# Experimental Study on the Soil Conditioning Materials for EPB Shield Tunneling in Silty Sand

Authors: Chi-Hao Cheng; Shao-Ming Liao; Xiao-Bo Huo; Li-Sheng Chen
Journal: Advances in Civil Engineering
 (2020)

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## Abstract

Earth pressure balance (EPB) shield tunneling in a silty sand stratum is frequently faced with the wear of rotary cutter disc, clogging, or even collapse of workface due to its noncohesive and discrete properties of silty sand material. Soil conditioning is an effective way to reduce the discrete and friction properties of silty sand and to increase its rheology and fluidity, thus improving the cutting performance of EPB machines. However, soil conditioning materials were generally prepared and injected based on past limited field experiences or lab tests which were far from reality. In this article, a ground suitability test system for simulating shield tunneling in a conditioned ground was specially developed and used in a series of tests to investigate the influences of key factors of soil conditioning on the shield cutting performance. In addition, a field experiment of shield tunneling in silty sand of Wuhan Metro was conducted for verification. The major findings were obtained as follows. (1) The proposed test system performed well in simulating and assessing the cutting performance of EPB shield in conditioned soils, and the test results agreed well with the field test. (2) The soil conditioning materials can significantly reduce the cutting torque of shield tunneling in silty sand by up to 60%–70%. (3) The optimal foam and slurry parameters are suggested in the paper for shield tunneling in silty sand, respectively. (4) The test results reveal that the slurry conditioning is better than the foam in decreasing the cutter torque in silty sand. To achieve the same effect of soil conditioning, the injection ratios of foam and slurry should be 45% and 10%, respectively, to achieve the torque reduction ratio of 60%. These findings can provide a practical reference for engineers to determine the best-fit conditioning materials and construction parameters in the silty sand stratum.


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## Body

## 1. Introduction

Tunnel engineering has played a very important role in the rapid development of infrastructure construction [1–4]. Massive earth pressure balance (EPB) shields have been put into building comprehensive networks of underground transportation in megacities of China and around the world [5–8] . However, the widespread unfavorable geologic conditions often severely hamper the normal operation of this mechanized tunneling equipment [3, 9–11]. For instance, the shield tunneling in sand is prone to serious abrasion of cutter disk, downtime of advancement, and even collapse of the workface [12–14]. The use of soil conditioning material is an effective way to improve the applicability of EPB shields in different geological conditions [15–17].Many researchers and engineers have paid much attention to the soil conditioning materials and their injection parameters, as well as their effects on the shield driving performance [18–29]. Wei [30] proposed that excavated soils should ideally have a “paste flow” (e.g., low inner friction, preferable consistency, low permeability, and compressibility). Massive projects [31–35] considered the injection of conditioning agents such as foams, bentonite slurry, and polymers ahead of the cutter head, into the working chamber, and along the screw conveyor, to mix with the original soil during the excavation process. Peila et al. [36] performed a slump test to evaluate the flowability of conditioned soils, reaching a consensus that the ideal slump value is 100–250 mm [37–40]. Although the slump test is easy to perform and can reflect an overall plastic index on the behavior of the conditioned soil, the state of the prepared conditioned sample in the test is quite different from its actual states at the shield workface or in its working chamber. Several experimental devices have been developed to simulate the operation of shield machines [39, 41–48]. Sotiris et al. [46] utilized a mixing apparatus to simulate the mixing process in a shielded chamber so that the friction properties of conditioned soils can be assessed based on the difficulty in mixing. Merritt [42], Peila et al. [39], Rivas et al. [45], and Thewes and Budach [47] developed microscrew conveyor models for laboratory tests to investigate the performance of conditioned soils based on different screw speeds and tank pressures. Nevertheless, most of these experiments were carried out at atmospheric pressure without considering soil confining pressure and thus still far from application to field construction. In fact, the soils at the excavation face and in the working chamber or screws are under highly variable confining pressures. Therefore, it is necessary to perform a close-to-field dynamic simulation test on the relationship between the shield cutting performance and soil conditioning.In this paper, a special ground suitability test system for simulating shield driving was developed, and a series of tests were conducted to investigate the effect of different conditioners on the shield cutting performance. The simulation test considered four key factors (moisture content, soil pressure, soil conditioner type, and injection ratio) that affect soil conditioning performance the most. Finally, the laboratory test results were verified by the field observations of a real tunneling project in Wuhan, China.


## 2. Test Setup

The test system consists of a main test device and two soil conditioning devices (a foaming device and a slurry device), as shown in Figure1. The main test device was designed and manufactured to simulate the cutting and driving process of shield machine and automatically to measure and record the interactions between the cutter head and soils. The foaming device was used for foam generation and injection for different foaming materials. The slurry device was used for preparation and injection of bentonite slurry into the cutter head.Figure 1
Ground suitability test system for EPB tunneling.


### 2.1. Foaming Device

Figure2 shows the working principle of the foam generator, which consists mainly of a feeding system, air control system, foam mixer, and foam generator connected in series with a reducing valve, pressure gauge, flowmeter, and stop valve to gather and regulate the pressure and flow in the air and liquid mixer. Firstly, the foaming material is stored in the storage silo and connected to the air and liquid mixer. Then, it is delivered to the air-liquid mixer by the air compressor. Finally, the foam is produced by the foam generator after thorough mixing. Figure 3 shows an image of the assembled foaming device, and the dimension is 0.6 meters long, 0.4 meters wide, and 0.6 meters high.Figure 2
System schematic of foam device. 1, air compressor; 2, reducing valve; 3, pressure gage; 4, storage silo; 5, flowmeter; 6, stop valve; 7, air and liquid mixer; 8, foam generator; 9, air flow sensor.Figure 3
Image of foam generator.


