## 6 Influencing Factors of the Strength Recovery
### Relationship between the Vertical Stress and the Strength Recovery
At low pressure, the strength recovery value increased less, while the strength recovery value increased significantly at high pressure. With increasing vertical stress, the curves of the strength recovery values for each healing time show an increasing trend, which indicates that the vertical stress has an effect on the strength recovery. Meanwhile, the relationship between the healing time and the strength recovery value given the same vertical stress indicates that, for increasing healing time, the strength recovery value increases; the strength recovery value reached the highest value for a healing time of 72 h. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Vertical Stress (kPa)**} & \multirow{2}{*}{**Indices**} & \multicolumn{5}{c}{**Healing Time (h)**} \\ \cline{3-6}  & & **3** & **6** & **12** & **24** & **72** \\ \hline \multirow{3}{*}{50} & Strength recovery value & \multirow{3}{*}{3.55} & \multirow{3}{*}{1.20} & 6.18 & 12.35 & 32.60 \\  & & & 0.52 & 0.51 & 0.45 \\ \cline{2-6}  & Recovery rate & 1.15 & 0.85 & 0.52 & 0.51 & 0.45 \\ \hline \multirow{3}{*}{100} & Strength recovery value & \multirow{3}{*}{0} & 7.03 & 15.75 & 21.11 & 45.13 \\ \cline{2-6}  & Recovery rate & 0 & 1.17 & 1.31 & 0.88 & 0.63 \\ \hline \multirow{3}{*}{200} & Strength recovery value & \multirow{3}{*}{18.20} & 30.45 & 38.42 & 49.13 & 67.23 \\ \cline{2-6}  & Recovery rate & & 6.20 & 5.07 & 3.26 & 2.05 & 0.93 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Recovery strength value and recovery rate indices. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Healing Time (h)**} & \multirow{2}{*}{**Vertical Stress (kPa)**} & \multirow{2}{*}{**50**} & \multirow{2}{*}{**100**} & \multirow{2}{*}{**200**} \\ \hline
6 & 61.80 & 75.62 & 143.45 \\
12 & 66.78 & 84.34 & 151.82 \\
24 & 72.95 & 89.70 & 162.13 \\
72 & 93.10 & 113.72 & 180.23 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Peak strength (kPa). The relationship between the vertical stress and the strength recovery rate given different healing times is shown in Figure 11, which shows that the strength recovery rate increases with increasing vertical stress given the same healing time and exhibits a positive correlation. This is mainly due to the increase in the vertical stress, which increases the density of the soil particles on the shear surface and the occlusion force between the particles. With increasing vertical stress and healing time, the slope gradually becomes smaller. When the vertical stress increases from 50 kPa to 200 kPa, the recovery rate increases by 4.22 kPa/h, 2.72 kPa/h, 1.53 kPa/h, and 0.48 kPa/h given healing time of 6 h, 12 h, 24 h, and 72 h, respectively. This indicates that the increment value of recovery rate gradually decreases with increasing vertical stress and healing time. The increase in the vertical stress and the healing time inevitably leads to an increase in the contact area and the contact time between the soil particles, an enhancement of the interactions between surfaces, an increase in the friction between particles, and an increase in the shear strength. On the other hand, the pores between the soil particles are determined by the minerals and the property of the soil itself. So, the soil porosity ratio and the degree of compactness of the contact surface are limited. Even given a higher vertical pressure and longer healing time, the strength recovery value is limited under a gradual change in compactness. When the soil particles at the shear surface reach absolute compactness, the contact area of the particles reaches saturation. Even if the healing time is longer, the strength recovery value is saturated and the strength recovery rate will be reduced or even stop. In summary, with the increase in vertical stress, the strength recovery rate shows an increasing trend, but the increasing amplitude tends to be stable with the increase in healing time. Figure 12 shows the SEM (50\(\times\)) characteristics of the shear surface after completing the \"shear-pause-shear-pause-shear\" cycle.