## 2 Slurry Penetration Mechanism and Test  
During the construction of excavation engineering projects, bentonite slurry mainly has the following functions [18]: (1) serve as a support medium to balance the underground earth and water pressure and ensure adequate face stability; (2) exchange much from the excavation face to the ground surface; (3) improve the physical properties of soil after slurry penetration; and (4) reduce the friction between soil and excavation tools or machines, and decrease the temperature of excavation tools or machines. The function (1) that originates from oil exploration technology is the most important for bentonite slurry in excavation engineering.  
By means of experiments and theoretical analysis, [PERSON] [19] realized that there are three main types of slurry-soil interaction. The first one describes the stagnation of bentonite particles at the beginning of slurry penetration, which results in the formation of a whole filter cake mode (Type I); the second describes the migration of slurry particles at the beginning of slurry penetration and the following formation of filter cake, which is called the permeability zone plus filter cake mode (Type II); and the third describes the migration of slurry particles with slurry penetration always but without a filter cake on the excavation surface, which is called the whole permeability zone mode (Type III) [20; 21]. Type I mode usually appears in low-permeability cohesive soil. We tried to avoid Type III mode, which means the wrong choice of slurry materials. Type II mode is the common phenomenon in the sand stratum.  
Furthermore, [PERSON] et al. [22] investigated the pressure transfer mechanisms with Type II mode, which distinguished two phases of the process. The first is characterized as the viscous mud spurt, since the slurry penetrates the soil very rapidly in this phase. In the following second phase, when the penetration rate slows down, the bentonite slurry begins to consolidate and the associated formation of filter cake helps to seal the face and transfer the slurry support pressure onto the soil skeleton. This mechanism was also verified by [PERSON] et al. [23].  
Although the slurry easily penetrates into pores in the highly permeable coarse soil, bentonite particles in the slurry may clog the pore space in stratum, which is called the filtration effect [24]. The clogged bentonite particles in the soil can promote the formation of the filter cake. Obviously, slurry penetration, as the fundamental stage, has a great impact on the formation process and properties of the membrane or filter cake.  
Figure 1 shows a filtration column test devices that is widely used to study the process of slurry penetration. The slurry is placed upon a soil sample in a filtration column and penetrated into the soil by applying the pressures via piston or air pressure. The volume of water drainage and pore water pressure along the soil column can be obtained during slurry penetration by measuring the cylinder and pressure gauges installed on the side of the filtration column. Utilizing the filtration column test (Figure 1b), [PERSON] et al. [25] investigated the ability of slurry with various additives to form the lower-permeable filter cake in the highly permeable ground. [PERSON] et al. [20; 21] conducted a series of studies to investigate the filter cake formation during the construction of shield tunnels. [PERSON] et al. [26] eventually demonstrated the relationships between the bentonite concentration of slurry, slurry penetration velocity, and the range of slurry penetration depth. [PERSON] et al. [27] also employed filtration column tests to explore the distribution of bentonite concentration along the soil column and the range of slurry penetration depth by measuring the electrical conductivity of leachate and scanning electron microscope (SEM) qualitatively. However, it is difficult to observe the process of slurry penetration directly in all of the above tests.