### 2.2. Slurry Device

The slurry device mainly includes a storage silo, syringe, and cylinder (Figure4). The dimension of length, width, and height is 0.7 meters, 0.53 meters, and 1.3 meters. The prepared slurry is transferred from the storage silo to the syringe by controlling the ball valve. Then, the slurry is injected into the drain pipe by the extension of the cylinder. The injection pressure and rate are automatically recorded.Figure 4
Slurry device. 1, storage silo; 2, syringe; 3, cylinder; 4, ball valve; 5, drain pipe.


### 2.3. Main Device

The main device is composed of a propulsion system, cutter head, and soil chamber system used to simulate the driving process of EPB shield machine (Figures5 and 6). The external dimension is 3.1 meters long, 1.2 meters wide, and 1.2 meters high. The response data from the main device are automatically recorded to assess the performance of cutter head under different soil conditioning situations. Firstly, the soil sample in the soil chamber is pressurized to a required value equivalent to the target ground pressure. By regulating oil pressure cylinder and monitoring pressure gauge, the ground overload can also be taken into consideration. Then, the soil is cut using the cutter head driven by a propulsion system under a set advance rate and rotation speed. At the same time, soil conditioning materials are injected into the front of cutter head at a rate set by the foaming device and slurry device. The torque and thrust of cutter head, earth pressure, are automatically recorded in the driving process.Figure 5
Propulsion and cutting system.Figure 6
Soil chamber system.According to the similarity principles of scaled test, the cutter torque for actual shield machine can be determined by the simulation test as follows:(1)T=αk3T0,where T is the cutter torque of actual shield machine, T0 is the cutter torque from the simulation test, k is the similarity ratio of the cutter diameter between the test device and shield machine, and α is the calibration coefficient of the test system (α = 1.1 ∼ 1.3).


## 2.1. Foaming Device

Figure2 shows the working principle of the foam generator, which consists mainly of a feeding system, air control system, foam mixer, and foam generator connected in series with a reducing valve, pressure gauge, flowmeter, and stop valve to gather and regulate the pressure and flow in the air and liquid mixer. Firstly, the foaming material is stored in the storage silo and connected to the air and liquid mixer. Then, it is delivered to the air-liquid mixer by the air compressor. Finally, the foam is produced by the foam generator after thorough mixing. Figure 3 shows an image of the assembled foaming device, and the dimension is 0.6 meters long, 0.4 meters wide, and 0.6 meters high.Figure 2
System schematic of foam device. 1, air compressor; 2, reducing valve; 3, pressure gage; 4, storage silo; 5, flowmeter; 6, stop valve; 7, air and liquid mixer; 8, foam generator; 9, air flow sensor.Figure 3
Image of foam generator.


## 2.2. Slurry Device

The slurry device mainly includes a storage silo, syringe, and cylinder (Figure4). The dimension of length, width, and height is 0.7 meters, 0.53 meters, and 1.3 meters. The prepared slurry is transferred from the storage silo to the syringe by controlling the ball valve. Then, the slurry is injected into the drain pipe by the extension of the cylinder. The injection pressure and rate are automatically recorded.Figure 4
Slurry device. 1, storage silo; 2, syringe; 3, cylinder; 4, ball valve; 5, drain pipe.


## 2.3. Main Device

The main device is composed of a propulsion system, cutter head, and soil chamber system used to simulate the driving process of EPB shield machine (Figures5 and 6). The external dimension is 3.1 meters long, 1.2 meters wide, and 1.2 meters high. The response data from the main device are automatically recorded to assess the performance of cutter head under different soil conditioning situations. Firstly, the soil sample in the soil chamber is pressurized to a required value equivalent to the target ground pressure. By regulating oil pressure cylinder and monitoring pressure gauge, the ground overload can also be taken into consideration. Then, the soil is cut using the cutter head driven by a propulsion system under a set advance rate and rotation speed. At the same time, soil conditioning materials are injected into the front of cutter head at a rate set by the foaming device and slurry device. The torque and thrust of cutter head, earth pressure, are automatically recorded in the driving process.Figure 5
Propulsion and cutting system.Figure 6
Soil chamber system.According to the similarity principles of scaled test, the cutter torque for actual shield machine can be determined by the simulation test as follows:(1)T=αk3T0,where T is the cutter torque of actual shield machine, T0 is the cutter torque from the simulation test, k is the similarity ratio of the cutter diameter between the test device and shield machine, and α is the calibration coefficient of the test system (α = 1.1 ∼ 1.3).


## 3. Material Preparation

Material preparation involves as series of material tests on soil samples, foam, and slurry, which should be all prepared and optimized before the main test begin.


### 3.1. Soil

The soil samples for the test were silty sand taken from the tunnel site of Ling-Xiang tunnel section, of Wuhan Metro Line 3, which is 1200 m, mainly located in silty sand, as shown in Figures7–9. The natural moisture content of the silty sand ranges from 10% to 30% with internal friction angle of 30°, as shown in Table 1. The silty sand from the tunnel site was remixed with water and reconsolidated to be the same moisture content and friction angle as its natural state in the pressurized soil chamber. The processing steps begin with the air drying of the soil sample, then followed by grinding, sieving, mixing, sealing, and storing. Finally, the soil samples are layered compacted to achieve the required moisture content and density. Otherwise, considering the difference in mechanical property between original soil and remolded soil, the calibration coefficient α is adopted in equation (1). According to the natural particle grading curve of the silty sand (marked by red line in Figure 10), the recommended soil conditioning material for the soil is either foam or slurry [49, 50].Figure 7
Metro lines of Wuhan city and the location of field test site.Figure 8
Geological profile of the experimental tunnel.Figure 9
Test sample of silty sand from the tunnel site.Table 1
Physical and mechanical parameters of soils.
No.Soil layersγ (kN/m3)ω (%)c (kPa)φ (°)Es (MPa)3–2Clay18.018.618105.03–3Muddy clay17.522.51043.03–4Silty clay18.029.213145.53–5Silty sand18.530.503014Note.γ is the unit weight; ω is the water content; c is the cohesion; φ is the friction angle; Es is the constrained modulus.Figure 10
Particle grading curves of soils and suitable conditioning materials.


