Experimental Study On Effect Of Joint Roughness On Compressive Dynamic Properties Of Rock Mass

As in the typical discontinuous rock mass, joints are the key point to the dynamic properties of the rock mass, and the roughness is the main factor of the joint’s property. In this study, in allusion to the unevenness, the roughness of the joint was described by three quantitative indices, namely contact area (represented in the form of joint matching coefficient-JMC), thickness, and distribution. Given this, the modified Split Hopkinson Pressure Bar (SHPB) apparatus was used to study the one dimentional stress wave propogating in the jointed rock mass, and combined with the dynamic model of the jointed rock mass with a roughness, this thesis systematically revealed the influencing mechanism of the joint roughness on stress wave propagation and the compressive dynamic properties of the jointed rock mass. The main achievements of this thesis are shown as following.(1) Based on the method of characteristics and the displacement discontinuity method, the dynamic model of the jointed rock mass with a roughness was established, which put JMC as a main factor. Input the incident wave into the model, the reflected wave and transmitted wave were output. Then the stress wave propagation throught the jointed rock mass was obtained. Finally the parameters like stress, strain, normal closure and others representing the dynamic properties of the jointed rock mass were acquired.(2) By using the self-developed modified Split Hopkinson Granite Pressure Bar (SHGPB) apparatus, an experimental study was carried out to learn the effect of joint roughness on stress wave propagation in the jointed rock mass, and also verified the calculation results of the dynamic model. All the bars and specimens were made of very high quality granite, and one end face of the specimen was sawn to form the artificial joint with different JMC and joint thickness. By the modified two-point wave separation method, using the test data collected by the strain gauges, the incident wave, reflected wave and transmitted wave through the joint were worked out, and then the dynamic mechanical parameters of the jointed rock mass were obtained, e.g. pressure-closure relation, specific stiffness, overall elastic modulus, etc. Thus, the effects of JMC and joint thickness on stress wave transmission when propagating in jointed rock mass and its compressive dynamic properities were got. In addition, comparisons were made between the SHGPB experiment and the dynamic model calculation results, and found out that the effect laws about the joint roughness effect were the same, while the two results were much closer with a larger JMC.(3) As in the calculation and analysis, accounting the rock mass as an elastic medium, the homogenous aluminium was used to do the modify experiment. Using the self-developed modified Split Hopkinson Aluminium Pressure Bar (SHAPB) apparatus, three quantitative indices of the joint roughness (JMC, thickness and distribution varied from dispersed to aggregated, in the centre to at the edge) were studied to reveal their effects on stress wave propagation and the compressive dynamic properties of the jointed rock mass. All the pressure bars and specimens were made of the same alyminium alloy. Artificial joints were sawn on the surface of the specimen contacted to the output bar forming different roughness. The test data processing method was almost the same as in the SHGPB experiment. Comparing calculation results, including the transmission coefficients, dynamic pressure-closure relation, specific stiffness, overall elastic modulus, the effects of three quantitative indices of the joint roughness on the stress wave transmission and compressive dynamic properties of the jointed rock mass were gained, which verified the rules got from the SHGPB experiment to some extent.To sum up, as the JMC decreased, joint thickness increased, the joint distribution changed from dispersed to aggregated, in the centre or at the edge, the joint transmission coefficient and specific stiffness reduced, the overall elastic modulus of the jointed rock mass decreased, indicating that the stress wave transmission and compressive dynamic properties of the jointed rock mass became weakened, and all these effects were much more outstanding when JMC is smaller, while the calculation results of the dynamic model of the jointed rock mass with a roughness got much closer to the experiment results with a larger JMC.