Shield balance mud for a shield tunneling machine and a method for preparing the same

Abstract

The application provides a shield balancing mud for a shield machine and a preparation method thereof, and belongs to the field of shield tunnel engineering. The shield balancing mud comprises the following raw materials: modified clay base material: 100 parts, composite flocculating agent: 6-15 parts, tackifying water-retaining agent: 1.5-4 parts, water-dispersing agent: 0.5-2.5 parts, organic crosslinking agent: 0.3-1.2 parts, thixotropy adjusting agent: 0.2-1.5 parts, and pH adjusting agent: 0.5-4 parts. In the application, the high-activity modified clay is used as a skeleton, the inorganic flocculation network is used to provide a dense structure resistant to water erosion, the borax-cellulose reversible crosslinking is used to realize the thixotropic property, the polyacrylamide is used to enhance the water-dispersing resistance, and the thixotropy adjusting agent is used to balance the dispersion and flocculation, so that the mud material simultaneously has excellent water-dispersing resistance and reversible thixotropy in a high-water-pressure water-rich stratum.

Description

Technical Field

[0001] This application relates to the field of shield tunnel engineering technology, and in particular to a shield mud for shield tunneling machines and its preparation method. Background Technology

[0002] During tunnel excavation, tunnel boring machines (TBMs) often need to simultaneously inject mud materials to help fill the gaps at the shield tail, control ground settlement, and stabilize the tunnel face. These materials are collectively referred to as shield mud. Shield mud is typically made primarily of bentonite, supplemented with flocculants, thickeners, and other additives. After on-site preparation, it is injected into the strata surrounding the TBM. Its core functions include: forming a dense mud film at the excavation face to balance water and soil pressure; filling the gaps between the tunnel segments and the ground to prevent surface settlement; and lubricating the shield to reduce frictional resistance. Ideally, shield mud should possess good pumpability, resistance to water dilution, and appropriate structural strength. Especially in high-water-pressure, water-rich strata, the mud material needs to withstand external groundwater pressure without being washed away, while maintaining sufficient viscosity and cohesion in a static state and flowing smoothly during pumping.

[0003] However, existing commercially available bentonite mud and similar products (such as Ke Ni Xiao) have significant drawbacks in the application of high-pressure, water-rich formations. On the one hand, their resistance to water dispersion is insufficient: ordinary bentonite mud is easily diluted and lost when exposed to groundwater. Once the mud film is damaged, it cannot effectively balance water pressure, leading to face instability, excessive surface subsidence, or even sudden surges. While existing technologies promote flocculation by adding inorganic salts such as calcium chloride, the network structure formed by a single flocculant is loose and easily disintegrates under dynamic water pressure. Some solutions introduce organic polymers for thickening, but these lack a stable cross-linking mechanism and swell and degrade. On the other hand, thixotropic properties and resistance to water dispersion are difficult to achieve simultaneously: excessive flocculation or thickening to enhance resistance to water dispersion leads to excessively high yield stress in the mud after settling, requiring extremely high pressure or even causing pipe blockage when restarting pumping; conversely, reducing viscosity and structural strength to improve pumpability results in rapid loss of the mud in water-rich formations, rendering it ineffective as a support. Existing technologies have not yet solved the technical challenge of how to make shield tunneling slurry materials possess both excellent water dispersibility and reversible thixotropy in high water pressure and water-rich strata. Summary of the Invention

[0004] This application provides a shield mud for tunnel boring machines and its preparation method to solve the following technical problem: how to make the mud material for tunnel boring machines simultaneously possess excellent water dispersibility and reversible thixotropy in high water pressure and water-rich strata.

[0005] In a first aspect, this application provides a shield mud for tunnel boring machines, which, by weight, comprises the following raw materials: modified clay base material: 100 parts, composite flocculant: 6-15 parts, thickening and water-retaining agent: 1.5-4 parts, anti-water dispersing agent: 0.5-2.5 parts, organic crosslinking agent: 0.3-1.2 parts, thixotropic modifier: 0.2-1.5 parts, pH adjuster: 0.5-4 parts; The composite flocculant is composed of calcium chloride and polyaluminum chloride; The organic crosslinking agent is borax; The modified clay base material is sodium-based bentonite modified by composite extrusion of sodium carbonate and sodium citrate.

[0006] Optionally, the mass ratio of calcium chloride to polyaluminum chloride is (2~5):1.

[0007] Optionally, the thickening and water-retaining agent is carboxymethyl cellulose; The water-dispersible agent is polyacrylamide; The thixotropic modifier is sodium pyrophosphate; The pH adjuster is sodium carbonate.

[0008] Optionally, the preparation method of the modified clay matrix includes the following steps: Sodium-based bentonite is mixed evenly with sodium carbonate and sodium citrate, and then water is added to adjust the solid content to 70%~85% to obtain a mixed slurry. The mixed slurry is extruded 1 to 3 times under a pressure of 10 to 30 MPa, then aged in a sealed container for 12 to 24 hours, and finally dried and pulverized to obtain the modified clay base material.

[0009] Optionally, the mass of the sodium carbonate is 3-6% of the mass of the sodium-based bentonite; The mass of the sodium citrate is 0.5-2% of the mass of the sodium-based bentonite.

[0010] Secondly, this application provides a method for preparing shield mud for tunnel boring machines as described in any one of the first aspects, the method comprising the following steps: S1. Mix the modified clay base material with water and stir at a speed of 800~1500 r / min for 30~60 min to obtain prehydrated clay slurry; S2. Add a pH adjuster to the prehydrated clay slurry to adjust the pH to 8.5~9.5, and continue stirring for 20~40 minutes; S3. Under stirring conditions, first add calcium chloride from the composite flocculant, maintain the rotation speed at 600~1000 r / min and react for 10~20 min, then add polyaluminum chloride and continue stirring for 20~40 min to form an inorganic flocculation network and obtain the first composite slurry. S4. Under stirring conditions, the aqueous solutions of thickening and water-retaining agent and anti-water dispersing agent are slowly added to the first composite slurry. After the addition is complete, stirring is continued for 40-60 minutes to obtain the second composite slurry. S5. At a stirring speed of 400~600 r / min, the aqueous solution of the organic crosslinking agent is slowly added dropwise to the second composite slurry over 30~60 min. After the addition is completed, stirring is continued for 30~60 min to obtain the third composite slurry. S6. Add a thixotropic modifier to the third composite mud and continue stirring at a speed of 400~600 r / min for 20~40 min to obtain the fourth composite mud. S7. The fourth composite mud is spray-dried to a moisture content of ≤5%, then crushed and passed through a 200-mesh sieve to obtain the shield mud for tunnel boring machines.