### 3.2. Foam

The foam conditioning material for shield tunneling is usually prepared by a foaming device to mix water with foaming agent, which is a surfactant that reduces surface tension of water to create the foam or increases its colloidal stability by inhibiting coalescence of bubbles. In order to find the best fit foaming and injection parameters for the silty sand, the foams were prepared by adjusting the foaming device to have a different foam expansion ratio (Fer, see equation (2)) and foam stability (Fs, see equation (3)), which were described in detail by Wu et al. [51].The recommended value ofFer for shield tunneling is 20–40:(2)Fer=VfVl,where Vf is the volume of foam and Vl is the volume of the foaming solution.Fs can be measured by the foam dissipation test using the following equation:(3)Fs=mdmo,where md is the weight of the dissipated foam and mo is the initial weight of the foam. In shield tunneling practice, the weight of dissipated foam is changing over time, and thus, the half-dissipation time (T1/2) (i.e., the time during when Fs = 0.5) of foam is generally used to assess the stability of foam. The value ranges from 15–20 min for shield tunneling.In practice, theFer and T1/2 are greatly influenced by concentration of the foam solution, air flow, liquid flow, air pressure, and liquid pressure. Considering the complex influences of these five factors, the multivariable tests (orthogonal tests of 4 levels and 5 variables) were performed firstly to determine the possible set of optimal foaming parameters, with test conditions shown in Tables 2 and 3. And then, single-variable tests were performed further to verify and confirm the foaming parameters.Table 2
Test levels and foaming parameters.
LevelConcentration of foam solution (A) (%)Air pressure (B) (MPa)Air flow (C) (L/min)Liquid pressure (D) (MPa)Liquid flow (E) (mL/min)12.00.21.00.25023.00.32.00.310034.00.43.00.415045.00.54.00.5200Table 3
Scheme of orthogonal test.
No.Concentration of foam solution (A) (%)Air pressure (B) (MPa)Air flow (C) (L/min)Liquid pressure (D) (MPa)Liquid flow (E) (ml/min)LevelValueLevelValueLevelValueLevelValueLevelValue112.020.333.020.33150234.040.511.020.32100323.040.533.030.44200445.020.311.030.4150512.030.411.040.54200634.010.233.040.5150723.010.211.010.23150845.030.433.010.22100912.010.244.030.421001034.030.422.030.431501123.030.444.020.31501245.010.222.020.342001312.040.522.010.21501434.020.344.010.242001523.020.322.040.521001645.040.544.040.53150According to the multivariable test results shown in Figures11 and 12, the possible set of optimal foaming parameters is 3.0% foam solution concentration, 3.0 L/min air flow, 150 mL/min liquid flow, 0.3 MPa air pressure, and 0.4 MPa liquid pressure. To further verify and amend the results of multivariable test, the single-variable tests are conducted, and the results are summarized in Figure 13. As shown in Figures 13(a) and 13(b), when the concentration of foam solution increases to 3%, the half-dissipation time and expansion ratio nearly reach the maximum values of 1200 s and 40, respectively. Figure 13(c) illustrates that the foam has the best stability when the air pressure is 0.3 MPa, and the expansion ratio is controlled in 20–40 (see Figure 13(d)). Figures 13(e) and 13(f) show the relationship between the foam properties and air flow; the preferred air flow is 3 L/min. Additionally, the liquid pressure and liquid flow should be 0.4 MPa and 150 mL/min, respectively, as shown in Figures 13(g)–13(j). Hence, the optimized foaming parameters were confirmed as follows: 3.0% foam solution concentration, 3.0 L/min air flow, 150 mL/min liquid flow, 0.3 MPa air pressure, and 0.4 MPa liquid pressure. The corresponding half-dissipation time and the foam expansion ratio are recommended as 1217 s and 39.81, respectively.Figure 11
Effect of foaming parameters on half-dissipation time.Figure 12
Effect of foaming parameters on foam expansion ratio.Figure 13
Effect of foaming parameters on foam properties.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)


