Thixotropic slurry, preparation method and application of thixotropic slurry in pipe jacking construction of water-rich mud-containing gravel pebble stratum

Abstract

The invention discloses double-thixotropic slurry, a preparation method and application of the double-thixotropic slurry in water-rich mud-containing gravel pebble stratum pipe jacking construction, bentonite, a dispersing agent, a thickening stabilizer CMC and 70-80% of total water are mixed according to a preset proportion, stirring is conducted for 15-20 minutes at the rotating speed of 250-350 r / min, and uniform suspension liquid is formed; mixing and dissolving the residual water with xanthan gum, a flocculating agent and an anti-collapse agent, adding into the suspension liquid in the step S1, and stirring for 10-15 minutes at the same rotating speed; and adding the nano lubricant, stirring for 5-10 minutes at the rotating speed of 250-350r / min, and standing for 24 hours to form the thixotropic slurry. The lubricant is one or a combination of more of nano silicon oxide, nano aluminum oxide, nano zinc oxide, nano cellulose and nano carbon. The nano material is used as the lubricant, so that the friction coefficient can be obviously reduced, the energy loss in the friction process is reduced, the flowing property of the slurry can be improved, and the stability of the thixotropic slurry is enhanced.

Description

Technical Field

[0001] The present invention belongs to the technical field of thixotropic slurry technology in pipe jacking construction, and specifically relates to a thixotropic slurry, a preparation method thereof, and an application thereof in pipe jacking construction in a water-rich muddy gravel stratum. Background Art

[0002] Pipe jacking construction is a construction technology for completing pipeline laying without or with little excavation of the ground surface. This method has the advantages of little impact on ground traffic, fast construction speed, low comprehensive cost, etc., and is widely used in urban pipeline construction projects. Thixotropic slurry is a special slurry, and its characteristic is that it presents gel and sol states under static and disturbed conditions respectively. In pipe jacking construction, thixotropic slurry is often used as a lubricant to reduce the frictional resistance between the pipeline and the surrounding soil mass, which helps to reduce the energy consumption during the jacking process and improve the construction efficiency. In addition, thixotropic slurry can form a stable slurry jacket around the pipeline, playing a supporting role for the soil mass and the pipeline, which helps to prevent soil settlement and the pipeline from sinking or moving due to uneven stress during the jacking process.

[0003] The soil mass in the water-rich muddy gravel stratum has more pores, a relatively loose structure, and the water content varies greatly with the fluctuation of the groundwater level. The applicability of traditional proportioned thixotropic slurry materials is poor under such stratum conditions. On the one hand, the slurry is easy to diffuse and lose in the soil layer, and it is difficult to form a stable slurry jacket. On the other hand, the slurry has poor stability and is easily eroded by groundwater and loses its friction and resistance reduction performance. These two phenomena will both lead to problems such as low jacking efficiency and difficult construction. Developing a thixotropic slurry suitable for this stratum is beneficial to ensuring project safety, improving construction efficiency, reducing construction costs, and ensuring project quality, and has very important engineering significance.

[0004] However, most of the existing thixotropic slurries are only applicable to clay strata or sandy soil strata with relatively low water content, and cannot play an effective role in the water-rich muddy gravel stratum. In addition, most of the existing research on slurry proportioning focuses on the stability and wall protection properties of the slurry, and less optimizes the slurry proportioning for the friction and resistance reduction performance of the slurry. Summary of the Invention

[0005] The purpose of the present invention is to provide a thixotropic slurry, a preparation method thereof, and an application thereof in pipe jacking construction in a water-rich muddy gravel stratum to solve the above technical problems.

[0006] In order to achieve the above purpose, the technical solution adopted by the present invention is as follows: A thixotropic mud comprises the following components by weight: 6-10 parts of bentonite; 0.3-0.5 parts of dispersant; 0.05-0.4 parts of thickening stabilizer; 0.5-1.0 parts of lubricant; 88-94 parts of water; wherein the lubricant is one or more combinations of nano silicon oxide, nano aluminum oxide, nano zinc oxide, nano cellulose and nano carbon.

