###### Abstract  
In the restoration of ancient water engineering, the loss of fine soil particles from the ancient riverbank can easily cause seepage instability problems such as piping during the flood transient process. This paper explores the influence mechanism of flood fluctuation on soil seepage stability based on indoor experiments, field monitoring and saturated-unsaturated soil seepage theory. The paper obtains the soil-water characteristic curve of unsaturated soil using laboratory tests, builds the transient seepage finite element model of porous media and modifies parameters monitoring data to verify the numerical analysis results. The results showed that the groundwater level, pore water pressure and seepage hydraulic gradient had changed. The maximum pore water pressure between the ancient riverbank and antiseepage structure increased by 13.4%, the maximum hydraulic gradient at the toe of the riverbank increased by 49.3% and the instability of seepage significantly increased. Through the modified lime grouting between the ancient riverbank and the antiseepage structure, the structure of the soil mass was changed, and the maximum hydraulic gradient was reduced by 55.6%, which restrains piping damage. This study can be used in the restoration of ancient riverbanks to solve piping problems. transient seepage stability; saturated-unsaturated seepage model; ancient riverbank; antiseepage wall; pore water pressure; seepage hydraulic gradient; modified lime grouting +
Footnote †: journal: Applied sciences  
problems of ancient riverbanks, it is necessary to explore the seepage instability mechanism through in-depth theoretical analysis. In this paper, a numerical model of unsaturated soil parameter variation is proposed to study the seepage stability of bank slope soil, in order to obtain more practical results and provide a theoretical basis and methods for the repair of ancient riverbanks. The paper uses the Lanxi ancient riverbank as an example to illustrate the contents and methods of this study. In recent years, with the improvement of flood control safety requirements and flood control standards, the protection and reinforcement of ancient riverbanks are facing many problems that need to be solved urgently. The conservation and repair of historical water project cultural relics require that the original appearance of the cultural relics must be maintained, and the repair materials must be raw materials. Hydrogeology and human settlements have an important impact on the safety of the ancient riverbank, especially the seasonal river water level fluctuation, and human settlements have a large impact on the seepage stability of the ancient riverbank. Many domestic and overseas scholars had carried out research on the influence of seepage characteristics on bank seepage stability. According to the relationship between the water content of unsaturated soil, matric suction and seepage coefficient, the seepage equation of saturated-unsaturated soil is proposed [7; 8; 9]. Under the effect of seepage force, the microstructure arrangement of soil particles has changed. The change rule of the permeability coefficient of unsaturated soil is studied through tests, the pore characteristics of soil are described by fractal function and the seepage development process is studied through coupling equations of computational fluid mechanics and discrete elements [10; 11; 12]. The coupling equation of seepage and erosion shows that the pore water pressure of unsaturated soil increases until the hydraulic gradient is greater than the critical hydraulic gradient, and the fine particle phase migrates, resulting in deformation and instability [13; 14; 15; 16]. Based on the principle of \"combination of prevention and drainage\", the curtain antiseepage and drainage technologies are applied in riverbank engineering, the antiseepage body is used to weaken the seepage of the riverbank groundwater, and the drainage facilities are used to reduce the groundwater, so as to improve the stability of the slope seepage [17; 18; 19; 20].