### 3.3. Bentonite Slurry and CMC

The other conditioning material for the test is the bentonite with addition of carboxyl methyl cellulose (CMC, [C6H7O2(OH)2CH2COONa]n) which can significantly increase the viscosity of bentonite slurry as thickener when it is dissolved in water. The influences of the bentonite (Figure 14) and CMC (Figure 15) concentration were investigated via an optimization experiment of the slurry properties (density and funnel viscosity) (Table 4). In the 12 initial try tests shown in the table, only the concentration of bentonite was adjusted, and the results show that the funnel viscosity is more sensitive to the bentonite concentration than the density. As shown in Figure 16, the funnel viscosity increases with the bentonite concentration. When the concentration reaches 11%, the viscosity increases sharply and part of bentonite tends to be insoluble in water. To maintain the viscosity between 60 and 80 s, which is suitable for this type of ground, bentonite concentrations of 5%, 8%, and 10% are mixed with CMC (test no. 13 to test no. 24 in Table 4). According to Figure 17, the addition rate of CMC has an obvious effect on the funnel viscosity for bentonite concentrations of 8% and 10%. Hence, part of bentonite can be replaced with CMC to improve the solubility of the slurry. The recommended slurry proportioning is 8% bentonite and 0.5% CMC, resulting in a density and funnel viscosity of 1.054 g/cm3 and 79.36 s, respectively.Figure 14
Bentonite.Figure 15
Carboxyl methyl cellulose (CMC).Table 4
Results of slurry optimization experiment.
TypeNo.WaterBentoniteCMCDensityViscosity(g)(%)(g)(%)(g)(g/cm3)(s)Initial tests14000140001.01715.2624000280001.02215.63340003120001.02716.50440004160001.03217.61540005200001.03719.01640006240001.04422.81740007280001.04827.96840008320001.05332.81940009360001.05949.9610400010400001.06377.2311400011440001.071138.5612400012480001.074238.00Combination tests13400052000.150.31.03622.7314400052000.30.61.03723.8815400052000.511.03625.5416400052000.71.41.03730.5817400083200.150.481.05647.4618400083200.30.961.06259.4619400083200.51.61.05479.3620400083200.72.241.062117.43214000104000.150.61.06288.18224000104000.31.21.071133.30234000104000.521.072185.30244000104000.72.81.071245.60Figure 16
Effect of bentonite concentration on funnel viscosity.Figure 17
Effect of addition rate of CMC on funnel viscosity.


## 3.1. Soil

The soil samples for the test were silty sand taken from the tunnel site of Ling-Xiang tunnel section, of Wuhan Metro Line 3, which is 1200 m, mainly located in silty sand, as shown in Figures7–9. The natural moisture content of the silty sand ranges from 10% to 30% with internal friction angle of 30°, as shown in Table 1. The silty sand from the tunnel site was remixed with water and reconsolidated to be the same moisture content and friction angle as its natural state in the pressurized soil chamber. The processing steps begin with the air drying of the soil sample, then followed by grinding, sieving, mixing, sealing, and storing. Finally, the soil samples are layered compacted to achieve the required moisture content and density. Otherwise, considering the difference in mechanical property between original soil and remolded soil, the calibration coefficient α is adopted in equation (1). According to the natural particle grading curve of the silty sand (marked by red line in Figure 10), the recommended soil conditioning material for the soil is either foam or slurry [49, 50].Figure 7
Metro lines of Wuhan city and the location of field test site.Figure 8
Geological profile of the experimental tunnel.Figure 9
Test sample of silty sand from the tunnel site.Table 1
Physical and mechanical parameters of soils.
No.Soil layersγ (kN/m3)ω (%)c (kPa)φ (°)Es (MPa)3–2Clay18.018.618105.03–3Muddy clay17.522.51043.03–4Silty clay18.029.213145.53–5Silty sand18.530.503014Note.γ is the unit weight; ω is the water content; c is the cohesion; φ is the friction angle; Es is the constrained modulus.Figure 10
Particle grading curves of soils and suitable conditioning materials.


## 3.2. Foam

The foam conditioning material for shield tunneling is usually prepared by a foaming device to mix water with foaming agent, which is a surfactant that reduces surface tension of water to create the foam or increases its colloidal stability by inhibiting coalescence of bubbles. In order to find the best fit foaming and injection parameters for the silty sand, the foams were prepared by adjusting the foaming device to have a different foam expansion ratio (Fer, see equation (2)) and foam stability (Fs, see equation (3)), which were described in detail by Wu et al. [51].The recommended value ofFer for shield tunneling is 20–40:(2)Fer=VfVl,where Vf is the volume of foam and Vl is the volume of the foaming solution.Fs can be measured by the foam dissipation test using the following equation:(3)Fs=mdmo,where md is the weight of the dissipated foam and mo is the initial weight of the foam. In shield tunneling practice, the weight of dissipated foam is changing over time, and thus, the half-dissipation time (T1/2) (i.e., the time during when Fs = 0.5) of foam is generally used to assess the stability of foam. The value ranges from 15–20 min for shield tunneling.In practice, theFer and T1/2 are greatly influenced by concentration of the foam solution, air flow, liquid flow, air pressure, and liquid pressure. Considering the complex influences of these five factors, the multivariable tests (orthogonal tests of 4 levels and 5 variables) were performed firstly to determine the possible set of optimal foaming parameters, with test conditions shown in Tables 2 and 3. And then, single-variable tests were performed further to verify and confirm the foaming parameters.Table 2
Test levels and foaming parameters.
LevelConcentration of foam solution (A) (%)Air pressure (B) (MPa)Air flow (C) (L/min)Liquid pressure (D) (MPa)Liquid flow (E) (mL/min)12.00.21.00.25023.00.32.00.310034.00.43.00.415045.00.54.00.5200Table 3
Scheme of orthogonal test.
No.Concentration of foam solution (A) (%)Air pressure (B) (MPa)Air flow (C) (L/min)Liquid pressure (D) (MPa)Liquid flow (E) (ml/min)LevelValueLevelValueLevelValueLevelValueLevelValue112.020.333.020.33150234.040.511.020.32100323.040.533.030.44200445.020.311.030.4150512.030.411.040.54200634.010.233.040.5150723.010.211.010.23150845.030.433.010.22100912.010.244.030.421001034.030.422.030.431501123.030.444.020.31501245.010.222.020.342001312.040.522.010.21501434.020.344.010.242001523.020.322.040.521001645.040.544.040.53150According to the multivariable test results shown in Figures11 and 12, the possible set of optimal foaming parameters is 3.0% foam solution concentration, 3.0 L/min air flow, 150 mL/min liquid flow, 0.3 MPa air pressure, and 0.4 MPa liquid pressure. To further verify and amend the results of multivariable test, the single-variable tests are conducted, and the results are summarized in Figure 13. As shown in Figures 13(a) and 13(b), when the concentration of foam solution increases to 3%, the half-dissipation time and expansion ratio nearly reach the maximum values of 1200 s and 40, respectively. Figure 13(c) illustrates that the foam has the best stability when the air pressure is 0.3 MPa, and the expansion ratio is controlled in 20–40 (see Figure 13(d)). Figures 13(e) and 13(f) show the relationship between the foam properties and air flow; the preferred air flow is 3 L/min. Additionally, the liquid pressure and liquid flow should be 0.4 MPa and 150 mL/min, respectively, as shown in Figures 13(g)–13(j). Hence, the optimized foaming parameters were confirmed as follows: 3.0% foam solution concentration, 3.0 L/min air flow, 150 mL/min liquid flow, 0.3 MPa air pressure, and 0.4 MPa liquid pressure. The corresponding half-dissipation time and the foam expansion ratio are recommended as 1217 s and 39.81, respectively.Figure 11
Effect of foaming parameters on half-dissipation time.Figure 12
Effect of foaming parameters on foam expansion ratio.Figure 13
Effect of foaming parameters on foam properties.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)


