The effects of initial stress conditions and rock breaking parameters on rock fragmentation were investigated through indentation tests and numerical analysis with a discrete element method (DEM). Different initial stress conditions and the thrusting velocity of the cutter were the focus in the design of indentation tests. Some indicators were used to reflect the instability of the thrusting force, rock breaking efficiency, failure patterns, and the fluctuation of force-penetration response. The indentation results show that rock failure patterns have a closed correlation with initial stress conditions and loading rate. All tests showed several distinct chipping phases, and obvious chiseled pit and crushed rock could be observed on the top surface of each rock specimen. Moreover, rock brittleness was evaluated by the fluctuation of force-penetration curves; this study also revealed the changing law of peak and mean thrusting force. In addition, the numerical simulation of indentation tests reproduced the development process of rock fragmentation and crack propagation. Increasing confining stress restrained the propagation of vertical and lateral cracks, whereas the axial stress had the effect of restraining the propagation of radial cracks. However, there is a critical confining stress that causes different results of fractured depth. Therefore, this investigation is beneficial for optimizing the cutting parameters to promote rock breaking efficiency.

Rock properties (e.g., brittleness, fracture toughness, hardness, compressive strength and tensile strength), cutting parameters (e.g., cutting depth, attack angle, wedge angle, cutting velocity) and confining stress are closely related to rock fragmentation behaviors in the design and application of excavation and mining machinery. To investigate the influence mechanism of those factors on rock fragmentation, a series of linear cutting numerical tests were conducted using the discrete element method. Numerical models with different rock mechanical parameters were calibrated by changing the cohesion strength-to-tensile strength ratio of the linear parallel bond model in PFC2D. The effects of cutting parameters on rock fragmentation were studied using orthogonal test and range analysis. The numerical results indicated that the strength ratio had a dominant effect on the specific energy and that the cutting depth and attack angle had a dominant effect on the mean cutting force. The cutting velocity significantly influenced the specific energy, and a higher cutting velocity should be adopted in actual rock cutting. In addition, a cutting force prediction model was proposed that considered factors such as the compressive strength, wedge angle, attack angle and cutting depth. Moreover, the introduction of more rock mechanical parameters into the evaluation indicators can reflect a greater number of fragmentation characteristics.

To prevent serious shaft deflection disasters under asymmetric mining conditions, it is urgent to solve the problem of designing shaft protection rock pillar (SPRP) sizes in thick soil and thin rock strata. In this paper, based on the parallel mining model and the perpendicular mining model, a dynamic prediction model that can describe the horizontal movement of the shaft was established by the probability integration method and the Knothe time function. Next, according to the measured data of the shaft deflection in the Guotun Coal Mine, a back analysis was used to calculate the prediction parameters that were suitable for the deep soil strata. Based on the mining model, the variation law of the horizontal deflection displacement of the shaft and SPRP size was obtained. The results showed that the final displacements of the shaft under the two ideal mining models were equal, while the parallel mining model was superior to the perpendicular mining model at the initial stage of mining. The horizontal displacement of the shaft head had a nonlinear negative correlation with the SPRP, and the SPRP size in thick soil and thin rock strata calculated by the parallel mining model was more reasonable. For the Guotun Coal Mine, when the soil movement angle was 57.8% of the actual value, the horizontal displacement of the main shaft head was reduced by 87%. The results have important theoretical and practical value in preventing shaft deflection in thick soil and thin rock strata.

Rocks are less resistant to tension than to compression or shear. Tension cracks commonly initiate compression or shear failure. The mechanical behavior of layered rocks under compression has been studied extensively, whereas the tensile behavior still remains uncertain. In this paper, we study the effect of layer orientation on the strength and failure patterns of layered rocks under direct and indirect tension through experimental and numerical testing (RFPA^2D: numerical software of Rock Failure Process Analysis). The results suggest that the dip angle of the bedding planes significantly affects the tensile strength, failure patterns, and progressive deformation of layered rocks. The failure modes of the layered specimens indicate that the tensile strength obtained by the Brazilian disc test is not as accurate as that obtained by the direct tension test. Therefore, the modified Single Plane of Weakness (MSPW) failure criterion is proposed to predict the tensile strength of the layered rocks based on the failure modes of direct tension. The analytical predictions of the MSPW failure criterion agrees closely with the experimental and numerical results. In rock engineering, the MSPW failure criterion can conveniently predict the tensile strength and reflect the failure modes of layered rocks (such as shale, slate, and layered sandstone) with satisfactory accuracy.