Composite foam-bentonite spoil modifier for shield tunneling in sandy cobble strata and preparation method thereof

Abstract

This invention belongs to the technical field of shield tunnel construction engineering, and relates to a composite foam-bentonite slag conditioner and its preparation method for use in shield tunneling in gravel strata. The conditioner has two components, A and B: component A is a composite flocculant slurry containing sodium-based bentonite, soda ash, stabilizer, polyacrylamide, and polyaluminum chloride; component B is a composite foam generated from a foaming stock solution containing a composite foaming agent, foam stabilizer, hydroxyethyl cellulose, and ultrasonically pre-dispersed nano-silica. The synergistic effect of components A and B significantly improves the fluidity, overall stability, and impermeability of the slag, thereby effectively suppressing common risks in gravel strata such as increased cutterhead wear, abnormal torque fluctuations, slag cake formation, and slag heaving, ensuring the smoothness and safety of shield tunneling.

Description

Technical Field

[0001] This invention belongs to the field of shield tunnel construction engineering technology, and relates to a composite foam-bentonite soil conditioner and its preparation method for use in shield tunnels in sand and gravel strata. Background Technology

[0002] With the rapid development of urban rail transit construction, shield tunnels inevitably need to traverse complex sand and gravel strata. These strata are loosely structured, highly permeable, and have a large internal friction angle, leading to a series of technical challenges during shield tunneling, including severe wear of the cutterhead and cutting tools, large torque fluctuations, soil cake formation, and a tendency for soil ejection under high water pressure. Currently, single foaming agents or bentonite slurries are commonly used for soil improvement, but the results are unsatisfactory. Foaming agents have poor stability under coarse particle shearing and are prone to breakage and failure; while bentonite slurries require large-scale injection, easily leading to excessively sticky soil and increased costs, and the improvement effect is difficult to sustain under flowing water conditions.

[0003] Existing technology discloses a soil conditioner for shield tunneling, comprising the following raw materials: a composite foaming agent, modified bentonite, carboxymethyl cellulose, and lignin sulfonate. However, this soil conditioner is prone to foaming in strata with high pebble content, cannot effectively encapsulate coarse particles, and has a permeability coefficient of only 1.57 × 10⁻⁶. -6 The slump is 150-200 mm, and there are also contradictions such as insufficient improvement or excessive stickiness and cake formation of the slag. The stability of the improved slag is insufficient. Existing technology discloses the improvement of Chengdu sand and gravel strata by using foaming agents, bentonite and polymers. It is found that when sand and gravel soil reaches the fluid plastic state, its slump value should be 150-200 mm. The new foaming agent for improving sand and gravel soil is easy to break bubbles in strata with high stone content and has a high material injection ratio. However, this technology only focuses on the fluid plasticity of the slag foundation and does not have a special anti-seepage design, so it cannot cope with the problem of water-rich strata gushing. Summary of the Invention

[0004] To address the aforementioned issues, this invention provides a composite foam-bentonite slag conditioner and its preparation method for use in shield tunneling in gravel strata. The aim is to achieve good fluidity, stability, and water-stopping properties in the slag, thereby ensuring the safe and efficient tunneling of shield tunneling in gravel strata.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a composite foam-bentonite soil conditioner for use in shield tunneling in gravel strata, comprising: material A and material B, wherein the volume ratio of the two is 1:2-4; Material A is composed of the following raw materials in parts by weight: 40-70 parts sodium bentonite, 1-3 parts soda ash, 0.5-2 parts stabilizer, 0.05-0.3 parts polyacrylamide, 0.1-0.5 parts polyaluminum chloride, and 80-120 parts water. Material B is a composite foam, and its foaming solution is composed of the following raw materials in parts by weight: 3-8 parts composite foaming agent, 1-4 parts foam stabilizer, 0.1-0.5 parts hydroxyethyl cellulose, 0.5-2 parts nano silica, and 90-110 parts water.

