Method for determining water pressure outside shield tunnel

Abstract

The invention relates to a method for determining external water pressure of a shield tunnel, and relates to the technical field of shield tunnel construction, and the method comprises the following steps: determining a static water line in a longitudinal section of the shield tunnel and surrounding rock classification of surrounding rock of the whole shield tunnel; determining the positions of medium-and-long-term observation holes in different surrounding rock grades according to the static water line, and setting the long-term observation holes; a static water level line and an external water level pressure water level line in a wet water period in different surrounding rock grades are obtained through observation of the long-term observation hole; calculating reduction coefficients of different grades of surrounding rocks according to the static water level line and the external water level pressure water level line in the wet water period; and according to the reduction coefficients of the different grades of surrounding rocks, calculating the external water pressure at different grades of the surrounding rocks. The method has the advantages that the accuracy of calculation of the external water level pressure value is improved, the reliability of data is improved, the construction cost is reduced, and the construction safety is guaranteed.

Description

Technical Field

[0001] The present application relates to the technical field of shield tunnel construction, and in particular to a method for determining the external water pressure of a shield tunnel. Background Art

[0002] When a shield tunnel passes under a high mountain, it will be subjected to great external water pressure during construction and operation. The magnitude of the external water pressure has a great impact on the waterproof design of the shield tunnel buried deep in bedrock. At present, the maximum pressure that the domestic shield segment water stop strip can withstand is generally 0.6-0.8MPa. When the actual pressure exceeds this value, the service life of the water stop strip will be greatly reduced, and it is easy to leak in a short operation period, which brings huge challenges to the design and later operation safety.

[0003] External water pressure is the groundwater pressure acting on the outer surface of the shield tunnel lining. The water pressure on the outer surface of the lining caused by groundwater seepage in the overlying rock (soil) is often not equal to the acting head (hydrostatic pressure head He) from the groundwater level to the centerline of the tunnel. If there is a reduction in the head, the reduction factor β is used. e The reduction factor β in the prior art e The confirmation is based on the current national standard "Specifications for Geological Investigation of Water Conservancy and Hydropower Engineering" (GB50487-2008, 2022 edition). In the early stage of investigation, the external water pressure reduction coefficient β can be determined by looking up the table based on the permeability grade of the rock and soil mass. e .

[0004] The above external water pressure reduction factor β e The following issues were identified: 1. Most survey and design units determine the reduction factor by referring to the water conservancy and hydropower industry. The basis is rather far-fetched and may even deviate greatly from the actual working conditions.

[0005] 2. Shield tunnels in the water conservancy and hydropower industry generally allow drainage, and rarely use full-fill lining. Shield tunnels are formed by pipe segments, which is a typical full-fill lining. It is more convenient to set up a drainage system for shield tunnels in the water conservancy and hydropower industry. If leakage occurs later, it is convenient to enter the hole for maintenance. However, it is difficult to design a drainage system for a shield tunnel, and it is necessary to consider increasing the hole diameter, which will greatly increase the cost; if leakage occurs later, it is very difficult to enter the hole for maintenance.

[0006] 3. The survey period in the survey phase is very short, and the hydrostatic pressure obtained is often a short-term observation result, and it has not even experienced the flood season. The reliability of the data is poor. The hydrostatic pressure provided at this time is relatively small, which is dangerous for the project. Summary of the invention

[0007] In order to solve the problem in the prior art that the existing method of determining the external water pressure reduction coefficient βe by looking up a table to calculate the external water pressure during the shield tunnel survey period may have a large deviation from the actual working conditions, the present application provides a method for determining the external water pressure of a shield tunnel.

[0008] The present application provides a method for determining the external water pressure of a shield tunnel, which adopts the following technical solution: A method for determining the external water pressure of a shield tunnel, comprising the following steps: Determine the hydrostatic line in the longitudinal section of the shield tunnel and the surrounding rock classification of the entire shield tunnel; Determine the location of long-term observation holes for different surrounding rock classifications according to the static water level and set up long-term observation holes; Through long-term observation holes, the static water level and external water pressure water level in the flood season of different surrounding rock classifications were obtained; The reduction coefficients of surrounding rocks of different grades are calculated according to the static water level line in the flood season and the external water level pressure water level line in the flood season; The external water pressure at different surrounding rock grades is calculated based on the reduction coefficients of surrounding rock grades.

