Abstract
The water inrush and large surface settlement encountered by EPB shield crossing water-rich gravel layer have attracted much attention. Based on the section tunnel engineering of the Inner Mongolia University South Campus Station-Xilin Park Station of line 2 of Hohhot Rail Transit, the optimum mix proportion of bentonite slurry and foam diluent for forming slurry membrane in water-rich gravel stratum was studied in this study at first. The characteristics of conditioned soil were analyzed in combination with slag soil. The water plugging effect and surface settlement with or without slurry membrane were investigated with numerical simulation. The numerical model has been effectively verified by comparing the field monitoring and numerical simulation of surface settlement value. Further, the water shutoff mechanism of the slurry membrane is revealed. The setting of the slurry membrane exhibited a good water plugging effect and effectively decreased the surface settlement. The construction method of “water plugging with a slurry membrane” proposed in this study could powerfully combine the respective advantages of EPB shield and slurry balance shield, ensuring the safety, high quality, and high efficiency of tunnel construction.
1. Introduction
The construction process of the slurry balance shield is complex, but the slurry membrane formed during construction can well block the water seepage of the tunnel face. At the same time, the slurry membrane can provide temporary support force for the tunnel face [1]. Compared with the slurry balance shield, the Earth pressure balance (EPB) shield covers a small area during construction, with simple construction and process. At the same time, an EPB shield with a wide range of applications could be applied to all kinds of strata. Therefore, it is more widely used in urban tunnel engineering [2]. However, when EPB shield passes through unfavorable geological sections, such as water-rich gravel stratum, problems such as water gushing and excessive surface settlement can easily occur during construction [3]. At this time, if a dense slurry membrane can be formed in front of the tunnel, it can create a supporting effect on the tunnel, balancing the water and soil pressure on the tunnel face and controlling the occurrence of such accidents [4].
At present, some scholars have done much research on shield slurry membrane-related problems. Vorobiev [5] studied how the slurry membrane formed and the effect of pressure redistribution in this process through the model and finally predicted the slurry membrane formation process. Chen et al. [6] conducted a permeability column test to deeply explore the characteristics of each stage in the formation process of the slurry membrane in the sandy soil layer and obtained the relationship between slurry membrane permeability and time. Yin [7] studied the mechanism of slurry membrane formation by indoor test method and analyzed the variation law of slurry membrane permeability with time and the influence of slurry membrane permeability on formation permeability coefficient. The results showed that the permeability of the slurry membrane determines the stability of the shield excavation surface. Niu [8] conducted an indoor model test to explore the formation mechanism and law of slurry membrane during slurry infiltration into the formation. It was found that slurry pressure, slurry density, and soil particle size would affect the formation quality of the slurry membrane. Min et al. [9] adopted the laboratory test method to observe the permeability of slurry membrane in the formation, monitor the change of formation pore pressure during the formation of slurry membrane, and put forward the action mechanism of the slurry membrane.
The above research on the slurry membrane is mainly based on the slurry balance shield tunnel project. Because the mechanical parameters, such as the opening rate of the slurry balance shield, are different from the EPB shield, the tunneling principles of the two shield machines are inconsistent. EPB shield generally does not produce a slurry membrane effect but reduces the slurry membrane's impact as much as possible [10]. However, in the water-rich gravel stratum, we need an EPB shield to produce a slurry membrane to prevent the inrushing of the tunnel face. Therefore, it is essential to study the slurry membrane of the EPB shield machine.
The Inner Mongolia University South Campus Station-Xilin Park Station (Nei-Xi) Section tunnel of Bid 03 of Hohhot Rail Transit Line 2 in Inner Mongolia mainly crosses the water-rich gravel stratum, which is very difficult to stabilize the tunnel face and is prone to slurry outburst and water gushing. Based on the shield tunnel project in Nei-Xi Section of Bid 03 of Hohhot Rail Transit Line 2, aiming at the characteristics of strong permeability, poor self-stability, and continuity of stratum distribution in water-rich round gravel stratum, this paper studied how to integrate the advantages of the two kinds of shield machines and improve a new construction method of EPB shield “water plugging with a slurry membrane.” Through the slurry membrane mix testing, the optimal mix of the slurry membrane during the construction of the EPB shield in the water-rich round gravel stratum was studied. Combined with slag soil, the characteristics of conditioned slag soil were studied analyzed. The water plugging mechanism and the application effect of the slurry membrane were investigated through numerical simulation and on-site monitoring.
