Experimental Study on the Effects of New Foam on the Improvement of Sandy Soil for Earth Pressure Balance Shield

Abstract

In this paper, starting from the foaming amount, half-life and other indicators, the surfactant with excellent performance is optimized, and the optimal compound ratio and appropriate foam stabilizer, viscosity enhancer, and additives are studied. A new type of foam agent formula for the EPB shield is developed. The performance of the new foam agent is compared with that of domestic foam and imported foam from the aspects of foaming rate, stability, micromorphology and temperature resistance, which verifies that it meets the parameter requirements and advantages of a foam agent for shield tunneling. Then, by means of laboratory tests, such as the mixing test, friction coefficient test, adhesion resistance test, slump test and direct shear test, the improvement effect of the new foam agent is compared with existing foam agent products at home and abroad, and the improvement effect of the new foam agent on soil is evaluated. The formulation of the new foam agent is as follows: 1.6% sodium dodecyl sulfate (SDS) + 8% dodecyl dimethyl betaine (BS-12) + 7% dodecanol + 0.06% guar gum + 0.6% ammonium chloride. The foaming rate, stability, microstructure and temperature resistance of new foam meet the requirements of shield construction. New foam, imported foam and domestic foam have advantages in sand and soil improvement experiments, but from an economic point of view, the new type of bubble is better than the other two. A new type of shield foam agent is prepared to meet the needs of construction. There is a big advantage in terms of price.

1. Introduction

With the rapid development of urban rail transit construction, the soil pressure balanced shield, as a construction method that has little influence on the surrounding environment [1], has become an important construction method for urban subway construction in China [2,3]. The soil is filled with excavated soil in the pressure compartment during the construction of the Earth’s pressure balance shield [4]. The soil water pressure on the excavation surface is balanced by applying pressure to the excavated soil [5]. The excavated soil is discharged from the shield by a spiral drainer [6], and the realization of these two effects requires the soil in the pressure compartment to have a “plastic flow state” [7]. At present, academic circles believe that the physical and mechanical index of soil with a “plastic flow state” should meet the following requirements: control of permeability coefficient of soil in order to avoid “spewing” problems [8,9], control of internal friction angle to avoid the “blocking” problem [10], control of slump in order to control the stability of the excavation surface [11,12], and control of the compression coefficient to avoid the “crumble” problem [13,14,15,16]. To deal with these problems comprehensively, the physical and mechanical indicators must be optimized. To improve the soil so that it has no easily consolidated drainage [17,18], a higher water content [19,20], lower intensity [21,22], good impermeability [23], lower internal friction angle [24] and other features are important. Thus, the excavated soil reaches the “plastic flow state” required by the shield.

The improved technology of soil is mostly used to inject mud into the excavation surface, earth pressure compartment and spiral conveyor [25,26]. Foam [27,28] and other modifiers can maintain the stability of the excavation surface and achieve soil pressure equilibrium shield propulsion; at the same time, it can help reduce the load of equipment, reduce the subsidence of the ground and increase the speed of excavation [29,30]. Foam modifiers are the most advanced of all the improved technologies and have a good improvement effect. A blowing agent mixed with water and compressed air bubbles (30-400 μm) creates a foam modifier through a foam generator that is injected into the excavation surface and the earth bunker to improve the soil to ensure the smooth discharge of the silt and maintain stability of the excavation surface [31,32]. At the same time, the torque of the knife disc and the wear of the tool are greatly reduced [33,34].

According to a preliminary study, foam modifiers are more suitable for soil with relatively good particle grade [35], large average particle size [36] and high-water content [37], which is in line with this study. However, most of the foam agents used in domestic shield tunnels are imported from abroad, and the cost is higher. There is no clear set of rules to detect the improvement effect of foam agents on soil. In this paper, a new type of foam agent formula was developed by optimizing the surfactant with excellent performance and studying the optimal compound ratio, suitable foam stabilizer, tackifier and additives. The performance of the new foam agent is evaluated to verify that it meets the parameter requirements of the foam agent for shield tunneling. Then, by means of laboratory tests, such as the mixing test, friction coefficient test, adhesion resistance test, slump test and direct shear test, the improvement effect of the new foam is compared with the existing foam agent products at home and abroad, and the improvement effect of the new foam on soil is evaluated.

2. Experimental Study on Foam Agent for Shield

A foam agent for soil pressure equilibrium shield is usually composed of a foaming agent, stabilizing foam agent and auxiliary agent. At present, most of the foam agents used in the shield machine are composed of anionic surfactants and non-ionic surfactants. The main foaming agents are combined with foam stabilizers. Some foam agents also add additives and solvents that have a synergistic effects on the surfactants. In this paper, surfactants are applied to the engineering field based on the combination of surfactants to develop foam agents suitable for shield construction.

