Water Saturation Effects on Shear Mechanical Behavior of Rough Structural Planes in Sandstone

Abstract

Water exerts a significant influence on the mechanical integrity of rock masses through water–rock interaction, which is directly related to the stability of deep underground engineering projects. In this study, laboratory direct shear tests and discrete element method (DEM) numerical simulations were combined to investigate the effect of water saturation on the shear mechanical behavior of rough structural planes of sandstone. Laboratory experiments were conducted on sandstone samples with different levels of roughness (Joint Roughness Coefficient, JRC) under a constant normal stress in both dry and water-saturated conditions. The results showed that an increase in roughness significantly enhanced the shear strength, residual strength, and pre-peak stiffness, while facilitating more pronounced post-peak softening and shear dilatation, and promoted a progressive shift from predominantly interfacial frictional sliding toward greater involvement of asperity and rock matrix fracture. Water saturation obviously impaired these mechanical properties, and notably, such an impairment effect intensified with increasing roughness. Further DEM simulations indicated that water saturation promoted the initiation of tensile microcracks along the shear direction, alleviated local stress concentration, and dispersed fracture formation. The mechanism underlying the water effect stems from the synergistic action of rock matrix strength degradation and interfacial lubricating effect. In addition, the increasingly intense fracturing with roughness indicates that the contribution of matrix degradation becomes progressively more important for rough structural planes. These findings provide insights into the shear mechanism of rock mass structural planes under water–rock interaction, and offer support for the stability assessment and control of deep underground engineering in water-rich geological environments.