[0006] A second aspect of the present invention provides a method for preparing a composite foam-bentonite soil conditioner for use in shield tunneling in gravel strata, comprising: Preparation of Material A: Dissolve soda ash and stabilizer in some water and mix evenly; under continuous stirring, first add polyaluminum chloride solution and mix evenly; then add polyacrylamide and stir thoroughly to dissolve; finally add sodium bentonite and make up the remaining water, mix evenly to form a homogeneous slurry, and let it stand for 12-24 hours to obtain Material A; Preparation of foaming stock solution for component B: Nano-silica is mixed with a portion of water and ultrasonically dispersed to obtain a nano-silica dispersion; hydroxyethyl cellulose, composite foaming agent, and foam stabilizer are dissolved in the remaining water and mixed evenly; the nano-silica dispersion is mixed evenly with the latter solution to obtain the foaming stock solution; Generate composite foam: The foaming liquid is delivered to the foam generator, compressed air is introduced, the gas-liquid ratio is controlled at 10-15, and the foaming pressure is 0.3-0.5 MPa to generate composite foam of material B; The composite foam of material A and material B is pumped into the soil chamber or in front of the cutterhead. The injection volume ratio of material A to material B is 1:2~4, so that it is evenly and densely mixed with the excavated soil in the soil chamber to obtain improved soil.

[0007] A third aspect of the present invention provides the application of the above-mentioned composite foam-bentonite slag soil conditioner in urban rail transit construction.

[0008] Beneficial effects of the present invention The inorganic-organic composite flocculation system in component A of this invention can efficiently encapsulate pebbles and agglomerate fine particles to form a dense slag structure. The liquid-phase thickening-nano-solid-phase enhanced foam-stabilizing system in component B can generate ultra-high stability foam that can be maintained for a long time under high pressure and dynamic water conditions. The synergistic effect of the two components can significantly improve the fluidity, overall stability, and impermeability of the slag, thereby effectively suppressing common risks in sandy and gravelly strata such as accelerated cutterhead wear, abnormal torque fluctuations, soil cake formation, and slag heaving, ensuring the stability and safety of tunnel boring machine (TBM) excavation. The preparation method of this invention is simple, practical, and easy to promote. Detailed Implementation

[0009] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0010] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of skill in the art. The reagents and raw materials used in this invention are readily available through conventional means, and unless otherwise specified, they are used in accordance with conventional methods in the art or product instructions. Similarly, unless otherwise specified, the test methods of this invention are performed in accordance with conventional methods in the art or industry-standard methods or practices. Furthermore, any methods and materials similar to or equivalent to those described herein may be applied to the methods of this invention. The preferred embodiments and materials described herein are for illustrative purposes only.

[0011] This invention mainly proposes a composite foam-bentonite soil conditioner for shield tunneling in gravel strata, comprising: material A and material B, with a volume ratio of 1:2-4; Material A is composed of the following raw materials in parts by weight: 40-70 parts sodium bentonite, 1-3 parts soda ash, 0.5-2 parts stabilizer, 0.05-0.3 parts polyacrylamide, 0.1-0.5 parts polyaluminum chloride, and 80-120 parts water. Material B is a composite foam, and its foaming solution is composed of the following raw materials in parts by weight: 3-8 parts composite foaming agent, 1-4 parts foam stabilizer, 0.1-0.5 parts hydroxyethyl cellulose, 0.5-2 parts nano silica, and 90-110 parts water.

[0012] Preferably, the sodium-based bentonite has a particle size ≥ 200 mesh, a montmorillonite content ≥ 85%, and a gel value ≥ 90%.

[0013] Preferably, the soda ash is industrial-grade sodium carbonate.

[0014] Preferably, the stabilizer is selected from one or two of sodium carboxymethyl cellulose and xanthan gum.

[0015] Preferably, the polyacrylamide is an anionic or nonionic type with a molecular weight of 8-15 million.

[0016] Preferably, the basicity of the polyaluminum chloride is 70%-85%.

[0017] Preferably, the composite foaming agent is prepared by compounding sodium α-olefin sulfonate and fatty alcohol polyoxyethylene ether in a mass ratio of 1 to 3:1.

