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Petro, M. J. (Montana Tech of the University of Montana) | Sordo, B. (Montana Tech of the University of Montana) | Berry, S. M. (Montana Tech of the University of Montana) | MacLaughlin, M. M. (Montana Tech of the University of Montana) | Berry, K. G. (Freeport-McMoRan)
ABSTRACT: Current standard multi-stage direct shear test methods for rock joints do not consider the individually unique asperity profile and nature of the testing specimens. These techniques require either the shearing of a single sample under multiple normal loads, or the shearing of individual “similar” specimens also under multiple normal loads. “Similar” specimens of natural rock joints with identical asperity profiles are extremely difficult or impossible to locate, and continuous shearing of the same specimen causes damage to the asperity profile which will impact successive test data. Seeking to minimize this testing-inherent error, previous research at Montana Tech has demonstrated the alternate method of limited displacement multi-stage direct shear (LDMDS) testing to yield more accurate strength parameters when shearing a single unique specimen under multiple normal stresses. The LDMDS test procedure minimizes initial shear displacement and corresponding specimen surface damage, allowing the specimen to more closely retain its original asperity and strength profile as observed via undamaged single-stage testing. Replicas of a granitic rock joint (Specimen M3) were cast with Type III cement and sheared at normal stresses of 90, 180, and 270 kPa via Standard multi-stage, LDMDS, and undamaged single-stage methods. Average friction angle (φ) and joint shear intercept (Sj) values yielded by LDMDS tests (φ = 45°, Sj = 49 kPa) were significantly more similar to values calculated by undamaged single-stage tests (φ = 51°, Sj = 22 kPa), as compared to the Standard multistage parameters which, as expected, produced a significantly underestimated friction angle (φ = 22°) and overestimated joint shear intercept (Sj = 100 kPa).
Obtaining accurate shear strength parameters for rock mass discontinuities through the use of direct shear tests is of critical importance to the design of safe slopes and underground openings. Evaluation of the standard Mohr-Coulomb strength parameters (joint friction angle and joint shear strength intercept) requires determination of the shear strength at a minimum of three different normal stress values.
Hedayat, A. (School of Civil Engineering, Purdue University) | Bobet, A. (School of Civil Engineering, Purdue University) | Pyrak-Nolte, L.J. (Department of Physics, Department of Earth and Atmospheric Sciences, School of Civil Engineering, Purdue University)