2.1. Choice of Foaming Agent

In shield construction, the mechanism of action of bubbles is achieved by a large amount of tiny foam, which is a typical gas–liquid two-phase system consisting of less than 10% foaming agent solution and more than 90% air. Of commonly used foaming agent solutions, 90–99% are water, and the rest are foaming agents. In order to achieve the purpose of improving the soil, the foaming agent must have excellent foaming ability; in other words, the foam agent must first have a high foaming rate, which is understood to mean that the foam produced by the foam, the larger the foam volume, and the smaller the residual foam solution, the better. According to the field construction of the shield machine, there is a time interval from the foaming system sending out foam to mixing with the soil for improvement, which is about 2–3 min. Therefore, shield bubbles must have some stability, failure to decay excessively before mixing with the excavated soil, and loss of the purpose of improving the excavated soil; therefore, the stability of shield bubbles must also achieve certain effects to meet the requirements of shield construction.

In this paper, the following surfactants (

Table 1. Surfactants of Experimental Choice.

Type	Name of Surfactant	Specifications	Place of Origin
Anionic surfactant	Sodium dodecyl sulfate (SDS)	Chemically pure	Sinopharm Chemical Reagents Co., Ltd. (Shanghai, China)
	Sodium dodecyl sulfonate (AS)	Chemically pure	Sinopharm Chemical Reagents Co., Ltd.
	Sodium dodecyl benzene sulfonate (ABS)	Chemically pure	Sinopharm Chemical Reagents Co., Ltd.
Non-ionic surfactant	OP-10 (Qulatong X-100)	Chemically pure	Beijing Chemical Reagent Company (Beijing, China)
Cationic surfactant	Cetyltrimethylammonium chloride	Chemically pure	Tianjin Nankai Yungong Synthetic Technology Co., Ltd. (Beijing, China)
	Cetyltrimethylammonium bromide	Chemically pure	Sinopharm Chemical Reagents Co., Ltd.
Zwitterionic surfactant	Dodecyl dimethyl betaine (BS-12)	Industrial	Tianjin Xianguang Chemical Co., Ltd. (Tianjin, China)

) were selected based on the relevant surfactant properties.

2.2. Performance Evaluation of a Single Surfactant

The water-based foam agent and the foam solvent are water. According to the current laboratory conditions, tap water is selected as solvent. The experimental method is a simple, convenient and fast mixing method. The main index of evaluation is foaming volume and half-life. A single surfactant was evaluated in this experiment. The evaluation system is as follows: the surfactant is dissolved in 200 mL water, placed in a graded beaker, and stirred 2 min at 3000 r/min, and foam volume and half-life are recorded at room temperature. The selected mixer is a D90-A electric mixer produced by the Qingdao Haitongda Instrument Factory (Figure 1). Figure 2 is a map of the amount of foaming of different surfactants.

It can be seen from Figure 2 that the overall foaming performance of SDS is excellent. When the concentration of SDS is 1 g/L, the foaming amount is very low. When the concentration of SDS is greater than 2 g/L, the foaming amount is larger and tends to be stable. The solubility of AS at room temperature is small, and it can achieve a good foaming effect at a low concentration, but its half-life is very short, which is worse than that of SDS. The overall foam volume of ABS fluctuates little, and the foam volume is the largest when the concentration is about 4 g/L. When the concentration of OP-10 exceeds 2 g/L, the foaming amount is the largest and then tends to be stable; Cetyltrimethylammonium chloride has excellent foaming performance. When the concentration is low, it can reach a good foaming amount. When the concentration is greater than 0.6 g/L, the foaming amount tends to be stable; the foaming performance of cetyltrimethylammonium bromide is also relatively good. When the concentration is about 3 g/L, it reaches the maximum value. When the concentration is greater than this, the foaming amount decreases slightly but finally tends to be stable. The overall foaming performance of dodecyl dimethyl betaine is excellent. When the concentration is more than 3 g/L, the foaming amount is large and tends to be stable gradually.

According to foaming volume (Figure 2), the foaming properties of different kinds of surfactants vary greatly, and the foaming properties of the same surfactants vary at different concentrations. The results of the surfactants are as follows:

• Two surfactants, hexadecyl trimethyl ammonium chloride and hexadecyl trimethyl ammonium bromide, have the best foaming properties. Both are cationic surfactants, but they are expensive and do not conform to the principle of economy.
• Compared with several anionic surfactants, AS can achieve good foaming properties at very small concentrations. However, because of its short half-life, it cannot be used as a foaming agent alone;
• SDS and ABS have relatively good bubbles and half-lives. OP-10, dodecyl dimethyl betaine (BS-12), itself is a liquid; it is very conducive to the preparation of a water-based foam agent.
• Therefore, the selected experimental drugs are SDS, OP-10, ABS, AS and (BS-12).

