Shield tunnel construction method for preventing settlement and collapse of upper-soft lower-hard stratum

Abstract

The invention relates to the technical field of shield tunneling methods, in particular to an anti-sedimentation and anti-collapse shield tunnel construction method for an upper-soft and lower-hard stratum, which comprises the following steps: after a shield tunneling machine is launched, grouting construction is performed in sequence, a section layer of the shield tunneling machine is (2) 2c silt silty clay, (3) 2 silty clay and (4) 1b silty clay which are all flow plastic soil, and the section layer of the shield tunneling machine is composed of (2) 2c silty clay, (3) 2 silty clay and (4) 1b silty clay; and then a small amount of foam needs to be injected once every five rings, and then an anti-settling and anti-collapse stratum is formed. The device has the advantage of preventing the stratum from settling and collapsing, and in the actual using process, targeted construction measures such as grouting construction and foam filling are adopted according to the characteristics of the stratum with the soft upper portion and the hard lower portion, so that the stratum is effectively prevented from settling and collapsing; and secondly, by adjusting the foam filling amount, the water adding amount and the advancing speed in stages, the soil body is improved step by step, the workability of the soil body is improved, the screw conveyor is smooth in unearthing, the slipping situation of the belt conveyor is reduced, and the tunneling efficiency is improved.

Description

Technical Field

[0001] The invention relates to the technical field of shield methods, in particular to a shield tunnel construction method for preventing subsidence and collapse of upper soft and lower hard strata. Background Art

[0002] With the large-scale development of urban rail transit, shield tunneling has become one of the main methods for excavating urban subway tunnels. The stability of the shield tunnel excavation face is an important control factor related to construction safety. The instability of the excavation face is likely to induce major risk accidents such as surface collapse and building damage. The support pressure provided by the shield soil cabin is directly related to the stability of the shield tunnel excavation face. When the support pressure is insufficient, the excavation face will be at risk of instability. In engineering, the upper and lower rock and soil bodies above and below the tunnel excavation face are usually called "upper soft and lower hard strata" when the strength difference is 5 to 10 times. The instability characteristics of its excavation face are quite different from those in homogeneous strata, and it is difficult to control the support force. Therefore, determining the ultimate support force of the shield tunnel excavation face in upper soft and lower hard strata is of great significance to prevent the instability of the excavation face.

[0003] In a certain special-shaped shield tunnel section project, problems such as poor fluidity of the soil bin and poor soil discharge occurred in the soft upper and hard lower clay soil layers. Among them, the soft soil layer was thick, the soil distribution was relatively uniform, and it had strong compressibility. There was no special design on the market for the characteristics of the soft upper and hard lower strata, which made it difficult to effectively prevent the settlement and collapse of the strata during the construction process. Secondly, the method of adjusting the amount of foam and water added and the advancement speed in stages was not adopted, resulting in poor soil improvement effect and low excavation efficiency. Furthermore, the thrust of the shield machine and the torque of the cutter head were not effectively controlled, resulting in excessive wear and failure of the equipment, increasing construction costs and risks.

[0004] Therefore, there is an urgent need for a shield tunnel construction method that can prevent subsidence and collapse in soft upper and hard lower strata to solve the above problems. Summary of the invention

[0005] The purpose of the present invention is to provide a shield tunnel construction method for preventing subsidence and collapse of soft upper and hard lower strata, which has the advantages of preventing strata subsidence and collapse and solves the problems raised by the above-mentioned background technology.

[0006] To achieve the above object, the present invention provides the following technical solution: a shield tunnel construction method for preventing subsidence and collapse in upper soft and lower hard strata, the method comprising the following steps:

[0007] S1: After the shield machine was started, grouting construction was carried out in sequence. The cross-section layers of the shield machine were ②2c silty clay, ③2 silty clay, and ④1b silty clay, all of which were plastic soils (see Figure 1 ), then a small amount of foam injection is required every 5 rings to form a stratum that prevents subsidence and collapse;

[0008] S2:aThe first stage:

