# Experimental Investigation of Crystal Blocking in Drainage Pipes for Tunnels in the Kast Region
## 1 Introduction  
With the substantial increase in tunnel construction, the problems of tunnel drainage systems have become increasingly prominent, such as tunnel lining cracking and water leakage in side wall, which have seriously affected the use and safety of tunnels [1, 2]. Once the drainage system is blocked, the water pressure behind the lining rises, leading to increases in the axial thrust and bending moment of tunnel lining and a severe threat to the long-term safety of tunnels [3, 4, 5, 6, 7]. A large expenditure has to be made to monitor and maintain the tunnel drainage system continuously [8, 9]. Therefore, it is of great significance to conduct experimental research on the development law of internal crystal blockage of the drainage system to ensure the safety and stability of the tunnel lining structure. A great amount of effort has been devoted to investigating the blocking problem in drainage pipes for tunnels. [PERSON] and [PERSON] conducted a large number of on-site surveys in the field of drainage and found through microscopic analysis that the main component of crystallization is calcite crystal calcium [10, 11]. It is proposed that the continuous deposition and blockage of calcium carbonate minerals such as calcite (CaCO\({}_{3}\)) will seriously affect the water transport in tunnel water channels and pipeline systems [12]. It is revealed by a study of old tunnels in South Korea that the sediments in the drainage system of tunnels are mainly due to the chemical reaction between the tunnel concrete lining and groundwater. [PERSON] analyzed the scale sample of tunnel drainage pipe, and pointed out that calcite crystal is the main component of the scaling sample of tunnel drainage pipes [13]. The substance and formation mechanism of the blockage of the tunnel drainage system is another issue which concerns the researchers most. A large number of experiments have found that square carbonate is the main substance of the blockage of the tunnel drainagesystem. Some scholars analyzed the influencing factors of the formation mechanism of carbonate. [PERSON] conducted a series of laboratory experiments to obtain the scaling mechanism of calcium carbonate based on the crystal growth kinetic theory [14,15]. [PERSON] and [PERSON] found that the release of CO\({}_{2}\) gas caused the pH of the solution to rise, thereby promoting the precipitation of CaCO\({}_{3}\)[16,17]. The precipitation of CaCO\({}_{3}\) changed the ratio of the total carbon concentration in the solution to the total calcium concentration, leading to a change in the CO\({}_{2}\) precipitation rate. [PERSON] found that the increase in pH increases the thermal resistance of progressive fouling. However, the flow rate plays a more critical role in the thermal resistance of progressive fouling compared with pH [18]. [PERSON] found that installing a magnetization device and quantum stick in the tunnel drainage pipe can effectively reduce the formation of blockages and that the quantum stick shows a reasonable descaling effect under conditions that exceed the speed limit [19]. According to the physicochemical mechanism of CaCO\({}_{3}\) precipitation, the fluid-solid interaction of highly soluble concrete components, silicate, for instance, enhances the exchange of CO\({}_{2}\) between the drainage solution and tunnel atmosphere [20,21]. [PERSON] et al concluded that the experimental conditions such as the solution chemistry, the flow rate, the temperature, and the saturation index may have significant impacts on CaCO\({}_{3}\) deposition kinetics. They found that flocked polyvinyl chloride (PVC) pipes have a certain effect on the removal of crystallization of tunnel drainage pipes [22, 23, 24]. [PERSON] found that calcite depends not only on the hydraulic conductivity and CaCO\({}_{3}\) content of surrounding rocks but also on the lining materials and the geometry of tunnels [12].