# Monitoring and Data Analysis in Small-Scale Landslide Physical Model
## 4 Results
### Water-Related Sensors  
In the series of tests applied to the sandy slope, only soil moisture sensors and one soil potential sensor, i.e., TEROS 21, were used to observe VWC. Positions of the embedded system of TEROS sensors and MEMS accelerometers are presented in Figure 14. The sensors are positioned along the central line of the slope to avoid the possible effect of plexiglass sides of the model on measured values. Locations of the sensors are chosen as the critical points in the slope model for observation of slope movements and changes in moisture content (Figure 14). Sensors are built-in on different depths along the same profile to provide data at the same cross-section that enables validation and numerical analysis of observed landslide initiation. The boundary water drainage conditions in all three tests allowed the forming of the water table in the foot of the slope. The failures occurred in the same way, by rising of the ground water table and the reduction in the effective stresses till the foot of the slopes became submerged. The times to the failures in the models at different slope inclination are different because of different initial moisture contents in the slopes and different rainfall intensities that impacted on reaching of failure slope conditions, as seen in Figure 15. That is most obvious in presented results observed at the Test 1; the first sign of the landslide initiation was observed more than two hours after rainfall was started. In Tests 2 and 3, more similar rainfall intensities are applied, and much closer time periods to the failure were needed because of much higher inclination of the slope and consequently earlier establishment of ground water level in the slope. Figure 14: Positions of the embedded sensor network in the slope model: yellow triangles—TEROS 12, red dots—TEROS 21, blue squares—TEROS 10, and black squares—MEMS accelerometers. The antecedent rainfall conditions are reflected in the initial water content (or pore water pressure) distribution at the beginning of the rainfall event. In physical tests with small-scale landslide models, material is usually placed to achieve the desired initial dry density and water content conditions. For slopes consisting of sandy soils, as is the case here, tests are typically conducted starting with a low initial gravimetric soil water content (e.g., 2% in Tests 1, 2, and 3). Reduced water content results in increased matric suction in a small-scale sandy slope, resulting in an increase in effective stress and soil shear strength. At the same time, the reduced water content decreases the hydraulic conductivity of the sandy slope with respect to the water phase. In this sense, the initial water content distribution affects both the hydraulic and mechanical response of the slope. Figure 15: Diagrams of rainfall intensity during the tests with slope inclination at: (**a**) 30\({}^{\circ}\); (**b**) 35\({}^{\circ}\); (**c**) 40\({}^{\circ}\). Sensors in all conducted tests were installed in the same manner and at the same positions as is presented in Figure 14. Sensors are positioned at depths of 6, 12, 18, and 24 cm below the surface, but the results at the depth of 12 cm are not presented as the least important, Figure 16. In comparison to initial moisture content of 2% that corresponds to VWC of 0.03 m\({}^{3}\)/m\({}^{3}\) at the moment of sand material preparation, all sensor measurements showed slightly higher initial values of VWC from the start of the test, most noticeably in the lowest part of the slope (see Test 2 at the sensor at 24 cm depth, Figure 16f). In the early stages of the tests, faster rising of the VWC was observed in shallow layers in all three parts of the slope (Figure 16a,d,g). Rising of the ground water level in the foot of the slope is noticeable at the deepest sensors (L--24 cm), in Test 1 after 28 min of rainfall (Figure 16c), in Test 2 after 14 min of rainfall (Figure 16f), and in Test 3 after 28 min of rainfall (Figure 16i).