5.2.3　Film Formation Law of High Permeability Form...

5.2.3　Film Formation Law of High Permeability Formation Mud

Due to the complex formation conditions of shield tunneling，the slurry ratio will be adjusted according to the actual situation in the actual construction.Although the different proportion leads to the difference of the basic properties of the mud，the law of the penetration film is consistent.When bentonite is used as the base slurry，clay minerals generally form flocs in the mud.When this floc is larger than the formation pore，the flocs are blocked on the surface of the formation under mud pressure.The subsequent pressure makes the floc dehydration and produces a similar effect as consolidation.The water in the floc is discharged and infiltrated into the formation，and the solid particles gather on the surface of the formation to form a dense mud.When the formation pore is large（when the average pore diameter is about 60μm and k is greater than 1.0 x 10-1 cm/s），the floc can also be squeezed into the pore by pressure.However，these flocs may be intercepted and blocked in the small part of the pore diameter.If there are more（amount）of such flocs in the mud，the pores will be blocked first，and then the flocs will gather on the surface to form a mud film.Therefore，the film-forming type of mud is related to the particle size and formation pore of mud.In order to obtain the universal film-forming law of mud in sand stratum，the orthogonal experimental design is adopted to conduct the film-forming experiment of mud，and the film-forming law is summarized by factor analysis.

5.2.3.1　Mud Parameters

The mud ratio is designed according to three levels and four factors orthogonal table，there are nine groups.Orthogonal arrangements are shown in Tables 5.7 and 5.8.

5.2.3.2　Strati Graphic Parameters

The high-permeability sand is used in the stratum，and the sand used in the test is sand with a single particle size range，which is obtained by natural sand screening.The sand was stratified into the test vessel in strict accordance with the controlled dry density of 1.5 g/cm3，and the permeation test of the normal head was carried out.The formation permeability coefficient is shown in Table 5.9.

5.2.3.3　Test Analysis

Nine kinds of mud in the orthogonal table were used to test in five formations，a total of 45 groups.The changes of filter water with time，the type of mud film and the thickness of mud film were recorded.

1）No.1 formation

Nine kinds of mud in No.1 formation seepage test process and results are very similar，that is，after each stage load is applied，the filter water is very small，and the filter water is stable quickly，and the total filter water under the sixth stage load is also very small.Finally，nine kinds of mud are formed on the surface of the formation.The curve of filter water per unit area versus time in No.1 formation is shown in Fig.5.27.

In this kind of mud can only form the formation of mud film，as long as the mud has the following two conditions can form high quality mud film.

（1）There are enough powder-stick particles in the mud that can silt the pores on the surface of the formation，and at the same time have a lower sand content.

（2）The mud has high viscosity and good physical stability，which makes the solid particles and water in the mud have strong binding force，and the mud is not easy to be separated and dehydrated under pressure.

2）No.2 formation

The curve of filtration water per unit area with time in No.2 formation is shown in Fig.5.28.When No.2 mud is used for mud filtration test，a large amount of filtration water is produced when the primary load is added，and some mud penetrates into the formation.With the increase of mud pressure，the production of filtration water is less and the stability speed is faster.Finally，some mud particles penetrate into the formation，and some mud particles are blocked on the surface of the formation，forming a thick mud film.The other eight kinds of mud form solid particle thin layers on the formation surface.

3）No.3 formation

The curve of filter water per unit area of No.3 formation with time is shown in Fig.5.29.No.3 mud after the sixth stage load，completely lost，filtered water turbidity，mud failed to form any form of mud film in the formation.During the test ofNo.6 and No.9 mud，a thin layer of solid particles was formed on the formation surface.Some particles in the other mud could penetrate into the formation，while some particles could be silted up on the formation surface to form a thin mud film.

The permeability coefficient of No.3 formation is large，and if the mud does not match the formation，the mud will appear excessive filtration and even unable to film.In order to avoid this situation and ensure that the mud can form a good quality mud film on the excavation surface，the mud must meet the following requirements：

（1）The mud must contain a certain amount of powder or silt particles to ensure that the effective particle size of the mud is large enough to block the formation pores.

（2）Due to the large effective particle size of the mud and the large particle size of the solid particles in the mud，it is necessary to make the mud have a certain viscosity to ensure that the larger solid particles in the mud can be uniformly dispersed in the mud for a long time without precipitation.

4）No.4 formation

No.1-No.5 muds were completely filtrated under the second stage load，and no mud film was formed.Some particles of mud No.6-No.9 penetrated into the formation，and some particles were silted up on the surface of the formation，forming a mud film with zone（Fig.5.30）.

The permeability coefficient of No.4 stratum is large，and the mud is not easy to form film in this stratum.In order to ensure that the mud can form a good mud film on the excavation surface，the mud must meet the following requirements：

（1）The mud must contain a certain amount of particles with a large particle size as a leak-blocking particle to block the largepores in the formation.

（2）The mud must contain a certain amount of powdered particles to further silt the formation pores that have been smaller than the leak particles.

（3）Because the mud contains a large particle size of large leakage particles，and whether the leak particles can be spread evenly in the mud for a long time to determine whether the mud in the formation can be filmed，so it is necessary to ensure that the mud has a large degree of viscosity，so that the particle size of large leakage particles for a long time uniform lysing in the mud，no precipitation phenomenon.

5）No.5 Formation

No.1，No.2，No.3，No.4 and No.6 mud were completely filtered under the first load，No.5 and No.7 mud were completely filtered under the second load，No.1-No.7 mud failed to form a mud film，No.8 and No.9 mud formed a mud film with a permeable zone，and the mud film was thin（Fig.5.31）.