[0007] The present invention forms a nano-scale lubricating film between the pipe joint and the formation by adding nano-lubricants and taking advantage of the high specific surface area and surface activity of nano-materials, which can significantly reduce the friction coefficient. At the same time, the high specific surface area of ​​nano-materials adsorbs free water, which can inhibit the agglomeration of bentonite particles, so that the water separation rate of the mud remains at 0% after standing for 24 hours; and the nano-particles fill the gaps between the bentonite lamellae, enhancing the strength of the colloidal structure, and the mud viscosity remains stable when the shear rate changes, thereby enhancing the stability of the thixotropic mud. In addition, nano-particles fill the pores of the mud skin to form a dense mud skin, reducing the thickness of the mud skin from the traditional 1.5-3.0mm to ≤1.0mm, and the filtration loss from 20-30mL / 30min to **≤15mL / 30min; the mud casing permeability coefficient is ≤1×10⁻ 5 cm / s, effectively preventing groundwater intrusion and preventing mud dilution from failing, thereby improving impermeability and wall protection.

[0008] Further optimization, the bentonite is sodium-based bentonite, the montmorillonite content of sodium-based bentonite is ≥90%, and the cation exchange capacity CEC is ≥80mmol / 100g. Compared with calcium-based bentonite, sodium-based bentonite has a greater water absorption rate and expansion multiple, good slurrying performance, good dispersibility in water, excellent colloidal suspension viscosity, good thixotropy, good lubricity, high pH value, good thermal stability, high plasticity and adhesion. The drag reduction effect of sodium-based bentonite in sandy soil layers is more obvious, ensuring the filtration loss of mud, reducing mud loss, and better gelling after solidification.

[0009] Further optimization, the dispersant is sodium carbonate, with a purity of ≥99% and a particle size of ≤100 mesh. The role of sodium carbonate is to provide ions for the mud, promote ion exchange, increase the alkalinity coefficient of bentonite, change the surface adsorption of bentonite, promote the dispersion and hydration of clay particles, enable the mud to better form a sol state, and enhance the lubrication performance.

[0010] Further optimization, the thickening stabilizer is a compound of carboxymethyl cellulose CMC and xanthan gum, and the mass ratio of the two is 1:0.5-1:2. The use of CMC and xanthan gum compound thickening, combined with potassium humate anti-collapse agent, enhances the suspension stability and wall protection of the mud.

[0011] Using CMC and xanthan gum as thickening and stabilizing agents, hydrogen bonds are formed between the carboxyl groups in their molecular structures and water molecules, providing intermolecular interaction forces, thereby increasing the viscosity of the mud. At the same time, CMC and xanthan gum can adsorb on the clay surface to form a thin film, which can effectively reduce the aggregation settlement between soil particles, greatly improve the suspension stability of bentonite, and reduce the water loss of the mud in the gravel and cobble stratum; it helps to maintain the uniform distribution of clay particles in the mud, enhance the stability of the mud, and thus maintain the performance for a long time.

[0012] Further optimization, the lubricant is nano-zinc oxide, its particle size is 20 - 50nm, and the specific surface area ≥ 50m² / g.

[0013] Further optimization, the thixotropic mud also contains 0 - 0.2 parts of flocculant and 0 - 0.2 parts of anti-collapse agent. The flocculant is anionic polyacrylamide APAM, and the anti-collapse agent is potassium humate.

[0014] Polyacrylamide, through the strong adsorption of many adsorption groups (amide groups) on the molecular chain to solid-phase particles, adsorbs several solid-phase particles on one or several polymer chains. The polymer chains form a connecting bridge between the solid-phase particles, and rely on the cohesion of the long-chain molecules themselves to make the dispersed solid-phase particles coalesce to form a network structure, thereby effectively reducing the water separation rate of the mud, reducing the water loss of the slurry, and reducing the dissipation amount of the mud in the gravel and cobble stratum.

