## 4 Numerical Simulation of Damage Mechanism under the Original Support of II5 Rail Rise
### Crack Evolution Law of Roadway Surrounding Rock  
According to Figure 12, in the initial stage (e.g., on the 5 th day), there were few cracks around the roadway surrounding rock, with merely shear cracks distributed on the surrounding rock surface, and the cracks at the roof and shoulder corners were more significant than those at the roadway ribs and floor. These cracks were mainly caused by the instantaneous deformation of the roadway. On the 50 th day, shallow cracks in the ribs extended to the deep area; tensile cracks appeared on the floor; and the roadway section shrank overall, accompanied by rib bulge on the fault side (left rib) and floor heave. With the passage of time, the total number of cracks and the number of tensile cracks at the roadway left rib, the floor, and the shoulder corner increased continuously. On the 200 th day, the deep cracks began to expand, and the left rib had been transformed from a straight wall shape to an arc shape. On the 300 th day, the roadway presented obvious asymmetric damage. Cracks at the left and right ribs extended to a depth of 3 m and 2.5 m into the ribs, respectively. Severe tensile damage occurred at the roof on the left side where the blocks extruded upward, and the floor on the left side deformed more significantly than that on the right side. Massive tensile cracks were also distributed at the roof shoulder corners, resulting in slight separation at the junction of sandy mudstone and fine sandstone, the stripping of blocks in the middle of the roof, as well as the protruding and falling of blocks at the shoulder corners. Figure 13 contrasts the on-site damage characteristics with the simulated damage characteristics of the roadway. Clearly, these two are basically consistent. Figure 14 shows the evolutions of the number of cracks and damage degree over time in the monitored area. In the simulation, the total length of the contact surface of the surrounding rock in the monitored area was counted using FISH. The shear crack length and tensile crack length generated during the rheological process were monitored, and the damage degree of the test sample was defined by the ratio of the crack length to the total length of the contact surface. The development of shear cracks, the total damage, and the shear damage displayed obvious rheological characteristics. With reference to the development of shear cracks and tensile cracks, the whole process can be divided into three stages. In Stage I (0-40 th day), i.e., the decelerated deformation stage of rheology, massive  
Figure 11: Average displacement and displacement rate of two ribs of roadway. (**a**) average displacement; (**b**) average displacement rate. shear cracks were generated. The damage of the surrounding rock was mainly caused by shear damage. The shear damage curve and total damage curve coincided greatly. In Stage II (40 th-125 th day), i.e., the constant-rate deformation stage of rheology, shear cracks increased at a constant rate first; then, as the cracks expanded to the deep area, the shear cracks grew faster, and the number and damage of tensile cracks jumped. In Stage III (125 th-300 th day), i.e., the accelerated deformation stage of rheology, the number of shear cracks and the corresponding damage continued to increase at basically the same rate as that in Stage II, and the number of tensile cracks and the corresponding damage rose notably, causing obvious roadway destruction. On the 300 th day, the number of cracks totaled 2002, including 1765 shear cracks (accounting for 88.2%) and 237 tensile cracks (accounting for 11.8%). It can be seen that the cracks in the surrounding rock were mainly shear ones. The cumulative number of shear cracks in Stages I and II is 1331, accounting for 75.4% of the total number of shear cracks. In Stage III, the number of new shear cracks is 434, accounting for 24.6% of the total number of shear cracks. In Stages I and II, the cumulative number of tensile cracks was 83, accounting for 35% of the total number of tensile cracks.