The influence of strain rate on the mechanical behavior of unsaturated granular soils is a critical consideration in geotechnical contexts such as fault rupture and earthquakeinduced ground deformation. However, existing suction-controlled double-cell triaxial studies on unsaturated soils have largely been restricted to very low strain rates (creep regime), primarily due to the challenges of accurately capturing volumetric strains with conventional measurement systems. To address this gap in the quasi-static strain-rate regime, the present study investigates the strain rate–dependent shear response of dense unsaturated silty sand collected from the Dasht-e-Bayaz fault zone in eastern Iran. Nine unsaturated consolidated drained (UCD) triaxial tests were conducted at axial strain rates of 0.0006, 0.1, and 2.5%/s, under constant matric suction (corresponding to the natural in-situ water content) and net confining pressures of 60–240 kPa. To overcome limitations in volumetric strain measurement at higher rates, a novel hybrid technique combining high-precision LVDT with dual-camera 2D Particle Image Velocimetry (PIV) was employed. The results show that increasing strain rate led to higher peak deviatoric stress (up to 16.3%) and internal friction angle (by 3.3°), accompanied by reductions in unsaturated cohesion (up to 13.5%). The failure mechanism transitioned from ductile to quasi-brittle, characterized by stronger dilative behavior, reduced axial strain at failure, a marked increase in initial Young’s modulus (up to 100% at 2.5%/s), and stiffer stress–strain response. An empirical model based on equivalent strain energy density (SED) was calibrated and successfully predicted rate-dependent peak strength and failure envelope parameters within the quasi-static regime. Complementary analyses of dilatancy, strain-softening onset, and energy dissipation further confirmed increasing brittleness and greater post-failure energy absorption at higher strain rates. Overall, these findings underscore the importance of explicitly incorporating strain rate effects and suction-related mechanisms when assessing the shear performance of unsaturated soils in dynamic geotechnical applications.

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