A Critical Laboratory Investigation of Multi-Stage Direct Shear Testing Procedures on Rock Joints Using Synthetic Replica Specimens

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Authors
MacDonald, Nicholas
Keyword
Rock Mechanics , Laboratory Direct Shear Testing , Synthetic Specimens , Rock Joint , Multi-Stage Direct Shear Testing
Abstract
Direct shear testing is a common laboratory method used to define the geomechanical parameters of rock fractures for numerical inputs and rock engineering design. Of the standardized procedures, multi-stage direct shear testing is a controversial practice of repeatedly shearing the same rock joint specimen under increasing normal stresses to define its failure envelope. The limitations associated with multi-stage direct shear testing have been addressed in previous research and demonstrate that additional stages will reduce the measured peak shear strengths and impact the interpreted linear Mohr-Coulomb shear strength parameters. This research presents the results from 50 single stage, 8 multi-stage (with repositioning), and 8 limited displacement multi-stage direct shear tests completed on synthetic specimens. To create the synthetic specimens, a rigorous workflow was developed to transform rock joint specimens into synthetic replica specimens cast in cement grout using 3-dimensional Structure from Motion (SfM) photogrammetry and 3D printing technologies. In this research, four unique synthetic sample designs were generated using two parent rocks and two water: cement grout mixture ratios. Four unique sample designs enabled the author to observe how isolating a change in the joint roughness coefficient and/or a change in the joint compressive strength impacts the measured geomechanical parameters. Furthermore, using the single stage results as a baseline for comparison, the impact multi-stage direct shear testing procedures have on the measured geomechanical parameters are evaluated. The results presented in this research demonstrate that surface roughness characterization and its contribution to shear strength is more complex than currently practiced. The current practice of generalizing a rock joint specimen’s roughness based on the intensity in the relief of its asperities in cross-section can be misleading. Rather, joint roughness should consider the 3-dimensional geometry and dip angle of the asperities dipping subparallel to the shear direction. Furthermore, the results demonstrate that multi-stage direct shear testing procedures significantly impact the measured geomechanical parameters when compared to single stage test results. Finally, this research demonstrates that the residual joint friction angle of the tested synthetic cement grout specimens is not a material constant and has a reliance on the specimen’s topology and shear failure mechanisms.
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