# Study on Filling Material Strength and Dam Failure Characteristics of Loess Dam
## 3 Results
### Numerical Modeling and Field Data of Dam Progressive Failure  
The study object is located in Ansai District, Yan'an city, Shaanxi Province. Figures 12 and 13 show the initial state of the dam. In the above indoor triaxial test, we explored the shear strength characteristics of loess accumulation dam filler, which provided a basis for the subsequent analysis of the progressive failure law of the dam. Figures 14-16 show the situation of failure at three different time points and describe the progressive failure process of the loess accumulation dam during its whole life cycle. The first damage is caused by the rising water level behind the dam. Under the action of hydrodynamic force, the water forms a seepage channel in the dam body, which flows to the slope side of the airside. The shear strength of the slope foot is reduced by water soaking, so the first damage occurs. Since a large amount of rainwater infiltrates into the cracks in the top of the dam before the second damage, the pore pressure inside the dam crest increases, the effective stress decreases, the shear strength weakens, and the dam crest soil slides along the first damage surface; meanwhile, the rising water level of the reservoir causes the slope surface overflow point to move up, and the soil at the overflow outlet continues to soak and soften, the shear strength decreases, and the second damage occurs. Under the continuous erosion of rainwater, the topsoil of the dam is continuously slipping down along the rupture surface, and this is the third instance of damage. After this, the width of the dam roof gradually decreases and almost disappears completely. Figure 16: Third instance of dam damage: (**a**) Panorama of the third failure of dam body; (**b**) Dam third damage crack expansion diagram. Figure 14: First instance of dam damage: (**a**) Panorama of the first dam damage; (**b**) Dam first damage crack expansion diagram. Figure 15: Second instance of dam damage: (**a**) Panorama of the second failure of dam body; (**b**) Dam second damage crack expansion diagram. Figure 17 shows the morphological changes in the three instances of damage on the dam slope. The different colors in the figure represent three instances of damage of the dam slope sliding along the sliding surface. The number of accumulated objects produced by the three instances of damage is gradually reduced, the shearing outlet gradually moves upward, the angle between the shearing surface and the horizontal plane (an acute angle) is gradually reduced, and the length of the shearing surface is gradually increased. This result seems different from the laboratory test and can be attributed to the uneven distribution of water inside the dam and unilateral infiltration. The infiltration of water leads to the continuous erosion of the internal structural plane of the dam, resulting in this phenomenon, which is consistent with the actual situation. The author has described the analysis of dam site monitoring and failure characteristics in detail in another paper ([PERSON], 2020), so no more elaboration is necessary. Under the condition of rainfall, the continuous rise in the water level of the reservoir leads to a gradual rise in the saturation line, which leads to an increase in the soil moisture content inside the dam, leading to damage. Therefore, FIC3D is used to perform a numerical inversion for the three instances of damage to the loess stacked dam to observe the damage patterns of the dam slope and the change in external displacement and internal pore pressure. The numerical model is shown in Figure 18 and is divided into two parts. The upper part is the loess accumulation dam body. The seepage model is isotropic seepage, the lower part is the dam foundation, and the non-seepage model is selected. At the same time, through experimental research and analysis, it can be found that loess in the dam body is a typical elastic-plastic material, so the Mohr-Coulomb elastic-plastic model is used for calculations in this simulation.