# A Siphon Drainage Method for Consolidation of Soft Soil Foundation
## 2 Theoretical Basis of Siphon Drainage in Soft Soil Consolidation  
As reported in the previous works of literature, the groundwater level can be successfully lowered by siphon drainage. Lowering the groundwater level can have both a direct effect and an indirect effect on the acceleration of the consolidation. The direct effect is that lowering the groundwater level can increase the effective stress. When the groundwater level decreases, the weight of soils in the areas of reduced water level change from the original buoyant weight \(\gamma^{\prime}\) to the natural weight \(\gamma\). This change can accelerate the consolidation of not only the soils above the groundwater level, but also those below the groundwater level. For the former, the effective stress has been increased. As for the soils in the deep, the loads applied to them have increased. To better describe the influence of siphon drainage on soil consolidation, a diagram of the drainage method by the siphon tube is provided in Figure 1. As shown in the figure, there is a descending funnel during the siphon drainage. Point C and D in Figure 1 are two position points with different heights. For position C, the vertical effective stress before the drainage \(\sigma^{\prime}_{zC}\) is:  
\[\sigma^{\prime}_{zC}=h_{AB}\gamma+h_{BC}\gamma^{\prime}\]  
where \(h_{AB}\) and \(h_{BC}\) are the vertical heights between A and B, as well as B and C, respectively. When the groundwater level decreases, the vertical effective stress \(\sigma^{\prime}_{zC}\) is:  
\[\sigma^{\prime}_{zC}=h_{AB}\gamma+h_{BC}\gamma^{\prime}\]  
Therefore, the increment of vertical effective stress at C is:  
\[\Delta\sigma^{\prime}_{zC}=h_{BC}(\gamma-\gamma^{\prime})\]  
As for position D, the effective stress before the drainage is:  
\[\sigma^{\prime}_{zD}=h_{AB}\gamma+h_{BD}\gamma^{\prime}\]  
When the groundwater level decreases, the corresponding effective stress at D is:  
\[\sigma^{\prime}_{zD}=h_{AB}\gamma+h_{BC}\gamma+h_{CD}\gamma^{\prime}\]  
Therefore, the increase in vertical effective stress at D is:  
\[\Delta\sigma^{\prime}_{zD}=h_{BC}(\gamma-\gamma^{\prime})\]  
The increase in effective stress at C and D are the same. Since the natural weight is always larger than the buoyant weight, the effective stress can be increased if the groundwater level decreases. Compared with other drainage methods, the siphon drainage method is not affected by the slow seepage rate of groundwater. It can slowly transport the groundwater in soft soil into the pump well and then discharge it quickly by the pump. Thus, it is an energy-saving, environment-friendly, and easy-operated method. A schematic of the application of the siphon drainage method in soft soils treatment is provided in Figure 2.  
Figure 1: Diagram of groundwater level in soil mass before and after siphon drainage (Point A is at the surface, point B is at the initial groundwater level, point C is at the groundwater level after drainage, and point D is below the groundwater level after drainage). As shown in Figure 2, the siphon drainage system includes the following parts: siphon drainage hole, siphon pipe, a pump well, pumping pipe, and a water pump. The working principle of the siphon drainage method for soft soil foundation is as follows: Insert one end of the siphon pipe into the siphon drainage hole formed in soft soil foundation, and put the other end of the siphon pipe into the pump well. After the siphon is activated, the water in the soft soil will collect into the siphon drainage hole and be drawn out by the siphon pipe. If the depth of the siphon pipe penetrated into the soil exceeds the limit lift of the siphon, the siphon pipe will always be submerged in the groundwater without interruption. The water level in the pump well is controlled by an intelligent controller to keep it below the limit lift depth of the siphon, so there is a water level difference at both ends of the siphon pipe, and the siphon drainage process can continue.