## 3 Results
### Profile of the Volumetric Water Content in a Slope
#### 3.2.4 Failure Occurred (t = 0.78-1.2 h)  
In this part, the failure process is analyzed using the response of the tilt angle, pore water pressure, and VWC measurements in different locations in the slope. In case III and VII, the experimental flume was tilted to 15\({}^{\circ}\) and subjected to the same rainfall intensity. This section aims to evaluate the effects of an inclined angle on the water movement and slope stability.  
The main results of this part are reported in Figures 10 and 11. The initial pore water pressure values were negative at the locations of S-I and S-II. The pore water pressure and VWC increased during the infiltration and then stabilized when the infiltration reached a steady state condition. During the rainfall, the tilt angle of point G and point A almost maintained a constant value, while the soil in these locations maintained a negative value. A sudden change occurred when the pore water pressure approached 0 kPa at point L in all cases.  
Comparing the difference of the pore water pressure measured at S-II in case I and case V, presented in Figure 10a,b), the pressure grew from \(-4\) kPa at the beginning of the rainfall to around \(-1\) kPa after around 1.2 h above the coarse interface, while it reached \(-3.5\) kPa in case V. This suggests that there was a higher water content and pressure head above the coarse layer than at the same location in a single-layer slope, as the capillary barrier effect led to the storage of water and high water pressure head. In this respect, it is worth noticing that in case I, the pore water pressure at point L (S-I) increased suddenly after capillary barrier breakthrough at the interface.  
Figure 9: Profile of the VWC in section I in different cases (the VWC distribution after slope failure was verified by obtaining soil samples from various locations at the end of the experiments). (**a**) Case I; (**b**) Case II; (**c**) Case III; (**d**) Case V; (**e**) Case VI; (**f**) Case VII.  
The same behavior, in terms of the pore water pressure and VWC, was observed in tests with the tilt angle \(\alpha=15^{\circ}\) (Figure 11). In these two cases, the greater slope angle induced the most sudden variations of water content and pore water pressure, and also resulted in an earlier response of tilt sensors located at the toe of the slope. For case III with the tilt angle \(\alpha=15^{\circ}\), failure occurred in about 0.78 h, and the increase of VWC was more rapid than that of case VII, which was contrary to case VII. In the inclined group, the later failure occurred at 0.84 h in a single-layer slope, which proved that an inclined multi-layer slope is more dangerous than a single-layer one under this condition, which is contrary to the findings of the flat group.  
It is evident in Figures 10a and 11a that the increase of the pore water pressure can be divided into two periods. A negative pore pressure was measured at the beginning of rainfall (S-I), and then, around 30 min later, a sudden increase of the pore water pressure value \(\Delta\)u could be measured after capillary barrier breakthrough at point L (Figure 3b). Moreover, a few minutes after breakthrough, piping occurred for a pore pressure of 0 kPa at the toe, which seems to indicate a negative influence for the slope instability. Similar to case I, pore pressure sensor S-I in case III also showed the same trends after the breakthrough of the capillary barrier. It is clear from both data sets that the pore water pressure value increased from \(-1.5\) to \(-0.3\) kPa in case I and from \(-1.1\) to \(-0.21\) kPa in case III, respectively.  
Figure 10: Volumetric water content, pore water pressure, and tilt angle change trends at different locations during infiltration. (**a**) case I (tilt angle \(\alpha=0^{\circ}\), multi-layer), where the pore water pressure increased after capillary barrier breakthrough; (**b**) case V (tilt angle \(\alpha=0^{\circ}\), single layer). For the positions of sensors, see Figure 3b.