## 4 Results and Discussion
### Major Cracks and Stress Variations
#### 4.2.3 Rationality and Correctness of the Test Method to Monitor the Evolution of the Major Cracks  
FLAC3D was used in this study to conduct numerical simulation of the test loading process (as shown in Figure 7). The slope model was divided into three parts: slip mass, slip surface, and slide bed. The parameters are shown in Table 1. The shear strength parameters of these three parts met the requirements of the smallest slip surface, followed by the slip mass and the largest slide bed [61]. In Table 1, the parameters of the slip mass were obtained from the normally consolidated drained triaxial compression tests of remolded clay samples, and the parameters of the slide bed were referenced from [PERSON] et al. [62]. Before the occurrence of cracks, the soil slope can be regarded as a continuum without stress concentration [18], and its critical sliding surface is usually determined by the peak shear strength of the clay. The most influential factors on the characteristics of cracked clay are the direction and strength of cracking [63], and the available shear strength of cracks is equal to the peak strength of the normally consolidated drainage specimens [64]. In this study, the maximum difference between the simulated horizontal displacement of the slope shoulder and the measured displacement was 2.60 mm (140 kN), the maximum difference between the horizontal stresses at the slip surface was 8.68% (140 kN at point 31), and the difference between the horizontal stresses at the other measuring points was no more than 4.13%, indicating that the numerical simulation results were in good agreement with the measured results. Figure 7: Plastic zones in the model soil slope for (**a**) 100, (**b**) 120, (**c**) 140, and (**d**) 150 kN. The plus signs “+” indicate the stress monitoring points in the model test. Uniaxial and triaxial tests of clay samples have shown that fissured clays and unfissured clays have two fracture modes, namely opening and sliding, but they are different in diverse parts of the samples [49; 60]. As shown in Figure 7, in this test, the trailing edge crack was of the tension-shear type, while the front edge crack was of the shear type. In this experiment, the front edge of the bearing plate appeared as a diagonal crack with a biased downslope direction. This might have been caused by the rotation of the principal stress axes of the soil units during the loading process, where the crack deviated from the vertical direction with deepening. The M-C criterion is an ideal plastic criterion. When reaching tensile strength or shear strength, the soil yields. Although the development law of plastic zone in the soil slope is basically consistent with crack propagation, the experimental phenomena of crack generation and the numerical simulation in Figure 7 show that yielding is a necessary but insufficient condition for crack generation. The progressive failure process of clay slope is more sensitive to changes in horizontal stresses. In this study, stress saltation was obtained by horizontal stress increment analysis, and the cracking of soil was then analyzed. On this basis, the locations and depths of the major cracks were obtained. Taking into account the different changes in \(\Delta\sigma_{3}\) at different measuring points of diverse depths, the discontinuity of \(\Delta\sigma_{x}\) saltation in the stress transfer process between the two adjacent measuring points near the crack is represented by the changes from a negative value to an even smaller negative value, from a positive value to a negative value, or from zero to a negative value. This can be illustrated by the changes in horizontal stress increments between points 12 and 13, 22 and 23, and 32 and 33 during the front edge crack propagation. The location and depth of a crack are critical to slope stability, but the location of the crack has the greatest influence [28], and in most cases, the depth of the crack cannot be accurately estimated [27].