2. Project Background
As shown in Figure 1, Bid 03 of Hohhot Rail Transit Line 2 mainly includes Shuaijiaying Station-Inner Mongolia University South Campus Station-Xilin Park Station, with two stations and two sections; the length of Shuainei Section is approximately 1490 m. The size of the Nei-Xi Section is about 1149 m. The construction method is the shield method with a circular section. The spacing between the left and right lines of the tunnel is 12–14 m. This paper studies the right line tunnel in Nei-Xi Section.
Along with Nei-Xi Section, the stratum information is fine sand, silty clay, dense gravel, medium dense gravel, medium sand, and plain fill from bottom to top. The tunnel mainly passes through the water-rich gravel stratum. The gravel with a 2–20 mm particle size accounts for approximately 50%. The gravel with more than 20 mm particle size accounts for about 20%, of which the maximum particle size is 80 mm. The particle grading of the soil layer in this section is poor, the gap between particles is large, and the filler is mainly miscellaneous sand and soil. The mechanical properties are unstable, and the cohesion is small. See Figure 2 for the geological profile of the Nei-Xi Section. Field tests and laboratory tests were carried out during the survey. Field tests include borehole coring (see Figure 2), elastic longitudinal wave test, resistivity test, and standard penetration test. Laboratory tests include direct shear, triaxial consolidated fast shear, and unconfined compressive strength tests. During the exploration, taking the left line tunnel as an example, there are 13 boreholes (Black dotted lines) on the left and 12 on the right of Xiaoheihe River, and the depth of boreholes is 45–50 m. The number of boreholes is continuous, MN29-53 from left to right. The groundwater in the on-site area is generally rich and moderately rich, and the main aquifer is sandy soil and gravel stratum with strong water permeability. The elevation of groundwater level is 1033.4–1035.0 m, the buried depth of ground is 6.2–14.7 m, and the water level is above the tunnel vault.
3. Mix Test of Conditioned Materials of Slag Soil for the Slurry Membrane
During the slurry balance shield construction, the tunnel face was supported by bentonite slurry. The slurry membrane can be balanced theoretically when the slurry pressure is equal to the formation pore pressure. When the slurry pressure is greater than the formation pore pressure, the water and fine particles in the slurry permeate into the formation through the pores, resulting in the gradual decrease of the formation permeability coefficient. Then the slurry particles that have not permeated the formation are adsorbed and gathered on the tunnel face. Finally, a dense slurry membrane with low water permeability is formed.
The mechanism of the slurry membrane formed by the EPB shield in tunneling is different from that of the slurry balance shield. The formation of the slurry membrane in the EPB shield should be based on the good slag soil to be modified. With the action of soil pressure, owing to the existence of osmotic pressure, the conditioned slag soil permeates the tunnel face, and the slurry membrane effect occurs when the shield machine runs [11].
Whether the EPB shield can form a slurry membrane in tunneling is mainly related to the conditioning effect of slag soil [12, 13]. In this paper, based on on-site investigation, the conditioning test of slag soil was carried out with indoor test method, and the fluidity of slag soil under the condition of different ratios of bentonite slurry and foaming agent diluent was studied.
4. Test on the Mix of Bentonite Slurry
A slurry viscometer measures the viscosity of the mixture (see Figure 3), and the specific operation steps are as follows: ① Fill the slurry to the scribed line marked with “Slurry” on the measuring pipe, add water to the scribed line marked with “Water,” block the pipe orifice, and shake it. ② Pour the mixture into the filter cartridge, discard the liquid passing through the filter screen, and add clean water into the side pipe. Shake and pour into the filter cartridge. Repeat until the inside of the measuring tube is clean. ③ Wash the sand obtained on the screen with clean water and remove the residual slurry. ④ Put the funnel into the filter cartridge, then turn it over slowly, and insert the funnel nozzle into the measuring tube. Wash all the sand attached to the screen into the pipe with clean water. ⑤ After the sand settles, read the percentage of sand.
