Impact of Nonplastic Fines Content on the Small-Strain Shear Modulus and Water Retention Behavior of Unsaturated Sand–Silt Mixtures

Abstract

This study examines the small-strain shear modulus ( G max ) and water retention behavior of unsaturated sand–silt mixtures with a focus on the effects of fines content (FC), mean net stress ( p n ), and hydraulic hysteresis. A comprehensive experimental program was conducted, incorporating saturated and unsaturated bender element tests, soil water retention curve measurements, and scanning electron microscope imaging. The tests were performed on compacted specimens of Firuzkooh No. 161 silica sand mixed with varying silt contents (0-100%) as fines under p n of 50, 100, and 200 kPa along both drying and wetting paths. Two unsaturated triaxial systems equipped with bender elements and a hanging water column controlled matric suction ( ψ ) via axis translation and water head control techniques. The results revealed significant changes in soil structure and hydromechanical behavior with increasing FC. In contrast to other soil mixtures, clean sand exhibited distinct hydraulic and mechanical behaviors, with G max demonstrating a nonmonotonic variation with ψ . As FC increased, a pronounced reduction in G max emerged at FC of 20%, at which point the soil structure began to transform from a fines-in-sand to a sand-in-fines structure. Notably, the impact of hydraulic hysteresis became increasingly prominent with FC: for mixtures with 10%–100% FC, G max during wetting was higher than during drying at the same ψ levels, whereas an opposite trend was observed for clean sand. Additionally, for sand specimens, the suction stress concept effectively described the variation of G max with ψ , while for sand–silt mixtures (10%–100% FC), an extended framework incorporating the equivalent void ratio into a double hardening mechanism concept was adopted. The consistency between experimental observations and theoretical interpretations suggested that these concepts could provide a robust basis for predicting G max behavior in unsaturated sand–silt mixtures.