Experimental and numerical simulation study of crack coalescence modes and microcrack propagation law of fissured sandstone under uniaxial compression

Fissured rock mass is the most common construction object in geotechnical engineering and its mechanical properties are closely related to the stability of geotechnical engineering. A laboratory uniaxial compression test and a numerical simulation test were separately carried out on the fissured sandstone samples. Based on parameters, such as crack coalescence modes, the ratio of rise time to amplitude (RA) and the ratio of acoustic emission (AE) counts to duration (AF) of AE signals and spatial distribution of microcracks, failure modes and microcrack propagation laws of fissured sandstone were studied. The results demonstrate that the failure mode of fissured sandstone changes from being dominated by tensile failure into by shear failure with the increase of the dip angle of prefabricated fissures. The results of RA and AF of AE signals illustrate that it is difficult to produce tensile microcracks, but easy to generate shear microcracks in the samples with the rise of the dip angle of prefabricated fissures. In the numerical simulation test, when the dip angle of prefabricated fissures is small (θ ≤ 60°), microcracks initiate near the tip of prefabricated fissures. However, if the dip angle is large (θ ≥ 75°), the initiation location of microcrack is slightly affected by prefabricated fissures. In addition, the method for determining tensile and shear cracks was proposed based on the velocity field of particles. The results show that as the dip angle of prefabricated fissures increases, there are increasingly less tensile macrocracks, but increasingly more shear cracks at failure of the samples, which is basically consistent with conclusions obtained in the laboratory test of rock mechanics.