The specific gravity of the mixture is measured by the pycnometer method (see Figure 4). The particular measurement steps are as follows. Firstly, weigh the empty and dry pycnometer and weigh an appropriate sample in the pycnometer. Then inject the test solution (steam water) into the pycnometer to immerse the sample and clear the bubbles on the sample's surface. For example, put the pycnometer in a vacuum dryer to vacuum. After breaking the vacuum state, inject the test solution into the pycnometer. The temperature of the test solution should reach 23 ± 0.5°C in the liquid bath. Fill the pycnometer with the test solution to the limit volume. Dry the pycnometer and weigh it together with its contents. Then empty the pycnometer, fill it with the test solution, remove the air, and weigh the weight of the pycnometer and its contents at the temperature of 23 ± 0.5°C.
In this test, there are five groups of comparative trials. The abscissa is the mass ratio of bentonite to water in the figure, which is 1 : 6, 1 : 8, 1 : 10, 1 : 12, and 1 : 14, respectively, and the ordinate is the viscosity and specific gravity of bentonite slurry. The results are shown in Table 1 and Figures 5 and 6. According to Figure 5, it is obvious that when the mass ratio of bentonite to water is 1 : 6–1 : 8, the viscosity of the slurry increases gradually, and the slurry changes from paste to slow flow most prone to pipe plugging. When the mass ratio of bentonite to water is 1 : 8–1 : 10, the viscosity of the slurry is better, and the flow plasticity is good. When the mass ratio of bentonite to water is 1 : 10–1 : 14, the viscosity of the slurry decreases gradually to no viscosity until there is no puffing phenomenon.
It can be seen from Figure 6 that the specific gravity of the slurry decreases with the increase of the ratio of water in the bentonite slurry. When the mass ratio of bentonite to water is 1 : 6, the specific gravity of the slurry is 1.08 g/cm³. When the mass ratio of bentonite to water increases to 1 : 8, the specific gravity of the slurry decreases to 1.07 g/cm³. When the mass ratio of bentonite to water is 1 : 8, the specific gravity of the slurry is 1.07 g/cm³. When the mass ratio of bentonite to water increases to 1 : 10, the specific gravity of the slurry decreases to 1.06 g/cm³; at this time, the mix ratio can meet the construction requirements and achieve the effect of reducing cost. When the mass ratio of bentonite to water is in the range of 1 : 10–1 : 14, the slurry segregates, which cannot meet the application requirements. In summary, the bentonite slurry with a mass ratio of bentonite to the water of 1 : 8–1 : 10 has excellent performance and can save cost, which is adopted in the follow-up test.
5. Test on the Mix of the Foaming Agent Diluent
The specific operation steps of foaming by mixing method are as follows: Stir 100 ml of foaming agent solution with specific concentration to be tested at a certain stirring speed for 1 min, pour it into the measuring cylinder (see Figure 7), and start timing after foaming. The resulting foam volume is recorded as V (foaming volume). The time used to record the precipitation of 50 ml liquid from the foam is recorded as Thl (half-life of the solution). By comparing the size of V and Thl, the foaming force and stability of foam formed under different foam formulations were evaluated. In this test, the concentration of foaming agent diluent was 1%, 2%, 3%, 4%, 5%, and 6% (a total of 6 groups) [14, 15]. The test results are shown in Table 2 and Figures 8 and 9.
