Abstract
With the deep mining of coal, the phenomenon of high ground stress is more likely to cause dynamic disaster. In view of the above problems, this paper takes the unloading process of coal mining as the background to study the effects of mining rates under different conditions on the mechanical properties and triaxial failure criterion of composite coal-rock, so as to provide a theoretical basis for the prevention and control of dynamic disasters in coal mines. The composite coal-rock models with a composite ratio of 1 : 1 : 1 were tested under different unloading rates or confining pressures. The results show that the triaxial unloading process of coal-rock can be divided into five stages: compaction, single elasticity, elastic-plastic unloading, partial fracture, and complete fracture. In this paper, the failure criterion of composite coal-rock triaxial unloading is derived. The unloading rate has an exponential relationship with the triaxial compressive strength, and the relationship between initial confining pressure and compressive strength is linear. The triaxial compressive strength is determined by both. The peak strains ε of all samples under different unloading conditions were around 0.01. And initial confining pressure had an influence on the strain variation trend during the unloading of composite coal-rock. The higher the initial confining pressure, the greater the elastic modulus. In addition, an increase of initial confining pressure led to the increase of the total energy converted into dissipated part in the process of fracture and caused the obvious increase of the rebound characteristics of the curve. However, the unloading rate had no influence on the strain trend.
Highlights
Han et al conducted experiments on rock unloading axial pressure and confining pressure, respectively, and the results showed that the crack inclination angle had the greatest influence, followed by the initial confining pressure, while the unloading speed of radial pressure had a relatively small influence [9]
Wang et al conducted a series of triaxial tests on gas-coal-sandstone assemblages under two unloading conditions [13], and the results showed that the Mohr–Coulomb strength standard is suitable for gas-coal-sandstone assemblages with unloading stress paths
Zhou et al studied the interaction between cracks in the rock mass and conducted numerical simulation using the dislocation model, and the results showed that the interaction between cracks was more prominent under axial unloading [21]
Summary
In the actual coal mine, the coal seam is mostly sandwiched in the rock layer, which basically presents the three-layer structure of “rock-coal-rock” According to this characteristic, the composite coal and rock samples used in the test are processed. According to the calculation, when the composite coal and rock samples are tested under the conditions of 10 MPa, 15 MPa, and 20 MPa respectively, unloading should start near 36.398 MPa, 47.778 MPa, and 56.650 MPa. e unloading test was carried out in combination with the results of the preliminary test.
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