Characterization of peak shear strength of rough rock joints using limited displacement multi-stage direct shear (LDMDS) tests

¹ Montana Technological University, Department of Geological Engineering, 1300 West Park St, Butte, Montana, 59701 USA
² Ransom Consulting Inc., Portsmouth, New Hampshire, 03801 USA
³ Freeport McMoRan, Safford, Arizona, 85546 USA
⁴ Swans Island, Maine, 04685 USA

^* Corresponding author: mmaclaughlin@mtech.edu

Abstract

Current standard direct shear test methods for rock joints do not account for damage to the specimens' asperity profiles; tests require shearing of a single specimen to large displacements under successive normal stresses (the multistage test), or the use of similar specimens in multiple tests. Due to the inherently unique nature of rock joints and corresponding difficulty in obtaining specimens with identical or even similar geometries, multistage tests are more common. A major issue with the multistage test is that successive shearing of the specimen damages the surface asperities and changes its overall roughness profile, reducing the peak shear stress and consequently resulting in underestimation of the friction angle and overestimation of the joint shear intercept (cohesion). The limited displacement multistage direct shear (LDMDS) test method minimizes these testing imperfections by allowing shearing of a single specimen without extensive asperity damage, accomplished by immediately pausing shear displacement once peak shear stress has been reached, then proceeding to shear the specimen under the following normal stress value, and shearing into the post-peak region only after identifying multiple values of peak shear strength. The authors have validated the LDMDS procedure using cement replicates of rock joints, demonstrating that it yields more accurate strength parameters than the standard multistage direct shear test.