53rd U.S. Rock Mechanics/Geomechanics Symposium,
New York City, New York
2019. American Rock Mechanics Association
2 in the last 30 days
19 since 2007
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ABSTRACT: The At high-stress conditions in hard rock masses, the behavior of intact rock plays a major role in crack initiation and propagation, defining damage conditions around excavations. For modeling brittle rock behavior, the Cohesion-Weakening-Friction-Strengthening (CWFS) linear model has proven to be a good alternative for estimating the damage evolution around underground openings through the peak-residual values and the mobilization range defined as Critical Plastic Strain (CPS), however, it has not been extensively applied to intact rock at laboratory scale tests. Besides, the parameters of the CWFS model are usually difficult to obtain and are estimated indirectly from laboratory tests or from back-analysis of excavations that have presented brittle failure. This study uses a reliable optimization method to determine the CWFS model parameters for intact rocks to recreate its complete stress-strain curve, with a focus on post-peak behavior. Using this approach, the CWFS model parameters are estimated for an extensive database of six different hard rock types at different confining stress conditions, in order to analyze post-peak behavior. Results show that the obtained parameters for plastic strain-dependent mobilization of cohesion and friction are not fixed as supposed in other studies and depends on the lithology, furthermore its variation also depends on the initial values. CPS values also define the Drop Modulus parameter in brittle failure, which is discussed and compared with literature values. Guidelines for estimating the strength parameters mobilization can be concluded, including comparisons to usual intact rock parameters such as mi or UCS. Based on the implications of this study further development of strength mobilization models in hard rock masses is addressed and discussed.
Prediction of progressive short and long-term instabilities in underground excavations is a major challenge for mining projects to ensure safe continuous operation and decrease costs. As underground operations advance deeper, these instabilities can be caused by high field stresses. Under these high field stresses, hard rock mass strength depends not only on in situ fracture networks but also on the creation of new fractures, which occur when the strength of intact rock is surpassed. This process, also known as cracking, can initiate and propagate in hard rock or in a media with non-persistent fractures (Hoek & Martin, 2014). Intact rock behavior is related to rock mass behavior as well as to the failure envelope if high field stresses and low fracture frequency ratio is present (Kaiser, 2016). Intact rock behavior, then, may be useful for scaling and estimating hard rock mass behavior and may be more practical and economically feasible for studying brittle behavior.
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