Experimental Study on Long-Term Strength and Creep Characteristics of Sandstone Under Different Water Content and Confining Pressure

Water plays a crucial role in underground engineering, significantly influencing the short-term and long-term strength and deformation behavior of rocks. Understanding the mechanics of rock deformation under water–rock coupling is vital for assessing the stability of underground structures. In this study, short-term strength compression and creep experiments were conducted on sandstone samples with varying water contents. Critical water contents (0%, 0.8%, 1.6%, 2.4%, and 3.3%) were determined through water absorption experiments, with corresponding tests performed under confining pressures of 0 MPa, 6 MPa, 9 MPa, and 12 MPa. Results revealed an exponential decrease in uniaxial compressive strength and elastic modulus with increasing water content, while confining pressure influenced radial deformation, leading to a linear increase in compressive strength. Water content also significantly impacted micro-cohesion and internal friction angle, with cohesion being more sensitive to water. The creep experiment demonstrated a deformation trend similar to compressive strength, with analysis revealing three stages of creep deformation: viscoelastic, plastic, and nonlinear acceleration. Critical strengths entering different stages were identified, with their ratio to compressive strength stabilizing between 0.44–0.59 and 0.59–0.74. A strain parameter ε_ac, related to steady-state and accelerated creep failure time, showed little correlation with water content but exhibited a linear relationship with confining pressure. Short-term and long-term failure modes of sandstone were analyzed, indicating a transition from “X-shaped” conjugate shear failure to single oblique shear failure with changing confining pressure and water content. Mitigation measures, such as groundwater isolation and the application of high confining pressure, were identified as effective means to mitigate long-term rock deformation's adverse effects on structural integrity in onsite engineering projects, ensuring structural stability throughout construction. The study's findings and data are crucial for informing the long-term stability of engineering structures.