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

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. +e major findings were obtained as follows. (1) +e 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) +e soil conditioning materials can significantly reduce the cutting torque of shield tunneling in silty sand by up to 60%–70%. (3) +e optimal foam and slurry parameters are suggested in the paper for shield tunneling in silty sand, respectively. (4) +e 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%. +ese findings can provide a practical reference for engineers to determine the bestfit conditioning materials and construction parameters in the silty sand stratum.


Introduction
Tunnel engineering has played a very important role in the rapid development of infrastructure construction [1][2][3][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][6][7][8] . However, the widespread unfavorable geologic conditions often severely hamper the normal operation of this mechanized tunneling equipment [3,[9][10][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][13][14]. e use of soil conditioning material is an effective way to improve the applicability of EPB shields in different geological conditions [15][16][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][19][20][21][22][23][24][25][26][27][28][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][32][33][34][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][38][39][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][42][43][44][45][46][47][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 ewes 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. erefore, 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. e 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.

Test Setup
e test system consists of a main test device and two soil conditioning devices (a foaming device and a slurry device), as shown in Figure 1. e 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. e foaming device was used for foam generation and injection for different foaming materials. e slurry device was used for preparation and injection of bentonite slurry into the cutter head. Figure 2 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. en, 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.

Slurry Device.
e slurry device mainly includes a storage silo, syringe, and cylinder ( Figure 4). e dimension of length, width, and height is 0.7 meters, 0.53 meters, and 1.3 meters. e prepared slurry is transferred from the storage silo to the syringe by controlling the ball valve. en, the slurry is injected into the drain pipe by the extension of the cylinder. e injection pressure and rate are automatically recorded.      Advances in Civil Engineering

Main Device.
e main device is composed of a propulsion system, cutter head, and soil chamber system used to simulate the driving process of EPB shield machine (Figures 5 and 6). e external dimension is 3.1 meters long, 1.2 meters wide, and 1.2 meters high. e 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. en, 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. e torque and thrust of cutter head, earth pressure, are automatically recorded in the driving process.
According to the similarity principles of scaled test, the cutter torque for actual shield machine can be determined by the simulation test as follows: where T is the cutter torque of actual shield machine, T 0 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).

Cutter head rust bearing
Injection port of soil conditioning Support rust cylinder

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. e 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 Figures 7-9. e natural moisture content of the silty sand ranges from 10% to 30% with internal friction angle of 30°, as shown in Table 1. e 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. e 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].

Foam.
e 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 (F er , see equation (2)) and foam stability (F s , see equation (3)), which were described in detail by Wu et al. [51]. e recommended value of F er for shield tunneling is 20-40: where V f is the volume of foam and V l is the volume of the foaming solution. F s can be measured by the foam dissipation test using the following equation: where m d is the weight of the dissipated foam and m o 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 (T 1/2 ) (i.e., the time during when F s � 0.5) of foam is generally used to assess the stability of foam. e value ranges from 15-20 min for shield tunneling.    Note. c is the unit weight; ω is the water content; c is the cohesion; φ is the friction angle; E s is the constrained modulus.

Advances in Civil Engineering 5
In practice, the F er and T 1/2 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    Table 3: Scheme of orthogonal test.
No.     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

Bentonite Slurry and CMC.
e other conditioning material for the test is the bentonite with addition of carboxyl methyl cellulose (CMC, [C 6 H 7 O 2 (OH) 2 CH 2 COO Na] n ) which can significantly increase the viscosity of bentonite slurry as thickener when it is dissolved in water. e 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. e recommended slurry proportioning is 8% bentonite and 0.5% CMC, resulting in a density and funnel viscosity of 1.054 g/cm 3 and 79.36 s, respectively.

Test Procedure.
e testing conditions and parameters are shown in Tables 5-7. e 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. e 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. e 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. e process of testing is shown in Figure 18.

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. Figures 19-22, 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. e 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 Advances in Civil Engineering 13 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. Figures 23-26, 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.

e Effect of Confining Pressure. As shown in
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 Figure 23.
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 Figure 24, the maximum reduction was only 1200-1500 N m under different soil pressures.

4.2.3.
e Effect of Conditioning Material. e 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  e results revealed that the effect of soil conditioning is limited when the injection ratio reaches a certain value. For example, Figure 27 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 Cutter torque-σ 0.1 Figure 25: Effect of slurry conditioning and soil pressure on cutter torque at water content 30%. 16 Advances in Civil Engineering injection ratios of the two types of soil conditioners were different. As shown in Figure 27, the injection ratios of foam and slurry should be 45% and 10%, respectively, to achieve the same maximum reduction of torque at 60%.

Field Test and Verification
To realize the onsite effect of soil conditioning on the cutting performance of the shield machine, a field experiment was Advances in Civil Engineering conducted on a real tunneling project in Wuhan with Φ6.26 m EPB shield (shown in Figure 31). e tunneling parameters of the cutter head are listed in Table 9. ere 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. e foaming parameters adopted in the field test were the same as the laboratory test. As shown in Figure 32, 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   18 Advances in Civil Engineering 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. e predictions of the shield cutter torque based on the laboratory test results were compared with those from the field test in Figure 32, which shows they were in good agreement and most of the measured values were within the range of predicted values.

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. e effect of four key factors was comprehensively investigated on the shield cutting performance and verified by field experiment of a real tunnel project. e major findings were obtained as follows: (1) e 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) e 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. e optimal slurry is recommended as 8% bentonite and 0.5% CMC with resulted density and funnel viscosity as 1.054 g/cm 3 and 79.36 s, respectively. (3) e shield cutter torque is also sensitive to the moisture content and confining pressure of silty sand and the injection ratio of conditioners. e 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. e 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%.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.