[0011] Optionally, the mass ratio of the modified clay matrix to water is 1:(4~6).

[0012] Optionally, the mass concentration of the aqueous solution of the thickening and water-retaining agent and the anti-water-dispersing agent is 2-5%.

[0013] Optionally, the mass concentration of the aqueous solution of the organic crosslinking agent is 1.5~2.5%.

[0014] Optionally, the inlet air temperature of the spray dryer is 110~120℃, and the outlet air temperature is ≤65℃.

[0015] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a shield mud for tunnel boring machines. Through a carefully designed combination of components, the shield mud material can simultaneously obtain excellent water dispersibility and reversible thixotropy in high water pressure and water-rich strata.

[0016] The modified clay matrix is ​​produced by compound extrusion modification of sodium-based bentonite using sodium carbonate and sodium citrate. Sodium carbonate provides sodium ions that replace calcium ions in the interlayer, while sodium citrate chelates free calcium ions to prevent them from re-entering the interlayer. Combined with the extrusion process, the clay crystals are fully expanded and dispersed, forming an active matrix with high specific surface area and high cation exchange capacity. This highly active clay produces a large number of negatively charged flake-like particles after hydration, which inherently possess a certain degree of resistance to water dilution.

[0017] The composite flocculant consists of calcium chloride and polyaluminum chloride, which work synergistically: calcium ions in calcium chloride rapidly compress the electric double layer of clay particles, reducing electrostatic repulsion and causing the particles to aggregate into tiny floc nuclei; while polyaluminum chloride's polynuclear hydroxyl ions connect the floc nuclei into a large and dense flocculation network through adsorption bridging. This inorganic flocculation network structure is dense and has high strength, effectively blocking the infiltration of external high-pressure water and preventing the slurry from being diluted and lost, thus giving the material excellent resistance to water dispersibility.

[0018] Thixotropic and water-retaining agents (such as carboxymethyl cellulose) and organic crosslinking agents (such as borax) form reversible chemical crosslinks in the mud system. Borax hydrolyzes under weakly alkaline conditions, producing borate ions that react with cis-o-dihydroxy groups on the cellulose molecular chain to form reversible borate ester coordination bonds. When the mud is subjected to shearing forces such as pumping or stirring, these coordination bonds break, significantly reducing the system viscosity and facilitating flow and transport. When shearing ceases, the coordination bonds reform, restoring the mud to a higher structural viscosity, effectively suspending solid particles and maintaining stability at the working face. This reversible crosslinking mechanism is the chemical essence of thixotropy. Simultaneously, the long polymer chains of anti-dispersing agents (such as polyacrylamide) connect clay particles, inorganic flocs, and crosslinked cellulose networks into a larger integral structure through adsorption bridging, forming a steric hindrance layer on the particle surface, further preventing water molecule penetration and enhancing the material's resistance to water erosion. Thixotropic modifiers (such as sodium pyrophosphate) moderately regulate the balance between dispersion and flocculation by forming precipitates with free calcium ions, preventing irreversible structural hardening caused by excessive flocculation or cross-linking, ensuring that the mud can quickly recover appropriate strength when stationary and flow smoothly when pumped, thereby optimizing the thixotropic recovery rate.

[0019] The pH adjuster maintains the system in a weakly alkaline range that is most favorable for borax hydrolysis and the formation of borate ester bonds, ensuring the stable progress of the reversible cross-linking reaction.

[0020] Therefore, this application uses highly active modified clay as the framework, inorganic flocculation network to provide a dense structure resistant to water erosion, borax-cellulose reversible crosslinking to achieve thixotropic properties, polyacrylamide to enhance water dispersibility, and thixotropic modifier to balance dispersion and flocculation, so that the mud material has both excellent water dispersibility and reversible thixotropic properties in high water pressure and water-rich formations. Attached Figure Description

[0021] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0022] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a schematic flowchart illustrating the method for preparing shield mud for tunnel boring machines provided in this embodiment of the application. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0025] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0026] This application provides a shield mud for tunnel boring machines. By weight, the shield mud comprises the following raw materials: modified clay base material: 100 parts, composite flocculant: 6-15 parts, thickening and water-retaining agent: 1.5-4 parts, anti-water dispersing agent: 0.5-2.5 parts, organic crosslinking agent: 0.3-1.2 parts, thixotropic modifier: 0.2-1.5 parts, pH adjuster: 0.5-4 parts; The composite flocculant is composed of calcium chloride and polyaluminum chloride. The organic crosslinking agent is borax; The modified clay matrix is ​​sodium-based bentonite modified by composite extrusion of sodium carbonate and sodium citrate.

[0027] In some embodiments, the mass ratio of calcium chloride to polyaluminum chloride is (2~5):1.

[0028] In some embodiments, the thickening and water-retaining agent is carboxymethyl cellulose; The water-dispersing agent is polyacrylamide; The thixotropic modifier is sodium pyrophosphate; The pH adjuster is sodium carbonate.

[0029] In some embodiments, the preparation method of the modified clay matrix includes the following steps: Sodium-based bentonite is mixed evenly with sodium carbonate and sodium citrate, and then water is added to adjust the solid content to 70%~85% to obtain a mixed slurry. The mixed slurry is extruded 1 to 3 times under a pressure of 10 to 30 MPa, then aged in a sealed container for 12 to 24 hours, and finally dried and pulverized to obtain the modified clay base material.

[0030] In some embodiments, the mass of sodium carbonate is 3-6% of the mass of sodium-based bentonite; The mass of sodium citrate is 0.5-2% of the mass of sodium bentonite.

