Air Accumulation Control And Engineering Application On Slope Siphon Drainage

Rainfall is a key environmental factor to landslides. Characteristically, a great many landslides show seasonal changes of sliding during rainy seasons and maintaing stable in dry seasons. These dynamic deformations are evidently associated with rainfall processes. Thus it is vatal to well control the rising groundwater level, so as to improve the slope stability. As a time-honored method of liquid transfer, siphon has been widely used in daily life. During its process, flow velocity is subjected to the changing water levels, whose characteristic highly agrees with slope drainage. However, a long-term effectiveness of siphon process is less than guaranteed accounting for the high-lift in most large-scale slopes and the intermittent rainfall. As a result, siphon techniques have not yet been widely adopted till now.Based on experimental researches, theoretical analyses and field monitoring, this paper not only analyzed the formation and control methods of air accumulation in high-lift siphon, but also further explored its fundamental factors and control conditions. Moreover, technical issues and design procedures to guarantee a long-term slope high-lift siphon system has been studied, and a world’s first maintenance-free slope siphon drainage demonstration project built. According to the above researches, some major achievements and understandings have been obtained as follow:(1) In siphon drainage, water pressure gradually decreases with the increasing elevation over the surface of the nozzle. Thus, the air dissolved in water separates out and generates bubbles during siphon processes. Consequently, air accumulated at the tops of hoses may lead to interruption of the siphon process. Experimental investigation demonstrates that bubbly flow and slug flow are the major two-phase flows in high-lift siphon hoses, mostly slug flow at the top. What’s interesting, patterns of slug flow differ in varied diameters of siphon hoses. In specific, water films appear between slug bubbles and the hose wall in large-diameter hoses, and plug flow serves as an insulation layer of the air column and the water column in small-diameter ones. It is the plug flow that keeps the simultaneous movement of bubbles and water columns, and thus avoids air accumulation in siphon hoses.(2) Based on the principle of minimum potential energy and thermodynamic derivation, it is concluded that the formation of plug flow is mainly related to the hose diameter, contact angle and the surface tension coefficient. As to a specific hose material, there exists a critical diameter to the conversion between slug flow and plug flow. Under normal atmospheric temperature, the largest horizontal diameter of a stable bubble in water is around 4.6mm. However, the critical diameter is around 4mm due to the boundary effect of the hose wall and the flow redirection at the hose top. Therefore, as long as a 4mm-diameter siphon hose is adopted, it is highly possible to ensure complete plug flow in slope high-lift siphon drainage, which can discharge the released bubbles out of the siphon hoses in real time and avoid air accumulation. Thus a long-term drainage process ensured.(3) Once the siphon flow is interrupted, air accumulation is inevitable, at the top of the siphon hoses, which to some extent may influence the next siphon initiation. Air accumulation may attribute to four reasons. Supersaturated air released and accumulated at the top of the siphon hose owing to decreased pressure in high-lift siphon hoses, generating a maximum of 1.5m air column. Temperature increase can create a maximum of 0.55m long air column. Air diffusion from water will lead to a less than 0.01mm increment of air column at the top per day, which is considered negligible compared with that induced by air release due to pressure and temperature variations. What’s more, air diffusion from the hose wall plays a principal role in air supplement to the vacuum area in siphon hoses. Therefore, at least a space of 2.05m below the safety level should be made for underground water rise to ensure the well-off initiation of siphon system at the start of the next rainy season.(4) A world’s first maintenance-free slope siphon drainage demonstration project has been built, which shows the great advantage of siphon drainage in field construction. The flow rate of siphon can hardly be affected by external disturbance in this project, indicating the engineering applicability of slope high-lift siphon system. The capability of siphon drainage is corelated to the slope groundwater level. It is possible to achieve rapid groundwater level decrease in intensive rainy period, and improve the slope stability. By comparing different groundwater responses under different rainfall pattern, the groundwater response prediction model of this slope project can be obtained, which may serve as a reliable basis of engineering disaster prediction.