Effects of Confining Pressure on Mode I Fracture Toughness Considering Jacketed and Unjacketed Conditions

Abstract Accurate assessment of mode I fracture toughness ( K Ic ) in rocks under in-situ stress conditions is critical for analyzing various geomechancial problems in underground rock engineering. Typically, K Ic is determined through unconfined tests. Under confining pressure, researchers often refer to the apparent mode I fracture toughness ( K Ic *), where specimens are typically jacketed to isolate from confining fluids. Although rocks are porous media that naturally contain fluids, relatively few studies have investigated the toughness behavior of unjacketed specimens under confining pressure, a scenario prone to rock-fluid interactions. In this study, we measured the K Ic and K Ic * of pseudo-compact tension ( p CT) specimens of granite, limestone, sandstone, and polymethyl-methacrylate (PMMA) under two conditions: covered with a waterproofing rubber (jacketed) or directly exposed to the confining fluid (unjacketed). PMMA served as an impermeable reference material. Tests were conducted at room temperature and confining pressures ranging from 0.1 to 25 MPa. Results show that K Ic * is material-dependent for both jacketed and unjacketed specimens. Jacketed specimens generally exhibit significantly higher K Ic * values compared to unjacketed ones. However, K Ic * values obtained from unjacketed specimens better reflect the actual stress state and fracture behavior under in-situ conditions, as both the mechanical and hydraulic properties of the rock interact concurrently during the development of the fracture process zone. These findings emphasize the importance of selecting appropriate testing conditions for accurately assessing fracture toughness in rocks, which has implications for the evaluation of stability, fracturing and fluid flow in rocks in underground environments.