Composite clay mixed solution and application thereof

Abstract

The invention belongs to the technical field of shield under-pressure bin opening, and particularly provides a composite clay mixed solution and application thereof, and the composite clay mixed solution is prepared from bentonite slurry, methyl cellulose and slaked lime; the bentonite slurry is obtained by mixing a bentonite solution and a sodium silicate solution, and the bentonite solution is obtained by mixing water and sodium bentonite. According to the composite clay mixed solution provided by the invention, methyl cellulose and slaked lime are introduced to synergistically enhance a system; specifically, methyl cellulose (MC) is used as a non-ionic thickening agent, and methoxyl (-OCH3) on a molecular chain of the methyl cellulose (MC) improves the consistency of a system through hydrogen-bond interaction. MC with the viscosity of 4000 mPa.s is selected in an experiment, and the addition amount of the MC is 0.3%-0.5% of the mass of the bentonite; the slaked lime and SiO2 in the bentonite are subjected to a pozzolanic reaction, and the shear strength of the system can be increased to 2.8 kPa within 6 h through the reaction.

Description

Technical Field

[0001] This invention belongs to the field of shield tunneling pressurized tunneling technology, and relates to a composite clay mixture and its application. Background Technology

[0002] In traditional shield tunneling pressurized tunneling technology, the sodium-based bentonite system has technical defects in water-rich strata, such as insufficient mud film stability, thin film thickness (only about 10mm), poor impermeability, and low construction efficiency, which leads to high risks and increased costs in tunneling operations.

[0003] Pressurized opening of the tunnel boring machine (TBM) is a crucial step in cutter replacement during tunnel boring. A stable mud film needs to be established at the tunnel face to isolate the cutter from water and soil pressure. Current technologies often use sodium-based bentonite (mixture ratio of 6 m³ water to 1 t bentonite), but this method has the following problems: ① The reaction between montmorillonite and silicates is insufficient, making the mud film prone to rupture; ② Traditional formulations result in insufficient film thickness and weak compressive strength; ③ Low construction efficiency, with mud film establishment time exceeding 60 minutes. Summary of the Invention

[0004] This invention provides a composite clay mixture, which is prepared from bentonite slurry, methylcellulose and slaked lime; The bentonite slurry is obtained by mixing a bentonite solution and a sodium silicate solution, wherein the bentonite solution is obtained by mixing water and sodium-based bentonite.

[0005] Furthermore, the ratio of water, sodium-based bentonite, and sodium silicate solution is set to 4:1:0.4; The ratio of bentonite slurry, methylcellulose and quicklime is set to 1:0.004:0.05.

[0006] Furthermore, the preparation process of the bentonite solution is as follows: Mix 400 kg of sodium bentonite with 1000 kg of water; Aging for 24 hours; The bentonite solution was obtained by stirring at 120 rpm for 24 hours at 25 ± 2 °C.

[0007] Furthermore, the specific process for obtaining the composite clay mixture based on the bentonite solution is as follows: In 1m 2 10 kg of methylcellulose, a lime water solution prepared from 140 kg of water and 70 kg of quicklime, and 33 L of water glass were added sequentially to the bentonite solution. Then, the mixture was stirred at a speed of 120 rpm and a temperature of 25±2℃ for 24 hours to obtain a composite clay mixture.

[0008] The present invention also provides an application of the composite clay mixture as described above in the pressurized tunneling construction of a tunnel boring machine.

[0009] Compared with the prior art, the present invention has the following beneficial effects: (1) The composite clay mixture provided by the present invention introduces methylcellulose and quicklime to synergistically enhance the system; specifically, methylcellulose (MC) is a non-ionic thickener, and the methoxy group (-OCH3) on its molecular chain increases the consistency of the system through hydrogen bonding. The experiment selected MC with a viscosity of 4000 mPa·s, and the amount added was 0.3%-0.5% of the bentonite mass; quicklime reacts with SiO2 in bentonite to form pozzolanic reaction, and this reaction can increase the shear strength of the system to 2.8 kPa within 6 h.

[0010] (2) In this invention, the strength of the mud film is improved by controlling the amount of sodium silicate solution. Specifically, the Al-OH groups of the montmorillonite lattice in bentonite and the silicon-oxygen tetrahedra in the sodium silicate solution form a three-dimensional network structure through supramolecular interaction. This process is significantly affected by the Baume degree of the solution. Experiments show that when the Baume degree is >35°Bé, the viscosity of the system increases by 300%.