[0015] The effective components of potassium humate are -COOK, -OK, and free K + , which can ionize to form negatively charged hydrated groups after dissolving in water, and has a large surface functional group, which can adsorb more free water, thereby increasing the electrokinetic potential and electrostatic repulsion of bentonite particles, making the mud obtain a lower water loss and forming a thin and elastic mud cake. Using the carboxyl and phenolic hydroxyl groups in its molecular structure to combine with the mud, increasing the mud consistency, so that the mud sheath formed outside the pipe wall has a stronger wall protection effect. In addition, K + The sealing effect on gravel and cobbles can effectively reduce the water loss of the mud and improve the applicability of the mud in the gravel and cobble stratum. At the same time, it can prevent free water from seeping into the mud, playing a role in inhibiting hydration and preventing collapse.

[0016] A preparation method of the above thixotropic mud, comprising the following steps: Step S1. Primary stirring: Mix bentonite, dispersant, thickening and stabilizing agent CMC with 70 - 80% of the total water volume according to a preset ratio, and stir at a speed of 250 - 350r / min for 15 - 20 minutes to form a homogeneous suspension, obtaining the primary stirred thixotropic mud; Step S2. Secondary Stirring: After mixing and dissolving the remaining water with xanthan gum, flocculant, and anti-collapse agent, add them to the suspension in Step S1 and stir at the same rotation speed for 10 - 15 minutes to obtain the secondary stirred thixotropic mud; Step S3. Tertiary Stirring: Add the lubricant and stir at a rotation speed of 250 - 350 r / min for 5 - 10 minutes to obtain the tertiary stirred thixotropic mud; Step S4. Static Aging: Let the prepared tertiary stirred thixotropic mud stand for 24 hours to form the thixotropic mud.

[0017] The present invention adopts a tertiary stirring + static aging process to ensure the uniform dispersion of nanomaterials and the formation of a stable colloidal structure, thereby improving the performance of the thixotropic mud.

[0018] For further optimization, the mixing temperature of bentonite and water in Step S1 is controlled at 20 - 30 °C, and the temperature is kept stable during the stirring process; in Step S3, the lubricant is added in the form of a nanomaterial dispersion, the solid content of the dispersion is 5 - 10%, and it is pretreated by ultrasonic dispersion for 30 minutes; the total stirring time for three times ≥ 30 minutes.

[0019] For further optimization, the performance parameters of the thixotropic mud are: Viscosity: 60 - 180 s (funnel viscometer); Filtration loss: ≤ 15 mL / 30 min; Mud cake thickness: ≤ 1.0 mm; Water separation rate: 0%; pH value: 10.5 - 11.5.

[0020] Application of the above thixotropic mud in pipe jacking construction in a water-rich muddy sand and gravel stratum. The thixotropic mud is injected into the annular gap between the outer wall of the pipe section and the stratum through a synchronous grouting process to form a mud jacket with a thickness of 5 - 10 mm, and the permeability coefficient of the mud jacket ≤ 1×10 -5 cm / s.

[0021] Compared with the prior art, the beneficial effects of the present invention are: The present invention uses nanomaterials as lubricants. Nanomaterials have characteristics such as a large specific surface area and strong diffusivity. When added as lubricants to the thixotropic mud, they can form an easily shearable thin film between the pipe jacking pipe section and the stratum. This thin film can significantly reduce the friction coefficient and reduce energy loss during the friction process. On the other hand, the nanomaterials have a small particle size and are extremely easy to disperse in water and adsorb water molecules, thereby improving the flow performance of the slurry and enhancing the friction reduction and resistance reduction effect of the mud. At the same time, the nanoparticles can also fill and repair the friction surface to a certain extent, playing an anti-wear role and helping to extend the service life of the equipment. Description of the Drawings

[0022] Figure 1 It is a schematic diagram of the preparation process of the thixotropic mud for pipe jacking construction in a water-rich muddy sand and gravel stratum described in the present invention; Figure 2 Schematic diagram of the physical model for testing the frictional resistance in the embodiment. Specific implementation manners

[0023] In order to further illustrate the technical solution of the present invention, the preferred implementation manners of the present invention will be described below in conjunction with the embodiments. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the scope of protection of the present invention. Embodiment 1:

[0024] As Figure 1 shown, a method for preparing thixotropic mud includes the following steps: Step 1: Weigh 6 parts of bentonite, 0.3 part of sodium carbonate, and 0.05 part of CMC, add them to 70 parts of water, and perform primary stirring at a rotational speed of 250 - 350 r / min for 10 min to obtain primary stirred thixotropic mud.