[0031] It should be noted that the functions of each component are as follows: Modified clay matrix (sodium-based bentonite modified by composite extrusion of sodium carbonate and sodium citrate): The main mineral component of bentonite is montmorillonite, a 2:1 type layered silicate containing exchangeable cations between its layers. This scheme uses sodium carbonate and sodium citrate to composite modify sodium-based bentonite. Sodium carbonate provides Na... + It can remove residual Ca from the interlayer. 2+ The displacement enhances the negative charge on the surface of clay particles, thereby improving their hydration and swelling capacity. Sodium citrate, as a strong chelating agent, has its citrate ions react with Ca... 2+ Forming a stable water-soluble complex, displacing Ca 2+ Immobilized in the liquid phase, preventing re-entry into the interlayer, thus significantly improving sodiumization efficiency. Semi-dry extrusion (70%–85% solids content, 10–30 MPa) uses mechanical shear force to uniformly penetrate the modifier into the clay interlayer, simultaneously breaking down agglomerates and increasing specific surface area and active sites. Sealed aging for 12–24 hours ensures sufficient ion exchange. The resulting modified clay matrix exhibits an expansion ratio ≥25 mL / g and a cation exchange capacity ≥85 meq / 100g, providing a highly active substrate for subsequent flocculation.

[0032] The high expansibility of modified clay matrix causes it to form a large number of negatively charged flake-like particles after hydration, which serve as a basis for subsequent inorganic flocculants (Ca). 2+ The presence of sodium citrate (PAC) provides ample reaction sites. It is important to note that sodium citrate should be removed as much as possible after modification via washing or centrifugation (residual amount ≤0.1wt%), otherwise its strong chelating ability will consume subsequently added CaCl2, leading to a decrease in the actual effective concentration of the flocculant.

[0033] Composite flocculant (calcium chloride + polyaluminum chloride, mass ratio 2~5:1): ① Calcium chloride: dissociates into Ca upon dissolving in water. 2+ Ca 2+ By compressing the double electric layer on the surface of bentonite particles, the zeta potential is reduced, weakening the electrostatic repulsion between particles, causing the particles to aggregate due to van der Waals forces to form tiny flocs.

[0034] ② Polyaluminum chloride (PAC): PAC is a pre-hydrolyzed polynuclear hydroxyaluminum complex with a high positive charge. It adsorbs onto the surface of negatively charged clay particles through charge neutralization, and its long-chain structure can bridge multiple particles, connecting tiny flocs into large and dense flocs.

[0035] The stepwise addition (CaCl2 first, then PAC) utilizes the different action scales of the two flocculants: CaCl2 2+ Rapidly compressing the electric double layer shortens the interparticle spacing, facilitating simultaneous contact between the bridging segments of the PAC and multiple particles, forming a denser network. Simultaneously, the Al in the PAC... 3+ It can undergo ionic complexation with the carboxyl groups in the subsequently added anionic polyacrylamide (HPAM), anchoring the PAM chains to the surface of the flocs and enhancing their resistance to water dispersibility.

[0036] Thickening and water-retaining agent (carboxymethyl cellulose, CMC): CMC is an anionic water-soluble cellulose ether containing carboxymethyl (-CH2COO) groups on its molecular chain. - CMC contains unsubstituted hydroxyl groups (-OH). When dissolved in water, CMC significantly increases the viscosity of the continuous phase, suspending solid particles through a viscous effect and preventing sedimentation. Simultaneously, CMC molecules can adsorb onto the surface of clay particles via hydrogen bonds, forming a hydration film that reduces filtration loss and improves water retention.

[0037] This application uses low-substituted CMC (DS=0.4~0.7), where the hydroxyl groups of CMC provide crosslinking sites for borax. The B(OH)4 produced by borax hydrolysis... - Reversible borate ester coordination bonds are formed with the cis-ortho-dihydroxy groups of CMC to construct an organic cross-linked network. This network imparts thixotropic properties to the mud: the cross-links recover during rest, increasing the structural viscosity; during shearing, the cross-links break, decreasing the viscosity and facilitating pumping. Furthermore, a semi-interpenetrating network is formed between CMC and high molecular weight PAM through hydrogen bonding and physical entanglement, further enhancing the mud's cohesion and anti-dilution ability.

[0038] Anti-dispersibility agent (polyacrylamide, PAM, molecular weight ≥ 8 million): PAM (anionic or nonionic, molecular weight ≥ 8 million) is a long-chain water-soluble polymer. Its anti-dispersibility mechanism mainly includes: ① Adsorption bridging: The amide group (-CONH2) or carboxyl group (-COO) of PAM - ① It can adsorb onto the surface of multiple clay particles or flocs, connecting them into a larger three-dimensional network, improving the overall cohesion of the mud and resisting water erosion. ② Spatial stability: The long chains of PAM adsorbed on the particle surface extend into the water, forming a steric hindrance layer, preventing water molecules from penetrating into the mud and reducing the loss rate. Al in anionic HPAM 3+ It can form ionic complexes with the carboxyl groups of HPAM, creating ionic crosslinking points and enhancing the strength of the polymer network.

[0039] Organic crosslinking agent (borax): Borax (Na2B4O7·10H2O) dissolves in water and hydrolyzes, further generating borate ions under alkaline conditions. B(OH)4 - It forms reversible five- or six-membered ring borate ester coordination bonds with polysaccharides containing cis-ortho-dihydroxy structures (such as CMC and xanthan gum). This reaction is reversible: mechanical shearing breaks the borate ester bonds, and the viscosity of the system decreases; upon standing, the bonds reform, and the structure is restored. This is the core chemical mechanism by which mud acquires thixotropy.

[0040] Borax primarily crosslinks with CMC and shows almost no direct reaction with PAM (which does not contain ortho-dihydroxyl groups), ensuring selectivity in network construction. A pH adjuster (sodium carbonate) maintains the system pH between 8.5 and 9.5, which is the optimal range for borax hydrolysis equilibrium and boronic acid ester bond formation (pH < 8 for B(OH)4). - Insufficient concentration leads to slow cross-linking; when pH > 10, metaborate may be generated, reducing the cross-linking ability.

