Study On Reinforcement Mechanism Of Rock Bolt To Intermittent Jointed Rock Mass And Application

Structure plane is a characteristic of rock engineering. The rock was cut to discontinuous medium by the structure planes and the situation was changed to be complicated. Joints and fissures prevalently exist in nature. When the joint cut the rock material in a non-persistent way, the rock becomes intermittent jointed rock mass, which is the commonest formation in rock engineering. The failure of intermittent jointed rock mass was usually caused by propagation of secondary crack and perforation of rock bridge. So, the study of this kind of rock mass is practically meaningful. Rock bolt is widely used and achieves great efficiency in practice. However, the anchoring mechanism was not very clear so far, especially to the intermittent jointed rock mass. Hence, in this thesis, the laboratory tests and theoretical analysis were adopted to study the reinforcement mechanism of rock bolt to intermittent jointed rock mass.Firstly, the uniaxial tests were performed based on bolted intermittent jointed rock mass sample in large scale. In Chapter 2, the joint length, spacing, persistence and dip angle were selected as variables to study the reinforcement effect of rock bolt to jointed rock mass. The differences of crack propagation and failure mode of rock mass between bolted case and unbolted case were comparatively analyzed. Based on the experimental phenomena and data, some primary reinforcement rules were drawn. Then, AHP method was chosen to study the sensitivity of joints geometry parameters to reinforcement effect. The sequences of sensitivity were obtained, regarding peak strength increment and deformation modulus increment as target function respectively.In Chapter 3, firstly, the stress state of rock bridge was studied when the rock mass were under two-dimension compression stress. The perforation mode of rock bridge was analyzed and summarized to two categories under different stress level. Then, the shear resistance of rock mass was obtained for each perforation mode. After rock bridge failure, the original joint and secondary crack form a clear shear plane, which could be regarded as a rough joint. Considering the dilation, the shear resistance of rock mass in this situation was derived.When shear deformation happened in joints, the rock bolts that run through the joints also deformed. The force mobilized in the bolts reacts to the joints and performs reinforcement effect. So, the deformation of rock bolts was an important respect to the reinforcement mechanism. In Chapter 4, the anchoring effect to joint was studied considering the bolt tensile deformation, shear deformation and rotation near joint face. In elastic domain, the evolution of the mobilized forces in the bolt as a function of the displacements is obtained with the help of a variation formula. The deformed shape of the bolt is described by a hyperbolic cosine function. The forces at the elastic limit are calculated by a plastic hinge formation criterion, established by taking into consideration the interaction of the bending moment and axial force. In plastic domain, it is assumed that the axial force mobilized in the bolt continues to increase. The displacements are calculated using an axial rigidity secant which progressively decreases as a function of the plastic lengthening of the bolt. At failure, the mobilized forces in the bolt flush with the joint and the shear force based on the Tresca criterion. The displacements are calculated by a large deformation formula assuming that the length delimited by the plastic hinges attains the material's failure strain. With the orientation and the intensity of the mobilized resultant force in the bolt, one can determine the reinforced joint's shear strength by dissociating the bolt cohesion and the confinement effect.In Chapter 5, the results obtained in Chapter 3 and Chapter 4 were combined. Considering the anchoring effect of bolt to rock bridge, the shear resistance of bolted intermittent jointed rock mass were derived. Before the perforation of rock bridge, the equivalent deformation modulus of bolted rock material was obtained assuming the deformation compatibility between bolt and rock. Moreover, the shear resistance of bolted rock mass was calculated in this case, combining the result of Chapter 4. After the perforation of rock bridge, the anchoring effect of bolt to joints was applied to the jointed rock mass based on the geometry characteristic of intermittent joints. Taking the reinforcement of bolt to rough joint into consideration, the anchoring effect of bolt to the rock mass was derived in this situation.The theoretical model was applied in the Sarma method and used to calculate the safty factor for the slope. Firstly, the failure mode of rock bridge was judged according to the stress state. The shear resistance of slice was calculated based on shear-tensile failure or shear failure mode of rock bridge. The reinforcement effect could be considered in anchoring case. The total resistance divided by the total driving force equaled to the safty factor. The engineering application showed that the method adopted in this thesis achieved good results.