It can be seen from Figure 8 that the maximum foaming ratio increases with the increase of the concentration of foaming agent diluent. When the concentration of foaming agent diluent is between 1% and 3%, the maximum foaming ratio is 25–55 times. In comparison, with the concentration of foaming agent diluent being between 3% and 6%, the maximum foaming ratio is maintained approximately 50 times. The foaming rate is almost no longer affected by its concentration. Therefore, the best foaming effect can be achieved with the foaming agent concentration set to 3-4%. It can be seen from Figure 9 that the bubble half-life increases with the increase of foaming agent diluent concentration. However, the rate of change decreases gradually. The mechanism of the foam decay is mainly the liquid drainage of the foam liquid membrane [16]. When the diluent concentration of the blowing agent increases to a certain level, the foam decaying machine causes the liquid membrane’s discharging effect to change into gas diffusion. When the diluent concentration rises from 1% to 3% and the bubble half-life increases from 7 min to 13 min, the bubble stability is enhanced. When the diluent concentration is 3% to 6%, the bubble half-life increases from 13 min to 15 min. The bubble stability increases little. Meanwhile, when the concentration is more than 5%, the bubble half-life no longer changes. Therefore, when the concentration of foaming agent diluent is set at 3–5%, the bubble can achieve the most long-lasting effect. In summary, the performance of the foaming agent diluent is the best when the concentration of the foam diluent is set at 3-4%, and this mix ratio is used in the follow-up test.
6. Test Study on the Conditioning Characteristics of Slag Soil Conditioned Materials for the Slurry Membrane
6.1. Slump Test with Different Bentonite Slurry Ratios
The photos of the slump test are shown in Figure 10. The specific test steps are as follows: ① First, wipe down the tools such as slump, shovel, and mixing board. ② Prepare corresponding samples according to the improved proportion of residue and soil, and mix them evenly on the mixing plate. ③ Place the slump drum on a nonabsorbent flat plate and the funnel on the slump drum. The mixture is loaded into the drum in three layers, and the filling height of each layer accounts for about one-third of the height of the drum. Tamp each layer with a tamping rod from the edge to the center. ④ After the filling of the slump cylinder, use a scraper to level the slump hole, clean up the surrounding excess samples, and then lift the slump cylinder along the vertical direction. ⑤ Measure the vertical distance from the steel ruler to the top of the sample with a square wooden ruler on the top of the cylinder, and the measured reading is the slump value of the sample.
In this test, the volume ratio of bentonite slurry to actual slag soil was 0.5 : 10, 1 : 10, 1.5 : 10, 2 : 10, 2.5 : 10, and 3 : 10, respectively (6 groups). The slump can well reflect whether the conditioned slag soil can form a slurry membrane, and the slump of local slag soil is the best in 160–185 mm [17, 18]. Therefore, the slump of the conditioned slag soil is taken as the evaluation index (see in Table 3 and Figure 11) to evaluate the advantages and disadvantages of each mixing ratio.
It can be seen from Figure 11 that before the conditioning, the actual slag soil had no slump, and the soil flow plasticity was poor. The conditioned slag slump increases gradually with the volume ratio of bentonite slurry to slag slurry. When the volume ratio of bentonite slurry to slag soil is 1.5 : 10–2 : 10, the conditioned slag slump is 150–170 mm, which is the optimal collapse value of EPB shield construction. Through comparative test and construction experience, the best parameters of bentonite slurry for EPB shield to set up slurry membrane through water-rich gravel stratum are obtained as follows: (1) bentonite type: sodium bentonite; (2) the mass ratio of bentonite to water is 1 : 8–1 : 10; (3) the expansion time of bentonite is 20 hours; (4) the volume ratio of bentonite slurry to slag soil is 1.5 : 10–2 : 10.
6.2. Slump with Different Foaming Agent Diluent Ratios
In this test, the mass ratio of foaming agent diluent to actual slag soil is 1 : 1, 1 : 2, 1 : 3, 1 : 4, 1 : 5, and 1 : 6, respectively (6 groups), and the slump of conditioned slag soil with the different mass ratio is compared, as shown in Table 4 and Figure 12.
