## 1 Introduction
[PERSON] [43] carried out a physical model test of the dam break based on an iron tailings dam in Sichuan Province of China, and studied the damage process of the dam during flood overtopping. [PERSON] [44] established the physical model test based on the Wadugou tailings dam, analyzed the overtopping-dam-breaking process of the tailings dam under natural conditions and artificial dam protection measures, and studied the impact of different dam surface conditions on the failure of the tailings dam. [PERSON] [45] studied the failure process of the tailings dam under high water level operation by model experiment and analyzed the failure process of the tailings dam slope. [PERSON] [46] analyzed the causes and regional distribution patterns of the dam failures of 342 tailings dams in the world from 1915 to 2021 using the statistics method. The results showed that most tailings dam failures in Asia and Europe were closely related to rainfall. Therefore, the research on the failure mode and failure mechanism of the tailings dam under rainfall needs to be paid more attention. [PERSON] [47] carried out the test on rainfall erosion, and analyzed the erosion characteristics and influencing factors of an iron tailings dam under six different slope conditions. [PERSON] [48] conducted the rainfall erosion test, and considered that rainfall infiltration was the main factor in the hydraulic erosion and deformation instability of the tailings dam slope. Furthermore, the environmental protection measures were put forward for the tailings dam slope. [PERSON] [49] analyzed the change law of the seepage line of the tailings dam slope, as well as the dam break process of flood overtopping induced by rainfall through the tests. [PERSON] [50] explored the mechanisms of damage and fluidization of the tailings dam by using the flume model test, and analyzed the impact of rainfall on dam body safety. At present, laboratory tests on failures of the tailings dam under rainfall mainly focuses on specific failure mechanisms and influence factors, such as flood overtopping, erosion, etc. Research is lacking on the influence of different test conditions on the failure mode and failure mechanism under rainfall, such as initial seepage field, slope ratio, and other factors. In addition, the research object is a tailings dam, while the research on fly-ash dams is less. Therefore, in order to close the gap, this paper has carried out the laboratory tests considering the changes of initial seepage fields and slope ratios, aimed at studying the failure modes, failure mechanisms, and landslide-triggering factors of fly ash dam slopes under rainfall. Throughout the tests, three failure mechanisms and three failure modes were found. Differently from the clay slope, flow slide is an important failure mechanism of fly ash slopes under rainfall. Local erosion damage caused by runoff erosion and flow slide are the important triggering factors of the fly ash dam slope failure under rainfall. Furthermore, the influence of the initial seepage field and slope ratio on failure mode and failure mechanism is analyzed, which provides a reference for slope protection of the fly-ash dam.