The permeability coefficient of No.5 stratum is large，and the mud is very difficult to form a film in the stratum.Only the mud is guaranteed to meet the following requirements.Mud may permit the formation of a mud film on the excavation surface.

（1）The mud must contain a large number of particles large enough to block the leak particles，in order to block the formation of large pores，in this type of osmosis coefficient of the formation，the effective particle size of the mud must be at least more than 180 m.

（2）The mud must contain a large number of powdered particles to further silt the formation pores that have been smaller than the leak particles.

（3）Since the mud contains plugging particles with large particle size，and whether the plugging particles can be uniformly dispersed in the mud for a long time determines whether the mud can form a film in the formation，it is necessary to ensure that the mud has a large viscosity so that the plugging particles with large particle size are uniformly suspended in the mud for a long time without precipitation.

Five groups of experiments show that the density，viscosity and grade of mud have an effect on the formation of mud film.In different formations，the main mud parameters affecting the formation quality of mud film are different.

（1）When the permeability coefficient of the formation is less than 1 x 10-2 cm/s，the solid particles in the mud are difficult to penetrate into the formation due to the small particle size of the formation，and can only be blocked on the surface of the formation to form a mud film.In this kind of formation，the formation of the mud film is easy and the formation speed is the fastest.The main factor affecting the formation quality of the mud film is the proportion of the mud，followed by the particle size of the solid particles in the mud and the viscosity of the mud.

（2）When the permeability coefficient of the formation is between 1 x 10-2 cm/s and 1 x 10-1 cm/s，the particle size of the formation increases slightly.If the particle size of the solid particles in the mud is small and the viscosity of the mud is small，a large amount of mud will penetrate into the formation when the film is formed，resulting in a large amount of filtration.However，once the specific gravity，viscosity of the mud or any index in the particle size of the solid particles is increased，the formation of the mud film will become easy，and the filtration of the mud will be greatly reduced.In this kind of formation，the proportion of mud，viscosity and particle size of solid particles have great influence on the quality of mud film.

（3）When the formation permeability coefficient is between 1 x 10-1 cm/s and 1 cm/s，the particle size of the formation increases further，and the dependence of the mud film forming quality on the particle size of the solid particles in the mud is prominent.The increase of the particle size of the solid particles in the mud（that is，the increase of the effective particle size of the mud）can effectively reduce the filtration of the mud，improve the formation speed and the quality of the mud film.In this kind of formation，the biggest influence on the formation type and quality of mud film is the particle size of solid particles in mud，followed by the proportion of mud，and finally the viscosity of mud（Fig.5.32）.

（4）When the permeability coefficient of the formation is greater than 1 cm/s，in this kind of formation，the general mud will form a large amount of filtration（or even complete filtration），and cannot effectively plug the formation pores or plug the formation surface to form a mud film.The formation of the mud film depends entirely on whether there are enough solid particles in the mud to plug the formation pores or plug the formation surface to form a mud film.In this kind of formation，the size and amount of solid particle size in the mud determine whether the mud can form a film and the quality of the film.The specific gravity and viscosity of the mud play an auxiliary role in the formation of the mud film.The viscosity of the mud plays a very important role，that is，in order to ensure that large solid particles in the mud can be uniformly dispersed in the mud without precipitation，segregation and dehydration.

5.2.3.4　Film Formation of Mud in Various Types of Strata

The formation law of mud film is to reveal the law of what kind of mud can form what kind of mud film in what formation.This section aims to obtain the law of mud film formation in various formations from the perspective of mud and formation matching through the analysis of mud film formation test results.

1）Formation pore diameter

For sand and gravel layers，the formation pore diameter L=0.2D15，whereD15 is the particle size corresponding to 15％of the cumulative curve of formation particle size.All the strata selected in this experiment are sand layers，so the pore diameter of each stratum can be calculated by this method，as shown in Table 5.10.

2）Effective particle size of mud

The effective particle size of the mud is the particle size corresponding to the 85％cumulative curve of the solid particle size in the mud.The effective particle size of the nine mud used in this test is shown in Table 5.8.

3）Film formation regularity of various types of formation mud In this study，forty-five experiments were conducted on five formations of nine kinds of mud shown in Table 5.8 and Table 5.9，respectively.The experimental results（filtration loss of mud during the formation of mud film，formation type of mud film and permeability coefficient of mud film）are shown in Sect.5.2.3.3.

Now taking the formation pore size as abscissa and the effective particle size of mud as ordinate，forty-five film-forming tests are described as shown in Fig.5.33.Each point（a，b）in the figure represents a test，that is，the film-forming test of mud with effective particle size of a in the formation with pore size of b.The square point represents the formation of mud film as mud skin type，the circular point represents the formation of mud film as mud skin+permeability zone type，and the triangle point represents the formation of mud film.

Fig.5.33 shows that the distribution regularity of the test results is strong，the square points are all distributed in the left side of the line G=L，the triangle points are almost all distributed in the right side of the line G=L/2，and the circular points are mostly distributed between the line G=L and the line G=L/2，which means：

When G＞L，the mud forms a muddy mud film in the formation；

When L/2≤G≤L，the mud forms a mud film+permeable belt type mud film in the formation；

When G＜L/2，the mud cannot form a mud film in the formation.

Among them：G is the effective particle size of the mud，that is，the particle size corresponding to the 85％cumulative curve of the solid particle size in the mud；L is the pore diameter of the formation，and for the sand and gravel layer，the pore diameter of the formation isL=0.2D15，where inD15is the particle size corresponding to the formation curve of the formation particle size of 15％.