Characterizing Shear Strength of Rock Discontinuities under the Patton Failure Criterion

Summary

Abstract

The Patton failure criterion (1966) provides a straightforward framework for characterizing the shear strength of rock discontinuities, such as joints and faults. This model defines a bilinear failure envelope that depends on surface friction dilatancy and the applied normal stress. Experimental evaluation of discontinuities under this criterion, combined with a robust geomechanical characterization of rock masses, enables the assessment of stability in civil engineering and mining projects. This study introduces a detailed methodology aimed at evaluating the parameters of the Patton criterion through laboratory-scale direct shear tests. Cubic rock model specimens with 20‑centimeter sides are employed to realistically capture the roughness and morphological features of discontinuity surfaces, thereby overcoming the limitations associated with smaller sample sizes. Specimens are meticulously prepared following standardized protocols to ensure controlled failure surfaces. The samples are subjected to a series of normal stresses within a direct shear apparatus. During each test, shear stress, tangential displacement, and vertical displacement (dilatancy) are recorded as the specimen undergoes sliding until failure. The resulting data allow for the empirical construction of the failure envelope. The findings highlight the transition in discontinuity behavior from pure sliding friction to friction accompanied by dilatancy, thereby validating the nonlinear nature of the Patton criterion, particularly under low normal stress conditions. Analysis of the experimental data yields key design parameters for open discontinuities, including the friction angle and the dilatancy angle.