## 3 Results and Discussion
### Analysis and Discussion on Failure Characteristics of Fly Ash Dam Slope under Rainfall  
#### 3.3.1 Failure Mechanism and Failure Mode Analysis of Fly Ash Dam-Slope under Rainfall  
It can be seen from the test phenomena that the failure mechanisms of fly ash dam slopes under rainfall can be divided into three categories: sliding failure, runoff erosion failure, and flow-slide failure. The failure process of S1 was a multi-stage sliding failure. There was obvious sliding surface at each stage, and the sliding developed gradually from downstream to upstream, with progressive failure characteristics, which was similar to the failure mode of an ordinary slope described in the literature [51; 52]. In the failure process of S2, there were flow-slide failures, local sliding failures, and runoff erosion. The three kinds of failures occurred repeatedly, which together led to the erosion failure of the dam body; among them, flow-slide failure played a major role. However, in S4, there was erosion failure caused by flow-slide failure and runoff erosion, and runoff erosion played a major role. In S3, the overall sliding failure occurred with an obvious arc-shaped sliding surface,  
Figure 10: Progressive erosion failure mode mainly by runoff erosion. (a) Gullies appeared after 1 h 55 min. (b) Gullies’ development after 2 h 10 min. (c) Gullies’ development after 2 h 55 min (test termination). which was the overall shear-sliding failure. The failure forms in the laboratory tests are shown in statistical Table 3.  
The failure process of S1 is divided into the following stages: (1) With the rainfall infiltration, the water content of the dam body increased, and runoff erosion occurred at the dam-slope toe, which made the upper soil lose its support and slide downward. This caused the forming of transverse cracks on the surface of the dam slope, and thus, the first local-sliding failure was formed, as shown in Figure 11a,b. (2) After sliding, the loose fly ash was taken away by the rain, the upper dam body soil lost its support, another local sliding occurred, and multiple transverse cracks were formed on the slope surface. (3) The sliding surface continuously developed to the dam-slope crest, forming the retrogressive landslide by multi-stage sliding failure, as shown in Figure 11c, and finally forming a shallow failure area in the dam body. In S2, there are three development stages of flow-slide damage caused by runoff erosion: (1) Firstly, runoff erosion occurred at the dam-slope toe and developed upward, which led to local steepening of the dam slope, as shown in Figure 12a. (2) Due to the cut-ting of the dam body by the runoff, the local soil mass lost the lateral restraint. It became loose and was liquefied under the effect of rainfall, triggering the local flow-slide failure, as shown in Figure 12b. (3) Runoff erosion and flow-slide failure were repeated, eventually leading to a larger-scale flow slide, forming the progressive erosion failure of the dam body, as shown in Figure 12.  
Similarly, the erosion failure of S4 can be divided into three stages; the first two stages were the same as S2, and the third stage was different from S2. There was hardly large-scale flow-slide failure, but mainly runoff erosion damage. S3's sliding failure was mainly affected by two factors: (1) rainfall infiltration led to the increase in soil saturation and the increase in sliding force, and caused the decrease in soil  
Figure 11: Backward failure caused by multilevel local-sliding damage; (a) the shallow sliding trend forms; (b) the first shallow sliding; (c) subsequent shallow sliding.