## 3 Results
### Influence of the Tilt Angle  
#### 3.3.1 Influence of the Tilt Angle on Water Movement  
To show the water movement and its lateral diversion length in a multi-layer slope, the slope surfaces were marked with blue colors before the rainfall. With the aid of the backlighting and the dye traces, it was simple to visualize the dyed streamlines with the water movement once a steady state was achieved. The lateral diversion length and breakthrough zone could also be measured directly. Figure 11 shows photographs of the dyed trace streamlines from single-layer and multi-layer  
Figure 11: Volumetric water content, pore water pressure, and tilt angle change trends at different locations during infiltration. (**a**) Case III (tilt angle \(\alpha\) = 15\({}^{\circ}\), multi-layer), where the pore water pressure increased after capillary barrier breakthrough; (**b**) case VII (tilt angle \(\alpha\) = 15\({}^{\circ}\), single layer). For the positions of sensors, see Figure 3b.  
slopes with different tilt angles. The coarse layer is shown in deep gray in the pictures. For all of the inclined experimental cases, the dye traces were diverted downslope (referred to as capillary diversion, see Figure 12) and, in most cases, penetrated the coarse layer at different points (breakthrough). The slope at 7\({}^{\circ}\) with a breakthrough zone maintained a constant value when infiltration reached a steady state. In these three cases, a clear lateral flow region without infiltrate water passing through the coarse layer formed near the toe of the coarse layer. An amount of water entered the bottom layer through the breakthrough zone. Additionally, the breakthrough region was measured as the total length along the interface through which breakthrough was observed. This will be referred to as the piping in the subsequent discussion.  
Figure 13 shows the lengths of the three cases observed, and the capillary diversion length occurring upslope on the fine-coarse interface. The length of capillary diversion was measured from the initial point of the blue dye trace closest to the upper interface to the point where the dye first penetrated into the coarse layer. Figure 14 shows the VWC contour maps at 0.5 h, when failure occurred, rainfall stopped (t = 3.5 h), and the drying process stopped (t = 36 h) for case I, II, III, and VII. The initial VWC values were about 0.06 in the slopes.  
In Figure 14a,b, comparing the VWC in the flat group at t = 0.5 h, the area above the interface reached a higher degree of saturation in the multi-layer slope than in the single-layer slope, since the capillary barrier prevented the water from infiltrating into the coarse layer, which made the slope more stable and caused a delay in the failure time. Comparing Figure 13, it was proven that lateral diversion occurred along the interface in the inclined slope, which resulted in a higher water content near the toe of the slope, and water could not infiltrate into the lower finer layer in Figure 13. This result may be associated with the sloping of the cover system [24]. The lateral diversion in the coarse layer was around 0.4 m, possibly because of the inclined angle, material properties, and rainfall intensity used in the test. Afterward, water could infiltrate into the deeper finer layer at the end of lateral diversion [26].  
Figure 12: Side view of model experiments with blue dye traces during rainfall for the slopes in the inclined group. (**a**) case II (tilt angle \(\alpha=7^{\circ}\)); (**b**) case III (tilt angle \(\alpha=15^{\circ}\)); (**c**) case VI (tilt angle \(\alpha=21^{\circ}\)); (**d**) case VI (tilt angle \(\alpha=21^{\circ}\)); (**e**) case VII (tilt angle \(\alpha=21^{\circ}\)); (**f**) case VIII (tilt angle \(\alpha=21^{\circ}\)). The capillary barrier diversion in the multi-layer slope is shown by the movement of blue dye traces. The white arrow shows the approximate flow direction (case II and case VI).