When evaluating the properties of a single surfactant, we adopted the method of mixing, which is easy to operate, and evaluated the properties of various surfactants. However, according to actual operation, the mixing method can distinguish the properties of various surfactants but is prone to error. In this way, a more accurate improved Ross–Mile method was used to perform the surfactant combination experiment. The basic principles for improving the Ross–Miles method are: after the 500 mL surfactant solution flows from a height of 450 mm to the liquid surface of the same solution, the resulting foam volume is measured. This experiment selected the Type 2152 Roche foam instrument (Figure 3). The material is B40 glass material, the main composition of which is: dividend funnels, jackets, metering tubes and marble brackets. There is also supporting equipment, a Super Constant Temperature Water Bath, with circulating water pump, which can control water temperature at 50 ± 0.5 °C; the aim is to keep the whole experiment at a constant temperature.

Evaluation methods:

(1)

Record the time corresponding to the solution level falling to 150 mL, then stop the solution flow, and measure the foam volume for 30 s, 3 min and 5 min.

(2)

If the upper center of the foam is low-lying, record the reading by the arithmetic mean between the center and the edge.

(3)

Repeat the test to achieve a minimum of 3 errors in the allowable range.

(4)

The results are expressed as the number of milliliters of foam formed after the flow stops at 30 s, 3 min, and 5 min, and the corresponding curve can be drawn if necessary. Using the arithmetic mean of the repeated measurement results as the final result, the difference between the repeated measurement results should not exceed 15 mL.

In this paper, the main foaming agent A is selected from five selected compound experimental drugs, and the other four surfactants are multiplied and designed. Among them, A is SDS, B is ABS, C is AS, D is OP-10, and E is BS-12.

2.2.1. Program I: A/B Multiplexing System

The concentration of the total surfactant in the solution is determined to be 3 g/L unchanged; the ratio between the components (mass ratio) is changed, and the bubble volume of the foam (30 s foam volume after the flow is stopped) is determined. The experimental results are shown in Figure 4.

Figure 4 shows that A/B has good synergies at A:B = 80%:20%–70%:30%, and blistering performance is best at 78%:22%.

2.2.2. Program II: A/C Multiplexing System

The concentration of the total surfactant in the solution is determined to be 3 g/L constant; the ratio between the components (mass ratio) is changed, and the bubble volume of the foam (30 s foam volume after the flow is stopped) is determined. The experimental results are shown in Figure 5.

As can be seen from Figure 5, at A:C = 70%:30%–60%:40% and 90%:10%–85%:15%, the foam volume is above 550 mL, and the co-foaming effect is good. Foaming is best when A:C = 90%:10% and 65%:35%.

As the concentrations of B and C increase, the blistering performance of the composite system increases gradually. Figure 4 and Figure 5 show that the bubble volume of the A/C complex system is significantly better than that of A/B; the main reason is that the C molecule can be inserted into the A molecule, reducing electrostatic repulsion between the A molecules. By increasing the adsorption of the surfactant molecule, the surface tension is reduced, and the blistering height is increased. In addition, the tight adsorption of surfactant arrangement increases, which increases the surface viscosity of the solution, which is beneficial to the stability of the foam. The foam does not burst easily in the early stages of formation, thus increasing the height of the bubble. In the B molecule, due to the steric resistance of the phenyl group, the blistering ability is affected. Therefore, it can be said that the performance of the A/C multiplexing system is due to the A/B multiplexing system.

2.2.3. Program III: A/D Multiplexing System

The concentration of the total surfactant in the solution is determined to be 3 g/L constant; the ratio between the components (mass ratio) is changed, and the bubble volume of the foam (30 s foam volume after the flow is stopped) is determined. The experimental results are shown in Figure 6.

As you can see from Figure 6, foaming is best when A:D = 83%:17% (approximately 5:1). Studies [38,39] suggest that hydrogen bonding between non-ionic surfactants and protons in water shows the properties of weak cationic surfactants. This has some electrical attractiveness with the molecular polar head of the anionic surfactant, making surfactants more tightly arranged. Thus, a dense surface adsorption layer is formed on the liquid surface, which greatly reduces the surface tension of the solution. Figure 6 shows that as the D concentration increases, the viscosity of the solution system increases, which is conducive to the foam not bursting in mechanical oscillation and increasing the height of the bubble. However, if the D content is too large, the solution viscosity will be too large, blistering will not expand easily, the content of A will decrease, the surface tension will become larger, and the blistering capacity will decrease again.

