# Percolation Threshold of Red-Bed Soft Rock during Damage and Destruction
## 3 Results and Discussion
### Numerical Simulation of Percolation in Compression Test of Red-Bed Soft Rock
#### 3.3.2 Numerical Simulation of Triaxial Compression Percolation of Red-Bed Soft Rock  
(1) Conventional triaxial compression numerical test  
Previous conventional triaxial compression tests of red-bed soft rock [49] have been numerically simulated to obtain its damage characteristics under different confining pressures, as shown in Figure 10.  
It can be seen from Figure 10 that when the confining pressure is 0, it becomes a common uniaxial compression test and there are five stages of damage characteristics as described in Section 3.2.1. When the confining pressure is greater than 0, the five stages are not obvious. The initial stage of damage is obviously shortened, the stage of stable development of damage occupies almost the entire process, the stages of sharp damage and peak damage are not obvious, and the stage of residual damage is prolonged. The damage caused by an external force is more evenly distributed in all stages, and the amount of damage in each stage is not extensive.  
Under the action of external force, the soft rock continues to suffer minor damage which accumulates and eventually leads to its failure. However, due to the existence of confining pressure, the rock's compressive strength is enhanced and its sudden damage prevented, such that there is no obvious moment of sudden damage in the whole damage process. However, the damage still showed a trend of first rising and then falling, and the position of maximum extent of damage occurred after the stress peaks.  
(2) Triaxial compression numerical test under the action of water-stress coupling  
The stress-strain curve of water-stress coupled triaxial compression test [50] was numerically simulated to obtain soft rock damage under different confining pressures, as shown in Figure 11.  
As can be seen in Figure 11, the damage characteristics of red-bed soft rock under coupled water-stress conditions differ from those achieved by the conventional triaxial compression test. On the whole, the damage characteristics obtained by the water-stress coupled triaxial compression test are similar to those found under uniaxial compression. There are five stages of damage but the extent of damage at each stage was found to be lower than that under uniaxial compression.  
Compared with the effects of the conventional triaxial compression test, the softening effect of water on soft rock was apparent when water is taken as the confining pressure and under the condition of 48 h saturation. Although the confining pressure increased the compressive strength of soft rock, it failed to provide sufficient support for soft rock to counteract the effects of water. There was still a sudden moment of rock damage.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Sample** & **Percolation Threshold from** & **Percolation Threshold from** & **Error (\%)** \\
**Number** & **Laboratory Compression Test** & **Numerical Compression Test** & **Error (\%)** \\ \hline
1 & 0.404 & 0.460 & 13.9 \\
2 & 0.334 & 0.300 & \(-\)10.2 \\
3 & 0.434 & 0.430 & \(-\)0.9 \\ Mean value & 0.391 & 0.397 & 1.5 \\ \hline \end{tabular}
\end{table}
Table 3: Comparison of percolation thresholds obtained by numerical simulation and by laboratory compression test.  
Figure 10: Damage characteristics and stress–strain relationships of red-bed soft rock in conventional triaxial compression numerical tests under different confining pressures (the confining pressure in (**a1**,**b1**) is 0, in (**a2**,**b2**) is 1 MPa, and in (**a3**,**b3**) is 3 MPa).