[0011] (3) The composite clay mixture prepared by the stepwise activation method in this invention, as shown by the MARS III rheometer test, has a shear rate of 100 s⁻¹. -1 The filter exhibited Bingham fluid characteristics, with a dynamic-plastic ratio (YP / PV) of 0.85 Pa / (mPa·s), a 62% improvement over the baseline group. Testing with an improved API filtration analyzer showed that the filtration loss decreased to 8 mL after 30 minutes, and the filter cake permeability coefficient was 3 × 10⁻⁶. -9 cm / s, meeting the construction requirements of water-rich diorite strata.

[0012] In addition to the objectives, features and advantages described above, the present invention has other objectives, features and advantages. Detailed Implementation

[0013] To make the above-mentioned objectives, features, and advantages of the present invention clearer and easier to understand, the specific embodiments of the present invention will be described in detail below.

[0014] Example 1: This invention provides a composite clay mixture, which is prepared from bentonite slurry, methylcellulose and slaked lime; The bentonite slurry is obtained by mixing a bentonite solution and a sodium silicate solution, wherein the bentonite solution is obtained by mixing water and sodium-based bentonite.

[0015] Furthermore, the ratio of water, sodium-based bentonite, and sodium silicate solution is set to 4:1:0.4.

[0016] Furthermore, the ratio of the bentonite slurry, methylcellulose, and quicklime is set to 1:0.004:0.05.

[0017] Furthermore, in this embodiment, sodium-based bentonite is first pre-hydrated to form a colloidal suspension bentonite solution, thereby improving the rock-carrying capacity and wall-hanging stability of the composite liquid. Then, the colloidal suspension bentonite solution is stirred and allowed to stand to determine the optimal hydration time and degree of hydration, so as to avoid insufficient hydration leading to excessively low viscosity or sedimentation.

[0018] Specifically, the prehydration treatment process for sodium-based bentonite includes: S1.1 Prepare a sample for prehydration treatment to obtain a prehydrated sample; Specifically, the pre-hydrated sample was obtained by mixing 400 kg of bentonite with 1000 kg of water; S1.2. Stir at 120 rpm at 25±2℃ for 24 hours to prehydrate the prehydrated sample and obtain a prehydrated bentonite solution. S1.3, Conduct performance tests on the prehydrated bentonite solution; specifically, the performance tests include tests on parameters such as viscosity, filtration loss, and colloidal stability; S1.4 Analyze the parameters obtained from the performance test to determine the optimal prehydration time and degree, so as to guide the on-site operation of the tunnel boring machine during pressurized tunneling.

[0019] In this embodiment, the sodium-based bentonite is pre-hydrated to achieve the following effect: (1) Avoid failure: Insufficiently prehydrated materials may result in substandard performance (bentonite particles may not be sufficiently dispersed, leading to insufficient viscosity and mud film collapse).

[0020] (2) Cost control: Optimize hydration conditions through experiments to reduce resource waste.

[0021] Specifically, the stepwise activation process of the pre-hydrated bentonite solution includes: S2.1, Prepare samples for stepwise activation treatment; A stepwise activated sample was prepared using 1 cubic meter of prehydrated bentonite solution, 10 kg of methylcellulose, a lime aqueous solution (prepared from 140 kg of water and 70 kg of slaked lime) and 33 L of water glass. S2.2. The step-by-step activation sample is subjected to step-by-step activation treatment to obtain a composite clay mixture; The specific method is as follows: (1) Stirring speed (120 rpm); Ensure that all components are mixed evenly to avoid uneven reaction caused by excessively high local concentrations; (2) Temperature (25±2℃); The reaction rate is moderate at room temperature, preventing high-temperature degradation of methylcellulose or premature hardening of CSH gel; (3) Order of addition; First, add methylcellulose to ensure the bentonite is dispersed, then add quicklime to provide Ca. 2+ Finally, water glass initiates gelation, and the sequence is irreversible; otherwise, the system may fail.