It can be seen from Figure 12 that when the ratio of foaming agent diluent to actual slag soil is 1 : 1, the slump of slag soil is larger, which is approximately 240 mm. At this time, the flow plasticity of slag soil is poor, which can easily lead to “slurry cake” in tunnel construction. When the ratio of foaming agent diluent to actual slag soil is 1 : 2–1 : 4, the slump of slag soil is 150–200 mm, and the slump is more reasonable than the above. The increase of the amount of foaming agent diluent reduces the friction effect between soil particles, decreasing the slump of the improved residue [19]. However, according to the characteristics of rich water and strong permeability in this formation, it is suggested that the mass ratio of foaming agent diluent to slag soil must be set at 1 : 2. When the content of foaming agent diluent increases from 1 : 4 to 1 : 6, the viscosity of the conditioned slag soil is enhanced with the flow plasticity greatly reduced, and the slump reduced from 150 mm to 30 mm. However, forming a slurry membrane with a low collapse in the formation is difficult. Therefore, it is not recommended that the mass ratio of foaming agent diluent to slag soil is 1 : 4–1 : 6. Through comparative test and construction experience, the optimum parameters of foaming agent diluent for EPB shield to set up slurry membrane through water-rich gravel stratum are obtained as follows: (1) the concentration of foaming agent diluent is 3-4%; (2) the mass ratio of foaming agent diluent to slag soil is 1 : 2.
6.3. Water Plugging Mechanism of the Slurry Membrane
Based on the above achievements, this section establishes a three-dimensional shield and soil layer model through ABAQUS software and then converts it into a FLAC3D calculation model through relevant plugins to simulate the change of pore pressure and stress of surrounding rock after shield tunnel excavation [20]. By changing the physical and mechanical parameters of the soil in front of the cutter head, pore pressure and surface settlement with and without slurry membrane were compared.
6.4. Model Building
In the Cartesian coordinate system, the semistructural model is adopted because of the symmetry of the structure, load boundary conditions, and the gravity acceleration points to the negative direction of the Z-axis. The axial direction of the tunnel is the Y-direction (longitudinal length), with the range being 0–64.5 m. The X-direction of the model (width) range is −34.1–0 m. The Z-direction (height) range is −34.1–16.45 m direction.
In the model, the X = 0 plane is the symmetry plane of the model, which restricts the translational degrees of freedom in the X-direction and the rotational degrees of freedom in the Y and Z directions. It does not restrict the pore-water pressure. The X = −34.1 plane is an approximate infinite field boundary, constraining the translational degrees of freedom in the X-direction and the pore-water pressure to simulate the permeable boundary. Y = 0 and Y = 64.5 planes are symmetrical boundaries, restricting the translational degrees of freedom in the Y-direction and the rotational degrees of freedom in X and Z directions, and do not restrict the pore-water pressure. Z = −34.1 plane is the bottom surface of the model. It is considered the top surface of the impervious bedrock layer, which restricts the translational degrees of freedom and rotational degrees of freedom in X, Y, and Z directions. It does not restrict the pore-water pressure. The plane with Z = 16.45 is the surface and does not constrain any translational and rotational degrees of freedom, but the pore-water pressure is constrained to zero.
The surrounding rock is simulated with elastic-plastic material, which accords with the Mohr–Coulomb failure criterion, and the seepage type adopts an isotropic fluid model. The shell of the shield and lining segment adopts an isotropic linear elastic model. The shell element realizes the shield shell, and the three-dimensional solid element realizes the lining segment. The shield shell adopts steel material parameters, and the lining segment adopts the material parameters of C40 concrete. The weight of water is ρw = 1000 kg/m3, and the bulk modulus of fluid is Kw = 104 Pa. The specific formation mechanical parameters and structural material parameters are shown in Tables 5 and 6, respectively.
The model uses hexahedral elements for meshing, and the mesh size increases with the increase of the distance from the tunnel face. Near the tunnel, the mesh size of the model is set to 0.2 m. The mesh size at the edge of the model is 5 m. The model consists of 47548 nodes and 45080 elements (43960 solid elements and 1120 shell elements), as shown in Figure 13.
6.5. Distribution of Pore-Water Pressure
Pore-water pressure can reflect the water plugging effect of the slurry membrane. The high pore-water pressure indicates that the dissipation degree of water pressure is low, which means that the water plugging effect is good. The pore-water pressure distribution with and without a slurry membrane is compared as shown in Figures 14 and 15.
