Influences of water saturation and strain rate on the mechanical and failure behavior of sandstone under direct shear test with constant normal pressure

Understanding the mechanical and failure behavior of rock subjected to dynamic shear load is essential for the scientific design and safe operation of rock engineering. To understand the coupled effect of water and strain rate on the mechanical and failure behavior of sandstone, direct shear tests with constant normal pressure were conducted on sandstone specimens under different water saturations with shear strain rate ranging from 10-5 s-1 to 10-2 s- 1. The acoustic emission (AE) and scanning electron microscope (SEM) techniques were applied to analyze shear behavior and reveal failure mechanism under different loading conditions. Results show that at a given strain rate shear strength decreases with increasing water saturation, while it increases with strain rate when the water saturation remains constant. Meanwhile, the water-saturated specimens are more sensitive to strain rate effect than dry specimens, which is mainly the result of the Stefan effect. Both water and strain rate affect AE activity and fracturing of the specimens during shearing process. AE activity was more intense and lasted longer at low water saturation condition. With increasing strain rate, the peak AE counts decreased, yet the peak cumulative energy of AE events increased. The SEM analysis reveals that the development of microcracks is fuller under the conditions of low water saturations and high strain rates. The microcrack network and fracture surfaces are more complicated at lower water saturation, while microcracks are wider and the fracture surfaces are smoother under higher strain rate shearing. The findings of this study contribute to a better understanding of mechanical and failure behavior of sandstone material under the coupled effect of water saturation and dynamic disturbance.