## 4 Discussion  
Considering that the time interval of normal re-jacking on site generally does not exceed 6 h, by comparing the data in Table 4, it was found that the amplification of the peak shear stress and friction coefficient of the interface with a slurry concentration equal to 18% was the greatest. However, excessive amplification of the friction coefficient and peak value of the shear stress will lead to a significant increase in the jacking force required for re-jacking, which is highly likely to lead to surging of the jacking force and increase the difficulty of re-jacking. Combined with the influence law of the concentration and standing time on the friction characteristics of the pipe-soil interface, the slurry with a concentration range of 14-16% showed a better lubrication effect and was less affected by the standing time. According to the test data, the friction coefficient of the pipe-soil interface first decreased and then increased with the increase in the slurry concentration. This is because when the slurry concentration was less than or equal to 12%, the slurry did not support the soil well, resulting in contact between the slurry soil mixture and the pipe wall during \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Standing**} & \multicolumn{2}{c}{**Amplification of Peak Shear Stress**} & \multicolumn{2}{c}{**Amplification of Friction Coefficient**} \\ \cline{2-7}  & \(\mathbf{\omega}\) = **14\%** & \(\mathbf{\omega}\) = 16\%** & \(\mathbf{\omega}\) = 18\%** & \(\mathbf{\omega}\) = 14\% & \(\mathbf{\omega}\) = 16\% & \(\mathbf{\omega}\) = 18\% \\ \hline
6 h & 5\% & 18.0\% & 28.3\% & 3.6\% & 7.0\% & 10.3\% \\
12 h & 17.3\% & 28.0\% & 36.2\% & 10.7\% & 15.8\% & 17.2\% \\
24 h & 34.4\% & 41.6\% & 44.8\% & 21.4\% & 22.8\% & 24.1\% \\
36 h & 36.34\% & 43.5\% & 46.2\% & 21.4\% & 22.8\% & 24.1\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Amplification of the peak shear stress and friction coefficient of the pipe–soil interface with different standing times. shearing, and the friction coefficient of the interface was larger. When the slurry concentration increased, the \"barrier-lubrication\" effect of the slurry was improved, so the interface friction coefficient decreased. With the further increase in the slurry concentration, the adhesion between the slurry and pipe wall increased, so the friction coefficient increased. Within a certain standing time range, the interface friction coefficient increased with the increase in the standing time. This is because slurry is thixotropic. The standing time changes the state of the slurry, changing the slurry from a flowing state to a flocculation state. The viscosity of the slurry increases and the adhesion degree with the pipe wall increases, so the friction coefficient also increases. This is also almost consistent with [PERSON]'s conclusion regarding the influence of the pipe jacking stagnation time on the pipe-soil interface [22]. [PERSON] (2006) proposed an equation to calculate the jacking force as follows [23]:  
\[JF_{frict}=\mu_{\text{int}}\frac{\gamma\cdot r\cdot\cos\Bigl{(}45+\frac{\phi_{r }}{2}\Bigr{)}}{\tan\phi_{r}}\cdot\pi\cdot d\cdot l \tag{1}\]  
where \(JF_{frict}\) is the frictional component of the jacking force, \(\mu_{\text{int}}\) is the soil pipe residual interface frictional coefficient, \(\gamma\) is the unit weight of the soil, \(\phi_{r}\) is the residual friction angle of the soil, \(d\) is the outer diameter of the pipe, \(r\) is the pipe radius, and \(l\) is the length of the pipe. According to the above formula, the frictional component of the jacking force is mainly related to the friction coefficient of the interface. The factors of the slurry concentration and slurry standing time studied in this paper have a significant effect on the friction coefficient of the interface, thus affecting the jacking force.