## 3 Results
### A Model of the Failure Process  
Based on the deformation characteristics and changes in the slopes during rainfall, the toes of the slopes were the first parts to fail, for all tested slopes, which initiated the failure process. During the failure process, slope erosion and local failures occurred for all tested slopes, but the failure modes of the slopes differed according to the mitigation measures. Based on the soil parameters and deformation processes of the tested slopes, the failure modes and differences of each slope can be described in the following stages. 1. **Toe erosion. ** After about 30 min of testing, slope surface soil gradually becomes saturated, and the infiltration rate of rainfall on the slope surface is lower than the rate of rainfall supply. The surface of the slope begins to accumulate water during this time, and slope erosion and local failures occur at the toe of the slope due to the continuous erosion of surface runoff and the softening effect of rainfall infiltration. Different mitigation measures showed different degrees of slope erosion and response times (as shown in Figure 8a). For the slope with a declining gradient mitigation, the slope erosion was more rapid and obvious due to its longer length, gentler gradient, and more obvious convergence flow. However, this gentler gradient also made it difficult for high and steep lateral faces to form at the toe of the slope. For the slope with a stepwise drainage channel mitigation, the effect of surface runoff was not obvious in the early stages of rainfall due to the collection effect of the slope drainage channel. Slope toe erosion and damage did not occur, and rainfall infiltration was slow. Therefore, there was no obvious deformation on the slope surface in the first 30 min. This was in stark contrast to the other two types of slopes where  
Figure 7: The process of failure for the slope with a stepwise drainage channel mitigation ((**a**–**d**) are the 40 min, 60 min, 120 min and 150 min after experimental). obvious slope toe erosion occurred after only 10 min of rainfall. This also shows that constructing drainage channels on a slope can effectively reduce erosion due to rainfall and therefore slope damage. 2. **Formation and expansion of slope surface cracks. ** The slope body generates tensile stress towards the direction of the free surface and undergoes continuous creep due to slope toe erosion. In our experiments slope deformation accumulated gradually, and tensile cracks appeared on the slope surface in the vicinity of the free surface of the slope toe. However, the crack evolution patterns differed between different slopes (Figure 8b). For the slope without any mitigation measures, the appearance of tensile cracks occurred in an obvious sequence. For the slope with a declining gradient mitigation, the tensile cracks appeared simultaneously at various positions on the slope surface, without an obvious sequence. For the slope with a stepwise drainage channel mitigation, the tensile cracks all appeared near the transverse drainage channels on the slope surface. 3. **Local failure. ** The expansion of cracks causes the local pore-water pressure to increase continuously, and the effective stress of the soil decreases. Connected cracks form a potential sliding surface, and the local failures occur along the cracks. The location and magnitude of local failures in our testing were affected by slope mitigation measures (Figure 8c). In particular, the local failure in the slope with a declining gradient mitigation was small, and mainly occurred in the soil layer no deeper than 5 cm. The local failure in the slope with a stepwise drainage channel mitigation was larger, and the sliding surface was deeper. However, the overall deformation was smaller, and the collapse only occurred at the toe of the slope. 4. **Overall failure. ** Continuous local failures cause the free surface of the toe of the slope to become steeper, and the tensile stress on the slope increases continuously.