6.3.2　Pressured Inlet Technology Based on Mud Film...

6.3.2　Pressured Inlet Technology Based on Mud Film Closed Air

In order to change the cutter for slurry shield with pressure into the chamber，it is usually necessary to reduce the liquid level of the slurry chamber to facilitate the operation of the workers with pressure into the chamber.At this time，the upper and middle soil layers in front of the shield excavation are all supported by gas.For the strong permeable strata，a large number of compressed air will carry fine particles through the pores between the strata to escape upwards.Increasing the pore size of the strata will cause instability of air pressure and cannot balance the water and soil pressure of the excavation face.In order to realize the stability of excavation face in strong permeable stratum，a layer of airtight mud film must be established on the surface of excavation face.This section takes Nanjing Yangtze River Tunnel Project as an example to introduce the pressurized cabin entry technology based on mud film air closure.

6.3.2.1　Air-Closing Technology of Mud Film

In the Nanjing Yangtze River Tunnel Project，after the excavation of the Nanjing Yangtze River Tunnel to the 650th ring，the excavation parameters are significantly increased，in which the torque is increased to 15 kN m，and the excavation speed is reduced to 10 mm/min.Giant granular soil，large blocks of ancient wood and pebbles above 20 cm are found in the discharged slag soil，and the excavation cannot be continued.In order to ensure the safety of equipment and long-distance construction，the shutdown inspection is carried out in the 659 ring.During the tunneling construction before the shield shutdown，the shield has entered the gravel land with strong permeability and high water pressure.In the layer，the polymer mud with high density and viscosity is used，and the mud filtration is less.The mud film is mud-skin mud film.However，the polymer slurry is not stable when it is shut down for a long time，and the slurry is prone to deterioration of quality such as aging，which will further affect the quality of slurry film formation.Therefore，the bentonite slurry with good stability should be used in the opening of the cabin.In other words，before opening the cabin，some adjustments must be made to the formula and basic parameters of the slurry，and the indoor permeability test and air tightness test of the slurry must be carried out to ensure the stability of the excavation face and the successful implementation of the cabin entry operation.

1）Laboratory permeability test of pure bentonite mud

In order to make the mud penetrate a certain distance in the stratum and improve the cohesive force of the soil on the excavation face，the Na grade bentonite was used to swell at the concentration of 3％，5％，8％and 12％respectively for 24 h.The specific gravity and viscosity of the mud are shown in Table 6.15.

In this experiment，the gravel sand layer that is the most unfavorable for film formation is selected as the test stratum，that is，the full-face gravel sand stratum is used to simulate the actual stratum.The coarse particles larger than 2 mm account for about 40％of the whole stratum content，and the permeability coefficient of the stratum is about 3.95 x 10-2 cm/s in the test，which is basically consistent with the permeability coefficient of gravel sand stratum in practical engineering.

Test instruments and test methods are shown in Figs.6.36 and 6.37，and the pressure is 0.3 MPa at one time.The stability of mud infiltration，the amount of filter water and the forming time of mud film are measured as the evaluation indexes of the quality of mud film，and the form of mud film is observed.The smaller the filter water，the shorter the film-forming time，the denser the mud film formed，and the better the quality of the mud film.

The basic properties of four kinds of mud and the results of film forming test in the formation are shown in Table 6.15.

It can be seen from Table 6.15 that the viscosity of bentonite mud with the concentration of 3％-5％is low and the specific gravity is less than 1.05，which can form a permeable zone mud film in the formation.However，the mud loss is relatively large and the film forming time is long.The viscosity of bentonite mud with 8％-12％concentration is more than 20 s.After the mud penetrates into the stratum for a certain distance，it quickly stabilizes，and the mud loss is less.The mud film of mud skin-permeability zone is formed（Fig.6.38），and the film forming time is very short.

2）Mud film forming test for increasing specific gravity and viscosity

After forming a mud-skin permeable zone mud film on the excavation surface，the specific gravity of the mud was increased to 1.15-1.20，and the viscosity was more than 25 s.The mud film-forming test was carried out.

Because the proportion of pure bentonite mud is generally small，if the proportion of pure bentonite mud is adjusted to 1.15-1.20，the viscosity will be large and it is difficult to meet the pumping requirements.Therefore，low specific gravity bentonite slurry with high viscosity and natural clay slurry（the excess slurry stored when shield passes through silty clay stratum）are mixed to configure the slurry with high physical stability with specific gravity of 1.15-1.20 and viscosity of more than 25 s.

（1）Physical stability of slurry.It is represented by the water evolution rate of 24 h slurry.The smaller the mud dewatering rate，the better its physical stability.The slurry was loaded into a measuring tube with a capacity of 100 mL，and the volume of water released after 24 h was 1 mL，i.e.，the water released rate after 24 h was 1％.The physical stability of the slurry was good.

（2）Mud film test.The test was carried out in the mixed layer of fine sand，gravel sand and pebble.The test method and procedure were the same as those of the above mud film test，but this test was graded pressure，and the variation curve of mud filtration water was plotted.The mud forms a mud film，as shown in Fig.6.39，and the relationship between the amount of filtration water and time and mud pressure is shown in Fig.6.40.

It can be seen from Fig.6.40 that under the action of each stage of pressure，the filtration volume of slurry is very small，and the filtration volume per minute is about 0.6 mL.The filtration volume on the real excavation surface is about 3 m3/2 h（meeting the empirical value of filtration volume：10-20 m3/2 h）.

