## 3 Test Result
### Influence of the Standing Time on the Friction Characteristics of the Interface  
By analyzing the test data of the last stage and considering the slurry performance parameters, 3 groups of slurries with an improved lubrication performance of 14%, 16%, and 18% were selected for the test of this stage. As shown in Figure 7, the standing time had little effect on the \"\(\tau\)-\(s\)\" curve of the interface, and the curve was still the strain softening type. Under the same concentration condition, the \"\(\tau\)-\(s\)\" curve of the interface of the slurry standing times of 24 and 36 h approximately coincided. According to the data shown in Table 3, under the same concentration, the peak shear stress and friction coefficient increased first and then stabilized with the increase in the slurry standing time. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Concentration**} & \multirow{2}{*}{**Standing**} & **Normal** & **Shear Dis-** & **Peak Shear** & **Friction** \\ \multicolumn{1}{c}{**“\(\omega\)”**} & & **Time (h)** & **Load \({}^{\prime\prime}\)\(s\)** & **placement** & **Stress \({}^{\prime\prime}\)\(\tau_{\text{max}}\)** & **Coefficient** \\  & & **(kPa)** & **“\(s\)\({}^{\prime\prime}\) (mm)** & **(kPa)** & **“\(\mu\)\(s\)\({}^{\prime\prime}\)** \\ \hline \multirow{6}{*}{\(14\%\)} & 0 & 100 & 3.562 & 39.467 & 0.28 \\  & 6 & 100 & 3.441 & 41.430 & 0.29 \\ \cline{1-1}  & 12 & 100 & 3.192 & 46.301 & 0.31 \\ \cline{1-1}  & 24 & 100 & 3.142 & 53.050 & 0.34 \\ \cline{1-1}  & 36 & 100 & 3.138 & 53.810 & 0.34 \\ \hline \multirow{6}{*}{\(16\%\)} & 0 & 100 & 3.509 & 37.900 & 0.285 \\  & 6 & 100 & 3.401 & 44.706 & 0.305 \\ \cline{1-1}  & 12 & 100 & 3.170 & 48.500 & 0.33 \\ \cline{1-1}  & 24 & 100 & 2.939 & 53.699 & 0.35 \\ \cline{1-1}  & 36 & 100 & 3.061 & 54.383 & 0.35 \\ \hline \multirow{6}{*}{\(18\%\)} & 0 & 100 & 3.175 & 37.231 & 0.29 \\  & 6 & 100 & 3.009 & 46.368 & 0.32 \\ \cline{1-1}  & 12 & 100 & 2.881 & 50.707 & 0.34 \\ \cline{1-1}  & 24 & 100 & 2.680 & 53.913 & 0.36 \\ \cline{1-1}  & 36 & 100 & 2.646 & 54.439 & 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 \({}^{\prime\prime}\)\(\tau\)-\(s\)”curve, and the friction coefficient is the ratio of the stable value of shear stress to the normal load.  
\end{table} 
Table 3: Friction test data of the pipe–soil interface at different standing times. Figure 7: Friction characteristics of the pipe–soil interface with different slurry standing times. (**a**) “\(\tau\)–s” curve of the pipe–soil interface with slurry with a concentration range of 14%. (**b**) “\(\tau\)–s” curve of the pipe–soil interface with slurry with a concentration range of 16%. (**c**) “\(\tau\)–s” curve of the pipe–soil interface with slurry with a concentration range of 18%. The reason for this change trend was that, on the one hand, the slurry is thixotropic. After standing for a period of time, the viscosity and static shear stress of the slurry increased. For the slurry with a concentration range of 14-18%, shear failure occurred between the pipe wall and slurry. With the further development of shear, part of the slurry damaged by shear overflowed from the upper shear box. The other part was mixed with the slurry that was not damaged by shear and continued to contact with the pipe wall and produce shear. When the shear rate was smaller and the shear displacement was limited, the peak shear stress and friction coefficient increased with time. On the other hand, the thixotropy of the slurry is related to the type and content of the solid particles and whether treatment agent is added. When the contents of bentonite and treatment agent are certain, the viscosity and static shear stress of the slurry will not increase indefinitely with time [21], so the peak shear stress and friction coefficient will tend to be stable. Under the same standing time, the friction coefficient increased with the increase in the concentration.