The joint arrangement in rock masses is the critical factor controlling the stability of rock structures in underground geotechnical engineering. In this study, the influence of the joint inclination angle on the mechanical behavior of jointed rock masses under uniaxial compression was investigated. Physical model laboratory experiments were conducted on jointed specimens with a single pre-existing flaw inclined at 0°, 30°, 45°, 60°, and 90° and on intact specimens. The acoustic emission (AE) signals were monitored during the loading process, which revealed that there is a correlation between the AE characteristics and the failure modes of the jointed specimens with different inclination angles. In addition, particle flow code (PFC) modeling was carried out to reproduce the phenomena observed in the physical experiments. According to the numerical results, the AE phenomenon was basically the same as that observed in the physical experiments. The response of the pre-existing joint mainly involved three stages: (I) the closing of the joint; (II) the strength mobilization of the joint; and (III) the reopening of the joint. Moreover, the response of the pre-existing joint was closely related to the joint’s inclination. As the joint inclination angle increased, the strength mobilization stage of the joint gradually shifted from the pre-peak stage of the stress–strain curve to the post-peak stage. In addition, the instantaneous drop in the average joint system aperture (aave) in the specimens with medium and high inclination angles corresponded to a rapid increase in the form of the pulse of the AE activity during the strength mobilization stage.
This paper investigates the influence of mineral structure on macromechanical behavior of coal under different loading conditions using X-ray CT scanning experimental and numerical methods. The three-dimensional (3D) reconstruction of coal was conducted to assess the spatial distribution characteristics of the mineral structure by AVIZO software. Based on fractal box dimension (BCD) and equivalent diameter, the mineral structures were quantitatively characterized. The 3D finite element models with three distribution characteristics of minerals were built, and the model was considered as a random heterogeneous two-phase material composed of coal matrix and mineral matter. The results show that the frequency of mineral structures decreases with the increase of equivalent diameter in the coal sample. The BCD of the original mineral structure in coal is greater than 2, but the BCD of each part of the minerals divided based on the screening principle is less than 2. Under uniaxial conditions, the elastic modulus, peak strength, and residual strength of coal are monotonically increasing with the size of the mineral structure. The larger the mineral structure size and the more complex the distribution are, the greater the area of stress concentration and the more uniformity the distribution will be. The failure (plasticity) first occurred at the interface between the matrix and the mineral, and the failure zone is significantly different due to the influence of different mineral structures. Under confining pressure, the presence of mineral structure decreases the brittleness of coal, and the variation of brittleness is related to the size and spatial distribution of mineral structures. The fitting relationship between confining pressure and brittleness index is linear, and the correlation coefficient exceeds 0.95.
Mechanical properties of rock masses are dominated by the nonlinear response of joints and their arrangement. In this paper, combined influences of joint spacing (s) and joint inclination angle (β) on mechanical behavior of rock mass models with large open joints under uniaxial compression were investigated by PFC modeling. With a large amount of local measurement circles placed along the pre-defined measurement lines (ML), stresses and joint response parameters at different scales (the measurement circles, the MLs and the whole specimen) were defined and calculated. It was found that macroscopic behaviors of the jointed specimens, such as four types of deformation behaviors, four failure modes, strength, deformability modulus and ductility index, are dominated by nonlinear response of the joint system, especially the interaction between the joints and rock bridges. The joints may experience three stages, i.e., starting to close, closed and opening again. On the joint plane, the peak stresses of the rock bridges and those of the joints may not be reached at the same time; i.e., joint strength mobilization happens with the loss of the rock bridges’ resistance. The influence of s on specimen behavior is little for β = 90°, obvious for β = 0° or 30° and significant for β = 45° or 60°, and this can be related to their different microscopic damage mechanisms.
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