Undrained SHPB experiments on calcareous sand with different saturation degrees

Extensive research has been conducted on the impact behavior of unsaturated sand at high strain rates. However, achieving the undrained boundary condition remains a persistent challenge, leading to an inconsistent understanding of the dynamic responses of sand with varying saturation degrees. In this study, a novel sleeve designed to conduct split Hopkinson pressure bar (SHPB) tests under undrained boundary conditions. Furthermore, drained SHPB tests were carried out by using the conventional steel sleeve as references. The absolute particle crushing distributions within various size ranges were investigated by utilization of dyed calcareous sand. Results revealed that, for the conventional drained sleeve, the locking-up phenomenon of full saturation sand was only observed at strain rate of 750 s⁻¹. However, locking-up occurs at all strain rates for undrained sleeve. The locking-up stiffness at strain rate of 1000 s⁻¹ was 1.6 times larger compared to that at strain rate of 500 s⁻¹ and 750 s⁻¹, respectively. It means that the locking-up stiffness increases with strain rates under the fully undrained boundary conditions. Furthermore, for the drained sleeve, negligible reductions up to 10.8% were observed in measured B_r if saturation degrees change from 0 to 100%. In contrast, for the undrained sleeve, the maximum reduction on B_r was 47.6% and increases rapidly with increasing strain rates. The particle crushing was more sensitive to saturation degree at higher loading strain rates under undrained boundary conditions. Meanwhile, the particle crushing probability \({P}_{\text{c}}\) of medium-sized (1.0–1.5 mm) particles decreases with decreasing saturation degrees and increasing strain rate. It leads that the probabilities of particle crushing across various size ranges become more uniformly distributed with lower saturation degree and higher strain rates.