Investigation On Failure Mechanism And Bumping Tendency Of Coal-rock Combined Body

As the shallow coal resources drying up and the coal mining depth increasing, there are more and more disaster appears. In the process of a large number deep coal mining, there are so many kinds of coal pillar reserved in the mine tunnel, the function of these coal pillars is support the tunnel. The disaster will be hard to imagine if the coal pillar stability are influenced. The coal pillar is mainly consists of the coal and the rock. When they combine together, the combination mechanical properties will affect the stability of coal pillar. The frequency of the rock burst disaster in coalmines is higher and higher, it has become a serious threat to the development of our country’s coal industry. In essence, the rock-burst must satisfy three conditions:the strength condition (when the stress suffered by the coal and rock is more than its strength, coal and rock will be damaged), the energy condition (the coal and rock combined body has the ability to accumulate and release energy suddenly), the coal-rock combined body has bumping tendency (it has the ability to emerge brittle failure with prominent stress decrease). The bumping tendency is a sufficient condition, if the coal and rock combined body does not destroy, the phenomena of rock-burst will not happen. But because the mechanical properties of the coal and rock combined body is different from the individual rock, thus taking the coal and the rock as a combination that becoming an important role for the prediction and prevention of the rock burst.This article uses the numerical analysis and calculation software RFPA to simulate the coal-rock combined body’s impact failure from three aspects:inclined angles、different combination methods and confining pressure. In order to explore the influence of the interaction between different inclined angles of the coal-rock combined body and confining pressure on the impact damage, thus there are some numerical tests to analyze strength and AE phenomenon of the different interface inclined angles coal-rock combined body severally under uniaxial and triaxial compression. In order to analyze the failure mechanism, the article chooses different combination conditions of the coal and the rock under uniaxial and triaxial compression, then used the results of the impact energy index and AE energy characteristics to do further research on bumping tendency. The research results show that:(1) In the study on influence of rock thickness and confining pressure for coal-rock stability indicates:the brittle failure and the bumping tendency of the coal-rock combined bodies will enhance with the increase of rock thickness and the confining pressure value and the peak strength、residual strength and AE energy peak increases as well.(2) Based on the numerical simulation results, the destruction of the coal-rock combination mainly occurs in the coal body. The failure mode is shear failure under the uniaxial compression, its crack shape appears as "V"; the failure mode is shear failure and extrusion under the triaxial compression, its crack shape mainly present for M type.(3) The increase of the confining pressure reduces the degree of the combination’s brittle failure. The strength of hard roof-coal body and hard roof-coal body-floor is gradually closing to each other. Thus it seems that the degree of influence of confining pressure on strength more than the combination.(4) According to the impact energy index and AE evolution rules, the bumping tendency of hard roof-coal body is more than hard roof-coal body-floor under the same confining pressure.(5) Simulation result was found that the AE energy characteristics and the strength of the coal-rock combined body indicated the same variation regulation. As the inclined angle becomes larger, the risk of the rock-burst becomes lower and the risk of interfacial sliding failure is higher.(6) It is respectively to study the influence of the inclined angles and confining pressure on the strength of coal-rock combined body deformation and destruction. If the inclined angles of the coal-rock combined body is smaller and the confining pressure is higher, the strength of the coal and rock combined body will become greater. Under the same confining pressure, the reduction rate of the coal and rock combined body strength is faster with the inclined angles increases. Under the high geostress and large inclined angles, the strength of the coal-rock combined body is mainly determined by confining pressure.(7) The research on different inclined angles of the coal-rock body that had been found with numerical simulation software RFPA reveals that the coal-rock combined body’s damage model under the different inclined angles is line with 45 °, it divided into two kinds of model:the shear compression failure and the interfacial sliding failure. When a<45°, coal-rock combined body presented the characteristic of shear compression failure; when α≥ 45, failure mode turned into interfacial sliding failure.