[0009] Optionally, the determination of the static water level line in the longitudinal section of the shield tunnel and the surrounding rock classification of the entire shield tunnel specifically includes: using comprehensive exploration means including one or more methods of geological mapping, drilling and geophysical exploration to determine the static water level line in the longitudinal section of the shield tunnel and the surrounding rock classification of the entire shield tunnel.

[0010] Optionally, the static water level line and the external water level pressure level line of the flood season are obtained by observing through long-term observation holes, specifically including: the long-term observation holes include static water level long-term observation holes and external water level long-term observation holes, the static water level of the flood season is observed through the static water level long-term observation holes, and then the static water level line of the flood season is fitted through the static water levels of multiple flood seasons; the external water pressure of the flood season is observed through the external water level long-term observation holes, and then the external water level pressure level line of the flood season is fitted through the external water pressures of multiple flood seasons.

[0011] Optionally, the setting method of the static water level long-term observation holes and the external water level long-term observation holes specifically includes: respectively setting static water level long-term observation holes and external water level long-term observation holes at several typical surrounding rock locations, and the static water level long-term observation holes and external water level long-term observation holes set at the same typical surrounding rock location are spaced at a set interval and are located in the same seepage field of the same hydrogeological unit.

[0012] Optionally, the method of setting up the long-term static water level observation hole specifically includes: drilling a hole with a hole diameter of 100-110 mm and a hole depth of 1-3 m below the bottom of the shield tunnel, washing the hole, pumping water and setting a steel casing according to the requirements of the hydrogeological test to avoid the occurrence of blockages in the hole wall causing blockage in the hole.

[0013] Optionally, the arrangement of the external water level long-term observation hole specifically includes: Drill holes with a diameter of 400-600 mm and a depth of 10-11 m to the top of the shield tunnel; Use a drill bit with a diameter of 100 to 110 mm to drill into the final hole 1 to 3 m below the bottom of the shield tunnel; Carry out hole washing and water pumping according to the requirements of hydrogeological tests; Configure water filter pipes and water risers. The water riser is above the flange and uses conventional steel casing. The filter pipe is below the flange. The pipe diameters are all 90-100mm. The spacing between the filter holes of the filter pipe is 15-25cm. The two adjacent rows are staggered. The diameter of the filter holes of the filter pipe is 0.5-1.5cm. Pour cement slurry from the top of the hole to the outside of the water riser until the cement slurry overflows from the hole mouth.

[0014] Optionally, the arrangement of the external water level long-term observation hole specifically includes: Drill holes with a diameter of 100 to 110 mm and a depth of 1 to 3 m below the bottom of the shield tunnel; Wash the borehole and pump water according to the requirements of the hydrogeological test.

[0015] According to the pore size, calculate the fine sand body and first put in a 4m long section of fine sand.

[0016] Two water pressure gauges are placed, and two water presses are set in an offset manner; Add fine sand and let it stand until the fine sand settles and stabilizes; Cement slurry is poured into the bottom of the hole until it overflows from the hole mouth, and the entire hole section above the sand is poured into concrete.

[0017] Optionally, the reduction coefficient β of the surrounding rock of different grades is calculated according to the static water level line in the flood season and the external water level pressure water level line in the flood season. e , specifically including: establishing the calculation formula of the external water pressure reduction coefficient βe: Among them, β e : external water pressure reduction coefficient; H w : The pressure head actually acting on the outer surface of the lining structure after being affected by seepage is the external water pressure head; H e : The acting head from the groundwater level to the center of the tunnel is the hydrostatic pressure head; Calculate the external water pressure reduction factor β according to the formula e .

