## 1 Introduction
(2010) studied the friction characteristics of different kinds of lubricants using a simple laboratory test. The test showed that when \"bentonite\" is used as the lubricant, the friction coefficient of the interface is reduced by about 23%; and when \"bentonite + plasticizer\" is used as the lubricant, the friction coefficient of the interface is reduced by about 63% [12]. [PERSON] and [PERSON] (2016) studied the influence of the slurry pressure on the lubricating effect using a laboratory test. The test results showed that when bentonite slurry is continuously injected, even under very low pressure, a lubrication interface is formed between the slurry and the pipe, and the friction coefficient of the interface is reduced by almost 90% [13]. [PERSON] et al. (2019) tested the friction coefficient between sandstone and a concrete pipe under seven different contact conditions using the direct shear test. The test showed that the friction coefficient of the pipe-soil under the contact condition of pure bentonite slurry is the smallest [14]. [PERSON] et al. (2022) performed a direct shear test to study the friction characteristics of seven kinds of soil and concrete pipes with or without lubrication slurry. The results showed that under slurry-lubricated conditions, the friction coefficient of the concrete-sand interface could be reduced by 60-80%, and the friction coefficient of the concrete-silty clay interface and the concrete-clay interface could be reduced by about 90% [15]. In summary, the existing research has mainly focused on the friction coefficient of the pipe-soil interface under different contact conditions, drag reduction slurry formula, and traditional performance parameters. Less research has focused on high-concentration bentonite slurry and the addition of a treatment agent to achieve an improvement, especially regarding the influence of the slurry concentration and standing time on the friction charac teristics of the pipe-soil interface. However, research on the mechanical characteristic of the interface has mainly concentrated on geotechnical engineering, such as pile foundations, retaining walls, and the soil behind the wall. The influence law of different normal loads, mud skin moisture contents, and mud skin thicknesses on the mechanical properties of soil and the structural surface have mainly been studied through direct shear and single shear tests [16; 17; 18; 19; 20]. However, during downgrade jacking of a curved pipe in a water resources transportation project, due to the good fluidity and low viscosity of the traditional slurry, the slurry flows into the tunnel boring machine. In the front pipe, under the action of gravity, and the suction effect of the slag discharge pump of the machine, the slurry is then pumped into the slag discharge pipe, which cannot form a dense mud cake around the pipe wall, resulting in a poor lubrication effect. As such, the compound bentonite is used to prepare a liquid drag reduction slurry and \"concentrated slurry\" on site. The \"concentrated slurry\" is injected into the outer pipe wall of two pipes behind the tunnel boring machine, and the liquid drag reduction slurry is injected into the outer pipe wall of a subsequent pipe to alleviate the flow of traditional bentonite slurry into the tunnel boring machine due to gravity to achieve a better drag reduction effect. After the above grouting method is adopted, although the flow of liquid slurry into the tunnel boring machine is effectively alleviated, the jacking force surges in the normal jacking stage and rejacking stage. The jacking force curve is shown in Figure 1. The above problems may be due to unreasonable control of the slurry concentration during slurry configuration and the long interval of the rejacking construction.