## 3 Results and Discussion
### Analysis and Discussion on Failure Characteristics of Fly Ash Dam Slope under Rainfall
#### 3.3.2 Influencing Factors of the Failure Characteristics of Fly-Ash Dam Slopes under Rainfall  
With regard to dam-slope materials, the density of fly ash is low, and runoff erosion is more likely to occur under rainfall. In the tests, erosion caused looseness of nearby soil, which was easily liquefied under rainfall, causing flow-slide damage. This is different from the general clay slope; the nearby clay after runoff erosion is more prone to collapse in a clay slope [53]. Therefore, for the fly ash dam slope, in the early stages of rainfall, the local erosion damage was often caused by runoff erosion and flow slide. With the progress of rainfall, the local erosion will trigger a series of failures, including overall sliding failures, multi-stage sliding failures, and further erosion. Therefore, the local erosion needs to be paid attention to as an early warning of slope damage [54], and the surface of the fly ash dam slope should be strictly protected.  
With regard to slope ratio, with the decrease in slope ratio, the failure mode evolves from a sliding failure to a flow-slide failure and a runoff erosion failure. The larger the slope ratio, the more obvious the sliding failure characteristics; the smaller the slope rate, the more obvious the runoff erosion damage. However, the above conclusion is different from the erosion effect on the slope under rainstorm conditions [47; 48]. In the tests, the slope ratios of S1, S2, S3, and S4 before rainfall were 1:2.1, 1:3.2, 1:2, and 1:3, respectively. As the slope ratios of S1 and S3 were large, sliding failure occurred under this condition. The slope ratios of S2 and S4, which were small, hardly had sliding failures occur; instead, flow-slide failures and runoff erosion failures occurred.  
From the initial seepage field of the fly ash dam slope, the water content of dam-slope soil is high due to the existence of the initial seepage field. In the tests, the saturated area of the dam-slope surface appeared first at the dam-slope toe under rainfall, due to the thinnest soil layer being at the dam-slope toe. Under the action of seepage and rainfall, the soil in this area was prone to liquefaction [50], and rainfall runoff erosion was generated at the dam-slope toe, causing local damage and erosion of the dam-slope toe. If the dam slope ratio were relatively large, the local damage could cause a series of shallow sliding failures (in S1); if the dam slope ratio were gentle and could not reach the sliding failure condition, the existence of a local failure area would intensify runoff erosion. The runoff cutting on the dam body would cause the local soil to lose its lateral constraint and become loose. Affected by the initial seepage field, the soil had high water content, and the loose soil was more likely to be liquefied by rainfall. This finally led to flow-slide failure, forming a flow-slide failure mode (in S2). When there was no seepage field before rainfall, runoff erosion still occurred first. The erosion damage and the increase in the soil moisture content reduced the anti-slide force of the slope. When the slope gradient was large, sliding failure more easily occurred (in S3). On the contrary, when the slope was gentle and there was no sliding failure, the erosion developed upstream. Compared with S2, the water content of the soil is low and it is difficult to form a large-scale flow slide. Therefore, only a small-scale flow slide is observed in the test, and the main failure mode is runoff erosion (in S4).