Study On Principal Of Inner-outer Bearing Structures Coupling Stabilization For Strata Control Around Deep Coal Drifts

Strata control around coal drift is one of the technological difficulties for deep coal mining. Combing the existing theoretical results, new concepts of "inner bearing structure" and "outer bearing structure" are put forward and an inner-outer bearing structure model is constructed based on the analysis to the characteristics of strata behaviors. On the basis, systematic research is made on the stability control mechanism of this kind of surrounding rock through lab tests, theoretical analysis, numerical simulation and field observations. The main content is as follows:1. New relationships between surrounding rock stress field evolution and rock properties are discovered. These properties contain softening, dilation, creep, relaxation, and long-term strength of the surrounding rock. The post-peak creep tests are conducted under different loading paths. The results show that: the post-peak creep properties are influenced by the damage degree. With the increase in damage degree, the creep velocity rises, while the creep collapsing time decreases. The creep properties will be changed fundamentally by the confining pressure increment. This leads to a decrease in the creep velocity and a prolongation of the creep collapsing time and hence makes the conversion of creep types from the unstable to the stable. This supplies a theoretical basis for stability analysis of the surrounding rock in the fractured zone.2. Observations of the base displacement in deep roadways show that the strain distribution express flexuous characteristics. The numerical simulations show that the spatial distributions of strain, stress and the strain softening coefficient express flexuous characteristics. Moreover, they are interrelated closely. Also the mechanical states concerning the stress, deformation, properties etc. of the inner-outer bearing structure are analyzed and got, which provides a good basis for the recognition and definition of the outer bearing structure in engineering applications.3. An elastic-plastic theoretical model of circular roadways considering the interaction between the inner and the outer bearing structure is presented, based on which the expressions of stress, displacement and the radiuses of the plastic zone and the loose zone in the surrounding rock are derived. Furthermore, another theoretical model considering the same interaction is derived for coal sides in coal drifts. Based on this model, the expressions of stress, horizontal displacement and the plastic zone width of coal side are derived. Also, the expressions of characteristic parameters of the outer bearing structure are derived from the aforementioned analytical results, which reveals the relationship between surrounding rock stability of roadways and the characteristic parameters of the outer bearing structure, as well as the relationship between the support strength of the inner bearing structure and characteristic parameters of the outer bearing structure .4. The spatial effects of excavation and support in a deep coal drift are studied with 3D numerical simulations, from which the formation mode of the outer bearing structure is found under three conditions, namely without support, with rockbolt support and with combined support of rockbolt and cable. The effect of the support parameters on the formation process of the outer bearing structure is analyzed. A viscoelastic plastic-softening analytical model of circular roadways is created, and the expressions about plastic zone radius, loose zone radius, surrounding rock stress, surrounding rock displacement, and characteristic parameters of the outer bearing structure are obtained. The evolution of the outer bearing structure and the role of the inner bearing structure plays during this process are studied. A rheological model of the outer bearing structure in coal sides is constructed. The expressions of plastic zone stress and the inner boundary location of the outer bearing structure are derived. The moving process of the outer bearing structure in coal sides and the influence exerted by the support strength of the inner bearing structure are analyzed.5. A stabilization condition of the outer bearing structure and surrounding rock is put forward, based on which the mechanism that the inner bearing structure makes the outer bearing structure stability is discussed. The temporal effects, the bearing capability, and the support effect of the inner bearing structures formed through bolt support, grouting, and brace support are analyzed. The influences of the cable on the inner and the outer bearing structures are analyzed. According to the differences of the inner bearing structure in function mechanisms, the coupling process of the inner and the outer bearing structure is divided into two stages, the support principles and methods of each stage are proposed.6. The research results were validated in the engineering practice of strata control around deep coal drifts in the fourth coal mine of Pingdingshan Coal Industry Group.