[0018] Preferably, the foam stabilizer is selected from lauroyl diethanolamine and dodecyl dimethylamine oxide.

[0019] The present invention will be further described in detail below with reference to specific embodiments. It should be noted that the specific embodiments are explanations of the present invention and not limitations thereof.

[0020] Example 1: This invention proposes a composite foam-bentonite soil conditioner for use in shield tunneling in gravel strata and its preparation method. The specific processing steps of this method are as follows: This embodiment provides a composite foam-bentonite slag soil conditioner and its application test.

[0021] Preparation of Slurry A: Weigh 50 parts of sodium bentonite with 88% montmorillonite content, 2 parts of industrial soda ash, 1 part of sodium carboxymethyl cellulose, 0.1 parts of anionic polyacrylamide with a molecular weight of 12 million, 0.3 parts of polyaluminum chloride with a basicity of 80%, and 100 parts of water. First, dissolve the soda ash and sodium carboxymethyl cellulose in 80 parts of water and stir until homogeneous. While stirring at 400 rpm, slowly add the polyaluminum chloride solution pre-dissolved in 10 parts of water and stir for 5 minutes. Then add the polyacrylamide and stir until completely dissolved. Finally, slowly add the sodium bentonite powder and increase the stirring speed to 600 rpm, continuing to stir for 30 minutes. Add the remaining water to obtain a homogeneous slurry. Let it stand for 18 hours for maturation before use.

[0022] Preparation and foaming of material B: Weigh 3.5 parts sodium α-olefin sulfonate, 1.5 parts fatty alcohol polyoxyethylene ether AEO-9, 2 parts lauroyl diethanolamine, 0.2 parts hydroxyethyl cellulose, 1 part nano silica, and 100 parts water. First, mix the nano silica with 20 parts water and sonicate at 400 watts for 15 minutes to obtain a dispersion. Dissolve the sodium α-olefin sulfonate, AEO-9, lauroyl diethanolamine, and hydroxyethyl cellulose in the remaining 80 parts water and stir thoroughly. Then, mix the nano silica dispersion with this solution evenly to obtain the foaming stock solution. In the shield tunneling simulation, inject this stock solution into the foam generator, control the gas-liquid ratio at 12, and the foaming pressure at 0.4 MPa to generate composite foam.

[0023] On-site simulation mixing and performance testing: Standard sand and gravel slag with a pebble content of 35% was used as the modification target. Its particle size distribution met the relevant requirements of the "Technical Specification for Slag Modification of Earth Pressure Balance Shield Tunneling Machine" and the natural moisture content was controlled at 4%~6%. The prepared material A and the foam material B generated on-site were injected into a closed mixing container at a volume ratio of 1:2.5. At the same time, the standard sand and gravel slag to be modified was added. The ratio of the total volume of the modifier injected to the volume of the slag was controlled at 20%. The mixture was stirred continuously at a speed of 60 r / min for 3 min to ensure that the material A, material B foam and sand and gravel slag were fully mixed and uniform, and the slag modification was completed. After standing for 2 min, various performance indicators were tested.

[0024] (1) Half-life of the modifier The airflow method commonly used in shield tunneling was employed for testing. After the foam of material A and material B of this invention were mixed evenly according to the volume ratio set in the example, the mixture was immediately injected into a 250 mL graduated bottomless measuring cylinder. The surface of the foam was leveled and the initial foam volume V0 was recorded. The mixture was then left to stand at a constant temperature of 25 ℃ and standard atmospheric pressure. The time it took for 100 mL of liquid to be released from the foam was recorded as the half-life of the modifier. The test was performed in parallel for 3 times, and the arithmetic mean was taken as the final result.

[0025] (2) Viscosity of material A Take the A material slurry after it has been allowed to stand and mature, and keep it at a constant temperature of 25 ℃ for 30 min in a constant temperature water bath. Use an NDJ-1 type rotational viscometer with rotor No. 2 and set the rotation speed to 60 r / min. Immerse the rotor to the specified mark of the slurry, start the viscometer and record the viscosity value after the reading stabilizes. Perform three parallel tests and take the arithmetic mean as the final result.

