Strain Rate-Dependent Behavior of Unsaturated Silty Sand in Fault Zone Alluvium: Experimental Insights into Fault Rupture Effects

The influence of strain rate on unsaturated granular soils plays a critical role in dynamic geotechnical responses, particularly in fault rupture and earthquake-induced deformations. Although matric suction significantly governs the mechanical behavior of these materials, most studies have primarily concentrated on the very low strain rate regime. However, fault rupture during a fling-step typically occurs at significantly higher strain rates. Therefore, to evaluate the effects of permanent displacement induced by fault rupture on structures overlying unsaturated alluvium in fault zones, further experimental investigations are essential to identify the strain rate-dependent response of unsaturated silty sand alluvium under higher strain rate conditions. This study examines the strain rate-dependent behavior of compacted unsaturated silty sand in the Khezri–Dasht Biyaz fault zone, southeastern Iran. A series of unsaturated consolidated drained (UCD) triaxial tests were conducted under controlled net confining pressures and constant matric suction at strain rates of 0.0006, 0.1, and 2.5% per second. Axial loading was applied via a servo-controlled actuator, and simultaneously, volumetric strains were precisely measured using a new Particle Image Velocimetry (PIV)-based technique. Furthermore, a previously developed strain rate-sensitive empirical model was calibrated and employed to predict the stress-strain response. The results indicate that increasing strain rate in the quasi-static regime enhances shear strength (up to 22.5%) and internal friction angle (up to 4.2°) while reducing unsaturated cohesion (up to 10%) and capillary effects. A transition from ductile to brittle failure, reduced contractive strains, increased dilative behavior, and changes in stress state at the onset of softening were observed. Moreover, energy absorption capacity up to failure decreased with increasing strain rate, highlighting the role of strain energy in the mechanical response. These findings enhance understanding of fault-proximal soil behavior, contributing to seismic hazard assessment and geotechnical modeling.