## 3.3. Bentonite Slurry and CMC

The other conditioning material for the test is the bentonite with addition of carboxyl methyl cellulose (CMC, [C6H7O2(OH)2CH2COONa]n) which can significantly increase the viscosity of bentonite slurry as thickener when it is dissolved in water. The influences of the bentonite (Figure 14) and CMC (Figure 15) concentration were investigated via an optimization experiment of the slurry properties (density and funnel viscosity) (Table 4). In the 12 initial try tests shown in the table, only the concentration of bentonite was adjusted, and the results show that the funnel viscosity is more sensitive to the bentonite concentration than the density. As shown in Figure 16, the funnel viscosity increases with the bentonite concentration. When the concentration reaches 11%, the viscosity increases sharply and part of bentonite tends to be insoluble in water. To maintain the viscosity between 60 and 80 s, which is suitable for this type of ground, bentonite concentrations of 5%, 8%, and 10% are mixed with CMC (test no. 13 to test no. 24 in Table 4). According to Figure 17, the addition rate of CMC has an obvious effect on the funnel viscosity for bentonite concentrations of 8% and 10%. Hence, part of bentonite can be replaced with CMC to improve the solubility of the slurry. The recommended slurry proportioning is 8% bentonite and 0.5% CMC, resulting in a density and funnel viscosity of 1.054 g/cm3 and 79.36 s, respectively.Figure 14
Bentonite.Figure 15
Carboxyl methyl cellulose (CMC).Table 4
Results of slurry optimization experiment.
TypeNo.WaterBentoniteCMCDensityViscosity(g)(%)(g)(%)(g)(g/cm3)(s)Initial tests14000140001.01715.2624000280001.02215.63340003120001.02716.50440004160001.03217.61540005200001.03719.01640006240001.04422.81740007280001.04827.96840008320001.05332.81940009360001.05949.9610400010400001.06377.2311400011440001.071138.5612400012480001.074238.00Combination tests13400052000.150.31.03622.7314400052000.30.61.03723.8815400052000.511.03625.5416400052000.71.41.03730.5817400083200.150.481.05647.4618400083200.30.961.06259.4619400083200.51.61.05479.3620400083200.72.241.062117.43214000104000.150.61.06288.18224000104000.31.21.071133.30234000104000.521.072185.30244000104000.72.81.071245.60Figure 16
Effect of bentonite concentration on funnel viscosity.Figure 17
Effect of addition rate of CMC on funnel viscosity.


## 4. Main Test

### 4.1. Test Procedure

The testing conditions and parameters are shown in Tables5–7. The soil samples were configured with two moisture contents of 10% and 30% and then pressurized at 0.1 MPa and 0.25 MPa, respectively. Based on the abovementioned optimized parameters of foam and slurry, the injection process was performed in several stages with different injection ratios, as shown in Tables 6 and 7. The driving parameters of the shield cutter head were set, as shown in Table 8, in compliance with the onsite construction parameters of shield. During the test process, the cutter torque was automatically recorded by the torque sensor installed on the ground suitability test system. The cutter torque is regarded as a comprehensive index in the test for accessing the cutting performance because the changes in cutter torque closely reflect the abrasion of cutters and cutting efficiency. The process of testing is shown in Figure 18.Table 5
Testing conditions for soil conditioning.
No.Soil pressure (MPa)Water content (%)Soil conditionerInjection ratio of volume10.110Foam0–100%20.110Slurry0–30%30.130Foam0–100%40.130Slurry0–30%50.2510Foam0–100%60.2510Slurry0–30%70.2530Foam0–100%80.2530Slurry0–30%Table 6
Relationship between foam injection ratio and advance distance.
Advance distance (mm)0–3031–6061–9091–120121–150151–180Injection ratio (%)010–1530–3550–5570–7590–100Table 7
Relationship between slurry injection ratio and advance distance.
Advance distance (mm)0–3031–6061–9091–120121–150151–180181–210211–240Injection ratio (%)03–57–810–1214–1618–2024–2628–30Table 8
Driving parameters of shield cutter head.
Opening ratio (%)Rotating speedAdvance speed501 r/min30 mm/minFigure 18
The testing process.