[0026] (3) Slump of the improved slag The test was conducted using a standard slump cone (100 mm upper diameter, 200 mm lower diameter, and 300 mm height). The improved sand and gravel slag was filled into the slump cone in three layers, with each layer being tamped evenly 25 times. After filling, the surface of the cone opening was leveled, and the slump cone was lifted vertically at a constant speed. The height difference between the highest point of the slumped slag and the top of the cone was measured, which was the slump value. The test was performed in three parallel trials, and the arithmetic mean was taken as the final result.

[0027] (4) Permeability coefficient of improved slag soil The constant head permeability test method was used. The improved slag was loaded into a TST-55 permeameter to prepare a cylindrical sample with a diameter of 61.8 mm and a height of 40 mm. The sample was saturated using the vacuum saturation method to ensure that no air bubbles overflowed. The head difference was adjusted to the set value, and after the head stabilized, the timer was turned on to measure the amount of water that permeated through the sample within the set time. The head difference and permeation time at both ends of the sample were recorded, and the permeability coefficient was calculated according to Darcy's permeability formula. The test was performed in parallel for 3 times, and the arithmetic mean was taken as the final result.

[0028] (5) Cutter head torque reduction The test was conducted using a shield tunneling muck improvement simulation test bench, with the cutterhead structure and rotation speed consistent with the parameters of the earth pressure balance shield tunneling machine used in the project. First, unimproved standard sand and gravel muck was loaded into the test chamber, and the cutterhead rotation speed was set to 1.5 r / min. After the torque stabilized, the average cutterhead torque T0 of the unimproved muck was recorded. After emptying the test chamber, standard sand and gravel muck of the same batch and mass was loaded, and the improver was injected according to the implementation plan to complete the muck improvement. Under the same test conditions, the average cutterhead torque T1 of the improved muck was recorded. The torque reduction was calculated using the formula [Cutterhead Torque Reduction = (T0-T1) / T0×100%]. Three parallel tests were performed, and the arithmetic mean was taken as the final result.

[0029] (6) Pebble Encapsulation Rate The sieving and weighing grading method was used for testing. 5 kg of the improved slag sample prepared according to the corresponding embodiment was taken and gently sieved using a 20 mm standard square-hole sieve. All pebble particles on the sieve were collected and their total mass was weighed. The surface coating of each pebble was checked for slurry. A particle was considered effectively coated if ≥ 90% of its surface area was continuously coated with slurry. The total mass of effectively coated pebbles was weighed, and the pebble coating rate was calculated as the ratio of the effective coating mass to the total pebble mass. The test was performed in triplicate, and the arithmetic mean was taken as the final result.

[0030] (7) Foam residue rate under dynamic water conditions The constant weight method of circulating dynamic water flushing was used for testing. Improved slag samples were prepared according to the corresponding embodiment ratio, placed in the sample box of the dynamic water flushing test device, and the density of the sample was controlled to be consistent with that of the slag in the shield tunnel soil chamber. The sample was continuously flushed with a constant water flow rate of 0.1 m / s for 30 min. The total effective foam mass in the sample before and after flushing was determined by the constant weight method of drying at 105 ℃. The foam residue rate was calculated according to the ratio of the retained mass to the initial mass. The test was performed in parallel for 3 times, and the arithmetic mean was taken as the final result.

[0031] (8) Surge suppression effect The pressure relief test was conducted using a closed pressure chamber simulation method. The modified slag prepared according to the corresponding embodiment was loaded into a closed pressure chamber simulating shield tunneling gushing, ensuring the sample's compaction was consistent with the slag inside the shield tunnel's soil chamber. A constant air pressure of 0.3 MPa was applied to the chamber and maintained for 5 minutes. The pressure relief port was then rapidly and completely opened, and pressure was continuously released for 60 seconds. The effluent was collected throughout the process, and the gushing state, water leakage, and solid phase gushing mass were recorded to comprehensively determine the gushing suppression effect. The test was conducted in three parallel trials, and the overall result was used as the final conclusion.

