5.2.2　Mud Preparation Technology

5.2.2　Mud Preparation Technology

Taking Nanjing Yangtze River Tunnel as an example，this section introduces the mud preparation technology for the typical high permeability fine sand and gravel composite strata crossing by slurry shield.

5.2.2.1　Normal Excavation Mud Preparation

1）Mud ratio and basic properties

According to the typical fine sand stratum，gravel stratum and round gravel stratum，two groups of mud ratio were designed and film-forming tests were carried out.Generally speaking，the preparation of mud is to mix the basic slurry with bentonite，and add clay and silty clay to adjust the particle size and density of solid particles in the mud.The addition of CMC increases the viscosity of mud and anhydrous Na2CO3 improves the physical stability of mud.Therefore，different contents of clay（silty clay），CMC and anhydrous Na2CO3 were added to the bentonite slurry during construction to prepare slurry with different specific gravity，viscosity and gradation.Among them，the slurry in the silty sand stratum is prepared with the old slurry and five slurry preparation agents provided by North China Oilfield.The old slurry is taken from the slurry used after the slurry circulation of the slurry shield in Nanjing Yangtze River Tunnel.However，due to the small content of fine particles in the gravel stratum，the slurry is prepared by base slurry，clay，fine sand and polymer.The slurry ratio is shown in Table 5.3.

It can be concluded from the basic properties that in the silty sand formation，the density of the first six groups of mud has little change，and the funnel viscosity and plastic viscosity have been improved after adding the slurry preparation agent，among which the physical stability of No.3 and No.5 mud is better.The physical stability of No.7 mud is the worst，and water absorption seriously produces flocculation.There is little difference in mud density between each group in gravelly sand and round gravel formation.The funnel viscosity and plastic viscosity of mud are greatly improved after adding fine sand and polymer，and the more the polymer is added，the greater the mud funnel viscosity.

2）Mud film formation test

（1）Test device.The test adopts self-made mud film forming test device，which is composed of organic glass column，flange plate，beaker，electronic scale，electronic scale data acquisition device，air compressor，pressure gauge and pressure regulating device，as shown in Fig.5.11.The organic glass column simulates the working chamber excavation surface stratum system in the construction，and the upper end is sealed with seal rings，bolts and flange plates.The beaker receives the water discharged from the test，namely the seepage flow.The water is weighed by an electronic scale，and the data acquisition device automatically records the change of seepage flow at different times.The data acquisition time interval can be set by itself.The air compressor provides air pressure to the organic glass column.The air pressure gauge shows the internal pressure of the device，and the pressure regulating device can regulate the air pressure.The organic glass column used in this experiment is 60 cm high，8.4 cm inner diameter，0.5 MPa maximum withstand pressure，0.01 MPa pressure control accuracy of pressure regulator，0.01 g electronic scale accuracy.

（2）Test method.A certain amount of water is added to the organic glass column to saturated stratum.Firstly，the filter layer with particle size of 2-5 mm is added，and then the test stratum is added.Open the drain valve so that the liquid level is aligned with the surface of the formation，and then slowly inject mud into the device along the guide rod to avoid disturbing the formation.The sealing device was used to adjust the pressure regulating device to the test osmotic pressure，and the film forming test was carried out for a certain time under a stable air pressure.The change of filter water was recorded by data acquisition.After the infiltration film is formed，the remaining mud in the organic glass column is poured out，the mud film is taken out，and the thickness of the mud film and the thickness of the infiltration zone are measured.Finally，sample the mud film into the oven to measure moisture content.

（3）The mud film form is shown in Fig.5.12.

Through the test results，the film formation effect of the mud was analyzed，and the mud plan suitable for different formations was finally determined，as shown in Table 5.4.