### 4.2. Test Results

According to the above test procedure, the influences of the moisture content and confining pressure of soil, the conditioner type, and the injection ratio on the cutting performance were experimentally investigated, respectively, and summarized below.


#### 4.2.1. The Effect of Moisture Content of Silt Sand

As shown in Figures19–22, the moisture content of the silty sand affects significantly the cutting torque, which increased by 5 to 7 times when the moisture content increased from 10% to 30% in case of no soil conditioning. However, it was only when the foam injection ratio reached 30% or when the slurry injection ratio exceeded 5% that the cutter torque began to reduce remarkably.Figure 19
Effect of foam conditioning and soil water contents on cutter torque at soil pressure 0.25 MPa.Figure 20
Effect of slurry conditioning and soil water contents on cutter torque at soil pressure 0.25 MPa.Figure 21
Effect of foam conditioning and soil water contents on cutter torque at soil pressure 0.1 MPa.Figure 22
Effect of slurry conditioning and soil water contents on cutter torque at soil pressure 0.1 MPa.The maximum reduction in the cutter torque was a result of combined effect of the moisture content and soil pressure. When the moisture content was 10% at a soil pressure of 0.1 MPa, the maximum reduction ratio of cutter torque occurred as 40%–60%, while at the same moisture content and higher pressure of 0.25 MPa, the maximum reduction in cutter torque was only 20%–30%. However, the maximum reduction ratio could reach 60%–70% when the moisture content was 30% and soil pressure was 0.25 MPa.


#### 4.2.2. The Effect of Confining Pressure

As shown in Figures23–26, the cutter torque in silty sand is positively correlated with the soil confining pressure. When the soil confining pressure increased from 0.1 MPa to 0.25 MPa, the cutter torque increased by 2 to 3 times in case of no soil conditioning.Figure 23
Effect of foam conditioning and soil pressure on cutter torque at water content 30%.Figure 24
Effect of foam conditioning and soil pressure on cutter torque at water content 10%.Figure 25
Effect of slurry conditioning and soil pressure on cutter torque at water content 30%.Figure 26
Effect of slurry conditioning and soil pressure on cutter torque at water content 10%.When the soil moisture content was 30%, even a high soil confining pressure could result in a larger decrease in the cutter torque with soil conditioning. For instance, a maximum decrease in the cutter torque of 60% could be obtained when the soil confining pressure was 0.25 MPa, while only a 15% decrease occurred when the soil confining pressure was 0.1 MPa, as shown in Figure23.However, the opposite was observed when the moisture content was 10%. A higher soil pressure resulted in smaller decreases in the cutter torque with soil conditioning. However, the differences in the reduction of cutter torque under the two pressure conditions were close. As shown in Figure24, the maximum reduction was only 1200–1500 N m under different soil pressures.


#### 4.2.3. The Effect of Conditioning Material

The effect of different kinds of soil conditioners on the cutting performance was compared under the same moisture content and soil pressure conditions, as shown in Figures27–30.Figure 27
Effect of two types of soil conditioners on cutter torque at water content 30% and soil pressure 0.25 MPa.Figure 28
Effect of two types of soil conditioners on cutter torque at water content 30% and soil pressure 0.1 MPa.Figure 29
Effect of two types of soil conditioners on cutter torque at water content 10% and soil pressure 0.25 MPa.Figure 30
Effect of two types of soil conditioners on cutter torque at water content 10% and soil pressure 0.1 MPa.The results revealed that the effect of soil conditioning is limited when the injection ratio reaches a certain value. For example, Figure27 shows that when the injection ratio of slurry exceeds 15% or that of foam exceeds 45%, the decrease rate of the cutter torque tends to be stable.Judging from the maximum decrease of the torque, slurry was a better option than foam for decreasing the cutter torque. In order to achieve the same effect of soil conditioning, the injection ratios of the two types of soil conditioners were different. As shown in Figure27, the injection ratios of foam and slurry should be 45% and 10%, respectively, to achieve the same maximum reduction of torque at 60%.


## 4.1. Test Procedure

The testing conditions and parameters are shown in Tables5–7. The soil samples were configured with two moisture contents of 10% and 30% and then pressurized at 0.1 MPa and 0.25 MPa, respectively. Based on the abovementioned optimized parameters of foam and slurry, the injection process was performed in several stages with different injection ratios, as shown in Tables 6 and 7. The driving parameters of the shield cutter head were set, as shown in Table 8, in compliance with the onsite construction parameters of shield. During the test process, the cutter torque was automatically recorded by the torque sensor installed on the ground suitability test system. The cutter torque is regarded as a comprehensive index in the test for accessing the cutting performance because the changes in cutter torque closely reflect the abrasion of cutters and cutting efficiency. The process of testing is shown in Figure 18.Table 5
Testing conditions for soil conditioning.
No.Soil pressure (MPa)Water content (%)Soil conditionerInjection ratio of volume10.110Foam0–100%20.110Slurry0–30%30.130Foam0–100%40.130Slurry0–30%50.2510Foam0–100%60.2510Slurry0–30%70.2530Foam0–100%80.2530Slurry0–30%Table 6
Relationship between foam injection ratio and advance distance.
Advance distance (mm)0–3031–6061–9091–120121–150151–180Injection ratio (%)010–1530–3550–5570–7590–100Table 7
Relationship between slurry injection ratio and advance distance.
Advance distance (mm)0–3031–6061–9091–120121–150151–180181–210211–240Injection ratio (%)03–57–810–1214–1618–2024–2628–30Table 8
Driving parameters of shield cutter head.
Opening ratio (%)Rotating speedAdvance speed501 r/min30 mm/minFigure 18
The testing process.