[0041] Thixotropic modifier (sodium pyrophosphate): Sodium pyrophosphate (Na4P2O7) mainly functions as a dispersant / deflocculant in bentonite slurry. Pyrophosphate ions react with free Ca in the solution. 2+ The reaction produces calcium pyrophosphate precipitate. This is achieved by lowering the Ca2+ level. 2+ Concentration, to prevent excessive Ca 2+ It can induce irreversible over-flocculation, thus maintaining the system in a moderately flocculated state. An appropriate amount of sodium pyrophosphate can prevent the flocculation cross-linking network from becoming overdeveloped, ensuring that the structure recovered by the mud after settling is sufficient to suspend particles (preventing sedimentation) and rapidly reduce viscosity under pumping shear (facilitating transport). If sodium pyrophosphate is added in excess, it will over-disperse or even completely negate the flocculation effect, preventing the mud from forming an effective structure; if insufficient, the flocs will be too dense, resulting in excessive resistance when restarting the pump.

[0042] pH adjuster (sodium carbonate): Sodium carbonate provides a weakly alkaline environment, adjusting the system pH to 8.5-9.5. This pH range has the following functional effects: ① It enhances the negative charge on the surface of bentonite particles, improving their initial dispersibility; ② It provides suitable alkaline conditions for borax hydrolysis, ensuring the stability of B(OH)4. - The concentration is sufficient and stable; ③ It inhibits the acid-catalyzed degradation of PAM.

[0043] Figure 1 This is a schematic flowchart illustrating the method for preparing shield mud for tunnel boring machines provided in this embodiment of the application.

[0044] Based on a general inventive concept, such as Figure 1 As shown, this application provides a method for preparing the shield mud for tunnel boring machines as described in any one of the above claims, the method comprising the following steps: S1. Mix the modified clay base material with water and stir at a speed of 800~1500 r / min for 30~60 min to obtain prehydrated clay slurry; S2. Add a pH adjuster to the prehydrated clay slurry to adjust the pH to 8.5~9.5, and continue stirring for 20~40 minutes; S3. Under stirring conditions, first add calcium chloride from the composite flocculant, maintain the rotation speed at 600~1000 r / min and react for 10~20 min, then add polyaluminum chloride and continue stirring for 20~40 min to form an inorganic flocculation network and obtain the first composite slurry. S4. Under stirring conditions, the aqueous solutions of thickening and water-retaining agent and anti-water dispersing agent are slowly added to the first composite slurry. After the addition is complete, stirring is continued for 40-60 minutes to obtain the second composite slurry. S5. At a stirring speed of 400~600 r / min, the aqueous solution of the organic crosslinking agent is slowly added dropwise to the second composite slurry over 30~60 min. After the addition is completed, stirring is continued for 30~60 min to obtain the third composite slurry. S6. Add a thixotropic modifier to the third composite mud and continue stirring at a speed of 400~600 r / min for 20~40 min to obtain the fourth composite mud. S7. The fourth composite mud is spray-dried to a moisture content of ≤5%, then crushed and passed through a 200-mesh sieve to obtain the shield mud for tunnel boring machines.

[0045] In some embodiments, the mass ratio of the modified clay matrix to water is 1:(4~6).

[0046] In some embodiments, the mass concentration of the aqueous solution of the thickening and water-retaining agent and the anti-water-dispersing agent is 2-5%.

[0047] In some embodiments, the mass concentration of the aqueous solution of the organic crosslinking agent is 1.5 to 2.5%.

[0048] In some embodiments, the inlet air temperature of the spray dryer is 110~120℃, and the outlet air temperature is ≤65℃.

[0049] It should be noted that this preparation method achieves efficient prehydration of modified clay matrix and water, precise adjustment of pH environment, orderly construction of inorganic flocculation network, uniform compounding of organic polymer, controlled realization of reversible cross-linking, and low-temperature drying and micronization of final product by precisely controlling the process parameters of each step.

[0050] In S1, modified clay matrix is ​​mixed with water at a mass ratio of 1:4 to 6 and treated at a high speed of 800 to 1500 r / min for 30 to 60 min. This high shear condition can fully break down the agglomerates of clay particles, causing the layered silicate crystal layers to hydrate and expand rapidly, forming a uniform pre-hydrated slurry with a certain initial structural viscosity. The high rotation speed helps to overcome the viscous resistance between clay particles, ensuring that water molecules can fully enter the interlayer, while the appropriate time range ensures sufficient hydration and avoids excessive shearing that could lead to particle breakage.

[0051] Add a pH adjuster to S2 to adjust the pH of the system to 8.5~9.5, and continue stirring for 20~40 minutes. This alkaline range is the optimal window for the subsequent formation of reversible borate ester bonds between borax and carboxymethyl cellulose (the concentration of borate ions is insufficient when the pH is below 8, and metaborate is easily generated when the pH is above 10, which reduces the crosslinking efficiency). At the same time, it promotes the enhancement of the negative charge on the surface of bentonite particles and improves dispersibility. The stirring time ensures that the pH is uniform and stable.

[0052] S3 is the stepwise inorganic flocculation step. First, calcium chloride is added and the reaction is maintained at 600~1000 r / min for 10~20 min. 2+ Rapidly compressing the electric double layer of clay particles and lowering the Zeta potential causes the particles to aggregate due to van der Waals forces, forming tiny flocs. Subsequently, polyaluminum chloride (PAC) is added and stirring continues for 20-40 minutes. The polynuclear aluminum hydroxyl ions of PAC bridge the tiny flocs into large, dense flocs through adsorption. This stepwise addition avoids competitive adsorption between the two flocculants, allowing Ca... 2+ After completing charge neutralization and PAC, it plays a bridging role, thereby forming a denser inorganic flocculation network.

[0053] In S4, the thickening and water-retaining agent (carboxymethyl cellulose) and the anti-dispersing agent (polyacrylamide) are prepared into a 2-5% aqueous solution and then slowly added. This concentration range ensures that the polymer is fully dissolved without being too viscous and difficult to transport. The slow addition method prevents the local concentration from being too high, which would cause polymer entanglement or precipitation. After the addition is complete, continue stirring for 40-60 minutes to ensure that the polymer chains are uniformly adsorbed on the surface of the flocs and form hydrogen bonds or physical entanglement.

[0054] S5 is the key reversible crosslinking step. The organic crosslinking agent (borax) is prepared into an aqueous solution of 1.5~2.5% and slowly added dropwise to the mud over 30~60 minutes at a low speed of 400~600 r / min. The control of the dropwise addition time is crucial. If it is too fast, borax and carboxymethyl cellulose will crosslink instantly to form local gel clusters, resulting in uneven mixing or even blockage of the pipeline. The slow dropwise addition of 30~60 minutes, combined with a moderate stirring speed, allows borate ions to gradually form reversible borate ester bonds with the cis-ortho-dihydroxy groups of carboxymethyl cellulose, resulting in uniform crosslinking and controllable degree of crosslinking.

