# Experimental Investigation of Crystal Blocking in Drainage Pipes for Tunnels in the Kast Region
## 4 Results and Analysis  
### Standard Condition  
Crystal obstructions are formed and accumulated on the inner wall of the drainage pipes and the drainage side ditch, causing blockage of the drainage system after 12 days of testing, as shown in Figure 6a,b, respectively. The yellow crystal obstructions are mainly CaCO3 and sediment, as shown in Figure 6c.  
The crystallization curves of the three circular drainage pipes (A, B, and C) in the standard condition are shown in Figure 7a. The curves illustrate that the development rule of crystals on the inner wall of the drainage pipe under working conditions can be mainly divided into two stages: (I) the growth stage and (II) the stabilization stage. In regard to the first stage (0-9 d), with the continuous flow of the ionic solution, the crystals formed inner side of the drainage pipe quickly for the rate of ion formation is greater than the rate of solution scouring the inner wall, leading to a continued increase in the weight of drainage pipe. With regard to the second stage (10-12 d). After 9 days, as the crystal formation rate gradually equals the solution scouring rate, the growth rate of crystals slowed down and the crystal weight reached the peak (A:1.95 g/m, B:1.82 g/m, C:1.94 g/m) at the 12 th day, resulting in a stable amount of crystallization.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Condition** & **Sand** & **LDP** & **Diameter** & **Spacing** & **Material** \\ \hline Standard condition & Yes & No & 6 mm & 50 cm & PA \\ No filling sand & No & No & 6 mm & 50 cm & PA \\ Install LDP & Yes & Yes & 6 mm & 50 cm & PA \\ Enlarge pipe diameter & No & No & 8 mm & 50 cm & PA \\ Narrow section space & No & No & 6 mm & 25 cm & PA \\ Change pipe material & No & No & 6 mm & 50 cm & PP \\ \hline \hline \end{tabular} Note: PA denotes polyamide, and PP denotes polypropylene.  
\end{table}
Table 1: Indoor test conditions of tunnel drainage pipe blockage.  
The variation curves of the three circular drainage pipes (A, B, and C) versus time in the experiment are shown in Figure 7b. The flow curve of the ABC pipeline fluctuates greatly, and there are many intersections, which cause the instantaneous crystallization rate and the scouring rate of each pipeline to be similar, so the crystallization curve of each circular drainage pipeline is basically consistent.  
An illustration of the crystallization of the arch (tubes 1 and 6 as shown in Figure 1c, spandrel (tubes 2 and 5 as shown in Figure 1c) and vault (tubes 3 and 4 as shown in Figure 1c) versus time under standard conditions is shown in Figure 8. It is illustrated from the crystallization time curve that the crystal weights are 1.93 g/m, 1.87 g/m, and  
Figure 6: Drainage pipes before and after the testing and the crystal sample. (**a**) Drainage pipes before the testing. (**b**) Drainage pipes after the testing. (**c**) The crystal sample.  
Figure 7: Variation of crystallization rate and flow rate of different circular drainage pipes with time. (**a**) Crystallization rate curves. (**b**) Flow rate curves.  
1.44 g/m for arch, shoulder, and vault, respectively, due to the following two reasons. Firstly, the sequence of the locations of flow rates is arch > spandrel > vault. When the flow rate is not enough to scour the tunnel crystal, with the increase in flow velocity, more ions pass through the drainage tube in unit time, leading to an increase in the amount of crystallization. Secondly, the unstable crystals formed on the upper part of the circular drainage pipe are vulnerable to being peeled off under the erosion of the water flow, causing the crystals at the bottom to increase. Therefore, the test results show that the bottom blockage should be given priority and prevented when considering the long-term operation of annular drainage pipelines.