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UBC Theses and Dissertations

Directional and 3-D confinement-dependent fracturing, strength and dilation mobilization in brittle rocks Rahjoo, Masoud


The continued demand for new resources and infrastructure in the civil, mining, and energy sectors has pushed these industries to greater depths. This has exposed projects to higher stresses and more complex failure modes involving massive brittle rock, which in turn has pushed engineers and designers to the limits of conventional empirical and numerical design capabilities, thus representing a significant challenge to the rock engineering profession. Massive rock becomes susceptible to brittle fracturing in high stress environments, surprising operators with behaviours ranging from progressive non-violent failure and dilative behaviour in the form of spalling to sudden and violent failure in the form of strainbursting. Needed are new solutions and engineering design tools tailored specifically to brittle fracturing mechanisms. However, the dominance of shear failure in weak/jointed rock masses under lower stress environments encountered during the development of rock mechanics as a discipline has led to a shear failure paradigm, for which σ₂-independent Mohr-Coulomb based strength /dilation models form the basis for present-day design tools. Not accounted for is the extensional fracturing observed under high stresses and low confinement conditions and its 3-D characteristics. This has led to serious consequences for deeper tunnelling and mining projects. Critical is the recognition of the dual nature of brittle failure, incorporating extensional and shear fracturing, and their 3-D directionality and 3-D confinement-dependency that affect the corresponding mobilization of strength and dilation. This thesis addresses these knowledge gaps with the objective to investigate and develop a series of new formulations specific to the brittle failure mechanism that more correctly models the progressive failure, strength mobilization, and bulking deformations experienced under complex stress paths for deep excavations in massive to moderately jointed brittle rock. First, a theoretical framework is developed that can describe the dual extensional/shear fracturing mechanisms, and their 3-D directionality and 3-D confinement-dependency. This understanding was then used to develop and formulate: i) an integrated 3-D confinement-dependent strength criterion that captures both extensional and shear fracturing, ii) a cohesion-weakening friction-strengthening strength mobilization model, and iii) a 3-D confinement-dependent dilation mobilization model capable of capturing the 3-D directional nature of spalling rock.

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