[0032] The test results are shown in Table 1: Table 1. Results of on-site simulated mixing and performance testing

[0033] Example 2: This embodiment focuses on testing the performance of the modifier under dynamic water conditions.

[0034] Preparation of Material A: The components were adjusted to 60 parts sodium bentonite, 1.5 parts soda ash, 0.5 parts xanthan gum, 0.15 parts nonionic polyacrylamide with a molecular weight of 10 million, 0.4 parts polyaluminum chloride with a basicity of 75%, and 110 parts water. The preparation process was the same as in Example 1.

[0035] Preparation of foaming stock solution (Component B): The adjusted composition is 4 parts sodium α-olefin sulfonate, 2 parts AEO-9, 1.5 parts dodecyl dimethylamine oxide, 0.3 parts hydroxyethyl cellulose, 1.2 parts nano-silica, and 105 parts water. The nano-silica is ultrasonically dispersed and then mixed with the other components, following the same process as in Example 1. The foaming parameters are set as follows: gas-liquid ratio 13 and pressure 0.35 MPa.

[0036] Foam material A and material B were mixed at a volume ratio of 1:3 and subjected to a flushing test in a dynamic water environment with a water velocity of 0.1 meters per second. The test method is as follows: The tests were conducted according to GB / T 50123-2019, "Standard for Geotechnical Testing Methods". 1. Test apparatus: A horizontal circulating water tank with a variable frequency water pump is used, along with an embedded sample tank and a flow rate monitoring system, which can accurately control the water flow rate; 2. Sample preparation: Standard sand and gravel soil with a stone content of 35% was used, consistent with the example, and improved according to the volume ratio of A to B materials of 1:3. The sample was compacted to a dry density of 1.8 g / cm³. 3 Then it is placed into the sample holder; 3. Core parameters: ambient temperature 25 ℃ ± 2 ℃, constant water flow velocity 0.1 m / s, continuous flushing for 30 min, maintaining stable water flow without turbulence throughout the process.

[0037] The test results are shown in Table 2: Table 2. Flushing Test Results

[0038] Example 3: This embodiment demonstrates the comprehensive performance and long-term stability of the modifier under different formulation ratios.

[0039] Preparation of Material A: The components are 55 parts sodium bentonite, 2.5 parts soda ash, 0.8 parts sodium carboxymethyl cellulose, 0.2 parts xanthan gum, 0.2 parts anionic polyacrylamide, 0.25 parts polyaluminum chloride, and 105 parts water. The preparation process is the same as before.

[0040] Preparation of foaming stock solution for component B: The components are 5 parts sodium α-olefin sulfonate, 1.7 parts AEO-9, 2.5 parts lauroyl diethanolamine, 0.4 parts hydroxyethyl cellulose, 0.8 parts nano silica, and 108 parts water. The preparation and foaming process are the same as before. The mixing volume ratio of component A to component B foam is 1:2.

[0041] The testing method is as follows: (1) Half-life of the modifier Under constant temperature and standard atmospheric pressure at 25 ℃, the composite foam of material A and material B was mixed evenly according to the volume ratio set in the example, and immediately injected into a 250 mL graduated bottomless cylinder. The surface of the foam was leveled and the initial foam volume was recorded. The foam was allowed to stand and the time it took for 100 mL of liquid to be separated from the foam was recorded, which is the half-life of the modifier. The test was performed in parallel for 3 times and the arithmetic mean was taken as the final result.

[0042] (2) Foaming ratio The airflow method commonly used in shield tunneling was employed for testing. The foaming liquid of material B prepared in the example was injected into a standard foam generator. Compressed air was introduced to generate composite foam according to the air-liquid ratio of 10-15 and the foaming pressure of 0.3-0.5 MPa set in the example. The total volume of the generated foam, V1, and the volume of foaming liquid consumed to generate the foam, V0, were recorded. The foaming ratio was calculated according to the formula [foaming ratio = V1 / V0]. The test was conducted in parallel for 3 times, and the arithmetic mean was taken as the final result.