[0009] In the blind area, eight holes (S13~S16, S32, S33, S11, S17) are filled with foam and kept open. Each ring is filled with 15m 3 , the water filling holes of breast plates S07, S30, S21, S10, and S18 are always open, and 15m3 of water is added to each ring of soil bin. 3 , open 2 water holes on the left and right cutter discs, add 5m of water per ring 3 , slow advancement (15mm / min) was adopted, water was added at intervals, foam was added synchronously and normally, and advancement was stopped after 10cm to improve the soil with an idle cutterhead. The improved soil had significantly improved workability, the screw machine was unearthed smoothly, and the thrust of the shield machine and the torque of the cutterhead were significantly reduced. However, more foam was added, and the belt conveyor was prone to slippage. Special personnel needed to be arranged to handle the advancement process, and the advancement speed was slow, and the improvement in work efficiency was significantly small (an average of 4 rings / d);

[0010] bThe second stage:

[0011] Add foam to the four holes S13, S16, S32, and S33 in the upper part of the blind area. Keep it open. Add 10m3 of foam to each ring. 3 The water adding holes of chest plate S07, S30, S21, S10 and S18 are always open. 20m3 of water is added to each ring of soil bin. Two water adding holes are opened on the left and right cutter discs respectively. 5m3 of water is added to each ring. 3 , appropriately increase the advancement speed (20mm / min), add water and foam synchronously, stop advancing after 40cm, and improve the soil with an idle cutterhead. After the improvement measures are adjusted, the soil has better workability, the screw conveyor is unearthed smoothly, and the belt conveyor does not slip. The total thrust and cutterhead torque have increased compared with the first stage, but are within a reasonable range, and the excavation efficiency has been significantly improved (5.5 rings / d on average);

[0012] cThe third stage:

[0013] Fill the holes S32 and S33 in the upper part of the blind area and one hole each on the left and right cutter discs, a total of 4 holes, with foam. Keep it open, and fill each ring with 8m3 of foam. 3 , the water filling holes of breast plates S07, S30, S21, S10, and S18 are always open, and 20m3 of water is added to each ring of soil bin. 3 , open 2 water holes on the left and right cutter discs, add 5m of water per ring 3 The advancement speed is maintained at 30mm / min, water and foam are added synchronously and continuously, and each ring of shield is continuously advanced. After the improvement measures are adjusted, the slag has better workability and the screw machine is unearthed smoothly. (See Figure 6), the belt conveyor did not slip, the excavation efficiency of each ring was significantly improved (7 rings / d on average), and the thrust and cutter torque of the shield machine were controlled within a reasonable range. By adding foam to improve the fluidity and stability of the soil, the viscosity of the slag in the soil bin was reduced, and the soil discharge efficiency was improved.

[0014] Furthermore, as a preferred embodiment of the present invention, in step S1, the ratio of foam stock solution to water is 1:19; and the ratio of foam composition to foam solution to compressed air is 1:8 to 1:12.

[0015] Furthermore, as a preferred embodiment of the present invention, before soil improvement, the total thrust of the shield machine is 29000-33000 kN, with a maximum of 33200 kN, the torque is 2500-3000 kN·m, the cutter head torque coefficient ranges from 0.8 to 1.0; the screw machine torque coefficient ranges from 9 to 11, and the advancement speed is slow at this stage.

[0016] Furthermore, as a preferred embodiment of the present invention, in the first stage of soil improvement, the total thrust of the shield machine drops to 15,000 to 18,000 kN, and the torque fluctuates in the range of 900 to 1,100 kN·m, with a large decrease. The cutter head torque coefficient ranges from 0.5 to 0.8; the screw machine torque coefficient ranges from 3 to 5; and the propulsion speed is improved in this stage.

[0017] Furthermore, as a preferred embodiment of the present invention, in the second stage of soil improvement, the number of foam holes and the amount of foam are reduced, the amount of water is increased, the total thrust of excavation varies in the range of 16000 to 20000 kN, the torque fluctuation range is 1000 to 1500 kN·m, the cutter head torque coefficient range is 0.6 to 0.8; the screw machine torque coefficient range is 5 to 6; the excavation parameters generally increase slightly, but are within a reasonable range.

[0018] Furthermore, as a preferred embodiment of the present invention, in the third stage of soil improvement, the hole position is further adjusted and improved, and the propulsion speed is increased. During the tunneling process of the shield machine, the total thrust fluctuates within the range of 19000 to 21000 kN, the cutter head torque varies within the range of 1300 to 1600 kN·m, and the cutter head torque coefficient ranges from 0.6 to 0.8.