[0022] Furthermore, in the stepwise activation process, sodium-based bentonite undergoes multi-stage physicochemical reactions with methylcellulose, slaked lime, and water glass, ultimately forming a composite material with specific functions. The detailed mechanisms and synergistic effects of each step are as follows: 1. The interaction between methylcellulose and sodium bentonite: (1) Physical adsorption and dispersion: Methylcellulose, as a high-molecular-weight polymer, adsorbs onto the surface of bentonite particles through hydrogen bonds and van der Waals forces, forming an organic coating layer. This coating effectively prevents bentonite particles from agglomerating and improves their dispersibility in the system.

[0023] (2) Rheological regulation: The aqueous solution of methylcellulose has high viscosity. Its addition significantly increases the viscosity of the system, forms a stable colloidal structure, delays bentonite settlement, and improves workability.

[0024] (3) Water retention effect: The water-retaining properties of methylcellulose can slow down the moisture loss of bentonite, preventing cracking caused by premature drying.

[0025] 2. The addition of slaked lime (Ca(OH)2) and ion exchange: (1) Calcium ions (Ca 2+ ) Replace sodium ions (Na) + ), slaked lime releases Ca after dissolving. 2+ Na between sodium-based bentonite layers + Cation exchange occurs, and it is partially converted into calcium-based bentonite. Na-montmorillonite + Ca 2+ →Ca-montmorillonite + 2Na+ This reaction leads to increased interlayer bonding of bentonite and reduced swelling, but may decrease colloidal stability.

[0026] (2) pH control: Ca(OH)2 raises the pH of the system to a strongly alkaline level (pH 12-13), activating the subsequent reactivity of water glass and providing an alkaline environment for the formation of calcium silicate gel.

[0027] 3. Cementation and structural strengthening of water glass (Na2SiO3): (1) Silicate (SiO3) 2- Reaction with calcium ions SiO3 in water glass 2- With the Ca provided by quicklime 2+ Combine, generate Calcium silicate hydrate (CSH gel): Ca 2+ +SiO3 2- +H2O→CaO·SiO2nH2O gel CSH gel is a major hydration product of cement, giving the material high strength and durability.

[0028] (2) Sodium ions (Na) + (re-exchange) Na released from water glass + It is possible that some of the Ca in the bentonite interlayer will be replaced. 2+ It restores some of its sodium-based properties, balancing swelling and colloidal stability.

[0029] (3) Bentonite surface modification Silicate ions can react with Al-OH or Si-OH groups on the surface of bentonite to form Si-O-Si or Si-O-Al bonds, which enhance the adhesion between particles.

[0030] 4. Synergistic effects and final performance: (1) Structural strengthening CSH gel forms a three-dimensional network structure with bentonite particles, which significantly improves the compressive strength and impermeability of the material.

[0031] (2) Viscosity and stability balance The thickening effect of methylcellulose complements the colloidal stability of bentonite, while the gelling effect of water glass further solidifies the system and prevents stratification.

[0032] In this embodiment, through stepwise activation, sodium-based bentonite undergoes adsorption, ion exchange, and gelation reactions sequentially with methylcellulose, quicklime, and water glass, ultimately forming a composite material with high viscosity, high strength, and high stability. This process fully utilizes the synergistic effect of organic and inorganic processes, and has significant application value in the fields of environmental engineering and building materials.

[0033] Example 2: This invention also provides an application of the composite clay mixture as described above in the pressurized tunneling construction of a tunnel boring machine, which specifically includes the following steps: Step 1: After replacing the slag in the earthen warehouse, inject composite grout; Step 2: Move the blade back 5cm and stir for 2 hours to form a 15mm thick mud film; Step 3: Replace the slurry in the chamber with air pressure to complete the preparation of the opening environment.

[0034] Experimental Example 1: Proportioning of Composite Clay Mixture Table 1: Experimental results of bentonite composite solution (based on 1m³) 2 (For example)

[0035] Experimental Example 2: Mud Film Construction The specific process of mud film construction includes the preparation of composite clay mixture and the replacement of mud film with slag in the soil storage area.

[0036] Specifically, mud film construction is a key process in the entire pressurized cutterhead replacement process. Through injection, filling and squeezing, replacement of excavated soil, and rotating the cutterhead to mix and squeeze, a pressure-retaining mud film with a certain thickness, strength, waterproofness, and durability is established on the outer wall of the shield and in front of the tunnel boring machine.

