Mechanical Behavior and Failure Mechanism of Rocks under Impact Loading: The Coupled Effects of Water Saturation and Rate Dependence

Rock masses in engineering applications are often simultaneously soaked in a moist environment and exposed to dynamic loads with different rates. It is therefore essential to understand the mechanical behavior and failure mechanism of rocks by considering the coupled effects of water saturation and rate dependence. In this study, numerous dynamic compression tests were conducted on marble and granite under oven-dried and water-saturated conditions using a split Hopkinson pressure bar apparatus. Some indices such as dynamic compressive strength, dynamic increase factor, and dissipation energy density, were unitized to investigate the mechanical behavior and failure mechanism of saturated rocks. The test results show that, regardless of the moisture state, the dynamic strength is positively sensitive to the strain rate. The existence of water contributes to the reduction of dynamic strength and elastic modulus. The dynamic strength reduction of saturated marble decreases with increasing strain rates, while that of saturated granite remains almost unchanged. As the strain rate increases, the dissipation energy density of both types of rock increases, and the ratio of energy utilization significantly decreases. Integrated investigations on the rate dependence of dynamic strength and water-weakening mechanisms suggest that the propagation and development of cracks through the region of mineral grains are responsible for strength enhancement under increasing strain rates. The overall strength reduction of saturated marble is mainly governed by the coupled effects of pore-water pressure and viscosity. With an increase in the strain rate, the strengthening viscosity effect diminishes the strength reduction primarily caused by the increasing effect of pore-water pressure. The dynamic strength of saturated granite is mainly controlled by the rate effect, and its various water-weakening effects remain almost unchanged with an increasing strain rate. The current work investigates the coupled effects of water saturation and rate dependence on the mechanical and fracturing behaviors of two different types of rock under dynamic loading conditions. It turns out that water saturation causes dynamic strength reduction, and this reduction, which depends on the rock type, is closely related to the loading rate. For carbonate rocks, the dynamic strength reduction induced by water can be suppressed by the increasing strain rate, but this is not applicable for siliceous rocks with low porosity. The results of this work highlight to engineers that lithology and microstructure are predominant factors affecting the dynamic mechanical and fracturing behavior of rocks in a water environment. The common water-weakening phenomenon in some rocks can be inhibited by the loading rate, resulting in a conservative engineering design. However, more academic and practical investigations on the coupled effects of water saturation and loading rate are required since the rock water-weakening affected by multiple factors is a complicated topic in vague.