[0019] Furthermore, as a preferred embodiment of the present invention, the cutter disc assembly gradually discharges the debris accumulated in the soil bin, and the cutter disc assembly includes a left spiral machine, a right spiral machine is arranged on one side of the left spiral machine, a left large cutter disc and a right large cutter disc are respectively embedded on one side of the left spiral machine and the right spiral machine, an upper small cutter disc and a lower small cutter disc are commonly arranged on one side of the left spiral machine and the right spiral machine, the upper small cutter disc and the lower small cutter disc are distributed up and down, and a blind area is arranged on one side of the left spiral machine and the right spiral machine.

[0020] The steps for implementing the cutterhead assembly are as follows:

[0021] Step 1: In view of the blockage of the left and right spiral machines in previous projects, the shield machine and soil improvement system are optimized and improved, and the left large cutter head, right large cutter head, upper small cutter head and lower small cutter head are rotated regularly. The left and right spiral machines are rotated forward to continuously excavate the soil, and finally the accumulated slag in the soil bin is gradually discharged. The shield machine is provided with injection ports on the excavation face and the soil bin to inject soil improvers, which can also be configured as drilling ports to enable it to have the ability of advanced drilling and grouting reinforcement. In addition, a plurality of fixed radial injection ports are provided on the outer periphery of the ring of the front shell of the shield machine, and bentonite and friction-reducing materials can be added to the soil around the shell. The foam injection and mud adding device mainly includes a stock liquid tank, a stock liquid tank, a foam solution tank, a bentonite tank, a foam and injection mud adding pump, a flow control unit, a foam gun, a foam control panel, a valve control panel and other control panels, pneumatic devices, etc. The mud adding and foaming system is configured with a single pipe and a single pump.

[0022] Step 2: The distribution of the slag improvement holes is as follows: there are 33 soil improvement holes in the soil bin, of which 10 are filling ports for stirring rods and 23 are filling ports for chest plates; there are 8 filling ports for the large blade disc, of which 6 can be filled on the front of the large blade disc and 2 can be filled on the back of the large blade disc (used to improve the mud cake on the front of the small blade disc and the poor excavation of the spiral machine); there are 2 reinforcement ports for the small blade disc, of which 2 can be filled on the front of the small blade disc. All filling ports can be switched between foam, bentonite and water.

[0023] Beneficial effects. The technical solution of the present application has the following technical effects: the present invention has the advantage of preventing ground settlement and collapse. In actual use, targeted construction measures are taken according to the characteristics of the upper soft and lower hard ground, such as grouting construction, foam injection, etc., to effectively prevent ground settlement and collapse; secondly, by adjusting the amount of foam and water injection and the propulsion speed in stages, the soil is gradually improved, the workability of the soil is improved, the screw machine is unearthed smoothly, the belt conveyor slips less, and the excavation efficiency is improved; further, the thrust of the shield machine and the torque of the cutter disc are controlled: by controlling the amount of foam and water injection, the thrust of the shield machine and the torque of the cutter disc are effectively controlled to keep them within a reasonable range, avoiding excessive wear and failure of the equipment; finally, by improving the fluidity and stability of the soil, the viscosity of the slag in the soil bin is reduced, the soil discharge efficiency is improved, and the construction period is shortened.

[0024] It should be appreciated that all combinations of the foregoing concepts, as well as additional concepts described in greater detail below, may be considered to be part of the inventive subject matter of the present disclosure, provided such concepts are not mutually inconsistent. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

[0026] Figure 1 It is a schematic cross-sectional diagram of the upper soft and lower hard soil layers of the interval tunnel of the present invention;

[0027] Figure 2 This is a diagram showing the main characteristics of the soil layer in the excavation section of the present invention;

[0028] Figure 3 It is a statistical table of physical and mechanical indicators of soil layers of the present invention;

[0029] Figure 4 This is a schematic diagram of the rotation blind area of ​​the cutter head assembly of the present invention;

[0030] Figure 5 This is a schematic diagram of the improved hole distribution of the cutter head of the present invention;

[0031] Figure 6 This is a schematic diagram of the improved hole distribution of the soil bin of the present invention.