[0055] Add the thixotropic modifier (sodium pyrophosphate) to S6 and continue stirring for 20-40 minutes. Sodium pyrophosphate neutralizes the positive charge by adsorbing onto the edges of clay particles, while simultaneously reacting with free Ca. 2+ The formation of calcium pyrophosphate precipitate regulates the dispersion-flocculation balance, prevents excessive flocculation that would make the slurry too thick, and ensures that the final product has both high structural viscosity (anti-settling) when at rest and good flowability when pumped.

[0056] S7 employs low-temperature spray drying, with an inlet air temperature of 110~120℃ and an outlet air temperature ≤65℃, reducing the moisture content to below 5%. Compared to conventional high-temperature drying, this condition avoids thermal degradation or cross-linking structure damage to carboxymethyl cellulose and polyacrylamide caused by prolonged heating above 80℃, while preserving the reversible cross-linking reactivity of borax-carboxymethyl cellulose. Controlling the outlet air temperature below 65℃ effectively protects the integrity of the polymer chains. Finally, the product is pulverized through a 200-mesh sieve to obtain a uniform dry powder, facilitating transportation and on-site pulping.

[0057] In summary, this application achieves excellent water dispersibility and reversible thixotropy in shield tunneling slurry materials in high-water-pressure, water-rich formations through the following six collaborative design aspects: (1) A composite extrusion-modified highly active clay matrix was used as the basic framework. Sodium-based bentonite was extruded with sodium carbonate and sodium citrate in a semi-dry state and aged, which fully replaced the exchangeable cations between the clay layers, making the layered structure more loose and open. This modification treatment significantly improved the swelling ratio and cation exchange capacity of the clay, enabling it to form a large number of negatively charged thin-film particles after hydration, providing abundant reaction sites for subsequent flocculation and cross-linking. The high swelling capacity itself also enhanced the mud's ability to resist the intrusion of external water.

[0058] (2) A dense, water-resistant network is constructed through stepwise inorganic flocculation. Calcium chloride is added first, where calcium ions rapidly compress the electric double layer on the surface of clay particles, reducing the electrostatic repulsion between particles and causing tiny particles to aggregate to form initial floc nuclei. Polyaluminum chloride is then added, whose polynuclear hydroxyaluminum ions have a high positive charge and long chain structure, enabling them to bridge and connect different floc nuclei, integrating loose small flocs into large, dense flocs. This sequential approach avoids interference between the two flocculants, resulting in a dense and strong flocculation network that effectively blocks the infiltration of external high-pressure water and prevents the slurry from being diluted and lost.

[0059] (3) The introduction of a reversible organic cross-linking network provides thixotropic properties. Borate ions generated by the hydrolysis of borax in a weakly alkaline environment form reversible borate ester coordination bonds with unsubstituted cis-o-dihydroxy groups on the carboxymethyl cellulose molecular chain. When the slurry is subjected to shearing action such as pumping or stirring, these coordination bonds break, the viscosity of the system decreases rapidly, and it is easy to flow and transport; when the shearing stops, the borate ester bonds reform, and the slurry recovers a high structural viscosity, which is sufficient to suspend solid particles and maintain the stability of the working face. This reversible reaction is the chemical nature of thixotropy.

[0060] (4) Adding high molecular weight polyacrylamide enhances the resistance to water dispersion. The long-chain molecules of polyacrylamide connect clay particles, inorganic flocs, and cross-linked cellulose networks into a larger integral structure through adsorption bridging. At the same time, the polymer chains form a steric hindrance layer on the particle surface, preventing water molecules from penetrating into the slurry. These two effects together improve the cohesion and resistance to water erosion of the slurry, maintaining a low loss rate even in high-pressure, water-rich formations.

[0061] (5) Utilizing sodium pyrophosphate to precisely regulate the dynamic balance between dispersion and flocculation. Pyrophosphate ions preferentially adsorb onto the positively charged edges of clay particles, neutralizing the edge charge and eliminating inter-particle surface attraction, thereby preventing irreversible structural hardening caused by excessive flocculation. Simultaneously, pyrophosphate ions react with free calcium ions in the system to form insoluble precipitates, appropriately reducing the calcium ion concentration and avoiding excessively vigorous flocculation. This results in moderate structural strength recovery (anti-settling) of the mud when at rest, lower yield stress and good fluidity during pumping, thus improving construction pumping performance while ensuring thixotropy.

[0062] (6) Strict control of process sequence and conditions ensures synergistic effect of each component. Specifically, this includes: pre-adjusting the pH to 8.5 to 9.5, which is the optimal range for the formation of borate ester bonds between borax and carboxymethyl cellulose; adding calcium chloride first and then polyaluminum chloride to ensure orderly flocculation; slowly adding the organic crosslinking agent in the form of a low-concentration aqueous solution over a longer period of time to avoid instantaneous crosslinking of borax and cellulose to form local gel clusters; and finally using low-temperature spray drying to prevent the degradation of polymer chains due to overheating during the drying process. These precise process controls ensure that the above chemical and physical mechanisms can be stably reproduced, thereby enabling the final product to achieve excellent water dispersibility and reversible thixotropy in high-water-pressure, water-rich formations.

[0063] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to industry standards. If there is no corresponding industry standard, then common international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0064] Example 1 I. Preparation of Modified Clay Base Material Take 1000g of sodium-based bentonite (montmorillonite content ≥85%), add 50g of sodium carbonate and 10g of sodium citrate, mix evenly, and then add water to adjust the mass solid content to 78% to obtain a mixed slurry. Place the mixed slurry in a roller extruder and extrude it twice under a pressure of 20MPa, then transfer it to a sealed container for aging for 18 hours. After extrusion aging, add water to make a pulp, centrifuge and wash 1-2 times, and then dry it in a 95℃ drying oven until the moisture content is ≤5%. Crush it with a pulverizer and pass it through a 200-mesh sieve to obtain the modified clay base material.

