## 3 Results and Discussion
### Results of the Test under S1 and S2 Conditions
#### 3.1.2 Rainfall Test of S1  
Before rainfall, the pressure measuring pipes were removed in order to avoid affecting the failure process of the fly ash dam slope. After the rainfall lasted for 1 h 34 min, some small gullies began to appear at the toe of the dam slope, and these small gullies gradually developed upstream, as shown in Figure 7a. After 2 h 6 min, due to the scouring action of rainwater, the gullies on the dam-slope surface developed rapidly, as shown in Figure 7b.  
Figure 6: Relationship of settlement over time; (**a**) slope ratio 1:2; (**b**) slope ratio 1:3.  
After 2 h 12 min, due to the runoff erosion at the dam toe, a small-scale flow slide occurred above the left side of dam-slope toe. This caused the upper soil to lose its support and slide downward, and induced the transverse tension cracks 1 and 2 on the dam-slope surface, as shown in Figure 7c.  
After 2 h 23 min, with the development of flow sliding, crack 1 disappeared, and crack 2 gradually widened, where a new free surface was formed. With the development of the free surface at crack 2, three new tensile cracks appeared successively above crack 2, including a tensile crack 3, a transverse crack 4 with a length of 30 cm, and a small tensile crack 5. The crack 4 was formed by the crack 6. The crack 7 was formed by the crack 8. The crack 9 was formed by the crack 10.5 mm thick layer (10.5 mm thick layer)  
crack 5. The formation of these tensile cracks indicated the occurrence of the retrogressive landslide. The first local sliding failure occurred at crack 2, as shown in Figure 7d.  
After 2 h 28 min, the first local sliding caused the soil above crack 2 to lose support, and a new free surface was generated at crack 3. With further development of the free surface, the sliding body formed, and the second local-sliding failure occurred at crack 3, as shown in Figure 7e. At the same time, due to the traction of the lower soil mass, crack 4 gradually widened and expanded to the right side, forming a large new free surface at the crack 4.  
After 2 h 31 min, crack 5 propagated rapidly, and a sliding body was formed between crack 4 and crack 5. With the slow sliding of the sliding body, the length and width of crack 5 gradually increased, and a free surface with a width of 35 cm was formed in the middle-upper part of the dam slope. Subsequently, a crack 6 with a length of approximately 20 cm appeared at the left side of the dam-slope crest, and the third local landslide occurred at crack 4, as shown in Figure 7f.  
After 2 h 33 min, crack 6 at the beach top rapidly developed and deflected towards crack 5, finally leading to coalescence. At this time, the soil mass between the two cracks gradually slid downward, and the fourth sliding failure occurred at crack 6, as shown in Figure 7g.  
After 2 h 35 min, due to the influence of the fourth sliding, the upper soil mass of the dam-slope crest lost its support, and a transverse crack 7 penetrating the entire surface of the dam slope appeared at the top. Several tensile cracks were rapidly generated on the right side of the dam-slope surface below crack 7, and the dam slope suffered the fifth sliding failure, as shown in Figure 7h.  
After 2 h 37 min, when the crack development reached the dam-slope crest, the dam slope had failed, the rainfall was stopped, and the test ended. Due to the multiple sliding damages of the dam slope, the sediment of the whole dam body slid in the downstream direction, and the total settlement of the dam-slope crest was 5 cm. According to the test phenomena, the failure mode of S1 was: runoff erosion failure triggered soil flow slide at the dam-slope toe, resulting in a multi-stage retrogressive landslide. After the landslides, ash was rapidly liquefied under the action of rainfall and discharged from the dam slope in the form of mud flow, as shown in Figure 7i.