## 3 Reference Case Study
### Numerical Model and Field Monitoring  
#### 3.2.1 Numerical Model  
In the process of water level fluctuation, piping often occurs in the original ancient riverbank of Lanxi, endangering the stability and safety of the ancient riverbank. In order to retain the original ancient dike, an antiseepage wall was set behind the dike, which changes the seepage characteristics of the structure. By establishing a two-dimensional finite element numerical model of saturated-unsaturated seepage in porous continuous media, the influence of antiseepage wall on the groundwater level, pore water pressure and hydraulic gradient of the ancient riverbank were studied.  
The average water level height of the Lanjiang River in winter is 23.14 m, and the water level of the Lanjiang River fluctuates sharply in rainy season. The typical flood fluctuation period and the corresponding hydrostatic pressure of the groundwater at the back side of the ancient dike were taken as the boundary conditions of the numerical model, as shown in Figure 9. The water level of Lanjiang River rose sharply from June 30, 2021, reached its highest point of 29.53 m at 6:00 on July 2, and dropped to 22.95 m at 18:00 on July 5, with a drop of 6.58 m, as shown in line 1 of Figure 9. The antiseepage wall is set behind the dike. The monitoring data shows that the peak value of the groundwater level behind the ancient dike decreases significantly, and its change with time is shown in line 2 of Figure 9.  
The impervious core at the back water side of the ancient dike has a diameter of 1.60 m and a depth of 6.61 m. The upper part of the antiseepage wall is a concrete wall,  
Figure 8: Change in soil microstructure before and after porous clay plate test. (a) Structure of soil sample before test; (b) structure of soil sample after test.  
Figure 9: Water level boundary condition of numerical model.  
and its bottom is located on the Sanhe soil foundation. The lower part of the concrete wall uses cement grouting to form a curtain to prevent seepage. The project implementation process is shown in Figure 10. The permeability coefficient of concrete cutoff wall is \(2.5\times 10^{-11}\) cm/s and the permeability coefficient of cement grouting curtain is \(1.6\times 10^{-8}\) cm/s.  
When establishing the numerical model, several oblique broken lines were used to simplify the slope surface of the ancient riverbank slope. The top elevation of the ancient riverbank is 33.05 m, the toe elevation is 26.88 m, the foundation elevation of the ancient dike is 24.80 m. The upper vertical section is 6.17 m high and 0.22 m wide at the top. The bottom of the upstream slope is 19.05 m high, and the slope gradient is 22\({}^{\circ}\) and 43\({}^{\circ}\), respectively, from bottom to top. The ancient dike is close to the building, the building foundation is deep foundation, and the upper load directly acts on the bearing layer, so its effect on the ancient dike could be ignored. The antiseepage wall is composed of concrete wall and a cement grouting curtain. The curtain penetrates the sand gravel layer and reaches the peilitic siltstone. The numerical model adopts a quadrilateral finite element with a side length of 0.5 m and generates 7023 nodes and 2584 elements. The numerical model of the ancient dike with antiseepage wall is shown in Figure 11.  
Figure 11: Numerical model of Lanxi ancient riverbank with antiseepage wall.  
Figure 10: Process of setting antiseepage wall. (**a**) Precast beam; (**b**) well circle formwork; (**c**) locking beam; (**d**) reinforcement binding; (**e**) placing concrete; (**f**) complete.