[0065] II. Preparation of Shield Clay for Tunnel Boring Machines Weigh the raw materials according to the following mass proportions: Modified clay base material: 100 parts; Composite flocculant: 10 parts (where the mass ratio of calcium chloride to polyaluminum chloride is 3:1); Thickening and water-retaining agent: Carboxymethyl cellulose (DS=0.6) 2.5 parts; Anti-water dispersing agent: Anionic polyacrylamide (molecular weight 9 million) 1.5 parts; Organic crosslinking agent: Borax 0.8 parts; Thixotropic modifier: Sodium pyrophosphate 0.8 parts; pH adjuster: Sodium carbonate 2 parts.

[0066] The specific preparation steps are as follows: S1. Wet prehydration: Mix 100 parts of modified clay base material with 500 parts of water, put them into a mixing tank, and stir at 1200 r / min for 45 min to obtain prehydrated clay slurry.

[0067] S2, pH adjustment: Add 2 parts sodium carbonate to the prehydrated clay slurry to adjust the pH to 9.0, and continue stirring for 30 minutes.

[0068] S3. Stepwise inorganic flocculation: Under stirring conditions (800 r / min), first add 7.5 parts of calcium chloride and react for 15 min; then add 2.5 parts of polyaluminum chloride and continue stirring for 30 min to form an inorganic flocculation network and obtain the first composite slurry.

[0069] S4. Addition of thickening and water-retaining agent and anti-dispersing agent: Dissolve 2.5 parts of carboxymethyl cellulose and 1.5 parts of polyacrylamide in water to prepare a mixed solution with a mass concentration of 3.5%. Slowly add this mixed solution to the first composite mud under stirring conditions. After the addition is complete, continue stirring for 50 minutes to obtain the second composite mud.

[0070] S5. Addition of organic crosslinking agent: Dissolve 0.8 parts of borax in water to prepare an aqueous solution with a mass concentration of 2.0%. At a stirring speed of 500 r / min, slowly add this solution dropwise to the second composite slurry over 45 min. After the addition is complete, continue stirring for 45 min to obtain the third composite slurry.

[0071] S6. Thixotropic adjustment: Add 0.8 parts of sodium pyrophosphate to the third composite mud and continue stirring at 500 r / min for 30 min to obtain the fourth composite mud.

[0072] S7. Drying and Crushing: The fourth composite mud is subjected to low-temperature spray drying, with the inlet air temperature set at 115℃ and the outlet air temperature at 60℃, until the moisture content is ≤5%. The dried product is collected, crushed by a crusher, and passed through a 200-mesh sieve to obtain the dry powder of shield mud for tunnel boring machines.

[0073] Example 2 I. Preparation of Modified Clay Base Material Take 1000g of sodium-based bentonite (montmorillonite content ≥85%), add 40g of sodium carbonate and 15g of sodium citrate, mix evenly, and then add water to adjust the solid content to 80% to obtain a mixed slurry. Place the mixed slurry in a roller extruder and extrude it three times under a pressure of 25MPa, then transfer it to a sealed container for aging for 24 hours. After extrusion and aging, add water to make a pulp, centrifuge and wash it 1-2 times, then dry and pulverize it, and then dry it in a 100℃ drying oven until the moisture content is ≤5%. After pulverizing and passing it through a 200-mesh sieve, the modified clay base material is obtained.

[0074] II. Preparation of Shield Clay for Tunnel Boring Machines Weigh the raw materials according to the following mass proportions: modified clay base material: 100 parts, composite flocculant: 14 parts (where the mass ratio of calcium chloride to polyaluminum chloride is 4:1), thickening and water-retaining agent: carboxymethyl cellulose (DS=0.5) 3.5 parts, anti-water dispersing agent: anionic polyacrylamide (molecular weight 10 million) 2.0 parts, organic crosslinking agent: borax 1.0 part, thixotropic modifier: sodium pyrophosphate 1.2 parts, pH adjuster: sodium carbonate 3.5 parts.

[0075] The specific preparation steps are as follows: S1. Wet prehydration: Mix 100 parts of modified clay base material with 600 parts of water, put the mixture into a mixing tank, and stir at 1400 r / min for 35 min to obtain prehydrated clay slurry.

[0076] S2, pH adjustment: Add 3.5 parts sodium carbonate to the prehydrated clay slurry to adjust the pH to 9.3, and continue stirring for 25 minutes.

[0077] S3. Stepwise inorganic flocculation: Under stirring conditions (900 r / min), first add 11.2 parts of calcium chloride and react for 12 min; then add 2.8 parts of polyaluminum chloride and continue stirring for 35 min to obtain the first composite slurry.

[0078] S4. Addition of thickening and water-retaining agent and anti-dispersing agent: Dissolve 3.5 parts of carboxymethyl cellulose and 2.0 parts of polyacrylamide in water to prepare a mixed solution with a mass concentration of 4.5%. Slowly add this mixed solution to the first composite mud under stirring conditions. After the addition is complete, continue stirring for 55 minutes to obtain the second composite mud.

[0079] S5. Addition of organic crosslinking agent: Dissolve 1.0 part of borax in water to prepare an aqueous solution with a mass concentration of 2.3%. At a stirring speed of 550 r / min, slowly add this solution dropwise to the second composite slurry over 50 min. After the addition is complete, continue stirring for 50 min to obtain the third composite slurry.

[0080] S6. Thixotropic adjustment: Add 1.2 parts of sodium pyrophosphate to the third composite mud and continue stirring at 550 r / min for 35 min to obtain the fourth composite mud.

[0081] S7. Drying and Crushing: The fourth composite mud is subjected to low-temperature spray drying, with the inlet air temperature set at 118℃ and the outlet air temperature at 63℃, until the moisture content is ≤5%. The dried product is collected, crushed by a crusher, and passed through a 200-mesh sieve to obtain the dry powder of shield mud for tunnel boring machines.

[0082] Example 3 I. Preparation of Modified Clay Base Material Take 1000g of sodium-based bentonite (montmorillonite content ≥85%), add 60g of sodium carbonate and 6g of sodium citrate, mix evenly, and then add water to adjust the mass solid content to 72% to obtain a mixed slurry. Place the mixed slurry in a roller extruder and extrude it once under a pressure of 15MPa, then transfer it to a sealed container for aging for 14 hours. After extrusion and aging, add water to make a pulp, centrifuge and wash 1-2 times, then dry and pulverize, and then dry it in an 85℃ drying oven until the moisture content is ≤5%. After pulverizing and passing it through a 200-mesh sieve, the modified clay base material is obtained.