[0037] The soil chamber was replaced using a composite clay mixture, the main components of which were sodium bentonite, quicklime, methylcellulose and water glass (sodium silicate solution), and the specific proportions are shown in Table 2.

[0038] Table 2: Proposed Mixture Ratio of Composite Clay Mixture

[0039] Furthermore, the specific process for preparing the composite clay mixture based on the proportions shown in Table 1 is as follows: After mixing water and bentonite in a 2.5:1 ratio, first add sodium silicate solution (cellulose CMC) according to the ratio, and stir evenly. Then add lime water (lime water is prepared by mixing the mud injection tank on the No. 6 trolley) and stir thoroughly. Finally, add water glass and stir until it becomes jelly-like.

[0040] Experiments showed that when the composite clay mixture prepared according to the proportions shown in Table 1 was applied to the pressurized opening of a tunnel boring machine, the mud film thickness reached 15 mm (an increase of 40%), the compressive strength was 2.8 kPa, the filtration loss was reduced to 8 mL / 30 min, and the permeability coefficient was 3 × 10⁻⁶. -9cm / s; mud film formation time shortened to 35 minutes (efficiency increased by 45%); cost reduced by 66% compared to traditional shield mud; rheological test: dynamic plastic ratio (YP / PV) reached 0.85Pa / (mPa·s), an improvement of 62% compared to traditional ratio; impermeability test: filtration loss of 8mL in 30 minutes, meeting the requirements of water-rich strata; taking its application in the 6.47m diameter shield tunnel of Nanjing Metro Line 6 as an example, the mud film formed stably under a pressurization rate of 0.3bar / min, and the success rate of opening operation was 100%.

[0041] Furthermore, the specific steps for replacing the mud film with the slag in the soil storage area are as follows: Step 1 (1829, 1830 ring tunneling); During the excavation of rings 1829 and 1830, the earth pressure in the upper part was increased by 0.2 to 0.3 Bar to improve the compaction of the soil in front. In the latter half of the excavation of ring 1830, one synchronous grouting pipe was moved to the middle shield, and inert grout was used for synchronous grouting and mud injection in the middle shield.

[0042] Step 2 (Inject shield tail grease and check screw conveyor seal); 2.1 Before stopping the machine, increase the amount of grease injected into the tail shield to fill each grease chamber with grease, and ensure that the grease pressure is greater than the synchronous grouting pressure (so that the tail shield sealing grease pressure reaches 4 bar) to prevent the grout from entering the tail shield brush. 2.2 The screw conveyor should be checked for sealing in advance and the bolts tightened. When using the screw conveyor to drain the liquid, the screw conveyor should be rotated at low speed and the gate should be opened slowly. Repeat the opening / closing action to ensure that the pressure fluctuation range of the soil chamber does not exceed 0.2 bar. If necessary, bentonite should be used for sealing.

[0043] Step 3 (Release the hinge); Before the tunnel boring machine is shut down and the chamber is opened under pressure, the articulation will be gradually released 1-3 rings, extending the articulation length by 100-130mm to reserve space for the subsequent retraction of the cutterhead to form a mud film.

[0044] Step 4: (Replacing slag with inert slurry); 4.1 After the 1830 ring tunneling is completed, connect the synchronous grouting pipe of the shield machine's grout tank to the ball valve of the manhole access panel with a reducer, ensuring that at least two pipes are connected to the soil chamber. Inert grout is injected into the soil chamber through these pipes, and the injection process is continuous. The soil chamber pressure is stabilized at 1.5 bar. When the soil pressure rises to 1.8 bar, slag replacement begins. 4.2 After the soil chamber pressure stabilizes at 1.8 bar, start the screw conveyor to slowly discharge slag, controlling the pressure in the upper soil chamber. At this time, the soil chamber pressure should be maintained at around 1.5 bar to avoid excessive screw conveyor pressure, which could cause air leakage. When the discharged slag clearly contains inert slurry, determine the location of slag replacement in the chamber by opening the ball valve on the soil chamber wall. At least 30-35 m³ of slag needs to be discharged. (The slag should be lowered below the center of the cutterhead, ensuring no air leakage when the screw conveyor discharges slag). (During the replacement process, strictly control the screw conveyor speed and soil chamber pressure, ensuring the soil chamber pressure is always greater than 1.5 bar, avoiding excessive pressure fluctuations, with a target of ±0.3 bar).