2.2.4. Program IV: A/E Multiplexing System

The concentration of the total surfactant in the solution is determined to be 3 g/L constant; the ratio between the components (mass ratio) is changed, and the bubble volume of the foam (30 s foam volume after the flow is stopped) is determined. The experimental results are shown in Figure 7.

As can be seen from Figure 7, the A/E composite system has excellent synergistic performance, with maximum foaming when A:E = 15%:85% (approximately 1:5). A molecule dissociates into an ion group with a negative charge in aqueous solution. There is a positive charge in E molecule, and there is a strong Kulen gravity between them, which greatly improves the surface activity of the composite system. In the A/E composite system, the mutual attraction between positive and negative ions of mixed surfactant molecules induces close arrangement among different surfactant molecules. The gap between polar head groups is smaller, and the inter-molecular force is equivalent to the bonding bond in Gemini-type surfactants. Increasing the adsorption of liquid membrane surfactants further reduces the surface tension of the solution, showing special activity similar to Gemini-type surfactants. Because the hydrophobic groups of the two surfactants are very similar, there is also an attraction between their oil-friendly groups. Compared to a single surfactant, hydrophobic groups are more potent because the molecules are more closely arranged and spaced apart.

In the above four schemes, it is not difficult to compare the experimental data to show that the best combination is the A/E composite system, which has the best synergistic effect when the mass ratio of A to E is 1:5 at a certain concentration.

2.3. Bubble Stabilizer

2.3.1. Effect of Polyacrylamide (W) on the Foaming System

The experimental setting is as follows: m (A):m (E) = 1:5, total surfactant concentration of 1%, total surfactant solution of 200 mL, change W, stir 2min under 8000 r/min with GJ-2S type high speed mixer, determination of foam volume and half-life. The results are shown in

Table 2. Foam amount and half-life of W at different concentrations.

Concentration of W (Mass Fraction)	0	0.05%	0.1%	0.15%	0.2%	0.25%	0.3%	0.35%	0.4%	0.45%	0.5%
Foam volume/mL	940	930	910	910	900	870	840	840	760	750	750
Half-life/s	450	560	640	700	720	820	840	930	1080	1210	1020

.

It can be seen from Table 2 that when the W concentration is 0.45%, the foam system has high stability, the half-life reaches 1210 s, and the blistering volume of the composite system decreases by about 20%. W can interact with the hydrophilic groups of surfactants due to its large hydroxyl, carboxyl and other hydrophilic groups. The structure of a polymer–surfactant complex can be seen as a polymer that is attached to or encircled by several micelles. It is firmly adsorbed in the surface mixed adsorption layer, where the long molecular chain runs through the surface adsorption layer, which increases the strength of the foam liquid film and improves the stability of the foam. However, the viscosity of the solution increases dramatically due to the addition of polymer substances, which makes it difficult for the gas to diffuse in the liquid to form foam, and the bubble height of the solution will drop rapidly. In summary, W can significantly increase the half-life of foam and improve the stability of foam, but at the same time reduce the amount of foaming of the solution.

2.3.2. Effect of Sodium Carboxymethyl Cellulose (X) on the Foaming System

The experimental setting is m (A):m (E) = 1:5, and the total surfactant concentration is 1%. The total surfactant solution is 200 mL, changing the concentration of X and stirring for 2min under 8000 r/min with a GJ-2S high speed mixer. The foam volume and half-life are determined. The results are shown in

Table 3. X Foam volume and half-life at different concentrations.

Concentration of X (Mass Fraction)	0	0.05%	0.1%	0.15%	0.2%
Foaming capacity/mL	940	840	750	680	540
Half-life/s	450 s	21 min	65 min	2 h 27 min	3 h 41 min

.

As can be seen from Table 3, X has a very good steady bubble effect, with a half-life of about one hour when the concentration reaches 0.1%. However, it also has the effect of increasing viscosity and decreasing the amount of foaming as the viscosity of the solution increases. When the X concentration is 0.1%, the amount of foaming of the solution decreases by 20.2%.

2.3.3. Effect of Dodecanol (Y) on the Foaming System

The experimental setting is as follows: m (A):m (E) = 1:5, total surfactant concentration 1%, total surfactant solution 200 mL, change the concentration of Y by stirring 2 min at 8000 r/min with a GJ-2S high speed mixer, and determine the foam volume and half-life. The results are shown in

Table 4. Foam amount and half-life of Y at different concentrations.

Concentration of Y (Mass Fraction)	0	0.05%	0.1%	0.15%	0.2%	0.25%	0.3%	0.35%	0.4%
Foam volume/mL	940	990	1020	1030	1030	1040	1000	980	950
Half-life/s	450	515	555	715	1080	1200	765	840	730

.

