at different values of matric suction to evaluate changes in the preconsolidation stress with suction. The Gmax values obtained previously on this silt matched well with a model using a hardening parameter independently derived from the isotropic compression tests, as well as the parameters of the soil water retention curve (SWRC). The model showed an increase in Gmax during drying from an initially saturated condition that was directly related to the increase in preconsolidation stress with suction, and the trends in Gmax followed transitions in the shape of the SWRC. The hardening parameter from these tests was also suitable for modeling the greater values of Gmax encountered during rewetting of the soil. The role of the preconsolidation stress in the model confirmed that changes in Gmax correspond to elastoplastic hardening mechanisms during drying rather than solely to changes in matric suction.