## 5 Conclusions  
Four groups of laboratory model experiments were performed to investigate the water movement, failure time, and modes in multi-layer and single-layer slopes caused by rainfall infiltration. In addition, the unsaturated hydraulic conductivity and SWCCs were also measured in the lab to clarify how the capillary barrier works under different conditions.  
The results of hydraulic conductivity show that the unsaturated hydraulic conductivity \(K\) in the coarse layer is lower than that in the fine layer in a lower suction condition (0.4 kPa), which results in the development of the capillary barrier at the interface of the fine-coarse layer.  
Different failure modes occurred in an inclined multi-layer slope and single-layer slope: sliding and piping failure. In the flat group, the capillary barrier was presented, which prevented the rainwater from infiltrating into the coarse layer for a while and caused a delay of the failure time. However, in the inclined group, the inclined intermediate coarser layer formed a capillary barrier, resulting in a significant amount of water being diverted to the downward slope and causing piping failure at the toe of the slope that resulted in earlier failure, which has a negative influence on the slope stability.  
An increased tilt angle has a drastic effect on the capillary diversion length, in which more infiltrate water will be diverted to the downslope side and then infiltrate into the bottom of slope, resulting in quicker failure times.  
The present study does not provide a model to be used in a specific site problem. Instead, the model is suitable for studies on hypothetical multi-layer hillsides to assess water movement patterns and general failure mechanisms. The results from such studies can prove useful in the development of an appropriate strategy for resolving problems in individual, site-specific multi-layer slopes.  
In the present manuscript, we have mainly focused on the influence of the tilt angle and the effect of the capillary barrier on water movement and slope failure modes compared with a single-layer slope. It should be noted that in multi-layer slopes, different geometric characteristics, such as different heights, slope ratios, and fronts, also have an influence on the water infiltration and water content distribution in the slopes, significantly affecting slope failure initiation. Model experiments with different geometries of the shallow soil cover will be conducted in further research [39].  
Conceptualization, J.T. and U.T.; investigation, D.H.; methodology, S.T.; resources, J.X.; supervision, U.T.; Writing--original draft, J.T. All authors have read and agreed to the published version of the manuscript. This research was supported by a Japanese Government Scholarship for PhD studies of the first author, the Sichuan Science and Technology Program (2018 JY0613), Grants-in-Aid for Scientific Research of Japan Society for the Promotion of Science (JSPS), and Research Fellowships of Japan Society for the Promotion of Science for Young Scientists. The authors would like to thank the reviewers for constructive comments that improved the paper. The authors declare no conflict of interest.