## 4.2. Test Results

According to the above test procedure, the influences of the moisture content and confining pressure of soil, the conditioner type, and the injection ratio on the cutting performance were experimentally investigated, respectively, and summarized below.


### 4.2.1. The Effect of Moisture Content of Silt Sand

As shown in Figures19–22, the moisture content of the silty sand affects significantly the cutting torque, which increased by 5 to 7 times when the moisture content increased from 10% to 30% in case of no soil conditioning. However, it was only when the foam injection ratio reached 30% or when the slurry injection ratio exceeded 5% that the cutter torque began to reduce remarkably.Figure 19
Effect of foam conditioning and soil water contents on cutter torque at soil pressure 0.25 MPa.Figure 20
Effect of slurry conditioning and soil water contents on cutter torque at soil pressure 0.25 MPa.Figure 21
Effect of foam conditioning and soil water contents on cutter torque at soil pressure 0.1 MPa.Figure 22
Effect of slurry conditioning and soil water contents on cutter torque at soil pressure 0.1 MPa.The maximum reduction in the cutter torque was a result of combined effect of the moisture content and soil pressure. When the moisture content was 10% at a soil pressure of 0.1 MPa, the maximum reduction ratio of cutter torque occurred as 40%–60%, while at the same moisture content and higher pressure of 0.25 MPa, the maximum reduction in cutter torque was only 20%–30%. However, the maximum reduction ratio could reach 60%–70% when the moisture content was 30% and soil pressure was 0.25 MPa.


### 4.2.2. The Effect of Confining Pressure

As shown in Figures23–26, the cutter torque in silty sand is positively correlated with the soil confining pressure. When the soil confining pressure increased from 0.1 MPa to 0.25 MPa, the cutter torque increased by 2 to 3 times in case of no soil conditioning.Figure 23
Effect of foam conditioning and soil pressure on cutter torque at water content 30%.Figure 24
Effect of foam conditioning and soil pressure on cutter torque at water content 10%.Figure 25
Effect of slurry conditioning and soil pressure on cutter torque at water content 30%.Figure 26
Effect of slurry conditioning and soil pressure on cutter torque at water content 10%.When the soil moisture content was 30%, even a high soil confining pressure could result in a larger decrease in the cutter torque with soil conditioning. For instance, a maximum decrease in the cutter torque of 60% could be obtained when the soil confining pressure was 0.25 MPa, while only a 15% decrease occurred when the soil confining pressure was 0.1 MPa, as shown in Figure23.However, the opposite was observed when the moisture content was 10%. A higher soil pressure resulted in smaller decreases in the cutter torque with soil conditioning. However, the differences in the reduction of cutter torque under the two pressure conditions were close. As shown in Figure24, the maximum reduction was only 1200–1500 N m under different soil pressures.


### 4.2.3. The Effect of Conditioning Material

The effect of different kinds of soil conditioners on the cutting performance was compared under the same moisture content and soil pressure conditions, as shown in Figures27–30.Figure 27
Effect of two types of soil conditioners on cutter torque at water content 30% and soil pressure 0.25 MPa.Figure 28
Effect of two types of soil conditioners on cutter torque at water content 30% and soil pressure 0.1 MPa.Figure 29
Effect of two types of soil conditioners on cutter torque at water content 10% and soil pressure 0.25 MPa.Figure 30
Effect of two types of soil conditioners on cutter torque at water content 10% and soil pressure 0.1 MPa.The results revealed that the effect of soil conditioning is limited when the injection ratio reaches a certain value. For example, Figure27 shows that when the injection ratio of slurry exceeds 15% or that of foam exceeds 45%, the decrease rate of the cutter torque tends to be stable.Judging from the maximum decrease of the torque, slurry was a better option than foam for decreasing the cutter torque. In order to achieve the same effect of soil conditioning, the injection ratios of the two types of soil conditioners were different. As shown in Figure27, the injection ratios of foam and slurry should be 45% and 10%, respectively, to achieve the same maximum reduction of torque at 60%.


## 4.2.1. The Effect of Moisture Content of Silt Sand

As shown in Figures19–22, the moisture content of the silty sand affects significantly the cutting torque, which increased by 5 to 7 times when the moisture content increased from 10% to 30% in case of no soil conditioning. However, it was only when the foam injection ratio reached 30% or when the slurry injection ratio exceeded 5% that the cutter torque began to reduce remarkably.Figure 19
Effect of foam conditioning and soil water contents on cutter torque at soil pressure 0.25 MPa.Figure 20
Effect of slurry conditioning and soil water contents on cutter torque at soil pressure 0.25 MPa.Figure 21
Effect of foam conditioning and soil water contents on cutter torque at soil pressure 0.1 MPa.Figure 22
Effect of slurry conditioning and soil water contents on cutter torque at soil pressure 0.1 MPa.The maximum reduction in the cutter torque was a result of combined effect of the moisture content and soil pressure. When the moisture content was 10% at a soil pressure of 0.1 MPa, the maximum reduction ratio of cutter torque occurred as 40%–60%, while at the same moisture content and higher pressure of 0.25 MPa, the maximum reduction in cutter torque was only 20%–30%. However, the maximum reduction ratio could reach 60%–70% when the moisture content was 30% and soil pressure was 0.25 MPa.


