# Study on Siphon Drainage Capacity of Slopes with Long-Horizontal Pipe Sections
## 2 Siphon Drainage Test  
### Test Model Setup and Scheme  
In order to study the variation in the efficiency of the slope siphon drainage system under the dual effect of pipe length and pipe diameter, several sets of controlled experiments were conducted to select the most suitable siphon pipe diameter for the engineering scenario by comparing the evaluation factors such as the working flow rate, the flow pattern in the pipe, and the restarting capacity of the siphon drainage system. The siphoning phenomenon is triggered by an entire tube of liquid under the effect of water head difference, which in turn is maintained under pressure difference between atmospheric pressure and negative pressure inside the tube, i.e., ambient air pressure is an essential factor affecting the siphoning characteristics. Further, the reduction of pipe diameter and length in the reduced size model also requires a proportional change in the test ambient air pressure and liquid properties. Therefore, in order to reduce test errors, the experimental setup uses a full-size model with pipes made of polyurethane (PU). The siphon drainage experimental set-up, as shown in Figure 3, was used to validate the proposed calculation method. The whole model is a complete siphon system, consisting of a constant lift water supply device, a siphon pipe (partitioned into rising fluid section, fluid horizontal movement section, and fluid falling section according to the function and fluid force characteristics), a constant lift water storage device, and a scale. The storage barrel set at the bottom of the rising fluid section of the siphon (later referred to as the rising section, horizontal section, and falling section) simulates the change of the groundwater level at the bottom of the borehole in different seasons of the actual project, and the change range is 3-12 m. The water storage bucket is a fixed-position constant lift device, and the height of the water surface in the bucket from the top of the siphon (horizontal section) is 17.2 m.  
The siphoning action starts when the siphon tube connecting the upper and lower water levels is filled with fluid, and the fluid in the drain tube at the lower water level moves under the action of gravity. The fluid in the inlet pipe then overcomes gravity to follow the flow under the action of intermolecular cohesion and negative pressure at the top, forming a continuous siphon flow, as shown in Figure 4. After the fluid movement in the siphon reaches the state of stable siphoning, the flow monitoring starts. Fifty-two experiments were performed, and the schemes are given in Table 1, where \(D\) represents the diameter of siphon pipe, \(L_{hori}\) represents the horizontal section length of siphon pipe,  
Figure 2: Schematic diagram of the inclined pipe siphon drainage method with long horizontal pipe sections (front view). and \(H\) represents the siphon lift. The flow velocity was measured by weighting the water collected in the water storage tank for a period. It should be noted that a single-phase flowmeter did not work well in these experiments since the flow pattern is multi-phase. Due to the inhomogeneity and randomness of the gas-liquid two-phase flow in the siphon, we performed repeated measurements until the fluctuation of the siphon flow rate under each condition did not exceed 2%, while the test measurements were obtained by removing the maximum and minimum values and taking the average value. We used the drainage velocity to measure the siphon drainage capacity under the corresponding conditions, while for its long-term applicability, it was evaluated by comparing the restarting capacity after the siphon stopped flowing. The same experimental model was used to simulate this process in this part of the study.