# Study on Siphon Drainage Capacity of Slopes with Long-Horizontal Pipe Sections
## 1 Introduction  
Landslide disasters mainly occur in the rainy season, related to the softening of slope soils, reducing shear strength, and increasing pore water pressure caused by increased groundwater levels due to rainfall [1, 2]. Therefore, timely reduction of the groundwater level is the most direct and effective control method with low cost [3], on consideration of the disaster-causing factors. Siphon drainage technology has remarkable advantages among the various methods to reduce the groundwater level. The siphon drainage method for slopes uses active suction and discharge of deeply buried groundwater from the slope by the siphon phenomenon under the action of atmospheric pressure difference, which means that the technology does not require additional continuous energy supply, as shown in Figure 1. In other words, siphon drainage technology requires no post-maintenance, which helps to reduce the overall project budget [4]. In addition, compared to traditional drainage methods such as horizontal drainage boreholes and surface drainage, siphon drainage technology is simple and very inexpensive to construct, requiring only boreholes for the slope and the placement of siphons in them; once the construction plan is selected, it can be constructed quickly to cope with the upcoming extreme rainfall [5]. Therefore, the initial drainage effect can meet or exceed the standard requirements during the preliminary engineering phase of siphon drainage technology application [6]. However, several studies and engineering practices have shown that the long-term effectiveness of siphoning still faces significant technical challenges [7; 8; 9; 10; 11] because slope siphon drainage technology still has drawbacks that limit its overall development. Most landslide treatment projects inevitably require using pipes with a total length of more than 100 m and horizontal sections longer than 50 m for drainage due to topographic constraints and increased economic costs due to farmland occupation, as shown in Figure 2. This increase in pipe length has serious adverse effects on the siphon drainage process. First, the siphon limit lift is significantly reduced, while the drainage capacity is limited when approaching the limit lift. The siphon discharge capacity changes dynamically depending on the water level. When the groundwater level is high, the siphon lift is small, and the siphon will have a high discharge power; conversely, when the groundwater level drops, the siphon head increases, and the siphon discharge capacity decreases. The local atmospheric pressure determines the value of the ultimate head of the siphon. If we ignore the along-range and local head losses generated by fluid flow in the pipe, the ultimate siphon head is the height of the water column equivalent to the local atmospheric pressure. Therefore, the drainage capacity of the siphon is further limited by the resistance of the siphon to the fluid. In addition, after a long dry season, it is difficult to restart siphon drainage during the rainy season when the groundwater level rises again, and the phenomenon of \"breakage\" tends to occur due to the accumulation of air bubbles at the top of the pipe, which occurs when the siphon stops flowing for a long time due to a variety of factors. The accumulation of air leads to the destruction of the vacuum characteristics at the top of the pipe, which eventually reduces the restarting ability of the siphon and even renders the siphon device useless. The increased difficulty of siphon restart after dry season drainage stagnation limits its long-term effectiveness and reduces the safety of landslide management projects. According to a study by [PERSON] et al. [7], reducing the pipe diameter to form a segmental slug flow can timely discharge the air bubbles from the pipe and prevent air accumulation during the siphoning action. However, the existing 4 mm pipe diameter has a small drainage capacity in protection projects with high water tables or large landslide volumes.