[0032] In the figure, the meanings of the various reference numbers are as follows: 1. Left spiral machine; 2. Right spiral machine; 3. Left large blade disc; 4. Right large blade disc; 5. Upper small blade disc; 6. Lower small blade disc; 7. Blind spot; 8. Filling port on the front of the large blade disc; 9. Filling port on the front of the small blade disc; 10. Filling port on the back of the large blade disc. DETAILED DESCRIPTION

[0033] In order to further explain the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the specific implementation mode, structure, characteristics and effects of the present invention are described in detail below in combination with the accompanying drawings and preferred embodiments.

[0034] As attached Figure 1 To Attachment Figure 6 As shown: This embodiment provides a shield tunnel construction method for preventing subsidence and collapse in soft upper and hard lower strata, and the method comprises the following steps:

[0035] S1: After the shield machine was started, grouting construction was carried out in sequence. The cross-section layers of the shield machine were ②2c silty clay, ③2 silty clay, and ④1b silty clay, all of which were plastic soils (see Figure 1 ), then a small amount of foam injection is required every 5 rings to form a stratum that prevents subsidence and collapse;

[0036] S2:aThe first stage:

[0037] Add foam to the 8 holes S13~S16, S32, S33, S11, S17 in the upper part of blind area 7. Keep it open. Add 15m3 of foam to each ring. 3, the water filling holes of breast plates S07, S30, S21, S10, and S18 are always open, and 15m3 of water is added to each ring of soil bin. 3 , 2 water holes are opened on the left cutter disc 3 and the right cutter disc 4 respectively, and 5m of water is added per ring. 3 , slow advancement (15mm / min) was adopted, water was added at intervals, foam was added synchronously and normally, and advancement was stopped after 10cm to improve the soil with an idle cutterhead. The improved soil had significantly improved workability, the screw machine was unearthed smoothly, and the thrust of the shield machine and the torque of the cutterhead were significantly reduced. However, more foam was added, and the belt conveyor was prone to slippage. Special personnel needed to be arranged to handle the advancement process, and the advancement speed was slow, and the improvement in work efficiency was significantly small (an average of 4 rings / d);

[0038] bThe second stage:

[0039] Add foam to the four holes S13, S16, S32, and S33 in the upper part of blind zone 7. Open the holes normally and add 10m3 of foam to each ring. 3 The water adding holes of chest plate S07, S30, S21, S10 and S18 are always open. 20m3 of water is added to each ring of soil bin. Two water adding holes are opened on left cutter disc 3 and right cutter disc 4 respectively. 5m3 of water is added to each ring. 3 , appropriately increase the advancement speed (20mm / min), add water and foam synchronously, stop advancing after 40cm, and improve the soil with an idle cutterhead. After the improvement measures are adjusted, the soil has better workability, the screw conveyor is unearthed smoothly, and the belt conveyor does not slip. The total thrust and cutterhead torque have increased compared with the first stage, but are within a reasonable range, and the excavation efficiency has been significantly improved (5.5 rings / d on average);

[0040] cThe third stage:

[0041] Fill the holes S32 and S33 on the top of blind area 7 and one hole each on the left cutter head 3 and the right cutter head 4, a total of 4 holes, with foam. Keep it open, and fill each ring with 8m3 of foam. 3 , the water filling holes of breast plates S07, S30, S21, S10, and S18 are always open, and 20m3 of water is added to each ring of soil bin. 3 , 2 water holes are opened on the left cutter disc 3 and the right cutter disc 4 respectively, and 5m of water is added per ring. 3 The advancement speed is maintained at 30mm / min, water and foam are added synchronously and continuously, and each ring of shield is continuously advanced. After the improvement measures are adjusted, the slag has better workability and the screw machine is unearthed smoothly. (See Figure 6 ), the belt conveyor did not slip, the excavation efficiency of each ring was significantly improved (7 rings / d on average), and the thrust and cutter torque of the shield machine were controlled within a reasonable range. By adding foam to improve the fluidity and stability of the soil, the viscosity of the slag in the soil bin was reduced, and the soil discharge efficiency was improved.