[0025] Step 2: Weigh another 0.01 part of xanthan gum, 0.01 part of potassium humate, and 0.01 part of PAM, add them to 23.12 parts of water, and perform secondary stirring at a rotational speed of 250 - 350 r / min for 10 min to obtain secondary stirred thixotropic mud.

[0026] Step 3: Weigh 0.5 part of nano-silica and add it to the secondary stirred thixotropic mud, and perform tertiary stirring at a rotational speed of 250 - 350 r / min for 10 min to obtain tertiary stirred thixotropic mud.

[0027] Step 4: Let the tertiary stirred thixotropic mud stand for 24 h to obtain a highly efficient drag-reducing thixotropic mud applicable to the water-rich muddy gravel stratum, denoted as A. Comparative Example 1:

[0028] Weigh 6 parts of bentonite, 0.3 part of sodium carbonate, and 0.05 part of CMC, add them to 70 parts of water, and perform primary stirring at a rotational speed of 250 - 350 r / min for 15 min to obtain primary stirred thixotropic mud. Then weigh another 0.01 part of xanthan gum, 0.01 part of potassium humate, and 0.01 part of PAM, add them to 23.62 parts of water, and perform secondary stirring at a rotational speed of 250 - 350 r / min for 15 min to obtain secondary stirred thixotropic mud. Let the secondary stirred thixotropic mud stand for 24 h to obtain thixotropic mud without adding nano-lubricant, denoted as A'. Embodiment 2:

[0029] Weigh 7.5 parts of bentonite, 0.3 parts of sodium carbonate and 0.05 parts of CMC, add them to 70 parts of water, and conduct primary stirring at a speed of 250 - 350 r / min for 10 min to obtain a primary stirred thixotropic mud. Then weigh 0.01 parts of xanthan gum, 0.01 parts of potassium humate and 0.01 parts of PAM, add them to 21.62 parts of water, and conduct secondary stirring at a speed of 250 - 350 r / min for 10 min to obtain a secondary stirred thixotropic mud. Finally, weigh 0.5 parts of nano-silica, add it to the secondary stirred thixotropic mud, and conduct tertiary stirring at a speed of 250 - 350 r / min for 10 min to obtain a tertiary stirred thixotropic mud. Let the tertiary stirred thixotropic mud stand for 24 h to obtain a highly efficient drag-reducing thixotropic mud suitable for water-rich muddy gravel and cobble strata, denoted as B. Example 3:

[0030] This example has the same steps as Example 2, except that 0.5 parts of nano-silica are replaced by nano-alumina. Example 4:

[0031] This example has the same steps as Example 2, except that 0.5 parts of nano-silica are replaced by nano-zinc oxide. Comparative Example 2:

[0032] Weigh 7.5 parts of bentonite, 0.3 parts of sodium carbonate and 0.05 parts of CMC, add them to 70 parts of water, and conduct primary stirring at a speed of 250 - 350 r / min for 15 min to obtain a primary stirred thixotropic mud. Then weigh 0.01 parts of xanthan gum, 0.01 parts of potassium humate and 0.01 parts of PAM, add them to 22.12 parts of water, and conduct secondary stirring at a speed of 250 - 350 r / min for 15 min to obtain a secondary stirred thixotropic mud. Let the secondary stirred thixotropic mud stand for 24 h to obtain a thixotropic mud without adding nano materials, denoted as B'.

[0033] Result comparison: Measure the viscosity, specific gravity, filtration loss, mud cake thickness, water separation rate and pH value of the four prepared muds, and the results are shown in Table 1.