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From the cloud picture of pore pressure distribution, it can be seen that the central pore pressure of the tunnel face without a slurry membrane is 25 kPa. In comparison, it is 91 kPa with slurry membrane, and the central pore pressure of tunnel face with the slurry membrane is approximately 65 kPa higher than that without slurry membrane. The results showed that the permeability coefficient of the tunnel face with a slurry membrane is low. The permeability rate is slow with the pore-water pressure dissipating slowly, which the axial variation curve pore pressure can also reflect. As shown in Figure 14, when the pore pressure in front of the tunnel face exists in the slurry membrane, a larger water pressure gradient is produced at the slurry membrane, which rises faster along the longitudinal direction. The inflection point can be reached in the range of 2 m, indicating that the slurry membrane exerts good water plugging effect at this time.
6.6. Formation Mechanism of the Slurry Membrane
Kong et al. [21] introduce a filtration model to simulate the slurry membrane during slurry shield construction. The model reveals the dynamic process of the slurry membrane and the mechanism change of the stratum. Yu et al. [22] believe that high-quality slurry can form a slurry membrane quickly, which makes the slurry pressure work effectively on the excavation surface. Wei et al. [23] show that with the increase of the diameter rate of formation soil and bentonite, the slurry membrane forms the transition from mud cake to cement cake area until the slurry membrane cannot be developed in the same formation soil. Jia et al. [24] believe that filter cake formation mainly depends on the particle size distribution of slurry and the pore size of the formation. Well-graded mud particles can form a more compact filter cake than poorly graded mud particles on the excavation surface. The supporting principle of the filter cake is the sum of the driving force applied by water to the filter cake, including the force applied to the filter cake particles through the pressure gradient and the interaction force between the filter cake particles and water, providing effective horizontal stress to balance the horizontal Earth pressure in the ground. Therefore, the denser the filter cake formed, the greater the pressure gradient induced in the filter cake, and the greater the effective horizontal stress generated in the formation to maintain the stability of the excavation surface. According to the research of this paper, the mechanism of EPB shield forming slurry membrane is similar to that of slurry balance shield. The pressure in the soil bin is greater than the pore-water pressure in the formation, and the water and fine-grained components of the residue in the soil bin penetrate the formation through the pores of the formation. Among them, mud particles fill the formation pores, making the formation pore stress smaller than before and the permeability coefficient smaller. The infiltration of water increases the pore-water pressure in the formation. Mud particles gather on the formation surface and form a slurry membrane of mud skin type. A small number of mud particles penetrate the formation and block the pores of the formation. The morphology of the formation and slurry membrane is shown in Figure 16. Because the slurry membrane is not completely impermeable, the formation of pore-water stress inevitably arises in slurry membrane formation. The rising pore-water stress is the excess pore-water stress. Therefore, through the action of the slurry membrane, the slurry pressure in the formation is transformed into pore pressure resisting formation hydrostatic pressure, excess pore-water stress, and effective slurry pressure resisting formation Earth pressure. In the actual construction, the slurry pressure of the pressure tank should be set on the basis of the formation Earth water pressure and the excess pore-water stress in the formation after film formation. By adjusting the mud properties, a dense slurry membrane is formed on the formation surface, reducing the permeability coefficient of the formation and increasing the resistance of mud infiltration, which is conducive to the conversion of slurry pressure into effective slurry pressure and balancing the formation Earth pressure, so as to maintain the stability of the excavation surface.
7. Application Effect of the Slurry Membrane
7.1. Numerical Simulation of Surface Settlement
The surface settlement during shield construction can reflect the quality of construction. The surface settlement is related to the stratum conditions, the water plugging, and the support effect of the tunnel face [25]. Therefore, the impact of the slurry membrane can be reflected by comparing the surface settlement. The comparison of surface settlement with and without a slurry membrane is shown in Figures 17–19.