5.2.2.2　Preparation of Shutdown Opening Mud

At present，the general tunneling distance of river crossing and subsea tunnels constructed at home and abroad is relatively long，and due to the complex geological and hydrological conditions，shield opening inspection is sometimes carried out.In general，the shield is mostly under the condition of high water pressure at the bottom of the river when opening the cabin.At the same time，in order to ensure the safety of the personnel entering the cabin，the setting value of the slurry pressure will be reduced.Therefore，it is very important to study how to effectively use the slurry film to ensure the stability of the excavation face when opening the cabin.In the construction process before the opening of the cabin，the density and viscosity of the slurry used are relatively large，and only mud-skin mud film can be formed on the excavation surface where the shield stops，and no permeable zone can be formed.This mud film is relatively weak，the opening risk is relatively large.In order to ensure the safety of cabin opening operation，it is necessary to adjust the slurry ratio so that the slurry can not only form the mud crust，but also form the permeable zone.The formula of new mud is based on bentonite mud and waste mud，and the reasonable ratio is determined through experiments.

1）Film formation test of pure bentonite slurry

Four groups of mud were used in the experiment，and sodium bentonite was used for expansion.The ratio was 3％，5％，8％and 12％，and the expansion time was 24 h.In this experiment，the most unfavorable gravel sand layer for film formation is selected as the test stratum，and the coarse particles larger than 2 mm account for about 40％of the whole stratum content，and the permeability coefficient of the stratum in the test is about 3.95 x 10-2 cm/s，which is basically consistent with the permeability coefficient of the gravel sand stratum in the actual project.For each formulation and film forming test results，see Table 5.5.

The bentonite used in the test had a general expansion effect.After 48 h，there were still particles not completely expanded in the first three groups of mud.It can be seen from Table 5.5 that the bentonite slurry with the concentration of 3％-5％has low viscosity and the density is less than 1.05 g/cm3，which can form a permeable zonetype mud film in the gravel stratum.However，the mud loss is relatively large and the film-forming time is long.The viscosity of bentonite mud with the concentration of 8％-12％is more than 20 s.After the mud penetrates into the formation for a certain distance，it quickly stabilizes，and the mud loss is less，forming a mud film with mud skin permeability zone，and the film forming time is very short.The results of slurry film-forming test in each group are as follows.

The first group of 3％bentonite mud can be infiltrated in the gravel stratum，forming a permeable zone of about 10 cm thickness.The filtration water is large，and almost no mud skin is formed，the test condition is shown in Fig.5.13.

The second group of 5％bentonite mud can be infiltrated in the gravel sand stratum，forming a muddy skin of about 1 mm thick and a permeation zone of 8 cm thick.The test is shown in Fig.5.14.

The third group of 8％bentonite mud can be infiltrated in the gravel sand stratum，forming mm thick mud and 3 cm thick infiltration zone.The test condition is shown in Fig.5.15.

The fourth group of 12％bentonite mud can also be infiltrated in the gravel sand stratum，forming a 2 mm thick mud，a thinner permeate zone，and the mud film is formed in a short time，the test situation is shown in Fig.5.16.

The test results show that the pure bentonite slurry with smaller concentration，density and viscosity is more likely to penetrate into the formation，forming a mud film with permeability zone，and the depth of mud infiltration，that is，the thickness of permeability zone increases with the decrease of concentration，density and viscosity.

2）Mixed mud preparation and film formation test

The volume of pure bentonite will increase after expansion，so the density of pure bentonite mud is small.When the density of pure bentonite mud is 1.15-1.20 g/cm3，the viscosity is very high and difficult to pump.Therefore，considering the mixing of low specific gravity bentonite slurry with high viscosity and natural clay slurry，waste slurry stored during shield tunneling through silty clay stratum can be added to obtain better mixed slurry.In the actual mixing process，the bentonite slurry in the slurry tank is directly mixed with waste slurry，and the density meets the requirements，but the viscosity cannot reach the standard.Therefore，the dry bentonite powder should be added to the slurry tank according to the mass ratio of 1∶20，and then the waste slurry and the slurry of the slurry tank should be mixed according to the volume ratio of 1∶2.Finally，the density of the slurry is 1.16 g/cm3 and the viscosity is 30.27 s，which meets the requirements.The water separation rate test and mud film test were carried out by using the mixed mud with this ratio.

