Abstract
Flaws existing in rock masses are generally unparallel and under three-dimensional stress; however, the mechanical and cracking behaviors of the specimens with two unparallel flaws under triaxial compression have been rarely studied. Therefore, this study conducted comprehensive research on the cracking and coalescence behavior and mechanical properties of specimens with two unparallel flaws under triaxial compression. Triaxial compressive tests were conducted under different confining pressures on rock-like specimens with two preexisting flaws but varying flaw geometries (with respect to the inclination angle of the two unparallel flaws, rock bridge length, and rock bridge inclination angle). Six crack types and eleven coalescence types in the bridge region were observed, and three types of failure modes (tensile failure, shear failure, and tensile-shear failure) were observed in experiments. Test results show that bridge length and bridge inclination angle have an effect on the coalescence pattern, but the influence of bridge inclination angle is larger than that of the bridge length. When the confining pressure is low, coalescence patterns and failure modes of the specimens are greatly affected by flaw geometry, but when confining pressure rose to a certain level, the influence of confining pressure is larger than the effect of flaw geometry. The peak strength of the specimens is affected by flaw geometry and confining pressure. There is a critical value for the bridge length. If the bridge length is larger than the critical value, peak strengths of the samples almost keep constant as the bridge length increases. In addition, as the bridge inclination angle increases, there is an increase in the probability of tensile cracks occurring, and with an increase in the confining pressure, the probability of the occurrence of shear cracks increases.
Highlights
Rock masses often contain flaws such as fissures, weak surfaces, and joints, which have a significant effect on the mechanical properties and failure modes of the rock mass
For the specimens with a geometry of 20-30-60-60, no coalescence was observed when the con ning pressure was 1 MPa, and an antitensile crack was initiated from the tip of aw 1, which led to tensile-shear failure of the specimen. e coalescence types were the same under con ning pressures of 3 MPa and 5 MPa; the failure modes were different. e coalescence pattern for the specimens with a geometry of 20-30-60-60 was di erent from that of the specimens with a geometry of 20-0-60-60, which proves σ3 = 1 MPa
Compressive tests were conducted to investigate the cracking behavior and mechanical properties of the precracked specimens. e following conclusions are obtained from the test results in this study: (1) Under a low con ning pressure (1 MPa), as bridge length increases, the coalescence pattern changed from di erent types of coalescence to no coalescence
Summary
Rock masses often contain flaws such as fissures, weak surfaces, and joints, which have a significant effect on the mechanical properties and failure modes of the rock mass. Based on the results of triaxial compressive tests, Yang et al [12] revealed that the peak strengths and failure modes depend on flaw geometry and on the confining pressure. The mechanical properties and failure behavior of red sandstone specimens with two unparallel flaws were studied by Yang et al [17], who found that the mechanical parameters of the specimens were affected by the flaw inclination angle and that the cracking process depended on the flaw inclination angle and on the heterogeneity of the rock material. Haeri et al [18] studied the influence of unparallel flaws on the strength and stress-strain curves of rock-like specimens, and their experimental and numerical results revealed that the crack coalescence and final crack propagating paths were mainly caused by wing cracks. Huang et al [19] carried out triaxial compressive tests on the rock-like specimens with two unparallel preexisting flaws and found that failure modes of the specimens were determined by both the flaw inclination angle and the confining pressure: when the confining pressure was small, failure modes were mainly affected by the flaw inclination angle, and when the confining pressure was large, failure modes were mainly influenced by the confining pressure. erefore, in this research, rock-like specimens with two unparallel preexisting flaws were prepared, and triaxial compressive tests were conducted on these specimens under different confining pressures. e effects of confining pressure, rock bridge length, rock bridge inclination angle, and flaw inclination angle on the strengths and crack behavior of specimens under different confining pressures were analyzed based on the test results. is paper provides a better understanding of the mechanical properties and failure behavior of rock masses with unparallel flaws and different flaw geometries under triaxial compression
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