[0042] Specifically, in step S1, the ratio of foam stock solution to water is 1:19; the foam composition is foam solution to compressed air is 1:8 to 1:12.

[0043] Specifically, before soil improvement, the total thrust of the shield machine was 29,000 to 33,000 kN, with a maximum of 33,200 kN. The torque was 2,500 to 3,000 kN·m, and the cutter head torque coefficient ranged from 0.8 to 1.0. The screw machine torque coefficient ranged from 9 to 11, and the advancement speed was slow during this stage.

[0044] Specifically, in the first stage of soil improvement, the total thrust of the shield machine dropped to 15,000 to 18,000 kN, and the torque fluctuated between 900 and 1,100 kN·m, with a large decrease. The cutter head torque coefficient ranged from 0.5 to 0.8; the screw machine torque coefficient ranged from 3 to 5; the advancement speed was improved in this stage.

[0045] Specifically, in the second stage of soil improvement, the number of foam holes and the amount of foam were reduced, the amount of water was increased, the total thrust of excavation varied within the range of 16,000 to 20,000 kN, the torque fluctuation range was 1,000 to 1,500 kN·m, the cutter head torque coefficient range was 0.6 to 0.8; the screw machine torque coefficient range was 5 to 6; the overall excavation parameters increased slightly, but were within a reasonable range.

[0046] Specifically, in the third stage of soil improvement, the hole position was further adjusted and improved, and the advancement speed was increased. During the tunneling process of the shield machine, the total thrust fluctuated within the range of 19,000 to 21,000 kN, the cutter head torque varied within the range of 1,300 to 1,600 kN·m, and the cutter head torque coefficient ranged from 0.6 to 0.8.

[0047] Specifically, the cutter disc assembly gradually discharges the accumulated debris in the soil bin, and the cutter disc assembly includes a left spiral machine 1, and a right spiral machine 2 is arranged on one side of the left spiral machine 1. A left large cutter disc 3 and a right large cutter disc 4 are respectively embedded on one side of the left spiral machine 1 and the right spiral machine 2. An upper small cutter disc 5 and a lower small cutter disc 6 are commonly arranged on one side of the left spiral machine 1 and the right spiral machine 2. The upper small cutter disc 5 and the lower small cutter disc 6 are distributed up and down, and a blind area 7 is arranged on one side of the left spiral machine 1 and the right spiral machine 2.

[0048] The steps for implementing the cutterhead assembly are as follows:

[0049] Step 1: In view of the blockage of the left spiral machine 1 and the right spiral machine 2 in previous projects, the shield machine and the soil improvement system are optimized and improved, and the left large cutter head 3, the right large cutter head 4, the upper small cutter head 5 and the lower small cutter head 6 are rotated regularly. The left spiral machine 1 and the right spiral machine 2 are rotated forward to continuously excavate, and finally the accumulated slag in the soil bin is gradually discharged. The shield machine is provided with injection ports on the excavation face and the soil bin to inject soil improver, and can also be configured as drilling ports to enable it to have the ability of advanced drilling and grouting reinforcement. In addition, a plurality of fixed radial injection ports are provided on the outer periphery of the ring of the front shell of the shield machine, and bentonite and friction-reducing materials can be added to the soil around the shell. The foam injection and mud adding device mainly includes a stock liquid tank, a stock liquid tank, a foam solution tank, a bentonite tank, a foam and injection mud adding pump, a flow control unit, a foam gun, a foam control panel, a valve control panel and other control panels, a pneumatic device, etc. The mud adding and foaming system is configured as a single pipe and a single pump.

[0050] Step 2: The distribution of the slag improvement holes is as follows: there are 33 soil improvement holes in the soil bin, of which 10 are filling ports for stirring rods and 23 are filling ports for chest plates; there are 8 filling ports for the large blade disc, of which 6 can be filled at the filling port 8 on the front of the large blade disc, and 2 can be filled at the filling port 10 on the back of the large blade disc (used to improve the mud cake on the front of the small blade disc and the poor excavation of the spiral machine); there are 2 reinforcement ports for the small blade disc, of which 2 can be filled at the front of the small blade disc, and all the filling ports can be switched between foam, bentonite and water.

