## 5 Conclusions  
In this study, through the indoor direct shear test, the influence law of the slurry concentration and standing time on the friction characteristics of the pipe-soil interface was explored, and the following five conclusions were obtained.  
The concentration changes the location of shear failure. When the concentration is greater than or equal to 25%, the shear failure occurs in the slurry interior; when the concentration is less than or equal to 12%, the shear failure occurs between the slurry soil mixture and the pipe wall; and when the concentration is more than 12% and less than 25%, the shear failure occurs between the pipe wall and the slurry.  
Different concentrations of slurry have different effects on the peak shear stress and friction coefficient of the pipe-soil interface. For the pipe-soil interface with a slurry concentration of less than or equal to 20%, the peak shear stress depends on the structural shear strength of the interface, and the friction coefficient depends on the degree of friction between the slurry and the pipe wall. For the pipe-soil interface with a slurry concentration greater than 20%, the peak shear stress depends on the gel strength of the slurry, which is the static shear stress, and the friction coefficient depends on the internal friction of the slurry.  
When the concentration is less than or equal to 20%, the peak shear stress and friction coefficient first decrease and then increase with the increase in the concentration. When the concentration is more than 20%, the peak shear stress and friction coefficient increase with the increase in the concentration.  
When the concentration is unchanged, within a certain standing time range, the peak shear stress and friction coefficient of the interface increase with the increase in the standing time, and then are unchanged. When the standing time is greater than or equal to 6 h, the peak shear stress and friction coefficient increase with the increase in the concentration. Under the same standing time, the amplification of the peak shear stress and friction coefficient of the interface with different slurry concentrations increase with the increase in the concentration.  
Unreasonable control of the slurry concentration and long standing times will increase the risk of the jacking force surging and adversely affect pipe jacking construction. Slurry with a concentration range of 14-16% has a better lubrication effect and is less affected by the standing time. Although the friction coefficient of the pipe-soil interface of the slurry within this concentration range belongs to the range of 0.1-0.3, the friction coefficient is slightly larger. Therefore, it is suggested that the slurry concentration should be within 16% and lubricants, such as polyacrylamide and graphite powder, should be added to further reduce the friction coefficient of the pipe-soil interface.  
Conceptualization, C.Z. and P.Z.; methodology, C.Z. and P.Z.; software, A.X.; validation, A.X., K.L. and Z.C.; formal analysis, A.X. and K.L.; investigation, Z.C.; resources, H.A.; data curation, K.L.; writing--original draft preparation, A.X.; writing--review and editing, A.X.; visualization, Z.C.; supervision, Z.C.; project administration, P.Z. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The authors declare no conflict of interest.