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

Mary Maclaughlin,Katherine Berry,Michael Petro,Steve Berry,Anders Bro,A Tarantino,E Ibraim

doi:10.1051/e3sconf/20199213011

Mary Maclaughlin, Katherine Berry + Show 5 more

Open Access

https://doi.org/10.1051/e3sconf/20199213011

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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.

Highlights

The strength of jointed rock masses is controlled to a large degree by the relatively weak shear strengths of the discontinuity surfaces
The shear strength of rock joints is typically characterized using the standard MohrCoulomb parameters : joint friction angle and joint shear strength intercept. These strength parameters are most often determined from direct shear tests conducted at a minimum of three different normal stress values according to ASTM and ISRM standards [1, 2]
Limited displacement multistage direct shear (LDMDS) test The limited displacement multi-stage direct shear (LDMDS) test method was developed by Bro [6] to address the testing-induced damage associated with the ‘standard’ multistage direct shear test procedure

Summary

Introduction

The strength of jointed rock masses is controlled to a large degree by the relatively weak shear strengths of the discontinuity surfaces. Theprimary problem with the multistage direct shear test is that specimens are sheared beyond the point of peak strength and the asperities on the specimen surface are increasingly damaged during each stage of shearing, resulting in lower shear strength This has been recognized in previous studies [for example, 2-4] and in the current standards [1]. The inherent issue is that combining data from the first shear stage (performed on an ‘undamaged’ surface) with data from subsequent stages (on surfaces ‘damaged’ by previous shearing) results in underestimation of the peak friction angle and overestimation of the peak joint shear intercept [5] To address this issue, an alternative procedure has been developed by Bro [6] which minimizes surface asperity damage by applying only extremely limited shear displacements until multiple values of peak shear strength have been achieved. The goal is to provide quantitative evidence that adjustments to the current test procedure can provide more accurate strength parameters

Direct shear test procedures

Findings

Experimental studies and results