## 3 Reference Case Study  
### Tests and Physical Parameters  
Taking the Lanxi ancient riverbank as an example, this paper studies the influence of matric suction of saturated-unsaturated soil on soil seepage deformation, and then proposes measures to protect the ancient riverbank from seepage piping. The ancient riverbank is located at the downstream of the intersection of Qujiang River and Jinhua River in Zhejiang Province. On the right bank of Lanjiang River, it is a provincial cultural relic. Its geographical location and the appearance of the ancient riverbank are shown in Figure 3. In the rainy season, Qujiang River and Jinhua River converge on the Lanjiang River, which often causes flood disasters. Due to the large fluctuation of water level and the impact of rainfall and human settlements, safety problems such as collapse, wall deformation, leakage and piping occur frequently. In order to protect the original structure of cultural relics and minimize the impact of repair on adjacent buildings, an antiseepage wall is set at the back of the ancient riverbank to prevent the escape of fine soil particles due to seepage pressure. The Lanxi ancient riverbank is a vertical retaining wall structure with rubble masonry external protection and internal filling. The maximum wall height is about 6 m and the bottom width is about 1.5 m. The buildings behind the wall are close, with the nearest distance of 2.0 m. The top of the ancient riverbank is a 1.15 m high black brick crenel, with a gap of about 1 m. The soil behind the wall is loose-filling, poor-uniformity and high water permeability, and the initial permeability coefficient was 3.6 \(\times\) 10\({}^{-2}\)-5.1 \(\times\) 10\({}^{-3}\) cm/s. The geology of the typical section can be divided into three formations from top to bottom: Layer1\({}_{1}\) is the artificial fill surface layer, with a thickness of 2.20-9.00 m, mainly composed of rubble, a small amount of gravelly sand and a small amount of cohesive soil, with poor homogeneity. There are sewage pipes, residential power cables and other underground pipe networks near the residential area. Layer1\({}_{2}\) is plain fill with a thickness  
Figure 3: Geographical location and appearance of Lanxi ancient riverbank. (**a**) Geographical location; (**b**) overall view of Lanxi ancient flood bank; (**c**) local appearance of ancient flood bank. of 2.50-4.40 m. It is mainly composed of sand and gravel and sand, with general uniformity. Layer II is a sandy gravel, mainly composed of volcanic rock gravel, with gravel, silt and clay between the gravels, with a thickness of 5.40-10.70 m. Layer III is the peiltic siltstone with complete lithology. The geological cross section is shown in Figure 4, and the physical and mechanical parameters of each soil layer are shown in Table 1.  
At the site of the ancient riverbank, soil samples I\({}_{1}\), I\({}_{2}\) and II were taken along the depth for the soil property test. According to the standard for the geotechnical testing method (GB/T50123-2019), the engineering classification of soil was determined through the fine particle gradation analysis test. Then, the relationship between the volume moisture content, permeability coefficient and matric suction of the soil sample was obtained by using the porous clay plate test, which provides parameters for the finite element numerical analysis. At the same time, the microscopic images of soil samples were obtained by means of an electron microscope scanning test to analyze the structural changes of soil samples under the effect of matric suction. The test process is shown in Figure 5.