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As shown from Figures 17–19, compared with the state with slurry membrane, the maximum surface settlement without a slurry membrane is approximately 3 mm higher than that without slurry membrane, accounting for about 30% of the total settlement. Moreover, the influence range of surface settlement without a slurry membrane is larger. There are two reasons for this phenomenon. One is that owing to the good water plugging effect of the slurry membrane, the dissipation degree of pore-water pressure is low. Thus, the increase of soil effective stress is smaller, and the formation settlement is smaller [26]. The other one is that the slurry membrane has a certain strength. It supports the tunnel face, decreases the soil's extrusion deformation near the tunnel face, and finally reduces the surface settlement. Therefore, during the construction of the EPB shield, setting a slurry membrane in front of the tunnel face can effectively decrease the surface settlement.
7.2. On-Site Monitoring of Surface Settlement
According to the on-site monitoring results, the duration curve of on-site surface settlement and the surface settlement trough characterization curve are obtained following data processing, as shown in Figure 20.
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As shown from Figure 20, with or without a slurry membrane, the change of surface settlement at the tunnel axis generally increases gradually with time. The farther away from the transverse symmetrical axis of the tunnel, the smaller the surface settlement with the maximum surface settlement occurring at the surface of the transverse axis of the tunnel. Compared with the case without slurry membrane, the surface settlement with the condition of the slurry membrane is smaller with the convergence time of surface settlement being shorter than before. The formation deformation tends to be stable earlier, which shows that it is more advantageous to improve the stability of the strata in front of the tunnel face by forming a slurry membrane and safer for the excavation of the tunnel face. At the same time, through the comparison between on-site monitoring and numerical simulation of surface settlement, the figures show a more consistent change trend and final cumulative settlement (see Figures 12 and 20), which verifies the accuracy of the numerical model. Further, it shows that the slurry membrane has a positive effect on the stability of the face.
8. Conclusions
The slurry membrane formed during the construction of the slurry balance shield has great advantages. In contrast, the construction technology of the EPB shield is simpler, with the slag discharge being convenient and the construction area being small. To synthesize the advantages of the two kinds of shield, based on the shield tunnel project in the Nei-Xi section of Hohhot Rail Transit Line 2, aiming at the strong permeability water-rich gravel stratum, the optimum mix ratio of slag and soil of conditioning agent for EPB shield to form slurry membrane was studied. The application effect of the slurry membrane was analyzed. The following conclusions are drawn:(1)It is necessary to inject a large amount of slag and soil of conditioning agent, including bentonite slurry and foaming agent diluent, into the soil bin and tunnel face to set up the slurry membrane of the EPB shield in the water-rich gravel stratum. Among them, sodium bentonite must be used in bentonite slurry. The mass ratio of bentonite to water must be in the range of 1 : 8–1 : 10, with the puffing time being 20 hours and the volume ratio of bentonite slurry to slag being 1.5 : 10–2 : 10. The concentration of the foaming agent diluent is set to 3-4%, and the mass ratio of the foaming agent diluent to the slag soil is 1 : 2.(2)The rationality of the model is verified by comparing the surface settlement values of field monitoring and numerical simulation. The water shutoff mechanism of the slurry membrane is revealed. Setting the slurry membrane in front of the tunnel face in the water-rich gravel stratum with EPB shield to exert good water plugging effect limiting pore-water pressure dissipation and balancing the soil and water pressure on the tunnel face effectively decrease the surface settlement.(3)The construction method of EPB shield “water plugging with slurry membrane-slag discharge with screw machine” was used in water-rich gravel stratum, which can significantly combine the respective advantages of EPB shield and slurry-water balance shield and ensure the safety, high quality, and high efficiency during tunnel construction.
Data Availability
The data used to support the findings of this study are included in the article. Some or all data, models, or codes generated or used during the study are available from the corresponding author by request.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
Authors’ Contributions
Jinpeng Zhao contributed to conceptualization, data processing, and validation; Zhongsheng Tan contributed to funding acquisition; Rongsen Yu and Aidong Jia contributed to investigation; Chao Qu contributed to supervision; Jinpeng Zhao and Rongsen Yu contributed to writing the original draft and review and editing the paper. All authors have read and agreed to the published version of the manuscript.
Acknowledgments
This work was supported by the National Natural Science Funds of China (Grant no. 51678034). The research and publication of this article were funded by the National Natural Science Foundation of China with associated Grant no. 51678041.