[0034] Table 1 Performance parameters of the mud

[0035] As can be seen from Table 1, the content of added bentonite has an impact on all performance parameters of the thixotropic mud, especially the viscosity and filtration loss. After adding nano lubricants, the viscosity and filtration loss of the mud change greatly, and other parameters are not affected. Adding different nano-oxides has different degrees of influence on the mud parameters. Generally speaking, the influence of zinc oxide is greater than that of alumina, and the influence of silica is the smallest. Example 5:

[0036] Weigh 7.5 parts of bentonite, 0.3 parts of sodium carbonate and 0.2 parts of CMC, add them to 70 parts of water, and conduct primary stirring at a speed of 250 - 350 r / min for 10 min to obtain a primary stirred thixotropic mud. Then weigh 0.2 parts of xanthan gum, 0.2 parts of potassium humate and 0.2 parts of PAM, add them to 20.9 parts of water, and conduct secondary stirring at a speed of 250 - 350 r / min for 10 min to obtain a secondary stirred thixotropic mud. Finally, weigh 0.5 parts of nano-zinc oxide, add it to the secondary stirred thixotropic mud, and conduct tertiary stirring at a speed of 250 - 350 r / min for 10 min to obtain a tertiary stirred thixotropic mud. Let the tertiary stirred thixotropic mud stand for 24 h to obtain a highly efficient drag-reducing thixotropic mud suitable for water-rich muddy gravel pebble stratum, denoted as C. Comparative Example 3:

[0037] Weigh 7.5 parts of bentonite, 0.3 parts of sodium carbonate and 0.2 parts of CMC, add them to 70 parts of water, and conduct primary stirring at a speed of 250 - 350 r / min for 15 min to obtain a primary stirred thixotropic mud. Then weigh 0.2 parts of xanthan gum, 0.2 parts of potassium humate and 0.2 parts of PAM, add them to 21.4 parts of water, and conduct secondary stirring at a speed of 250 - 350 r / min for 15 min to obtain a secondary stirred thixotropic mud. Let the secondary stirred thixotropic mud stand for 24 h to obtain a thixotropic mud without adding nano-materials, denoted as C'. Example 6:

[0038] Weigh 10 parts of bentonite, 0.3 parts of sodium carbonate and 0.2 parts of CMC, add them to 70 parts of water, and conduct primary stirring at a speed of 250 - 350 r / min for 10 min to obtain a primary stirred thixotropic mud. Then weigh 0.1 parts of xanthan gum, 0.1 parts of potassium humate and 0.1 parts of PAM, add them to 18.7 parts of water, and conduct secondary stirring at a speed of 250 - 350 r / min for 10 min to obtain a secondary stirred thixotropic mud. Finally, weigh 0.5 parts of nano-zinc oxide, add it to the secondary stirred thixotropic mud, and conduct tertiary stirring at a speed of 250 - 350 r / min for 10 min to obtain a tertiary stirred thixotropic mud. Let the tertiary stirred thixotropic mud stand for 24 h to obtain a highly efficient drag-reducing thixotropic mud suitable for water-rich muddy gravel pebble stratum, denoted as D. Example 7:

[0039] This example has the same steps as Example 6, except that 0.5 parts of nano-zinc oxide are replaced by nano-cellulose. Example 8:

[0040] This example has the same steps as Example 6, except that 0.5 parts of nano-zinc oxide are replaced by nano-carbon powder. Comparative Example 4:

[0041] Weigh 10 parts of bentonite, 0.3 parts of sodium carbonate and 0.2 parts of CMC, add them to 70 parts of water, and conduct primary stirring at a speed of 250 - 350 r / min for 15 min to obtain a primary stirred thixotropic mud. Then weigh 0.1 part of xanthan gum, 0.1 part of potassium humate and 0.1 part of PAM, add them to 19.2 parts of water, and conduct secondary stirring at a speed of 250 - 350 r / min for 15 min to obtain a secondary stirred thixotropic mud. Let the secondary stirred thixotropic mud stand for 24 h to obtain a thixotropic mud without adding nanomaterials, denoted as D'.