（1）The physical stability of the mud，expressed at the water-analysis rate of 24 h mud.The smaller the water analysis rate of mud，the better its physical stability.The water-analysis rate test shows Fig.5.17，the mud is loaded into a capacity of 100 mL，after 24 h the water-analysis volume of 1 mL，that is，24 h watersonography rate of 1％，indicating that the physical stability of the mud is very good.

（2）Mud film forming test.This test is carried out in gravel sand stratum，the test method and steps are the same as the above mud film test，but this test is grading pressure，drawing the change curve of mud filter water.The mud-skinned mud film formed by the mud is shown in Fig.5.18，and the relationship between the filtration rate and time and the mud pressure is shown in Fig.5.19.

It can be seen from Fig.5.19 that under the pressure of each stage，the amount of water filtered by the mud is very small，and the amount of filtered water per minute is about 0.6 mL.The filtered water volume change curve is converted to the actual excavation surface and the water loss is about every 3 m3 per 2 hours（the amount of water loss is satisfied：10-20 m3 per 2 hours）.

The above test results show that the bentonite slurry with a concentration of 8-12％can quickly form a mud film containing the osmotic zone on the excavation surface；the mixed mud of bentonite and natural clay with specific gravity of 1.15-1.20 and viscosity of 25 s or more is good in physical properties.The stability is high，and a micro-permeable mud-type mud film can be quickly formed on the excavation surface.

3）Evaluation of mud film formation after opening

After adjusting the mud by using the above method in the opening of the Nanjing Yangtze River tunnel，the mud of the original mud tank is replaced.The mud level of the slurry tank is lowered when opening，and only the air pressure is used to support the excavation surface.From the scene situation，a thick mud film is formed on the excavation surface，and the mud film is well sealed，and there is no gas leakage phenomenon，so that the air pressure can effectively act on the excavation surface and maintain open.The surface of the excavation is stable.The specific situation is shown in Fig.5.20.

5.2.2.3　Waste Mud Reuse

When shield tunneling passes through the silt silty clay stratum，a large number of abandoned clay will be produced.This stratum has strong self-made slurry ability，and it is difficult to separate mud and water.The cost of these abandoned soil treated by conventional methods is high，and the waste will cause environmental pollution.When the tunnel passes through the gravel and round gravel stratum，the coarse grain content is very high，and the fine grained soil with self-pulping function is very small.In this formation，in order to ensure the quality of mud，the need to add a large number of bentonite and pulping agent，high cost.If the two can be combined，the waste slurry containing a large number of clay particles originally produced in silty clay stratum can be used to supplement the slurry of gravel sand and round gravel stratum，which not only avoids the difficulty of fine particle separation，but also solves the raw material supply demand of slurry mixing in gravel sand stratum.In this section，through calculation and test，the reasonable proportion of recycled mud is determined，and the film forming quality of recycled mud is evaluated.

1）Disposal slurry reuse ratio test

The waste slurry used in the test was taken from the construction site of the Nanjing Yangtze River tunnel.When the slurry was mixed，the waste mud was first mixed with the expanded bentonite slurry.The mud density ranged from 1.15-1.2 g/cm3 and the viscosity requirement was greater than 25 s.If the viscosity does not meet the requirements，adjust with 1#and 3#pulp.

By calculating and repeating the comparison test of the slurry，the ratio of the mixed mud with a density of 1.15-1.2 g/cm3 is finally obtained.The waste slurry and 8％bentonite slurry can be mixed at a volume ratio of 0.4-1.1.

2）Mud and mud film quality evaluation

In order to evaluate the various indexes of the reclaimed mud，the mud and silt with high clay content stored on site were mixed in different proportions，and five groups of muds with different clay content and the same density were mixed to simulate the waste slurry with different quality.Then in CMC solution（Compared to the pulping agent，only change the mud viscosity，do not change the mud grading）to adjust the viscosity of the mud to the same.The gradation curve and basic properties of the mud are shown in Fig.5.21 and Table 5.6.