As can be seen from Table 4, Y has a steady bubble effect, as well as a significant bubble enhancement effect. Y itself is a surfactant that can act in concert with two other surfactants. The interaction between surface adsorption molecules is enhanced, the strength of the surface adsorption film is increased, and foam stability is improved.

By analyzing the experimental data of W, X and Y, it can be seen that X has the best bubble stabilization, with a much greater half-life than the other two bubble stabilizers. For the construction of the soil pressure equilibrium shield, it is not the longer the half-life, the better, as the foam’s half-life is too long. Although X has excellent steady bubble performance, it is obviously not suitable for shield construction. In addition, while W has a good foaming effect, it will affect foaming performance more or less. In addition to having a good foaming effect, dodecanol can also increase foaming in the compound system at a certain concentration.

2.4. Adhesives

Better solution viscosity is more conducive to the stability of the foam, so we chose another additive to increase the viscosity of the solution. According to the actual conditions, the modified guar gum produced by the Beijing General Institute of Mining and Metallurgy is selected.

Comparing the degree of influence of guar gum on foam properties with the viscosity of the configuration solution, the concentration of guar gum is 0.01% α.

2.5. Configuration of Foam Agent Finished Products

Since the foam agent developed in this project is used for shield construction, it must be configured as a product; the formula developed in the laboratory is only for the use of the formula. Therefore, the final foam agent must be concentrated to reduce transportation costs, and formulations are generally carried out at room temperature. In the A/E system, E is liquid, has good water solubility, and A has good water solubility; this provides a precondition for the concentration of the finished foam agent. The addition of some inorganic salts can reduce surface tension, increase surface activity, and improve the foaming force and stability of the compound system. The optimal content of ammonium chloride with a concentration of 0.6% is determined by a comparative experiment. At room temperature, a series of concentrations of foam agent formulations are carried out, and the final foam agent final formulations can be obtained after a large number of different proportions: 1.6% sodium dodecyl sulfate (SDS) + 8% dodecyl dimethyl betaine (BS-12) + 7% dodecanol + 0.06% guar gum + 0.6% ammonium chloride.

2.6. Foam Agent Performance Evaluation Study

Two important evaluation indexes of foam agent properties are foam rate and stability. The foam agent industrial products developed are evaluated using a self-developed foaming device.

2.6.1. Experimental Analysis of Foaming Rate and Stability

A self-designed laboratory-simulated foaming device is used to determine the foaming rate. This experimental device is foamed by an air flow shock foam solution. Compressed air enters the foam generator through valves and flowmeters. At the same time, a quantitative infusion pump is used to control the flow of the foam solution and to pump it into the foam generator; a large amount of homogeneous and fine foam is formed by mixing the special mesh structure in the foam generator (Figure 8).

The measurement of foaming rate is achieved by measuring the cup with scale. The test steps are as follows:

The pre-equipped foaming agent solution is bubbled with an indoor foaming device. The foaming device is adjusted during foaming until the required bubbles are obtained, and this state is kept stable (this is the key to the accuracy of this lab data). The scale of the beaker is observed when the foaming device remains in a stable foaming state. A starting scale is selected to start the foam in the 2000 mL measuring cup, and the beaker scale is recorded when the foam fills the measuring cup. At the same time, under the same experimental conditions, the foaming rates of an imported foam agent and a domestic foam agent are measured. The imported foam agent is diluted at a 3% concentration, the domestic foam agent is diluted at 5% concentration, and the new foam agent is diluted at a concentration of 10%. The stability of the foam is measured by the half-life of the foam, and the 200 mL solution is prepared in the GJ-2S high-speed mixer according to the dilution ratio of the preceding section. Clocking is started after stirring for 2 min with 8000 r/min, and half the time is recorded when the liquid is precipitated, which is the half-life of a foam. The results of the experiment on blowing rate and half-life are shown in

Table 5. Comparison of foaming agent half-life of the three foam agents.

	Imported Foam	Domestic Foam	New Foam
Foam volume Vf/mL	2000	2000	2000
Volume of the foam agent solution Vl/mL	120	170	190
Foaming rate ER/times	16.7	11.7	10.5
Half-life	14 min 32 s	8 min 10 s	7 min 35 s

.

2.6.2. Foam Microscopic Morphological Observation and Analysis

Using a BT-1600 Image Particle Analysis System for Test Instruments, three foam agents are diluted proportionally, and foam is emitted by a homemade foaming device. The newly emitted foam is placed onto the slide, covered with a slide and then observed at 10 times the objective under the microscope. Pictures are taken immediately with an objective 4 times the magnification. Then the foam image is processed, and finally the image is analyzed to get the particle size distribution table of the image particle. The image and particle size distribution of the three foam agents are shown in

Table 6. Particle size distribution of three different foams.