[0083] II. Preparation of Shield Clay for Tunnel Boring Machines Weigh the raw materials according to the following mass proportions: modified clay base material: 100 parts, composite flocculant: 7 parts (of which the mass ratio of calcium chloride to polyaluminum chloride is 2.5:1), thickening and water-retaining agent: carboxymethyl cellulose (DS=0.7) 1.8 parts, anti-water dispersing agent: anionic polyacrylamide (molecular weight 8.5 million) 0.8 parts, organic crosslinking agent: borax 0.4 parts, thixotropic modifier: sodium pyrophosphate 0.3 parts, pH adjuster: sodium carbonate 0.8 parts.

[0084] The specific preparation steps are as follows: S1. Wet prehydration: Mix 100 parts of modified clay base material with 450 parts of water, put the mixture into a mixing tank, and stir at 900 r / min for 55 min to obtain prehydrated clay slurry.

[0085] S2, pH adjustment: Add 0.8 parts of sodium carbonate to the prehydrated clay slurry to adjust the pH to 8.7, and continue stirring for 35 minutes.

[0086] S3. Stepwise inorganic flocculation: Under stirring conditions (650 r / min), first add 5 parts of calcium chloride and react for 18 min; then add 2 parts of polyaluminum chloride and continue stirring for 25 min to obtain the first composite slurry.

[0087] S4. Addition of thickening and water-retaining agent and anti-dispersing agent: Dissolve 1.8 parts of carboxymethyl cellulose and 0.8 parts of polyacrylamide in water to prepare a mixed solution with a mass concentration of 2.5%. Slowly add this mixed solution to the first composite mud under stirring conditions. After the addition is complete, continue stirring for 45 minutes to obtain the second composite mud.

[0088] S5. Addition of organic crosslinking agent: Dissolve 0.4 parts of borax in water to prepare an aqueous solution with a mass concentration of 1.6%. At a stirring speed of 420 r / min, slowly add this solution dropwise to the second composite slurry over 35 min. After the addition is complete, continue stirring for 35 min to obtain the third composite slurry.

[0089] S6. Thixotropic adjustment: Add 0.3 parts of sodium pyrophosphate to the third composite mud and continue stirring at 420 r / min for 25 min to obtain the fourth composite mud.

[0090] S7. Drying and Crushing: The fourth composite mud is subjected to low-temperature spray drying, with the inlet air temperature set at 112℃ and the outlet air temperature at 58℃, until the moisture content is ≤5%. The dried product is collected, crushed by a crusher, and passed through a 200-mesh sieve to obtain the dry powder of shield mud for tunnel boring machines.

[0091] Comparative Example 1 This comparative example is modified from the one disclosed in Example 1 as follows: In step S3, calcium chloride is not added, only 2.5 parts of polyaluminum chloride are added (the remaining components and steps are exactly the same as in Example 1).

[0092] Comparative Example 2 This comparative example is modified from the one disclosed in Example 1 as follows: In step S3, no polyaluminum chloride is added, only 7.5 parts of calcium chloride are added (the remaining components and steps are exactly the same as in Example 1).

[0093] Comparative Example 3 This comparative example is modified from the one disclosed in Example 1 as follows: Borax is not added in step S5 (the remaining components and steps are exactly the same as in Example 1).

[0094] Comparative Example 4 This comparative example is modified from the one disclosed in Example 1 as follows: In step S4, no polyacrylamide is added, only 2.5 parts of carboxymethyl cellulose are added (the remaining components and steps are exactly the same as in Example 1).

[0095] Comparative Example 5 (lacking the thixotropic modifier sodium pyrophosphate) This comparative example is modified from the one disclosed in Example 1 as follows: Sodium pyrophosphate is not added in step S6 (the remaining components and steps are exactly the same as in Example 1).

[0096] Comparative Example 6 This comparative example is modified from the one disclosed in Example 1 as follows: In step S3, calcium chloride and polyaluminum chloride are added simultaneously (the remaining components and steps are exactly the same as in Example 1).

[0097] Comparative Example 7 This comparative example is modified from the one disclosed in Example 1 as follows: In step S4, carboxymethyl cellulose, polyacrylamide, and borax are dissolved together in water to prepare a mixed solution, and then added simultaneously (borax is not added separately in step S5).

[0098] Comparative Example 8 This comparative example is modified from the one disclosed in Example 1 as follows: The modified clay base material was replaced with commercially available ordinary sodium-based bentonite (without any modification treatment and similar montmorillonite content).

[0099] Based on the preparation results of Examples 1-3 and Comparative Examples 1-8, the performance of the obtained Hengdun mud dry powder was tested.

[0100] The test method is as follows: Take dry powder of each sample of Hengdun mud and mix it at a water-to-solid ratio of 2:1 (i.e., 200g water to 100g dry powder) in a mixer at low speed for 10 minutes to prepare fresh mud slurry. After standing for 10 minutes, test various properties. For water dispersibility, a self-made high-pressure water jetting device was used: the mud slurry was placed on an 80-mesh sieve and vertically jetted with 0.3MPa water pressure for 5 minutes. The percentage of remaining mud slurry mass relative to the initial mass was measured and expressed as a loss rate (%). A lower loss rate indicates better water dispersibility. Thixotropic recovery was measured using a rotational viscometer. The thixotropic recovery rate was first measured at a shear rate of 100 s⁻¹. -1 The equilibrium viscosity η1 is taken as the initial value. After standing for 10 minutes, the viscosity η2 is measured again at the same shear rate. The recovery rate is calculated as η2 / η1 × 100%. The apparent viscosity for pumping is directly taken as the shear rate of 100 s. -1The equilibrium viscosity was determined. The static yield stress was measured using a stress-controlled rheometer. After a 10-minute settling period, the shear stress was increased at a rate of 0.1 Pa / s to determine the critical stress at which the slurry began to flow. The test results are shown in Table 1.

[0101] Table 1 Performance of Hengdun Clay Dry Powder

[0102] Note: "—" indicates that this item was not measured because the mud lost its structural stability or could not form a thixotropic network. Among them, the mud of Comparative Example 2 and Comparative Example 4 showed obvious stratification after standing for 10 minutes, and the thixotropic recovery rate could not be accurately measured. Comparative Example 7 had pipe blockage and a large number of gel particles during the preparation process, and a uniform mud could not be obtained.