The results of the above experiments show that the bentonite slurry with the concentration of 8％-12％can quickly form a mud film containing permeability zone on the excavation surface.The mixed mud of bentonite and natural clay with specific gravity of 1.15-1.20 and viscosity of more than 25 s has good properties and high physical stability，which can quickly form micro permeable mud film on the excavation surface.

Therefore，when the Nanjing Yangtze River tunnel is opened in gravel stratum，the mud of the pressure chamber is replaced in two steps.

①Firstly，8％-12％bentonite mud is used as seepage mud to form a certain distance seepage zone in the excavation surface stratum and increase the cohesion of the stratum.

②The mud with specific gravity of 1.15-1.20 and viscosity of more than 25 s is used to form a mud film with mud skin type of micro-permeable on the excavation surface，which effectively acts on the excavation surface and reduces the loss of mud pressure.

3）Air tightness experiment of mud film

In order to facilitate the work of changing cutters and repairing cutter heads in the pressure chamber，it is necessary to appropriately reduce the mud slurry level in the excavation chamber and the pressure chamber according to the geological conditions，and support the excavation face with air pressure.The key to the pressurized cabin operation is to maintain the stability of the air pressure in the soil cabin.If the air tightness of the mud film is not good，the leakage will lead to sudden changes in air pressure，groundwater infiltration，and a large amount of flow sand，resulting in instability and collapse of the excavation face，which will endanger the safety of the cabin construction personnel.Therefore，before the construction personnel enter the cabin，the air tightness of the pressure cabin must be tested first.

Testing the air tightness of the pressure chamber is divided into two parts.

（1）Indoor simulation test：In the self-made mud film-forming device，the air tightness test was carried out on the formed mud film to test whether the quality of the mud film can effectively prevent gas leakage and meet the air tightness requirements of the pressure chamber，and then the mud formula was determined.

（2）Field test：on the basis of laboratory test，the air tightness of pressure chamber after grouting plugging is directly tested on the spot.The purpose is to observe the air supply interval of the air compressor，the speed of gas leakage in the pressure chamber，the change of pressure chamber pressure，the amount of residual soil and ground monitoring after the pressure is maintained，so as to analyze whether the excavation face is unstable and whether the pressure in the chamber can remain stable，so as to determine whether it can enter the cabin.

During the test，if the air pressure in the pressure cabin is stable at the set value，the starting frequency of the air compressor is stable，the air supply is normal，and there is no deformation and air leakage on the ground，which proves that the formation can achieve gas pressure stabilization.If there is no excess slag and soil taken out after the cabin cleaning，it indicates that the soil layer on the excavation surface is stable and no collapse occurs.

6.3.2.2　Pressure and Decompression Process

1）Pressing process

In order to pass the pressurized working area，people and tools need to enter the auxiliary cabin through the gate，where the air pressure should be increased first.The pressure process of shield opening in Nanjing Yangtze River Tunnel is introduced in the form of schematic diagram.

The shield diagram is shown in Fig.6.41.It is defined that P0is the atmospheric pressure，P1 is the incision pressure，namely the working cabin pressure，and P2 is the pressure value between P0and P1.In the initial state，only the working chamber（excavation chamber and pressure chamber）is under pressure，the pressure is P1.If all pressure and decompression conditions are available，you can start working under pressure.

First step，the main cabin is pressurized.As shown in Fig.6.42，the main chamber pressure is increased from P0to P2by closing the main chamber gate and opening the main chamber valve，and then to the working chamber pressure P1.

The second step，cabin pressure.The secondary cabin pressurization procedure is similar to the main cabin pressurization procedure as shown in Fig.6.43.When the pressure of the main cabin is balanced with the pressure of the working cabin，the personnel enter the auxiliary cabin and begin to pressure the auxiliary cabin.In general，the main cabin is always pressurized first，and the auxiliary cabin always keeps the pressure relief state.Incoming personnel are pressurized in the secondary cabin to accommodate pressure in the main cabin and working cabin.The operation of adding personnel to the pressure operation area can be carried out by the pressure of the auxiliary cabin.

The third step，personnel into the cabin.When the pressure of the auxiliary cabin is balanced with the pressure of the main cabin and the working cabin，as shown in Fig.6.44，and there is no adverse reaction for the personnel entering the cabin，they can enter the working cabin to repair the cutterhead and other equipment.

2）Decompression process

The entry staff must return within the prescribed time.The pressure time depends on the person who finally reaches the pressure compensation in the auxiliary cabin，and the time required to enter the auxiliary cabin is also calculated within the working time of the site.If the pressure decreases very slowly during decompression，the released gas can be discharged through the blood circulation and lung.If the pressure is rapidly reduced，the bubbles will form water carbonate effect in body fluids and tissues.These can cause a series of decompression symptoms，releasing gas can cause temporary or permanent tissue damage，causing serious bodily harm to staff.Therefore，the decompression process and precautions should be strictly observed.

Similar to the pressure process，P0is defined as atmospheric pressure，P1is the working chamber pressure，and P2 is the pressure value between P0and P1.

In the initial state，as shown in Fig.6.45，the working cabin and the main cabin are under pressure，the pressure is P1，and the auxiliary cabin is under normal pressure.When all the conditions for staff and decompression are ready，decompression can be started.Normally，decompression is performed only through the main cabin.At the same time，the cabin is always in normal pressure.

As shown in Figs.6.46 and 6.47，the main cabin pressure is reduced from P1to P2，and then to P0，and the decompression operation is completed.