[0051] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Although the present invention has been disclosed as a preferred embodiment as above, it is not used to limit the present invention. Any technical personnel in this field can make some changes or modify the technical contents disclosed above into equivalent embodiments without departing from the scope of the technical solution of the present invention. However, any brief modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A shield tunnel construction method for preventing subsidence and collapse in soft upper and hard lower strata, characterized in that: The method comprises the following steps: S1: After the shield machine is started, grouting construction is carried out in sequence. The cross-section layers of the shield machine are ②2c silty clay, ③2 silty clay, and ④1b silty clay, all of which are plastic soils. Then a small amount of foam injection is required every 5 rings to form a stratum that prevents settlement and collapse; S2:aThe first stage: Add foam to the eight holes (S13 to S16, S32, S33, S11, and S17) in the upper part of the blind area (7), and keep them open. Add 15m3 of foam to each ring. 3 , the water filling holes of breast plates S07, S30, S21, S10, and S18 are always open, and 15m of water is added to each ring of soil bin. 3 , two water holes are opened on the left cutter disc (3) and the right cutter disc (4), and 5m of water is added per ring. 3 , slow advancement (15mm / min) was adopted, water was added at intervals, foam was added synchronously and normally, and advancement was stopped after 10cm to improve the soil with an idle cutterhead. The improved soil had significantly improved workability, the screw machine was unearthed smoothly, and the thrust of the shield machine and the torque of the cutterhead were significantly reduced. However, more foam was added, and the belt conveyor was prone to slippage. Special personnel needed to be arranged to handle the advancement process, and the advancement speed was slow, and the improvement in work efficiency was significantly small (an average of 4 rings / d); bThe second stage: Add foam to the four holes S13, S16, S32, and S33 in the upper part of the blind area (7), and keep them open. Add 10m3 of foam per ring. 3 The water holes of the breast plates S07, S30, S21, S10, and S18 are always open. 20m3 of water is added to each ring of the soil bin. Two water holes are opened on the left cutterhead (3) and the right cutterhead (4). 5m3 of water is added to each ring. 3 , appropriately increase the advancement speed (20mm / min), add water and foam synchronously, stop advancing after 40cm, and improve the soil with an idle cutterhead. After the improvement measures are adjusted, the soil has better workability, the screw conveyor is unearthed smoothly, and the belt conveyor does not slip. The total thrust and cutterhead torque have increased compared with the first stage, but are within a reasonable range, and the excavation efficiency has been significantly improved (5.5 rings / d on average); cThe third stage: Fill the holes S32 and S33 on the upper part of the blind area (7) and one hole each on the left cutter head (3) and the right cutter head (4), a total of four holes, with foam. Keep it open and fill each ring with 8m3 of foam. 3 , the water filling holes of breast plates S07, S30, S21, S10, and S18 are always open, and 20m3 of water is added to each ring of soil bin. 3 , two water holes are opened on the left cutter disc (3) and the right cutter disc (4), and 5m of water is added per ring. 3 The advancement speed was maintained at 30mm / min, water and foam were added simultaneously and normally, and each ring of the shield advanced continuously. The workability of the slag after adjustment was better, the screw machine unearthed smoothly, the belt conveyor did not slip, and the excavation efficiency of each ring was significantly improved (an average of 7 rings / d). The thrust of the shield machine and the torque of the cutter head were controlled within a reasonable range. Foam was added to improve the fluidity and stability of the soil, reduce the viscosity of the slag in the soil bin, and improve the soil discharge efficiency.

2. The shield tunnel construction method for preventing subsidence and collapse in soft upper and hard lower strata according to claim 1 is characterized by: In the step S1, the ratio of foam stock solution to water is 1:19; the foam composition is foam solution to compressed air is 1:8 to 1:

12.

3. The shield tunnel construction method for preventing subsidence and collapse in soft upper and hard lower strata according to claim 1 is characterized by: Before soil improvement, the total thrust of the shield machine was 29,000 to 33,000 kN, with a maximum of 33,200 kN. The torque was 2,500 to 3,000 kN·m, and the cutter head torque coefficient ranged from 0.8 to 1.