[0045] Step 5: (Injecting inert grout into the middle shield). 5.1 After the slurry replacement is completed, inert slurry is used. Each batch of slurry is about 8 m³ and transported to the tunnel by battery truck and pumped into the trolley synchronous injection tank. 5.2. Inert grout is injected into the shield shell through the synchronous grouting tank of the tunnel boring machine; 5.3 When the ball valve of the middle shield is opened and inert slurry flows out, it is determined that the middle shield slurry injection is completed.

[0046] Step 6: (Replace the inert slurry with a composite bentonite solution). 6.1 After the grouting of the central shield is completed, a composite bentonite solution is used. Each batch is made of about 8 m³ and transported to the tunnel by battery truck and pumped into the synchronous grouting tank of the trolley. 6.2 Mix lime water in the small mixing tank of the No. 6 trolley of the tunnel boring machine, and pump it evenly into the synchronous grouting tank of the trolley through the pipeline; 6.3 Add the designed amount of cellulose (CMC) to the synchronous grouting tank and mix thoroughly; 6.4 Add half the designed volume of water glass to the synchronous grouting tank and stir thoroughly to form a slightly viscous paste-like bentonite slurry. 6.5. Composite bentonite solution is injected into the soil chamber via the tunnel boring machine's synchronous grouting tank, with continuous injection throughout the process. The soil chamber pressure is stabilized at 1.5 bar. When the soil pressure rises to 1.8 bar, slag replacement begins. 6.6 After the soil chamber pressure stabilizes at 1.8 bar, start the screw conveyor to slowly discharge slag, controlling the pressure in the upper soil chamber. At this time, the soil chamber pressure should be maintained at around 1.5 bar to avoid excessive screw conveyor pressure causing air leakage. When the discharged slag clearly contains composite bentonite solution, take a slag sample from the screw conveyor outlet and place it in a graduated cylinder. By observing the slag sample color, sand content, and mixing with water glass, determine the location of slag replacement in the chamber. At least 30-35 m³ of slag needs to be discharged. (The slag should be lowered below the center of the cutterhead, ensuring no air leakage from the screw conveyor). (During the replacement process, strictly control the screw conveyor speed and soil chamber pressure, ensuring the soil chamber pressure is always greater than 1.5 bar, avoiding excessive pressure fluctuations, with a target of ±0.3 bar).

[0047] Step 7: (Construct a water-stop ring); After the slurry replacement is completed, pull the segments out 2-3 rings from the shield tail, and then apply 2 consecutive rings of double-liquid slurry to achieve the purpose of water stoppage. After the injection is completed, conduct an opening inspection to determine whether the water stoppage effect meets the standard (minor seepage (dripping) or no seepage).

[0048] Step 8 (Pressure Osmosis); 8.1 After the composite bentonite solution replaces the inert grout, bentonite is continuously injected into the soil chamber through the tunnel boring machine's synchronous grouting tank; 8.2. A three-stage pressure osmosis method is adopted, namely, the first stage is 1.5 bar to 2.0 bar, the second stage is 2.0 bar to 2.5 bar, and the third stage is 2.5 bar to 3.0 bar. The pressure stabilization time for each stage is not less than 120 minutes. After the pressure stabilization time of the third stage is reached, the pressure osmosis is completed.