## 4.2.2. The Effect of Confining Pressure

As shown in Figures23–26, the cutter torque in silty sand is positively correlated with the soil confining pressure. When the soil confining pressure increased from 0.1 MPa to 0.25 MPa, the cutter torque increased by 2 to 3 times in case of no soil conditioning.Figure 23
Effect of foam conditioning and soil pressure on cutter torque at water content 30%.Figure 24
Effect of foam conditioning and soil pressure on cutter torque at water content 10%.Figure 25
Effect of slurry conditioning and soil pressure on cutter torque at water content 30%.Figure 26
Effect of slurry conditioning and soil pressure on cutter torque at water content 10%.When the soil moisture content was 30%, even a high soil confining pressure could result in a larger decrease in the cutter torque with soil conditioning. For instance, a maximum decrease in the cutter torque of 60% could be obtained when the soil confining pressure was 0.25 MPa, while only a 15% decrease occurred when the soil confining pressure was 0.1 MPa, as shown in Figure23.However, the opposite was observed when the moisture content was 10%. A higher soil pressure resulted in smaller decreases in the cutter torque with soil conditioning. However, the differences in the reduction of cutter torque under the two pressure conditions were close. As shown in Figure24, the maximum reduction was only 1200–1500 N m under different soil pressures.


## 4.2.3. The Effect of Conditioning Material

The effect of different kinds of soil conditioners on the cutting performance was compared under the same moisture content and soil pressure conditions, as shown in Figures27–30.Figure 27
Effect of two types of soil conditioners on cutter torque at water content 30% and soil pressure 0.25 MPa.Figure 28
Effect of two types of soil conditioners on cutter torque at water content 30% and soil pressure 0.1 MPa.Figure 29
Effect of two types of soil conditioners on cutter torque at water content 10% and soil pressure 0.25 MPa.Figure 30
Effect of two types of soil conditioners on cutter torque at water content 10% and soil pressure 0.1 MPa.The results revealed that the effect of soil conditioning is limited when the injection ratio reaches a certain value. For example, Figure27 shows that when the injection ratio of slurry exceeds 15% or that of foam exceeds 45%, the decrease rate of the cutter torque tends to be stable.Judging from the maximum decrease of the torque, slurry was a better option than foam for decreasing the cutter torque. In order to achieve the same effect of soil conditioning, the injection ratios of the two types of soil conditioners were different. As shown in Figure27, the injection ratios of foam and slurry should be 45% and 10%, respectively, to achieve the same maximum reduction of torque at 60%.


## 5. Field Test and Verification

To realize the onsite effect of soil conditioning on the cutting performance of the shield machine, a field experiment was conducted on a real tunneling project in Wuhan with Φ6.26 m EPB shield (shown in Figure31). The tunneling parameters of the cutter head are listed in Table 9. There were totally 60 rings taken for the field experiment, in which the 1st to 30th rings were driven without soil conditioning while the 31st to 60th rings with foam conditioning at different injection ratios, as shown in Figure 8. The foaming parameters adopted in the field test were the same as the laboratory test.Figure 31
Picture of EPB shield.Table 9
Tunneling parameters of cutter head.
ModeEPBDiameter6260 mmOpen ratio50%Rated torque7206 kN mTotal thrust40000 kNSoil pressure0.16–0.22 MPaAdvance speed30 mm/minRotating speed1 r/minAs shown in Figure32, soil conditioning had an evident effect on shield cutting. During tunneling in the first 30 rings, the cutter torque fluctuated over a wide range from 3000 kN·m to 4500 kN m. When the soil was conditioned with foam and water from the 31th ring onward, the torque sharply decreased to 2200–3200 kN m, about 25%–50% drop.Figure 32
Comparison of laboratory test with field test result.The predictions of the shield cutter torque based on the laboratory test results were compared with those from the field test in Figure32, which shows they were in good agreement and most of the measured values were within the range of predicted values.


## 6. Conclusion

In order to realize the effect of soil conditioning on the shield cutting performance and to optimize the conditioning materials, a series of laboratory tests were conducted using a self-developed ground suitability test system. The effect of four key factors was comprehensively investigated on the shield cutting performance and verified by field experiment of a real tunnel project. The major findings were obtained as follows:(1)
The ground suitability test system performed well, and the prediction of the cutter torque based on the test agreed well with the field test. It can prospectively be served as an onsite real time testing equipment for shield tunneling.(2)
The optimal foam parameters are suggested for shield tunneling in silty sand as 3.0% foam solution concentration, 3.0 L/min air flow, 150 mL/min liquid flow, 0.3 MPa air pressure, and 0.4 MPa liquid pressure with half-dissipation time and expansion ratio as 1217 s and 39.81, respectively. The optimal slurry is recommended as 8% bentonite and 0.5% CMC with resulted density and funnel viscosity as 1.054 g/cm3 and 79.36 s, respectively.(3)
The shield cutter torque is also sensitive to the moisture content and confining pressure of silty sand and the injection ratio of conditioners. The torque at moisture content of 10% is 5 to 7 times more than that at moisture content of 30% in case of no soil conditioning. When the foam injection ratio exceeds 30% or the slurry injection ratio exceeds 5%, the shield cutter torque starts to significantly decrease. The maximum decrease in cutter torque by 40%–60% was achieved by optimized soil conditioning, moisture content, and soil pressure. However, the effect of soil conditioning is limited when the injection ratio reaches a certain value.(4)
In comparison, slurry is better than foam in decreasing the cutter torque in silty sand. To achieve the same effect of soil conditioning, the injection ratios of foam and slurry should be 45% and 10%, respectively, to achieve the maximum decrease in the torque at 60%.



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*Source: 8856569-2020-12-17.xml*