Abstract: In order to study the crack initiation and coalescence of the failure process of rock mass， the uniaxial compression of red sandstone samples with various rock bridge angles was numerically performed. Four failure patterns of red sandstone samples with single crack and hole is firstly summarized， then the radius of the plastic core and plastic extension area are defined， the single span radius (or the crack length， angle) and the size of the plastic damage area are characterized. It is found that the coalescence of plastic failure zone is well consistent with the coalescence and fracture of samples. Finally， a graphic discrimination method is proposed to discriminate the fracture mode of rock mass. The experimental results verify the correctness of the method of the failure mode， which is consistent with the results of similar physical tests.

Key words: rock mechanics, uniaxial compression, coalescence, fracture mode, discrimination method