[0049] Step 9 (Mud film establishment); 9.1. The tunnel boring machine's mortar tank contains 8 m³ of bentonite-mixed slurry. The cutterhead is retracted in stages (each retraction controlled at 4-5 cm), while simultaneously injecting the bentonite-mixed slurry and half the remaining volume of water glass according to the set ratio. After each stage of cutterhead retraction is completed, the cutterhead is started at 0.5 rpm and rotated in the same direction for 2 hours to form a first-stage pressure-maintaining mud film. 9.2. After stopping the machine for 1 hour, observe the rate of change of earth pressure. Once it stabilizes (the fluctuation range of the shield tunneling opening pressure is controlled within 0.4 bar / h), the next stage of cutterhead retraction can begin. This process is repeated to form a pressure-maintaining mud film approximately 5 cm thick. 9.3. Based on the total volume of bentonite-mixed grout injected into the soil chamber, calculate the amount (percentage) of water glass needed to supplement the remaining 2 / 3 volume. Use a secondary grouting machine to intermittently inject water glass to form a composite clay-mixed grout. Start the cutterhead at 1 rpm, keeping the cutterhead rotation in the same direction (determined based on the cutterhead change operation). Continue rotating the cutterhead for approximately 1 hour, then reduce the cutterhead speed to 0.5 rpm, keeping the cutterhead rotation in the same direction, and continue rotating for approximately 2 hours. Observe the changes in soil chamber pressure and torque fluctuations during this process. If the soil chamber pressure and torque are relatively stable, maintain this for at least 1 hour. Stop the machine for 1 hour and observe the rate of change in soil pressure; it should be relatively stable (the fluctuation range of the shield tunneling opening pressure should be controlled within 0.2 bar / h).

[0050] Step 10 (Slurry-gas replacement); 10.1. Establish air pressure in the soil chamber using the pressure-maintaining system to gradually replace the composite clay slurry within the chamber. Start the cutterhead at 0.5 rpm, rotating in the same direction. After 15 minutes of rotation, intermittently start the screw conveyor at 2-4 rpm to discharge the composite clay slurry. When the pressure in the soil chamber discharged by the screw conveyor is 0.2 bar lower than the set pressure for the shield tunneling pressurized opening, pause the discharge, activate the pressure-maintaining system, and restart the screw conveyor for discharge. Repeat this process until the liquid level in the soil chamber is reduced to half-fill (volume, ball valve activated), completing the air pressure replacement of the composite clay slurry in the chamber and providing an operating environment for the next opening operation. 10.2. Open the pressure holding system (1.7 Bar), and simultaneously open the ball valves at points 3 and 9 to displace the air in the soil chamber, reduce the temperature inside the soil chamber, and perform gas detection on the gas outlet of the ball valve.

[0051] Step 11 (Criteria for Opening a Position); Observe the pressure changes in the soil chamber (stabilized at 1.7 Bar) for at least 1 hour, while keeping the pressure-maintaining system on and the cutterhead stationary. Measure the air compressor loading and unloading every 10 minutes. If the loading time is less than the unloading time, it indicates that the mud film quality meets the requirements and the soil chamber has good air retention. The larger the difference between the loading and unloading times, the better the face stability and mud film quality. If the loading time is greater than or equal to the unloading time, it indicates that mud film establishment has failed and needs to be re-established.

[0052] Step 12 (Open the chamber and change the blade); Following the normal procedure, pressurized opening operations are carried out. The first personnel in the chamber mainly observe the stability of the mud film on the working face. If the mud film effect is good, the personnel can enter the chamber to start cleaning the toolbox, checking the wear of the tools, and then begin the subsequent tool replacement operation.

[0053] 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 clay mixture, characterized in that, The composite clay mixture is prepared from bentonite slurry, methylcellulose and slaked lime; The bentonite slurry is obtained by mixing a bentonite solution and a sodium silicate solution, wherein the bentonite solution is obtained by mixing water and sodium-based bentonite.

2. The composite clay mixture according to claim 1, characterized in that, The ratio of water, sodium bentonite, and sodium silicate solution is set to 4:1:0.

4. The ratio of bentonite slurry, methylcellulose and quicklime is set to 1:0.004:0.

05.

3. The composite clay mixture according to claim 1, characterized in that, The preparation process of the bentonite solution is as follows: Mix 400 kg of sodium bentonite with 1000 kg of water; Aging for 24 hours; The bentonite solution was obtained by stirring at 120 rpm for 24 hours at 25 ± 2 °C.

4. The composite clay mixture according to claim 3, characterized in that, The specific process for obtaining the composite clay mixture based on the bentonite solution is as follows: In 1m 2 10 kg of methylcellulose, a lime water solution prepared from 140 kg of water and 70 kg of quicklime, and 33 L of water glass were added sequentially to the bentonite solution. Then, the mixture was stirred at a speed of 120 rpm and a temperature of 25±2℃ for 24 hours to obtain a composite clay mixture.

5. The application of the composite clay mixture as described in any one of claims 1-4 in the pressurized tunneling construction of a tunnel boring machine.

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