Particle Size Interval (µm)	Imported Foam		Domestic Foam		New Foam
	Interval Distribution (%)	Cumulative Distribution (%)	Interval Distribution (%)	Cumulative Distribution (%)	Interval Distribution (%)	Cumulative Distribution (%)
1.00–10.92	7.52	7.52	3.01	3.01	0.27	0.27
10.92–20.83	22.65	30.18	1.34	4.36	0.61	0.88
20.83–30.75	24.95	55.13	0	4.36	10.36	11.24
30.75–40.67	22.84	77.96	0	4.36	8.42	19.66
40.67–50.58	15.83	93.8	0	4.36	2.83	22.49
50.58–60.50	2.26	96.06	0	4.36	15.9	38.38
60.50–70.42	3.94	100	0	4.36	26.55	64.93
70.42–80.33	0	100	95.64	100	14.61	79.54
80.33–90.25	0	100	0	100	20.46	100
90.25–100.17	0	100	0	100	0	100
100.17–110.08	0	100	0	100	0	100
110.08–120.00	0	100	0	100	0	100

and Figure 9.

It can be seen from Table 6 and Figure 9 that the particle size distribution of imported foam and new foam in three foams is uniform; the large size foam of domestic foam accounts for a large proportion.

2.6.3. Foam Resistance Study

In order to test the heat resistance of the foam agent, the three foam agents are kept in a refrigerator and a constant temperature water bath for a certain period of time. Then, according to the dilution ratio above, 100 mL foam solution is configured and is stirred for 2 min at 8000 r/min with a GJ-2S high speed mixer. The foam volume is determined. The amount of foam produced by the three foam agents at different temperatures is shown in Figure 10.

As shown in Figure 10, it starts with a gradual increase in temperature, and the bubble volume of the three foam agents also increases, mainly due to decreasing molecular attractiveness and reduced surface tension in the solution as the temperature increases; therefore, it is beneficial to increase the sparkling capacity of the solution. However, when raised to a certain temperature, the stability of the foam agent is poor because the uppermost side of the foam is always convex upwards. The bending film is sensitive to evaporation, and the higher the temperature, the faster the evaporation. The film becomes thin to a certain extent and bursts itself. Thus, at high temperature, because the foam produced is extremely unstable, bursting occurs instantaneously, making it difficult for the foam to accumulate and the foam volume decreases.

3. Test on Improvement Effect of New Foam Sand

This study mainly uses a stirring test, friction coefficient test, adhesion resistance test, collapse test and straight shear test to evaluate the effect of soil improvement.

In order to carry out the soil improvement test, there must be corresponding soil to ensure that the test is carried out. In order to ensure the effectiveness, repeatability and comparability of the test results, the self-prepared soil of the laboratory conforms to the following principles:

i

The particle grade of the formulated soil must conform to the particle grade of the actual soil.

ii

The prepared soil should be representative and typical.

iii

The preparation of 3 soil bodies should be repeatable, making the test process of soil body improvement repetitive.

This experiment simulated another typical stratigraphic formation in Beijing, namely silt/fine yarn/medium ~ coarse sand strata. The granular grade of sand soil is shown in Figure 11.

According to the grain grade configuration of sand soil above, the test soil is anhydrous soil, and the soil volume is 7.5 L. The foam agent is foamed, and then, foam is added to the soil for soil improvement and initial test analysis of foam agent addition. According to the calculation and analysis of the consumption of foam agent, the initial addition is determined to be 6% of the volume of the soil (450 mL). After each parameter is measured, the foam agent is added for soil improvement until the soil reaches the ideal plastic flow state. The results of the test are analyzed according to the type of soil improvement test.

3.1. Analysis of Results of Agitation Test

After soil improvement by adding foam to the formulated anhydrous sand soil, Figure 12 shows the change curve of net mixing power and mixing power rate of change after adding foam.

According to the curve in Figure 12, from the trend of the mixing power change rate curve, the mixing power rises at the beginning of the foam; that is, adding a small amount of foam increases the power, mainly because the foam emitted by the foam agent is gas-water. When less foam is mixed with the soil, the soil is clumped by the action of the water in the foam, which inevitably leads to an increase in mixing power. In general, however, the improvement effect of foam agents on sand soil is also obvious. When the amount of foam is added to a certain extent, the power of stirring decreases gradually and the improved soil becomes fluid plastic; however, the improved effects of the three kinds of foam show diversity, and the power change rate of the imported foam is higher in the improved sand and pebble soil. When the foam agent is first added to 6.0%, the mixing power drops; with the addition of foam, the power of stirring decreases. With the addition of foam, the amount of mixing power decreases and gradually stabilizes. When the amount of foam agent is 11.3%, the mixing power changes to −90.0%. A new type of foam and a domestic foam agent, which is homemade, have a tendency to increase the mixing power when 6.0% is added to the foam agent. The trend of decreasing only occurs when the amount of bubbles added exceeds 7.3%. At 11.3%, the rate of change in mixing power is −65% and 80%, respectively, compared to imported foam.

