# Experimental Investigation of Crystal Blocking in Drainage Pipes for Tunnels in the Kast Region
## 5 Conclusions  
In the present work, an experimental study is conducted to investigate the development law of crystalline blockage of tunnel drainage pipes. The experimental equipment composed of a testing chamber, drainage system, and water supply system is built, and the crystallization clogging tests are conducted. The effects of five influencing factors, such as the diameter of circular drainage pipe, spacing, material, longitudinal drainage pipe, and the presence or absence of overburden, on the development law of crystallization are studied. Some suggestions are proposed for preliminary design purposes. The conclusions drawn from this study are as follows:  
1. In the presence of the overburden layer, the development pattern of crystallization of drainage pipes can be divided into a growth stage and a stabilization stage. First, the crystallization showed a linear growth trend during the first 9 days. Subsequently, when the crystallization rate of the pipe wall is approximately equal to the scouring rate of the water flow on the inner wall of the pipe, the crystallization amount reaches a peak state, and the drainage performance of the tunnel remains stable. The flow rate of water is an important factor affecting crystallization. 2. The sequence of the locations where crystals are generated in descending order is the arch, the shoulder, and the vault. In the design of circular drainage pipes, priority should be given to the degree of blockage in the arch. The crystallization process without filling soil can be divided into the growth stage, the decline stage, and the stable stage. The higher the groundwater flow, the higher the crystallization rate. However, under the soil-free condition, the large water flow has a strong scouring effect on the crystallization, and the crystallization amount shows a decreasing trend in the short term. 3. The presence of the longitudinal drainage pipes has no significant effect on the blocking performance of the circular drainage pipes. Longitudinal drainage pipes are more likely to form crystals and sediment than circular drains due to high water flow and low slope, causing drain clogging. It is recommended that an appropriate slope of the longitudinal drainage pipes should be selected to reduce crystallization and sediment deposition in actual projects. 4. The crystallization curve and flow curve are basically consistent with the standard operating conditions when the diameter of the circular drainage pipe is increased from 6.0 mm to 8.0 mm. This indicates that when the drainage system does not reach  
Figure 15: Variation of crystallization rate and flow rates with time of drainage pipes of different materials. (**a**) Crystallization rate curves. (**b**) Flow rate curves. 100% full flow, the increase in drainage pipe diameter does not have a significant effect on the formation of crystallization and the drainage efficiency of the tunnel. Reducing the spacing between circular drainage pipes can effectively reduce the amount of crystallization of individual drainage pipes and improve the overall drainage efficiency of the tunnel. * The resistance of PP pipes to CaCO\({}_{3}\) crystallization is significantly greater than that of PA pipes. In a short period of time, the diameter of the circular drainage pipe and the spacing of the circular drainage pipe have little effect on the resistance to crystallization of the drainage pipes. However, by reducing the spacing of drainage pipes, the drainage efficiency of the tunnel can be significantly improved. It is suggested that reasonable parameters of circular drainage pipes should be selected in the design of drainage systems. Conceptualization, C.X. and P.L. ; methodology, Y.C. ; software, Y.Y. ; validation, C.X., P.L. and Y.C. ; investigation, S.W. ; data curation, C.X. and L.L. ; writing--original draft preparation, Y.C. and S.W. ; writing--review and editing, P.L. ; visualization, Y.C. ; supervision, P.L. All authors have read and agreed to the published version of the manuscript.