## 3 Test Result
### Influence of the Slurry Concentration on the Friction Characteristics of the Interface
#### 3.1.1 Concentration Less Than or Equal to 20%  
The test data listed in Table 2 show that the peak shear stress (\"\(\tau_{\text{max}}\)\") first decreased and then increased with the increase in the concentration, which is less than or equal to 20%. This was because the slurry with a concentration less than or equal to 12% did not support the soil well, resulting in contact between the slurry soil mixture and the pipe wall during shearing, and the occurrence of shear failure between the slurry soil mixture and the pipe wall (as shown in Figure 6a). With the increase in the slurry concentration, the contact degree between the sand and pipe wall was reduced and the supporting effect of the soil improved. The shear failure occurred between the pipe wall and slurry (as shown in Figure 6b,c), and the shear stress required to produce relative sliding decreased, so \(\tau_{\text{max}}{}^{\omega=10\%}>\tau_{\text{max}}{}^{\omega=12\%}>\tau_{ \text{max}}{}^{\omega=14\%}>\tau_{\text{max}}{}^{\omega=16\%}>\tau_{\text{ max}}{}^{\omega=18\%}\). However, with the increase in the concentration, the viscosity of the slurry increased and the degree of adhesion to the pipe wall increased, so \(\tau_{\text{max}}{}^{\omega=20\%}>\tau_{\text{max}}{}^{\omega=16\%}>\tau_{ \text{max}}{}^{\omega=18\%}\).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Normal Load** & \begin{tabular}{c} **Shear** \\ **Displacement** \\ **“\(\sigma\)” (kPa)** \\ \end{tabular} & \
\begin{tabular}{c} **Peak Shear** \\ **Displacement** \\ **“\(\sigma\)” (mm)** \\ \end{tabular} &
\begin{tabular}{c} **Friction** \\ **(kPa)** \\ \end{tabular} \\ \hline soil & 100 & 3.075 & 56.155 & 0.48 \\ \(\omega=10\%\) & 100 & 3.701 & 45.149 & 0.38 \\ \(\omega=12\%\) & 100 & 3.295 & 44.413 & 0.35 \\ \(\omega=14\%\) & 100 & 3.562 & 39.467 & 0.28 \\ \(\omega=16\%\) & 100 & 3.509 & 37.896 & 0.285 \\ \(\omega=18\%\) & 100 & 3.175 & 37.231 & 0.29 \\ \(\omega=20\%\) & 100 & 3.578 & 40.770 & 0.31 \\ \(\omega=25\%\) & 100 & 3.515 & 39.962 & 0.29 \\ \(\omega=30\%\) & 100 & 3.488 & 42.962 & 0.32 \\ \(\omega=35\%\) & 100 & 3.003 & 46.546 & 0.34 \\ \(\omega=40\%\) & 100 & 2.804 & 50.218 & 0.36 \\ \hline \hline \end{tabular} Note: the shear displacement in the table is the shear displacement corresponding to the peak shear stress in the “\(\tau\)-\(s\)” curve, and the friction coefficient is the ratio of the stable value of shear stress to the normal load.  
\end{table}
Table 2: Friction test data of the pipe–soil interface with different concentrations.  
The friction coefficient (\"\(\mu\)\") first decreased and then increased with the increase in the concentration, which is less than or equal to 20%. This occurred because the lower-concentration slurry did not support the soil and did not have a good lubrication effect. With the increase in the slurry concentration, the \"barrier-lubrication\" effect of the slurry improved, so \(\mu^{\omega=14\%}<\mu^{\omega=12\%}<\mu^{\omega=10\%}\). However, the further increase in the slurry concentration increased the viscosity of the slurry and reduced its lubricity. Therefore, for slurry to have a good lubrication effect, it must have a good supporting role.