Study on the macro-micro failure mechanism of granite and its geometry effect under the different conditions of confining pressure by discrete element

When the tunnel is excavated in the granite stratum,the surrounding rock will deform and fail,and the fail- ure modes of the surrounding rock are mostly rockburst,slab cracking,landslide,etc. The macroscopic mechanical pa- rameters of the granite and the progressive failure model can be obtained by indoor uniaxial and triaxial compression tests. The indoor compression test can analyze the failure nature of granite on macroscopic scale, while the failure process of granite under uniaxial and triaxial compression on microscopic scale can be simulated by PFC2D soft codes. By taking the Nanwan tunnel engineering along the Hong Kong-Zhuhai-Macao bridge connecting line as the study background,the macro-microscopic mechanical parameters,fracture mechanism and their shape effect of granite sample were analyzed by employing both of laboratory test and numerical simulation of discrete element. Research results show that: ① The results of numerical simulation are similar as the results form laboratory test in not only the macro scale mechanical parameters but also the final failure pattern. With the increase of confining pressure,the peak strength of rock increases accordingly,while the elastic modulus varies little. With the increase of specimen ratio between length and diameter L / D,the peak strength of rock decreases accordingly,while the elastic modulus increases. ② The statisti- cal method based on the comparative strain and crack number per some area is more effective to analysis the micro scale progressive failure mechanism with the increasing of specimen ratio between length and diameter L / D,the final failure pattern changes from shear to tensile progressively. The final crack number per some area will be largest when the L / D is equal to 1. 0. The percent of shear cracks will be largest when L / D is equal to 2. 0. ③ With the increase of the aspect ratio of rock specimens L / D,the peak strength decreased and the influence of confining pressure was obvi- ous. The elastic modulus also increased obviously but the relationship was not significant with confining pressure. ④ By statistically comparing the relationship between the number of fractures and the strain,the number of cracks inside the rock increases with the increase of axial deformation like the “S” curve under different confining pressure condi- tions. When the axial deformation is close to the peak strain,the crack grows abruptly,and the number of final cracks tends to converge only when the confining pressure is small. ⑤ With the increase of the length-to-diameter ratio of the specimen L / D,the number of cracks per unit area decreases gradually when the rock fractures,and the speed of de- creasing will increase. In general,the shape change of rock sample has significant influence on the whole peak strength and elastic modulus of granite. Therefore,the shape of the granite sample should be rationally designed in order to get the real parameters and strengths of rock during indoor test.