0. The screw machine torque coefficient ranged from 9 to 11, and the advancement speed was slow during this stage.

4. The shield tunnel construction method for preventing subsidence and collapse in upper soft and lower hard strata according to claim 1 is characterized by: In the first stage of soil improvement, the total thrust of the shield machine dropped to 15,000 to 18,000 kN, and the torque fluctuated between 900 and 1,100 kN·m, with a large drop. The cutter head torque coefficient ranged from 0.5 to 0.8; the screw machine torque coefficient ranged from 3 to 5. The advancement speed was improved in this stage.

5. The shield tunnel construction method for preventing subsidence and collapse in upper soft and lower hard strata according to claim 1 is characterized by: In the second stage of soil improvement, the number of foam holes and the amount of foam were reduced, the amount of water was increased, the total thrust of excavation varied within the range of 16,000 to 20,000 kN, the torque fluctuation range was 1,000 to 1,500 kN·m, the cutter head torque coefficient range was 0.6 to 0.8, and the screw machine torque coefficient range was 5 to 6. The overall excavation parameters increased slightly, but were within a reasonable range.

6. The shield tunnel construction method for preventing subsidence and collapse in upper soft and lower hard strata according to claim 1 is characterized by: In the third stage of soil improvement, the hole position was further adjusted and improved, and the advancement speed was increased. During the tunneling process of the shield machine, the total thrust fluctuated within the range of 19,000 to 21,000 kN, the cutter head torque varied within the range of 1,300 to 1,600 kN·m, and the cutter head torque coefficient ranged from 0.6 to 0.

8.

7. The shield tunnel construction method for preventing subsidence and collapse in upper soft and lower hard strata according to any one of claims 1 to 6, characterized in that: The cutter disc assembly gradually discharges the accumulated soil in the soil bin. The cutter disc assembly comprises a left spiral machine (1). A right spiral machine (2) is arranged on one side of the left spiral machine (1). A left large cutter disc (3) and a right large cutter disc (4) are respectively embedded on one side of the left spiral machine (1) and the right spiral machine (2). An upper small cutter disc (5) and a lower small cutter disc (6) are arranged on one side of the left spiral machine (1) and the right spiral machine (2). The upper small cutter disc (5) and the lower small cutter disc (6) are arranged in an upper and lower arrangement. A blind area (7) is arranged on one side of the left spiral machine (1) and the right spiral machine (2).

8. The shield tunnel construction method for preventing subsidence and collapse in upper soft and lower hard strata according to claim 7 is characterized by: The steps for implementing the cutterhead assembly are as follows: Step 1: In view of the blockage of the left spiral machine (1) and the right spiral machine (2) in previous projects, the shield machine and the soil improvement system are optimized and improved, and the left large cutter head (3), the right large cutter head (4), the upper small cutter head (5) and the lower small cutter head (6) are rotated regularly, and the left spiral machine (1) and the right spiral machine (2) are rotated forward to continuously excavate, and finally the accumulated slag in the soil bin is gradually discharged. The shield machine is provided with injection ports on the excavation face and the soil bin to inject soil improver, and can also be equipped with It is a drilling port, which enables it to have the ability of advance drilling and grouting reinforcement. In addition, a number of fixed radial injection ports are set on the outer periphery of the ring of the front shell of the shield machine, and bentonite and friction-reducing materials can be injected into the soil around the shell. The foam injection and mud adding device mainly includes a stock liquid tank, a stock liquid tank, a foam solution tank, a bentonite tank, a foam and injection mud adding pump, a flow control unit, a foam gun, a foam control panel, a valve control panel and other control panels, pneumatic devices, etc. The mud and foam adding system is configured with a single pipe and a single pump. Step 2: The distribution of the soil improvement holes is as follows: there are 33 soil improvement holes in the soil bin, of which 10 are filling ports for stirring rods and 23 are filling ports for chest plates; there are 8 filling ports for the large blade disc, of which 6 are available for filling at the front filling ports (8) of the large blade disc and 2 are available for filling at the back filling ports (10) of the large blade disc (used to improve the mud cake on the front of the small blade disc and the poor excavation of the spiral machine); there are 2 reinforcement ports for the small blade disc, of which 2 are available for filling at the front filling ports of the small blade disc. All the filling ports can be switched between foam, bentonite and water.

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