Experimental Study of Brittle Behavior of Clay Shale in Rapid Triaxial Compression |
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Authors: | Florian Amann Peter Kaiser Edward Alan Button |
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Institution: | (1) Institute of Geology, Engineering Geology, Swiss Federal Institute of Technology, Zurich, Sonneggstrasse 5, 8092 Zurich, Switzerland;(2) Center for Excellence in Mining Innovation, 936 Ramsey Lake Road, Sudbury, Canada;(3) Formally-Institute of Geology, Engineering Geology, Geoconsult Pvt. Ltd. India, Sonneggstrasse 5, 8092 Zurich, Switzerland |
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Abstract: | The brittle failure behavior of an over-consolidated clay shale (Opalinus Clay) in undrained rapid triaxial compression was
studied. The confining stress levels were chosen to simulate the range of confining stresses relevant for underground excavations
at the Mont Terri Underground Research Laboratory, and to investigate the transition from axial splitting failure to macroscopic
shear failure. Micro-crack initiation was observed throughout the confining stress range utilized in this study at a differential
stress of 2.1 MPa on average, which indicates that friction was not mobilized at this stage of brittle failure. The rupture
stress was dependent on confinement indicating friction mobilization during the brittle failure process. With increasing confinement
net volumetric strain decreased suggesting that dilation was suppressed, which is possibly related to a change in the failure
mode. At confining stress levels ≤0.5 MPa specimen rupture was associated with axial splitting. With increasing confinement,
transition to a macroscopic shearing mode was observed. Multi-stage triaxial tests consistently showed lower strengths than
single-stage tests, demonstrating cumulative damage in the specimens. Both the Mohr–Coulomb and Hoek–Brown failure criteria
could not satisfactorily fit the data over the entire confining stress range. A bi-linear or S-shaped failure criterion was
found to satisfactorily fit the test data over the entire confinement range studied. |
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