Date of Award
Master of Science (MS)
Current standardized direct shear test methods for rock joints do not account for damage caused to the individually unique asperity profiles of each sample. Current test methods require either a single sample to be sheared to large displacements under successive normal stresses, or the use of similar samples in multiple tests. Successive shearing of a single sample damages surface asperities and causes the overall roughness profile of the sample to change, reducing peak shear stress and consequently resulting in underestimation of friction angle and overestimation of joint shear intercept (cohesion). Obtaining joint surface specimens with identical roughness and geometry is extremely difficult or impossible considering the inherently unique nature of rock joints. To minimize these testing-induced errors, research at Montana Tech has demonstrated the limited displacement multi-stage direct shear (LDMDS) test method to yield more accurate peak shear stresses and strength parameters. The LDMDS test procedure eliminates the need for similar joint specimens and allows for the shearing of a single specimen without extensive asperity damage. This is accomplished by immediately pausing shear displacement once peak shear stress has been reached, then proceeding to shear the sample under the following normal stress value. Testing of the LDMDS procedure was accomplished using cement replicates of four rock joints with joint roughness coefficients (JRC) of 0-2, 4-6, 8-10, and 12-14. Average joint friction angle (φj) and joint shear intercept (Sj) values determined by LDMDS tests were more similar to baseline values determined by single-stage undamaged testing for joints of all roughness profiles. To validate direct shear test results, two dimensional numerical models were created using Itasca’s UDEC software in association with the Coulomb Slip (Area) and Barton-Bandis joint constitutive models. The UDEC models demonstrated peak shear stress values that were consistently within ±20% range of undamaged single-stage test peak shear stresses, verifying the results of the laboratory tests.
Petro, Michael, "CHARACTERIZATION OF PEAK SHEAR STRENGTH OF ROUGH ROCK JOINTS USING THE LIMITED DISPLACEMENT MULTI-STAGE DIRECT SHEAR (LDMSD) TEST METHOD" (2018). Graduate Theses & Non-Theses. 156.