## 4 Results and Discussion
### Major Cracks and Stress Variations  
#### 4.2.1 Evolution of Major Cracks with Horizontal and Vertical Stresses  
The purpose of EPC 51 was to monitor the stress changes in the slip surface at the toe of the slope. During the test loading process, EPC 42 was damaged, and no valid data were collected. Therefore, we took the horizontal stress changes in the first three layers and the slip surface for analysis, as shown in Figure 5. The development of stresses also showed progressiveness. In order to compare the four stages of displacements with the development stages of stresses, 40, 100, and 140 kN are given in Figure 5. In order to investigate the relationship between stress changes and displacement development more intuitively and to avoid repetition in the meantime, the slope shoulder displacement monitored by the displacement gauge is only marked in Figure 5a. As can be seen in Figure 5, as the load increased, the horizontal stress showed an overall increasing trend. The horizontal stress curves had relatively obvious inflection points at 100 and 140 kN (corresponding to inflection points A and B, respectively), and the curves displayed a more obvious three-stage process. In Table 2, analysis of the slopes of the horizontal stress curves of the first three layers showed that, except for the measuring points 11, 21, and 31 constrained by the left slip surface, overall, the closer to the center of the bearing plate, the faster the increase in horizontal stress. As shown in Figure 5a-c, compared to other measuring points, the horizontal stress values of the measuring points directly under the bearing plate were larger overall. These findings demonstrate that in the entire soil slope, the distribution characteristic of the horizontal stresses was large at the back and small at the front. For the convenience of the following description, the area directly under the bearing plate is defined as the \"loading zone\". Figure 5: Relationships between horizontal stresses and loads for (**a**) the first layer, (**b**) the second layer, (**c**) the third layer, and (**d**) along the slip surface. The first stage (corresponding to compression stage and uniform stage of displacement) was 0-100 kN. Before inflection point A, the horizontal stresses of each measuring point showed a linear increase. The third stage (corresponding to the failure stage of displacement) was 140-150 kN, and landslide occurred at this stage. After the inflection point B, the horizontal stresses of each measuring point dropped significantly, which is shown in Table 2 as negative slope of curve values. The second stage (corresponding to acceleration stage of displacement) was 100-140 kN. After inflection point A, horizontal stresses at this stage generally increased rapidly, as shown in Figure 5 and Table 2. In the 100-140 kN stage, the trailing edge crack continued to deepen and widen until it reached the slip surface. Yield failure occurred in the soils near the trailing edge crack. When loading continued, the stress tended to transfer from the loading zone to the soils without yield failure in the direction of the slope surface, and the front edge crack appeared later (at 120 kN). Therefore, only a few measuring points (such as measuring point 12 in Figure 5a, point 23 in Figure 5b, and points 32 and 33 in Figure 5c) inside the soil exhibited small inflection points at 120 kN. This phenomenon is mainly caused by stress concentration in the vicinity of the crack [42,60]. Compared to other measuring points inside the soil, measuring points 11, 21, 31, and 41 along the slip surface were more sensitive to the appearance of front edge crack, which is depicted as small inflection points at 120 kN in Figure 5d. Combining the displacement characteristics of the slope, it can be seen that the appearance of front edge crack aggravated the progressive failure of the slope along the slip surface to some extent, and the appearance of small inflection points on the horizontal stress curves along the slip surface was the response to the front edge crack.