[0042] Result comparison: Use a physical model to conduct frictional resistance tests on the 6 kinds of muds prepared under different pressure conditions. Each condition is tested 5 times, and finally the average value is taken. The physical model is shown in Figure 2 , and the results are shown in Table 2.

[0043] Table 2 Frictional resistance test results of six kinds of muds

[0044] It can be seen from Table 2 that as the pressure increases, the measured frictional resistance increases significantly. In Example 5, because nano - zinc oxide was added, the measured frictional resistance decreased significantly, which proves that adding nanomaterial lubricants to the thixotropic mud can form an easily - sheared thin film between the pipe jacking pipe joints and the formation. This thin film can significantly reduce the friction coefficient and reduce the energy loss during the friction process. On the other hand, the nanomaterials have a small particle size and are extremely easy to disperse in water and adsorb water molecules, thus improving the flow performance of the slurry and enhancing the friction - reducing and drag - reducing effect of the mud.

[0045] In addition, it can be seen from Table 2 that after increasing the bentonite content in the mud, the measured frictional resistance is greatly improved. Different nano - additives have different effects on enhancing the drag - reducing effect of the mud. Among them, nano - zinc oxide has the best effect, nano - carbon powder ranks second, and cellulose has the most general effect. Example 9:

[0046] Weigh 10 parts of bentonite, 0.3 parts of sodium carbonate and 0.2 parts of CMC, add them to 70 parts of water, and conduct primary stirring at a speed of 250 - 350 r / min for 10 min to obtain a primary stirred thixotropic mud. Then weigh 0.1 part of xanthan gum, 0.1 part of potassium humate and 0.1 part of PAM, add them to 18.45 parts of water, and conduct secondary stirring at a speed of 250 - 350 r / min for 10 min to obtain a secondary stirred thixotropic mud. Finally, weigh 0.75 part of nano - zinc oxide and add it to the secondary stirred thixotropic mud, and conduct tertiary stirring at a speed of 250 - 350 r / min for 10 min to obtain a tertiary stirred thixotropic mud. Let the tertiary stirred thixotropic mud stand for 24 h to obtain a highly efficient friction - reducing thixotropic mud applicable to water - rich muddy gravel pebble strata, denoted as E. Example 10:

[0047] Weigh 10 parts of bentonite, 0.3 parts of sodium carbonate and 0.2 parts of CMC, add them to 70 parts of water, and conduct primary stirring at a rotation speed of 250 - 350 r / min for 10 min to obtain a primary stirred thixotropic mud. Then weigh 0.1 part of xanthan gum, 0.1 part of potassium humate and 0.1 part of PAM, add them to 18.2 parts of water, and conduct secondary stirring at a rotation speed of 250 - 350 r / min for 10 min to obtain a secondary stirred thixotropic mud. Finally, weigh 1.0 part of nano - zinc oxide, add it to the secondary stirred thixotropic mud, and conduct tertiary stirring at a rotation speed of 250 - 350 r / min for 10 min to obtain a tertiary stirred thixotropic mud. Let the tertiary stirred thixotropic mud stand for 24 h to obtain a high - efficiency drag - reducing thixotropic mud applicable to the water - rich muddy gravel stratum, denoted as F.

[0048] Result comparison: Use a physical model to conduct frictional resistance tests on the four prepared muds under different pressure conditions, with each condition tested five times, and finally take the average value. The physical model is shown in Figure 2 , and the results are shown in Table 3.

[0049] Table 3 Frictional resistance test results of four muds

[0050] It can be seen from Table 3 that after increasing the nano - zinc oxide content in the mud, the measured frictional resistance decreases significantly. The greater the zinc oxide content, the smaller the measured frictional resistance.

[0051] Of course, the greater the nano - zinc oxide content is not necessarily better. If the nano - zinc oxide content exceeds the maximum value of the set range by too much, it will cause the mud viscosity and filtration loss to be too low, resulting in too much loss of the mud in the formation during the actual pipe - jacking process, increasing the mud loss. In addition, adding too many additives will increase the material cost and have a negative impact on the engineering economic benefits.