3.2. Analysis of Friction Coefficient Test Results

Soil improves by adding foam to formulated anhydrous sand. Figure 13 shows the change curve of the friction coefficient and the friction coefficient change after adding foam.

It can be seen from the curve in Figure 13 that the adhesion effect is increased in the early days of foam addition when the sand is clumped by the water in the foam so that the friction angle increases. However, as the amount of foam added increases, the friction angle begins to decrease again, and the downward trend is constant, which means that the foam must be added to a certain amount to reduce the friction coefficient of the soil. This can be found by the contrast of the three curves in the diagram, the improvement effect of the friction coefficient of imported foam on sand and pebble soil, as the amount of foam added increases, the friction coefficient decreases relatively greatly. The friction coefficient change rate is −18.3% when foam addition is 10.0% and −39.0% when foam addition is 11.3%. For new foam and domestic foam, the rate of change of the soil friction coefficient is relatively small under the same amount of foam. When all three foams are added at 11.3%, the foam continues to be added. The rub coefficient change has been relatively slow, which shows that the improvement effect of further adding foam on the soil friction coefficient is no longer very obvious. In general, imported foams are better than new and domestic foams in improving the friction coefficients of sand and soil.

3.3. Analysis of Adhesion Resistance Test Results

Figure 14 shows the change curve of adhesion resistance and the adhesion resistance rate of change after adding foam to the formulated anhydrous sand for soil improvement.

According to the curve in Figure 14, the improvement of the adhesion resistance of the three kinds of foam shows a tendency of first high and then low; the adhesion resistance of soil can be improved effectively with the addition of foam. The adhesion resistance of new foam-improved sand soils is effective. When foam is added at 8.7%, the adhesion resistance variable reaches −34.5%, the adhesion resistance change rate is −41.5% when foam is 11.3%. Domestic foam and imported foam also have an obvious effect in improving sand adhesion resistance. The rate of change in adhesion resistance is −14.4% and −35.8% when 8.7% and 11.3% are added to the foam. At 8.7% foam addition, adhesion resistance increases, and the rate of change is 68.9%, but when foam is 10% added, adhesion resistance decreases significantly and adhesion resistance decreases from 61.64 N to 27.34 N. When the addition of the three foams reaches 11.3%, the adhesion resistance change is no longer obvious even if the foam continues to be added. It is shown that the improvement effect of the further addition of foam on adhesion resistance is no longer significant.

3.4. Analysis of Collapse Test Results

Figure 15 shows the change curve of the rate of collapse and collapse after soil improvement by adding foam to the formulated anhydrous sand soil.

The slumping degree of the soil is the control parameter of the whole test. When the slumping degree of the soil reaches more than 15 cm, it is considered that the fluidity of the soil meets the requirement of an ideal plastic flow state. Based on the curve in the figure, it can be seen that as the amount of foam added increases, the subsidence of sand can be significantly increased, and the fluidity of soil is increased. When the amount of foam is added to the three types of foam, the collapse value of the soil decreased by a certain amount compared to the pre-improvement, but when the foam continues to be added, the collapse value increases significantly. Overall, the collapse value increases with the amount of foam added. According to the change rate curve, the effect of improved sand and soil collapse of imported foam is better than that of the new foam and imported foam. At 10% foam addition, the value of soil collapse after imported foam improvement is 20 cm and the rate of change is 17.6%. The value of soil collapse after new foam improvement is 14 cm and the rate of change is −17.6%. The value of soil collapse is 14 cm, and the rate of change is −17.6%.

3.5. Analysis of the Straight Shear Test

The soil is configured according to the foam addition amount in the previous test. Each set of test shears 4 identical soil samples, taking the mean. A vertical pressure of 200 kPa is added after the sample is fitted into a straight shear instrument, and a straight shear test is carried out after 3 min. Figure 16 shows the change curve of shear strength and shear strength change rate after adding foam.

