Study On Bearing Characteristic And Instability Mechanism Of Layered Rock Mass Reinforced With Bolts

Layered rock mass has obvious characteristic of non-continuum due to bedding plane, and its mechanical properties and failure properties is different from intact rock mass. The structural plane easy to sliding deformation and bed separation, and its harmful for stability of layered rock mass. Rock bolt is widely used in stability control of surrounding rock of roadway and achieves great efficiency in practice. However, the acting mechanism of bolts was not very clear so far, especially to layered rock mass. In this paper, layered rock mass of roadway roof was regarded as research object, basing on the combining method of physical simulation experiment, numerical simulation and theoretical analysis, a series of studies on the bearing characteristic of layered rock mass reinforced with bolts under different geometric characteristics and mechanical characteristics of structural plane and support conditions has carried out. In view of the layered rock mass failure mode and instability criterion of anchorage structure under different forced state are studied. Main results and conclusions are as below:(1) The influence of geometric characteristics and mechanical characteristics of structural plane and anchorage ways on the bearing characteristic of layered rock mass reinforced with bolts are studied based on physical simulation experiment. The larger the thickness of rock strata and cohesion of bedding plane, the higher the peak strength, elastic modulus and secant modulus of anchorage unit, and the post-failure brittle failure characteristics are more obvious. The thickness of rock strata and cohesion of bedding plane have smaller influence on the bearing capacity after the peak stress, while the support conditions have larger influence on the bearing capacity after the peak stress. The support conditions by a high value of bolt consistency, as well as by a high value of anchorage length, will give to higher bearing capacity after the peak stress of rock mass reinforced with bolts.(2) Experimental results show that the curve of axial force of bolts can be divided into five obvious stages that are steady increase stage, quick development stage, hardening stage, steady sliding stage and failure stage. The axial force of bolts at the peak point and its increasing rate show that the thickness of rock strata and cohesion of bedding plane have smaller influence on the bearing capacity after the peak stress, while the support conditions have larger influence on the anchoring effect of rock bolt before the peak stress, while the length of anchorage section have larger influence on the bolt anchoring effect before the peak stress. With the increase of anchorage section length, the propagation of fissures are restrained, and the rock mass reinforced with bolts present obvious plastic failure characteristic.(3) The result of numerical simulation indicates that the support conditions have notable influence on the failure mode of layered rock mass. Layered rock mass mainly takes buckling instability without support, and with the increase of bolt support density and bolt pretension, the sliding along bedding plane is restrained. Layered rock mass mainly takes general shear failure under a high support, and the bearing capacity and non-deformability of rock mass are enhanced.(4) The anchorage stress fields sharply change the internal stress states of rock mass, which is the root cause of anchor structure forming. Evolution characteristics of the anchorage stress fields are studied, the results indicated that the larger the bolt support density, the greater the anchorage stress fields, and the more uniform the stress distribution. With the increase of bolt pretension, the anchorage stress before the peak stress increases significantly, but the pretension have little effect on the anchorage stress after the peak stress.(5) Instability mechanism of anchorage structure under different stress states are studied based on catastrophe theory, and the instability criterion of anchorage structure are proposed under buckling and shear as well as bending.