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

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.

A Critical Review of Current States of Practice in Direct Shear Testing of Unfilled Rock Fractures Focused on Multi-Stage and Boundary Conditions

“…The total horizontal displacement is expressed by Equation (11). Observing the experimental curves as the joint is mated, Grasselli and Egger [44] affirmed that the joints deformed almost linearly up to unconstrained peak shear strength and described shear stiffness using the linear relationship expressed by Equation (12).…”

“…Currently, only the first two methods are described in detail by the ISRM Suggested Methods [1,2] and ASTM Standard [9]. Limited displacement multi-stage direct shear testing is a more recent development and is described by several authors [10][11][12]. The details of each staging methodology are further discussed in Section 5.…”

“…Once paused, the normal load/stress is increased to the next desired load/stress, and shear displacement is resumed. Once the unconstrained peak shear strengths are measured for each normal load/stress magnitude, residual shear strength can then be defined [10][11][12]. An example of a shear stress versus shear displacement curve of an LDMDS test is shown in Figure 6c.…”

“…To address some of the limitations associated with multi-stage direct shear testing, an alternative procedure was proposed by Barla et al [10] and Petro [11]. The proposed method limits the shear displacement within each stage of a multi-stage test to prevent extensive post-peak damage to the specimen and is referred to as limited displacement multi-stage direct shear testing (LDMDS) [11,12].…”

“…Over time, many researchers have questioned the validity of multi-stage testing as it has been observed to damage the surface of a specimen with successive shearing and impact the interpreted failure envelope strength parameters [11][12][13][14][15][16][17][18][19]. To evaluate the feasibility of multi-stage direct shear testing, test programs have been completed by various researchers to directly compare the results of single-and multi-stage tests and the interpreted Mohr-Coulomb parameters (e.g., [11,12,[17][18][19]).…”