The shear strength of the modified soil affects the wear of the cutting tool and the rotating parts of the shield. Reducing the strength of the excavated surface soil can reduce cutting resistance and thus the wear of components. One of the functions of foam is to lubricate cutting knives; this can reduce the operating temperature of parts and prolong the life of rotating parts, such as cutting tools and spiral ground dryers. The addition of foam can significantly reduce the strength of excavated soil, thus avoiding problems such as occlusion and crumbs. The shear strength of improved soil is an important measure of foam improvement. It is known from the curve in the figure that after adding foam, the shear strength of the soil is significantly reduced when the vertical pressure is 200 kPa. The shear strength of soil decreases significantly, and the shear strength of soil decreases with the addition of foam. Taking the new foam as an example, the shear strength changes by −11.8%, −13.5% and −30.8% when the foam addition ratio is 8.7%, 10.0% and 11.3%, respectively. This can be seen by the contrast of the three curves; the improvement effect of domestic foam on the shear strength of sand and soil is the best. The improvement effect of new foam is similar, and the shear strength of imported foam is slightly worse. In general, however, all three foams can significantly improve the shear stress of sand and soil.

3.6. Comprehensive Analysis of Soil Improvement Tests

Table 7. Comparison of the effect of improved sand and soil in foam.

Modifier	Mixing Test		Friction Coefficient Test		Adhesion Resistance Test		Slump Test		Direct Shear Test
	W (kW)	Rate of Change (%)	f	Rate of change (%)	F (N)	Rate of Change (%)	S (cm)	Rate of Change (%)	T (kPa)	Rate of Change (%)
New foam	0.140	−65.0%	0.34	−26.7%	21.36	−41.5.0%	16	−5.9%	97.96	−30.8%
Domestic foam	0.137	−80.0%	0.31	−35.0%	24.01	−34.2%	20	17.6%	86.9	−38.6%
Imported foam	0.135	−90.0%	0.29	−39.0%	23.42	−35.8%	23	35.3%	109.2	−23.0%

shows the comparison of mixing power, friction coefficient, adhesion resistance, slump value and shear strength of three foam agents in the sand improvement test when the foam content is 11.3%.

As can be seen from Table 7, all three foam agents can improve sand and soil. By comparing the test data, it can be found that the new foam is dominant in the friction coefficient test and adhesion resistance test, 0.34 and 21.36 N, respectively; imported foam is dominant in the mixing test and collapse test, 0.135 kW and 23 cm, respectively; the improvement effect of domestic foam in the straight shear test is dominant, 86.9 kPa. According to the comprehensive analysis, the combined effects of the three kinds of foam-improved sand and soil are basically the same, which can meet the requirements of shield construction. However, from an economic point of view, the new type of bubble is better than the other two. Figure 17 shows the effect diagram of new foam-improved sand soil when foam addition is 11.3%. The cost of materials required for preparing 1 t finished products is about 1100 yuan, while the domestic foam agent is priced at about 8000 yuan/t, and the price of the imported foam agent is higher, so the new foam is far lower than the market price.

4. Conclusions

• The new foam agent formulation is obtained by experimenting and combining with the actual product configuration method: 1.6% sodium dodecyl sulfate (SDS) + 8% dodecyl dimethyl betaine (BS-12) + 7% dodecanol + 0.06% guar gum + 0.6% ammonium chloride.
• The foaming rate, stability, microstructure and temperature resistance of the new foam are evaluated. Compared with the domestic foam and imported foam commonly used in shield construction at present, the performance of the new foam meets the requirements of shield construction.
• The effect of foam on soil improvement is also related to the amount of foam added. Because of the structural characteristics of the foam itself, too little foam is added to the opposite effect and too much is added to the foam to produce waste. The cost is too high for actual construction, which should be avoided resolutely in shield construction.
• In the test of foam-improved sand soil, all three foam agents have an obvious improvement effect on sand soil. By comparing the test data, it can be found that the new foam is dominant in the friction coefficient test and adhesion resistance test. Imported foam is dominant in the mixing test and collapse test, and the improvement effect of domestic foam in the straight shear test is dominant. According to the comprehensive analysis, the combined effects of three kinds of foam-improved sand and soil are basically the same, which can meet the requirements of shield construction.
• The foam soil improvement effect test was carried out, and the allocated soil was anhydrous soil. Although it had a certain reference value for soil improvement, the water content of each stratum was different in specific engineering practice. In the next test, it is necessary to improve the soil mass with different water contents, evaluate the impact of water content on the improvement effect of foam soil mass, and more closely link the new foam with the project.
• The soil parameters selected in this project are few, and the measurement of improved soil parameters needs to be expanded in the next step. For example, the permeability coefficient, compression coefficient, etc., for further design of an indoor simulation test is required.
• In the soil improvement test of this subject, the concentration of the foam agent only uses a preset value. In the next step, the improvement effect of different foam agent concentrations on soil can be studied through a simulation test.