Mechanical characteristics and crack propagation mechanism in rectangular and trapezoidal specimens of excavated pillars with various cavities: experimental and numerical investigations

In this research, the breakage behavior of rock pillars under the uniaxial compressive strength test is investigated using both experimental and three-dimensional discrete element methods. Gypsum samples with rectangular and hourglass hexagonal shapes are constructed to simulate underground mine pillars. Within the samples, various settings of created holes in different angles, numbers, and shape patterns are considered to design a total of 20 configurations for the failure test. Twelve layouts included horizontal rows of 5 holes (1, 2, or 3 rows) at different angles (0°, 30°, 60°, and 90°). The hole patterns in the other 8 arrangements involved some usual geometric shapes including vertical ellipse, vertical rectangle, triangle, horizontal ellipse, horizontal rectangle, diamond, trapezoid, and square. For the experimental tests, 60 specimens are prepared (3 samples for each configuration to increase reliability). For the PFC3D simulations, 20 models with similar setups are prepared to replicate the experiments. The loading rate was set to 0.016 m/s. Our results show that the hole parameters, i.e., angles, numbers, and shape configurations, are the key factors in the failure process. Our analysis helps reveal a correlation between the breakage pattern, the breakage mechanism of discontinuities, and the maximum applied force of the specimens. Increasing the hole angles and numbers add to the total crack number (TCN). The minimum load-carrying capacity of the samples is recorded for the sample with 15 holes at 30° and 60° angles.