[0052] In the above - mentioned examples and comparative examples, the corresponding components were weighed according to 100 grams per part.

[0053] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above - mentioned examples. What is described in the above - mentioned examples and the specification only illustrates the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A thixotropic mud, characterized in that, It includes the following components by weight parts: 6 - 10 parts of bentonite; 0.3 - 0.5 parts of dispersant; 0.05 - 0.4 parts of thickening stabilizer; 0.5 - 1.0 parts of lubricant; 88 - 94 parts of water; wherein, the lubricant is one or a combination of nano-silica, nano-alumina, nano-zinc oxide, nano-cellulose and nano-carbon.

2. The thixotropic mud according to claim 1, wherein The bentonite is sodium-based bentonite, the montmorillonite content of sodium-based bentonite is ≥90%, and the cation exchange capacity CEC ≥ 80 mmol / 100g.

3. The thixotropic mud according to claim 3, wherein The dispersant is sodium carbonate, its purity ≥ 99%, and the particle size ≤ 100 mesh.

4. The thixotropic mud according to claim 3, characterized in that, The thickening stabilizer is a compound of carboxymethyl cellulose CMC and xanthan gum, and the mass ratio of the two is 1:0.5 - 1:

2.

5. The thixotropic mud according to claim 4, wherein The lubricant is nano-zinc oxide, its particle size is 20 - 50 nm, and the specific surface area ≥ 50 m² / g.

6. The thixotropic mud according to claim 1, characterized in that, The thixotropic mud also contains 0 - 0.2 parts of flocculant and 0 - 0.2 parts of anti-collapse agent, the flocculant is anionic polyacrylamide APAM, and the anti-collapse agent is potassium humate.

7. A method for preparing the thixotropic mud according to any one of claims 1-6, characterized in that, It includes the following steps: Step S1. Primary stirring: Mix bentonite, dispersant, thickening stabilizer CMC with 70 - 80% of the total water amount according to a preset ratio, stir at a speed of 250 - 350 r / min for 15 - 20 minutes to form a homogeneous suspension, and obtain the primary stirred thixotropic mud; Step S2. Secondary stirring: Mix the remaining water amount with xanthan gum, flocculant, and anti-collapse agent and dissolve them, then add them to the suspension in Step S1, and stir at the same speed for 10 - 15 minutes to obtain the secondary stirred thixotropic mud; Step S3. Tertiary stirring: Add nano-lubricant, and stir at a speed of 250 - 350 r / min for 5 - 10 minutes to obtain the tertiary stirred thixotropic mud; Step S4. Static aging: Let the prepared tertiary stirred thixotropic mud stand for 24 hours to form thixotropic mud.

8. The preparation method of the thixotropic slurry according to claim 7, characterized in that, In Step S1, the mixing temperature of bentonite and water is controlled at 20 - 30°C, and the temperature is kept stable during the stirring process; in Step S3, the lubricant is added in the form of a nano-material dispersion liquid, the solid content of the dispersion liquid is 5 - 10%, and it is pretreated by ultrasonic dispersion for 30 minutes; the total stirring duration for three times ≥ 30 minutes.

9. The preparation method of the thixotropic mud according to claim 7, wherein The performance parameters of the thixotropic mud are: Viscosity: 60 - 180 s (funnel viscometer); Filtrate loss: ≤ 15 mL / 30 min; Mud cake thickness: ≤ 1.0 mm; Water separation rate: 0%; pH value: 10.5 - 11.

5.

10. Application of the thixotropic slurry according to any one of claims 1-6 in pipe jacking construction in water-rich muddy gravel and cobble stratum, characterized in that, The thixotropic slurry is injected into the annular gap between the outer wall of the pipe segment and the formation through the synchronous grouting process to form a slurry jacket with a thickness of 5 - 10 mm, and the permeability coefficient of the slurry jacket is ≤ 1×10 -5 cm / s.

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