[0043] (3) 28-day compressive strength Unconfined compressive strength was tested using the unconfined compressive strength test method. Improved sand and gravel slag was used to prepare standard cylindrical specimens with a diameter of 50 mm and a height of 100 mm. The compaction degree of the specimens was consistent with that of the slag in the shield tunnel's soil chamber. The specimens were placed in a standard curing chamber and cured for 28 days at a temperature of 20 ℃ ± 2 ℃ and a relative humidity of over 95%. After curing, a strain-controlled unconfined pressure gauge was used to uniformly load the specimen at a loading rate of 2 mm / min until failure. The maximum axial pressure at failure was recorded, and the unconfined compressive strength was calculated. Three specimens were prepared in parallel for testing, and the arithmetic mean was taken as the final result.

[0044] (4) Permeability coefficient of improved slag soil The constant head permeability test method was used. The improved slag was loaded into a TST-55 permeameter to prepare a cylindrical sample with a diameter of 61.8 mm and a height of 40 mm. The sample was saturated using the vacuum saturation method to ensure that no air bubbles overflowed. The head difference was adjusted to the set value, and after the head stabilized, the timer was turned on to measure the amount of water that permeated through the sample within the set time. The head difference and permeation time at both ends of the sample were recorded, and the permeability coefficient was calculated according to Darcy's permeability formula. The test was performed in parallel for 3 times, and the arithmetic mean was taken as the final result.

[0045] The test results are shown in Table 3: Table 3. Results of comprehensive performance and long-term stability tests of the modifier.

[0046] Comparative Example 1 The difference from Example 1 is that only material A is added, while the total amount of the modifier remains unchanged.

[0047] Comparative Example 2 The difference from Example 1 is that only component B is added, while the total amount of the modifier remains unchanged.

[0048] Comparative Example 3 The difference from Example 1 is that the foaming agent prepared in Example 1 of patent CN120248902A is used to replace material B in this application, while the amount of material B remains unchanged.

[0049] That is, the foaming agent composition is as follows: sodium dodecyl sulfate 2.4 parts, dodecyl dimethylamine oxide 0.8 parts, α- Sodium alkenyl sulfonate 1.8 parts, N,N 1.75 parts dimethylacetamide, 0.075 parts hydroxypropyl methylcellulose, 0.35 parts nano silica, 1.5 parts coconut oil diethanolamide, 3 parts polyvinyl alcohol, and 135 parts water.

[0050] Comparative Example 4 The difference from Example 1 is that the composite foaming agent prepared in Example 3 of patent CN120624029A is used to replace material B in this application, while the amount of material B remains unchanged.

[0051] That is: the composite foaming agent is composed of N Dehydroabenyl N,N It is prepared by compounding dimethylcarboxymethyl betaine with aminated nano-silica and adding okra gum; wherein, N Dehydroabenyl N,N The mass ratio of dimethylcarboxymethyl betaine to aminated nano-silica is 2:1.

[0052] The performance test results of Example 1 and Comparative Examples 1-4 are shown in Table 4.

[0053] Table 4 Summary of Performance Test Results for Example 1 and Comparative Examples 1-4

[0054] As shown in Table 4, the "A+B synergistic system" of this invention significantly improves the fluidity, overall stability, and impermeability of the slag, thereby effectively suppressing common risks in sandy and gravelly strata such as increased cutterhead wear, abnormal torque fluctuations, soil cake formation, and slag heaving. The foam half-life is >25 min, the slump of the improved slag is 185 mm, and the permeability coefficient of the improved slag is 5 × 10⁻⁶. - 7 The results showed improved performance in the present invention, including a slump of cm / s, a 35% reduction in cutterhead torque, a pebble encapsulation rate >90%, a foam residue rate >80% under flowing water conditions, and no gushing or bleeding. Comparative Examples 1 and 2, which used a single A or B material system, showed significantly worse results, validating the synergistic effect of A+B in this invention. Comparative Examples 3 and 4, which used existing foaming agents, performed worse than the present invention in most aspects, such as the improved slump of the excavated soil, the reduction in cutterhead torque, the pebble encapsulation rate, the foam residue rate under flowing water conditions, and the gushing suppression effect. This demonstrates that the present invention achieves superior results compared to existing technologies.

