## 6 Influencing Factors of the Strength Recovery  
### Relationship between the Vertical Stress and the Strength Recovery  
The undisturbed soil samples were sheared after the completion of consolidation at 50 kPa, 100 kPa, and 200 kPa under natural water content, and then, after sequential pauses of 3 h, 6 h, 12 h, 24 h, and 72 h, they were sheared again. The stress-displacement curves were then obtained with different vertical stress, as shown in Figures 7-9. The slide zone soil exhibits obvious signs of strength recovery; there is a clear peak at very small displacement when the sample starts shearing again, and when the displacement increases further, the strength decreases rapidly to the residual strength. The initial peak shear strength and residual strength under the three pressures are 65.20 kPa and 60.60 kPa, 74.13 kPa and 68.59 kPa, and 125.74 kPa and 113.00 kPa, respectively. The results indicate that the shear strength increases with increasing vertical stress. Under the same vertical stress, as the healing time increases, the shear strength becomes higher than the residual strength of the previous shear, indicating that the slide zone soil exhibits a strength recovery phenomenon. The recovered strength is brittle and disappears quickly after experiencing a very small displacement. The shear curves of the samples after a 3 h healing do not have consistent regularity. This may be because, after the initial shear is completed, a healing time of 3 h is relatively short and the pores of the soil particles and the cohesion at the shear zone are not fully recovered, such that the particles are still in a relatively disordered state, making the strength recovery not obvious. [PERSON] et al. (2016) showed that there was no obvious regularity between the recovery strength value and the healing time when the healing time was short. Therefore, the results of the test with a 3 h healing were not analyzed and discussed, whereas the data for healing of more than 3 h were analyzed and are discussed in detail. Figure 6: Sample morphology after shearing. the previous shearing, and the change in the vertical stress had an effect on the recovery of the shear strength. However, the recovery value of the strength did not depend entirely on the increase or decrease in vertical stress. If the vertical stress increases, the contact area between the soil particles increases. Simultaneously, there is a continuous reduction in the porosity as a result of the continual squeezing and filling of pores by fine soil particles. In addition, particle rearrangement occurs, optimizing the arrangement mode at the sliding surface, leading to a reduction in the porosity and an increase in the compactness. The interaction force between the particles continuously increases, resulting in a recovery of the shear strength upon reshearing. However, the compactness of the soil is limited and the number of pores and pore interconnectors first increases, then decreases, and finally stabilizes [27, 28]. Once the contact area and porosity are saturated, it is highly likely that the peak strength will increase with increasing vertical stress and then stop increasing and stabilize. Therefore, the effect of the vertical stress on the recovery of the soil strength is limited. Table 6 summarizes the strength recovery value and recovery rate corresponding to different healing times for different vertical stresses. Figure 10 shows the relationship between the vertical stress and strength recovery value given different healing times. It can be surmised that, under vertical pressure of 50 kPa, 100 kPa, and 200 kPa, the strength recovery values were 1.2 kPa, 7.03 kPa, and 30.45 kPa given 6 h of healing time; 6.18 kPa, 15.75 kPa, and 38.82 kPa given 12 h of healing time; and 12.35 kPa, 21.11 kPa, and 49.13 kPa given 24 h of healing time, respectively. The strength recovery values were 32.6 kPa, 45.13 kPa, and 67.23 kPa given 72 h of healing time. Given the same healing time, the strength recovery value increased with increasing vertical stress.