A Study of Strain Rate Effects on the Mechanical Behavior of Sand

A number of civilian and military applications are influenced by strain rate effects, such as: dynamic compaction, pile driving, projectile penetration, explosions, air blasts, mine blasts, among others. Strain rate effects are encountered when granular media experiences rapid monotonic loading. To investigate this phenomenon, a comprehensive battery of uniaxial and triaxial compression tests was carried out at different strain rates on sand. A custom-made uniaxial mold, and conventional triaxial chambers with in-house fixtures were used in the experimental program. Dry samples of siliceous and calcareous sand were studied in order to identify the role of material composition on high strain rate (HSR) and intermediate strain rate (ISR) behavior of sand. Four strain rates including 0.01, 0.1. 1, and 10%/s were employed to identify the effects of ISR and HSR loading. Specimens were prepared at two different densities. In triaxial compression specimens were confined at 100, 200 and 400 kPa.;Uniaxial compression results revealed that there is a distinct measurable strain rate effects for silica and calcareous sands, in which material composition plays a significant role. Higher strain rates contributed to up to 25% increase in strength and stiffness. In triaxial compression, the role of strain rate depends mainly on confining pressure and initial density, in which samples highly confined and dense exhibited more rate effects.;The role of particle breakage on the stress-strain behavior of uniaxial compression in sand was also studied. Samples with two aspect ratios loaded at two strain rates were arrested at various strains up to 30%. Particle size analysis of post-tests was performed using a unique particle size analyzer that employs dynamic imaging of the grains. The onset of non-linear strain-softening behavior and hysteresis observed in the stress-strain experiments correlated with the progressive size reduction observed in the arrested tests. It was also seen that smaller particles break more than larger particles, and that sand fractures typically following aspect ratios of the parent material.;A non-linear model to predict rate dependency of the stress-strain response of sand was also presented. The model is inspired by the equivalence of the strain energy density (SED) of the material at different strain rates, and was calibrated to model dry sand during uniaxial and triaxial compression. Predicted results were compared to experimental values from intermediate strain rate tests and from special tests loaded at HSR using drop-weight towers.;The present work contributes to the state-of-the-art in the behavior of granular media exposed to ISR and HSR loading, through test results performed at the macro scale level.