# Study on Siphon Drainage Capacity of Slopes with Long-Horizontal Pipe Sections
## 5 Conclusions  
Landslide disasters mainly occur in the rainy season, which is related to the softening of slope soils, reduced shear strength, and increased pore water pressure caused by the elevation of the groundwater level due to rainfall. Therefore, comprehensive disaster-causing factors and timely groundwater level reduction is the most direct and effective control method with low cost. Among the various methods of groundwater level reduction, the siphon drainage technique has distinct advantages because it requires no additional energy supply while its drainage capacity dynamically adjusts with water level changes. The research work obtained the following results and insights by comparing the performance of different pipe diameters concerning lift and length and restarting capacity through an original scale model.  
1. The flow rate of siphon drainage is influenced by several factors, including pipe diameter, pipe length, and lift, of which pipe length and lift are both uncontrollable factors in practical engineering. Hence, optimizing siphon pipe diameter is the most direct and effective way to address the efficiency and permanence of siphon drainage systems.
2. The typical pipe diameters of siphons are categorized and discussed according to the proportional difference in the size of the gravitational and viscous force influence caused by the pipe diameter. Among them, 5 mm pipe diameter as a transition pipe diameter between capillary and typical pipe diameter, under different capillary number and Reynolds number conditions, both capillary action and gravitational action dominant dual characteristics, can form a stable slug flow while maintaining the existence of liquid film to prevent increased friction losses along the way.
3. In the lateral comparison of five pipe diameters in the same single hole surge capacity under the conditions of lowering water level capacity, an 8 mm siphon pipe for lowering of water level capacity for the maximum, yet poor long-term effectiveness was not considered. Moreover, carefully considering each pipe diameter drainage capacity and single borehole cloth pipe program, 5 mm pipe diameter siphon total drainage capacity was of good performance. The combined pipe diameter test and drainage requirements show that the 5 mm pipe diameter, as a demarcating pipe diameter with the advantages of both surface tension and gravity, can meet the higher drainage requirements while ensuring the timely discharge of accumulated gas by the flow pattern in the pipe, guaranteeing the long-term effectiveness of long-distance siphon drainage. The results of the study will provide inspiration for the selection of optimal pipe diameters for future engineering applications.  
In future studies, the model performance of ultra-long tubes (siphon lengths greater than 200 m) will be further explored. In addition, the application of siphon drainage technology to different soil slopes or climatic conditions can also be a future research option.  
Conceptualization, formal analysis, H.S. and Y.S.; methodology, software, validation, investigation, resources, data curation, writing--original draft preparation, writing--review and editing, Y.Z. All authors have read and agreed to the published version of the manuscript.  
This study was supported by the National Natural Science Foundation of China (42230702).  
Not applicable.  
The authors declare no conflict of interest. The funders had no role in the design of the study, in the collection, analyses, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.