52nd U.S. Rock Mechanics/Geomechanics Symposium,
2018. American Rock Mechanics Association
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ABSTRACT: The strength and deformation characteristics of jointed rock masses before and after excavation were investigated using the discrete element method (DEM). A special loading path was adopted: firstly, the sample was subjected to anisotropic compression until the target stress level was reached; in the successive step, a circular tunnel was excavated in the center of the sample; then, biaxial compression was performed on the excavated sample until failure took place. The influence of joint inclination angle is explored and the development of cracks and its spatial distribution are presented and linked to the macro-scale responses. The simulation results show that the strength and modulus of jointed rock masses decrease significantly due to excavation. The peak strength after excavation returns to the largest axial stress value before excavation regardless of the joint inclination angle. The postexcavation modulus seems to increase with increasing inclination angle but the difference is less obvious in comparison to the preexcavation case. Cracking is anisotropic pre-excavation, but it becomes more and more isotropic until failure happens. The transition of cracks from an anisotropic pattern to an isotropic pattern is attributed to the variation of stress concentration and the evolution of stress arching zones.
The cracking behavior of jointed rock masses has been a hot research topic over decades. Past efforts were mostly devoted to revealing the cracking behavior (crack types, initiation, propagation and coalescence characteristics and their influential factors) of jointed rock masses either through laboratory element tests with the aid of highspeed camera and digital image analysis techniques (Bobet and Einstein, 1998; Prudencio and Van Sint Jan, 2007; Park and Bobet, 2010) or using numerical tools, e.g., the discrete element method (Bahaaddini et al., 2013). However, as noted by Lisjak et al. (2014), only very few studies were related to the excavation-induced cracking behavior, which is of greater significance to engineering practice. Furthermore, the successive cracking behavior and the residual bearing capacity of jointed rock masses after excavation have been rarely reported.
In this paper, the cracking behavior of jointed rock masses before and after excavation was investigated using particle-based discrete element software PFC 2D. The influences of joint inclination angle on the cracking behavior, post-excavation peak strength, residual bearing capacity and failure pattern after excavation were discussed. In particular, the formation of arching effect due to excavation and its relationship with cracking behavior were also explored. The macro stress- deformation responses were linked to the micro-scale observations.
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