[0055] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A composite foam-bentonite soil conditioner for shield tunneling in gravel strata, characterized in that, include: Material A and Material B, with a volume ratio of 1:2-4; Material A is composed of the following raw materials in parts by weight: 40-70 parts sodium bentonite, 1-3 parts soda ash, 0.5-2 parts stabilizer, 0.05-0.3 parts polyacrylamide, 0.1-0.5 parts polyaluminum chloride, and 80-120 parts water. Material B is a composite foam, and its foaming solution is composed of the following raw materials in parts by weight: 3-8 parts composite foaming agent, 1-4 parts foam stabilizer, 0.1-0.5 parts hydroxyethyl cellulose, 0.5-2 parts nano silica, and 90-110 parts water.

2. The composite foam-bentonite soil conditioner for shield tunneling in gravel strata as described in claim 1, characterized in that, The sodium-based bentonite has a particle size of ≥200 mesh, a montmorillonite content of ≥85%, and a gel value of ≥90%.

3. The composite foam-bentonite soil conditioner for shield tunneling in gravel strata as described in claim 1, characterized in that, The soda ash is industrial-grade sodium carbonate.

4. The composite foam-bentonite soil conditioner for shield tunneling in gravel strata as described in claim 1, characterized in that, The stabilizer is selected from one or two of sodium carboxymethyl cellulose and xanthan gum.

5. The composite foam-bentonite soil conditioner for shield tunneling in gravel strata as described in claim 1, characterized in that, The polyacrylamide is an anionic or nonionic type with a molecular weight of 8-15 million.

6. The composite foam-bentonite soil conditioner for shield tunneling in gravel strata as described in claim 1, characterized in that, The basicity of the polyaluminum chloride is 70%-85%.

7. The composite foam-bentonite soil conditioner for shield tunneling in gravel strata as described in claim 1, characterized in that, The composite foaming agent is prepared by compounding sodium α-olefin sulfonate and fatty alcohol polyoxyethylene ether in a mass ratio of 1 to 3:

1.

8. The composite foam-bentonite soil conditioner for shield tunneling in gravel strata as described in claim 1, characterized in that, The foam stabilizer is selected from lauroyl diethanolamine and dodecyl dimethylamine oxide.

9. A method for preparing a composite foam-bentonite soil conditioner for shield tunneling in gravel strata, characterized in that, include: Preparation of Material A: Dissolve soda ash and stabilizer in a portion of water and mix thoroughly; Add the polyaluminum chloride solution while stirring continuously, and mix thoroughly. Then add polyacrylamide and stir thoroughly to dissolve; finally add sodium bentonite and make up the remaining water, mix evenly to form a homogeneous slurry, and let it stand for 12-24 hours to obtain material A; Preparation of foaming stock solution for component B: Nano-silica is mixed with a portion of water and ultrasonically dispersed to obtain a nano-silica dispersion; hydroxyethyl cellulose, composite foaming agent, and foam stabilizer are dissolved in the remaining water and mixed evenly; the nano-silica dispersion is mixed evenly with the latter solution to obtain the foaming stock solution; Generate composite foam: The foaming liquid is delivered to the foam generator, compressed air is introduced, the gas-liquid ratio is controlled at 10-15, and the foaming pressure is 0.3-0.5 MPa to generate composite foam of material B; The composite foam of material A and material B is pumped into the soil chamber or in front of the cutterhead. The injection volume ratio of material A to material B is 1:2~4, so that it is evenly and densely mixed with the excavated soil in the soil chamber to obtain improved soil.

10. The application of the composite foam-bentonite slag conditioner according to any one of claims 1-8 in urban rail transit construction.

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