Numerical study on coalescence mode of rock bridge with double parallel pre-existing cracks

In the failure process of rock specimen with double cracks, the stress field between different cracks will interact, and the rock bridge area will coalesce. In the research, the strain strength criterion was embedded into the expanded discrete element program UDEC, which was secondarily developed to simulate the propagation of tensile crack and shear crack of specimens with prefabricated double cracks. According to the results of the numerical simulation, four basic coalescence modes of rock bridges will appear in the double-crack specimen during uniaxial compression: (1) Non-coalescence mode: the wing cracks at the tips of the two prefabricated cracks gradually expand independently, but the discontinuous mode of penetration does not occur in the rock bridge area; (2) Shear coalescence mode: in the rock bridge area, the main stress field and the shear stress field are concentrated, resulting in shear cracks coalescence in the rock bridge. At this time, the shear stress plays a leading role in the shear coalescence mode; (3) Tensile coalescence mode: tensile cracks penetrate the rock bridge, and the rock bridge is highly concentrated by the principal stress field. At this time, the rock bridge penetration has the characteristics of instantaneousness, and the tensile crack penetrates the tensile coalescence mode of the rock bridge; (4) Mixed coalescence mode: under the combined action of tensile force and shear force, the rock bridge gets the mixed tensile and shear coalescence mode after the specimen reaches the highest peak strength. Through comparison with laboratory tests, it is concluded that applying the strain strength criterion to the numerical simulation analysis can more accurately describe the changes of stress and strain and the penetration of rock bridges during mesoscopic failure, and enrich the mechanism penetration of multi-crack rock specimen in mesomechanics. The research provides a reference for numerical simulation in studying the actual engineering of rock mass meso-damage.