## 4 Seepage Control System in the Xigou Tailings Dam with an Increased Height
### Numerical Results
#### 4.4.3 Local Failure Analysis of the Drainage Pipe  
During the construction and long-term operation of the tailings dam, the drainage pipes may be blocked, which results in a local blockage and low permeability of some drainage pipes in the seepage system [35; 36]. Therefore, to further confirm the rationality of the new drainage system, a stochastic simulation analysis was carried out for a new drainage system due to partial blockage resulting in a decrease in the permeability coefficients. It was assumed that the blockage probability of all drainage pipes was random and had a uniform distribution. The simulation method and steps of the local failure of the drain pipe are as follows.  
1. Select the object elements, including the horizontal and upward bending drainage pipes for the three-dimensional finite element model, and then set the percentage of the local failure.  
Figure 13: The comparison of the burial depth of the phreatic surface (**a**) and the maximum seepage gradient (**b**) in various cases.  
2. Introduce the concept of random numbers, and sample them randomly according to the uniform distribution. When the ratio, which denotes the sum of the volume of the damaged elements to the total volume of drainage elements, reaches the set percentage of local failure, the sampling is stopped.
3. Modify the permeability coefficients of the random sampling elements to be consistent with the nearby tailings material.  
A total of 20 groups of samples were generated for the 5% random local blockage rate of the drainage pipes, and the seepage calculation was carried out. Two positions in the tailings dam were selected to record the elevation of the phreatic line, and a statistical analysis was conducted. As shown in Figure 14, when the number of samples reached 13 groups, the average elevation of the phreatic line at two positions tended to be stable. Therefore, it could be considered that the selected random sample number was representative. The average value of the phreatic surface of the tailings dam could converge under the condition of a random local blockage. The 20 groups of samples were calculated and analyzed, and the upper and lower envelope graphs of the phreatic line of the tailings dam were drawn, as shown in Figure 15a. Compared with the seepage field without blockage, a 5% random local failure had some influence on the phreatic line of the tailings dam. The local phreatic line had a larger deviation than that without the blockage, and the maximum height difference between the upper and lower envelopes reached 6.5 m, occurring near the initial dam. The maximum seepage gradient reached 1.33, but it did not exceed the dam body's allowable seepage gradient.  
Similarly, 20 sets of samples were generated for the calculation and analysis of the 10% random local failure. The numerical results are shown in Figure 15b. The variation rule of the seepage field was consistent with the 5% blockage condition, except that the degree of deviation of the local free surface was increased, and the maximum height difference between the upper and lower envelopes reached 9.6 m. The maximum seepage gradient of the dam increased by 0.14 of that of the 5% failure and was still within the permissive seepage gradient.