[0103] As can be seen from Table 1, the loss rate of Examples 1 to 3 is between 7.5% and 9.3%, the thixotropic recovery rate is between 85% and 88%, the pumping viscosity is 2.1 to 2.5 Pa·s, and the static yield stress is 32 to 38 Pa. They have excellent comprehensive performance and can meet the construction requirements of high water pressure and water-rich strata.

[0104] The loss rate of Comparative Example 1 (deficient in calcium chloride) and Comparative Example 2 (deficient in polyaluminum chloride) was significantly increased (>22%), indicating that a single flocculant cannot form a dense inorganic network and its resistance to water dispersibility is greatly reduced.

[0105] Comparative Example 3 (borax-deficient) had a thixotropic recovery rate of only 42% and a static yield stress as low as 9 Pa. The mud lost its thixotropic basis and was prone to settling and stratification.

[0106] The loss rate of Comparative Example 4 (lacking polyacrylamide) increased to 19.2%, indicating that the adsorption bridging and steric stabilization of polyacrylamide play an important role in improving water dispersibility.

[0107] Comparative Example 5 (lacking sodium pyrophosphate) had a static yield stress as high as 68 Pa and a pumping viscosity of 3.8 Pa·s, indicating that excessive flocculation led to an overly thick slurry, making actual pumping difficult (high starting pressure and easy pipe blockage).

[0108] Comparative Example 6 (with CaCl2 and PAC added simultaneously) performed worse than the example with stepwise addition, with a loss rate of 16.5% and a thixotropic recovery rate of 72%, verifying the necessity of stepwise addition.

[0109] The fact that Comparative Example 7 (premixed with borax and CMC) could not be pulped normally directly proves the indispensability of the process step of adding the organic crosslinking agent.

[0110] Comparative Example 8 (using ordinary unmodified sodium-based bentonite) was inferior to Example 1 in all aspects, especially in terms of loss rate of 14.0% and thixotropic recovery rate of 76%, indicating that composite extrusion modification significantly improved the activity of clay matrix.

[0111] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0112] Furthermore, in the description of this application, the terms "comprising," "including," etc., mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations.

[0113] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A type of shield mud for tunnel boring machines, characterized in that, By weight, the Hengdun mud comprises the following raw materials: modified clay base material: 100 parts, composite flocculant: 6-15 parts, thickening and water-retaining agent: 1.5-4 parts, anti-water dispersing agent: 0.5-2.5 parts, organic crosslinking agent: 0.3-1.2 parts, thixotropic modifier: 0.2-1.5 parts, pH adjuster: 0.5-4 parts; The composite flocculant is composed of calcium chloride and polyaluminum chloride; The organic crosslinking agent is borax; The modified clay base material is sodium-based bentonite modified by composite extrusion of sodium carbonate and sodium citrate.

2. The shield mud for tunnel boring machines according to claim 1, characterized in that, The mass ratio of calcium chloride to polyaluminum chloride is (2~5):

1.

3. The shield mud for tunnel boring machines according to claim 1, characterized in that, The thickening and water-retaining agent is carboxymethyl cellulose; The water-dispersible agent is polyacrylamide; The thixotropic modifier is sodium pyrophosphate; The pH adjuster is sodium carbonate.

4. The shield mud for tunnel boring machines according to claim 1, characterized in that, The preparation method of the modified clay base material includes the following steps: Sodium-based bentonite is mixed evenly with sodium carbonate and sodium citrate, and then water is added to adjust the solid content to 70%~85% to obtain a mixed slurry. The mixed slurry is extruded 1 to 3 times under a pressure of 10 to 30 MPa, then aged in a sealed container for 12 to 24 hours, and finally dried and pulverized to obtain the modified clay base material.

5. The shield mud for tunnel boring machines according to claim 4, characterized in that, The mass of the sodium carbonate is 3-6% of the mass of the sodium-based bentonite. The mass of the sodium citrate is 0.5-2% of the mass of the sodium-based bentonite.

6. A method for preparing shield mud for tunnel boring machines according to any one of claims 1 to 5, characterized in that, The method includes the following steps: S1. Mix the modified clay base material with water and stir at a speed of 800~1500 r / min for 30~60 min to obtain prehydrated clay slurry; S2. Add a pH adjuster to the prehydrated clay slurry to adjust the pH to 8.5~9.5, and continue stirring for 20~40 minutes; S3. Under stirring conditions, first add calcium chloride from the composite flocculant, maintain the rotation speed at 600~1000 r / min and react for 10~20 min, then add polyaluminum chloride and continue stirring for 20~40 min to form an inorganic flocculation network and obtain the first composite slurry. S4. Under stirring conditions, the aqueous solutions of thickening and water-retaining agent and anti-water dispersing agent are slowly added to the first composite slurry. After the addition is complete, stirring is continued for 40-60 minutes to obtain the second composite slurry. S5. At a stirring speed of 400~600 r / min, the aqueous solution of the organic crosslinking agent is slowly added dropwise to the second composite slurry over 30~60 min. After the addition is completed, stirring is continued for 30~60 min to obtain the third composite slurry. S6. Add a thixotropic modifier to the third composite mud and continue stirring at a speed of 400~600 r / min for 20~40 min to obtain the fourth composite mud. S7. The fourth composite mud is spray-dried to a moisture content of ≤5%, then crushed and passed through a 200-mesh sieve to obtain the shield mud for tunnel boring machines.

7. The method for preparing shield mud for tunnel boring machines according to claim 6, characterized in that, The mass ratio of the modified clay matrix to water is 1:(4~6).

8. The method for preparing shield mud for tunnel boring machines according to claim 6, characterized in that, The mass concentration of the aqueous solution of the thickening and water-retaining agent and the anti-water-dispersing agent is 2-5%.

9. The method for preparing shield mud for tunnel boring machines according to claim 6, characterized in that, The mass concentration of the aqueous solution of the organic crosslinking agent is 1.5~2.5%.

10. The method for preparing shield mud for tunnel boring machines according to claim 6, characterized in that, The inlet air temperature of the spray dryer is 110~120℃, and the outlet air temperature is ≤65℃.

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