Instability And Control Mechanism Of Layered Compound Roof In Consideration Of Shear Behavior Of Bedding Plane

Roadways with compound layered roof have an extensive distribution in China. In view of low cohesion characteristic of bedding planes, compound roofs tend to suffer from roof strata separation, sagging and even roof failure, which results in layered caving from the bottom strata above. Thus, the support technology of compound layered roof is one of the toughest problems in roadway support. The bedding plane, as a fragile part in compound layered roof, exercises key influence on roof stability. The determination of shear strength of bedding plane is primary issue of the stability analysis of compound roof and side slope. Therefore, it is of great importance to conduct a research on shear behavior of compound layered roof.In this study, combining lab experiment, theoretical analysis and numerical simulation the stability and control mechanism are investigated in consideration of the shear behavior of bedding plane. Firstly, the joint roughness cohesion(JRC) is measured. Then, in order to represent the bedding plane and simulate its shear behavior the profiles of bedding plane are imported into PFC2 D to create samples which shear tests are submitted to under different normal stress. Finally, with linear regression analysis the internal friction angle of the joint is calculated. The mechanical analysis on the stability of compound layered roof is carried out. Based on built-up beam theory analysis, it is manifested that through increasing the bedding plane shear strength, the resistance to deformation and failure is improved. The concept “Key Strata of Compound Layered Roof(KSCLR)”, one or several layers in overlying strata of roof playing control roll in roof stability, is introduced and mechanical analysis based on KSCLR theory is conducted. In consideration of shear stress, the KSCLR caving span is calculated. The concept “ Flexible Control Strata(FCS)” is proposed. The discrimination and its effect on stability of compound layered roof are analyzed. Finally, Software implementation of the discrimination of FCL is carried out on the platform of Visual Studio 2005 with the programming language of C#.With UDEC numerical simulation software, the effect of bedding plane parameters(internal friction angle, cohesion and tensile strength) on the stability of compound layered roof is investigated. It is shown that with the increase of internal friction angle or cohesion, the deformation of roadway surrounding rock, the number of fissures and failure zone decrease and parameter sensitivity in the deformation of roadway surrounding rock decreases. The values of bedding plane tensile strength have no obvious impact on failure zone. The main failure modes of the roof are shear failure and tensile failure which tend to take place together. The failure zone, also dominated by shear and tensile failure, exhibits an inverted triangle distribution. Besides, the floor mainly suffers from tensile failure. The roof fissure development process can be divided into two stages: the rapidly growing stage and stabilization stage while that of the ribs can be divided into three stages: fissure emergence, fissure closing and fissure re-growing stages.Taking Laoshidan coal mine as a research object, the support mechanism and countermeasures of compound layered roof are studied. The shear resistance of rock bolt, overall effect of rock bolt and the enforcement effect of anchor cable are investigated with UDEC numerical simulation software. The surrounding rock failure evolution regularity is summarized into four stages: the first stage is failure emergence stage while at the second stage failure zone develop into deeper areas in the roof. After that, at the third stage, shear and tensile failure materialize substantially in the ribs resulted from the roof compression action. At the last stage, failure evolution enters the stabilization stage. Through statistical analysis of the ratio of contact length of different failure modes to total contact length, the main failure modes in different areas of the surrounding rocks are determined. Based on the failure modes in different areas, different support countermeasures are proposed.