（1）The effect of the mucous grain content on the physical stability of the mud.Good physical stability is one of the necessary characteristics of mud，the physical stability of mud refers to the solid particles in the mud to maintain the suspension and dispersion state of the ability，mainly reflects the mud to prevent particle subsidence and transport，slag，is one of the important indicators of mud.Usually with a certain amount of mud placed in the barrel in a period of time to express the water analysis rate，generally selected 2 h and 24 h water-reducing rate，to reflect the shield excavation process in the assembly of pipe pieces of the short-term shutdown and a longer period of downtime.The smaller the water analysis rate，the better the physical stability of the mud，the less likely the mud to occur separation，precipitation，the larger the water analysis rate，the poor physical stability of the mud，mud is prone to separation，precipitation，the stability of the excavation surface，mud circulation and slag discharge are not conducive.The results of the water analysis rate test are shown in Fig.5.22.

When the density and viscosity are the same，with the increase of clay content in the mud，the water release rate of the mud decreases significantly，that is，the physical stability of the mud is getting better and better.Slurry with clay content of more than 30％has little 2 h precipitation rate，less than 10％.When the clay content is less than 20％，the 2 h water separation rate is relatively large，more than 30％，which will affect the quality of mud film and the transport of slag.The physical stability of the mud is good when the clay content is high，because the clay particles are small and the settlement is very slow.At the same time，it has large specific surface area，negative charge and good dispersion.

（2）The effect of the clay content on the amount of additives.

The CMC（carboxymethyl cellulose sodium）selected for the mud tackifier can stabilize the mud.The action between CMC content and clay content was analyzed by experiment，as shown in Fig.5.23.During the test，CMC is added in the form of a solution.In the mud，it can increase the viscosity of the mud without affecting the mud grading.

Under the premise of reaching the same viscosity，the higher the clay content of natural mud，the less CMC content is needed.The CMC content of mud with clay content less than 20％is more than 2 times higher than that of mud with clay content more than 30％.This is because the tackifier plays the role of tackifying，mainly relying on the bridging effect between polymer and soil particles.Due to the large specific surface area and negative charge，clay particles are the most active particles in the mud.The mud with high clay content is conducive to the tackifying effect of tackifier.Therefore，when preparing the slurry，the slurry with high clay content（generally above 20％）should be selected as the basic material of the slurry.The less the tackifier is needed for preparing the slurry，the lower the cost is.

（3）The effect of mud clay content on the film quality of slurry shield.In the scenario of high permeability and high pressure in the gravel sand stratum crossing the simulated mud water shield，five layers of clay with different clay content were used for the osmosis film formation test in the self-made mud osmosis film forming test device.Fig.5.24 is five sets of mud，the curve of water filtration as a function of time.

It can be seen that when No.1-5 mud is filmed，the filter water increases in turn.The filtration capacity of No.1-4 mud is small and stable in a short time，while the filtration capacity of No.5 mud is much larger than the previous four groups of mud，and the mud permeability tends to be stable after a long time.Fig.5.25 is the form of mud film formed after the test of Sample 1 and Sample 5 mud.It can be seen that the mud film formed by Sample 1 mud is only 1-2 mm thick and relatively dense，while the mud film formed by Sample5 mud is about 5 mm thick and slightly loose.

Fig.5.26 is the curve of permeability coefficient of mud film formed by five groups of mud with mud clay content.It can be seen that with the increase of mud clay content，the permeability coefficient of mud film formed by mud decreases gradually.The permeability coefficient of the mud film formed by Sample 5 mud（clay content of 6％）is about 5.36 x 10-5 m/s，which is close to the permeability coefficient of common silty sand.The permeability coefficient of the mud film formed by Sample 1 mud（clay content of 47％）is about 2.86 x 10-7 m/s，and the permeability coefficient of the mud film is reduced by nearly two orders of magnitude.Therefore，the larger the clay content of the mud，the smaller the filter water，and the shorter the time when the mud reaches the seepage stability，the denser the mud film is formed and the better the quality is.When the clay content is less than 20％，the filtration amount of the mud film is significantly increased，and the time to achieve stable permeability is also prolonged，so the quality of the formed mud film is also poor.

This large-scale use of slurry with high clay content to adjust the properties of slurry not only improves the film-forming quality and physical stability of slurry，reduces the use of pulping agent，reduces the economic cost，but also reduces the adverse environmental impacts such as large area of waste slurry，which has good engineering effect.