[0018] Optionally, the hydrostatic pressure head H e The calculation method specifically includes: using a level meter to measure the static water level depth value, subtracting the shield tunnel axis depth value, and obtaining the static water pressure head He ; The external water pressure head H w The calculation method specifically includes: using a water level meter to measure the external water level depth value, subtracting the shield tunnel axis depth value, and obtaining the external water pressure head H w .

[0019] Optionally, the calculation of the external water pressure at different surrounding rock classifications according to the reduction coefficients of surrounding rocks of different classifications specifically includes: determining the surrounding rock classification at the location where the external water pressure head needs to be calculated, obtaining the hydrostatic pressure head at the location where the external water pressure head needs to be calculated according to the hydrostatic level line, and calculating the external water pressure head at the location according to the following formula: H w =H e ×β e Among them, β e : external water pressure reduction coefficient; H w : The pressure head actually acting on the outer surface of the lining structure after being affected by seepage, i.e. the external water pressure head, m; H e : The acting head from the groundwater level to the center of the tunnel is the hydrostatic pressure head.

[0020] In summary, the present application includes at least one of the following beneficial technical effects: In the present application, the external water pressure reduction coefficients in different surrounding rock layers are calculated by setting long-term static water level observation holes and external water level observation holes on different surrounding rock grades, thereby obtaining accurate external water pressure reduction coefficients corresponding to the surrounding rock layers, and then calculating the external water level pressure value at the corresponding position through the external water pressure reduction coefficient, thereby improving the accuracy of the calculation of the external water level pressure value, enhancing the reliability of the data, reducing the construction cost, and ensuring the safety of the construction. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the steps of a method for determining the external water pressure of a shield tunnel in an embodiment of the present application.

[0022] Figure 2 It is a schematic diagram of the static water level line and the external water level pressure water level line during the flood season of a method for determining the external water pressure of a shield tunnel in an embodiment of the present application.

[0023] Figure 3 It is a schematic diagram of the structure of the external water level long-term observation hole of a method for determining the external water pressure of a shield tunnel in Example 2 of the present application.

[0024] Figure 4 It is a schematic diagram of the structure of a long-term observation hole for external water level in a method for determining external water pressure in a shield tunnel in Example 1 of the present application.

[0025] Figure 5 It is a schematic diagram of the static water level long-term observation hole structure of a method for determining the external water pressure of a shield tunnel in an embodiment of the present application.

[0026] Among them, 1. Shield tunnel; 2. First type of external water level long-term observation hole; 21. Water pressure gauge; 22. Fine sand layer; 23. First cement slurry layer; 24. Data signal line; 25. Sine reading instrument; 3. Second type of external water level long-term observation hole; 31. Riser; 32. Filter tube; 33. Flange; 34. Expansion waterstop tape; 35. Second cement slurry layer; 36. First water level gauge; 4. Static water level long-term observation hole; 41. Steel casing; 42. Second water level gauge. DETAILED DESCRIPTION

[0027] The following is an explanation of the implementation of the present application by means of specific embodiments. People familiar with the art can easily understand other advantages and effects of the present application from the contents disclosed in this specification.

[0028] See also Figure 1-5 . It should be noted that the structures, proportions, sizes, etc. illustrated in the drawings of this specification are only used to match the contents disclosed in the specification for people familiar with this technology to understand and read, and are not used to limit the restrictive conditions for the implementation of this application, so they have no substantial technical significance. Any modification of the structure, change in the proportional relationship, or adjustment of the size, without affecting the effects and purposes that can be achieved by this application, should still fall within the scope of the technical content disclosed in this application. At the same time, the terms such as "upper", "lower", "left", "right", "middle" and "one" cited in this specification are only for the convenience of description, and are not used to limit the scope of the implementation of this application. Changes or adjustments in their relative relationships should also be regarded as the scope of the implementation of this application without substantially changing the technical content.

[0029] The following is combined with Figure 1-5 This application is described in further detail.

[0030] Embodiment 1: This embodiment discloses a method for determining the external water pressure of a shield tunnel.

[0031] Reference Figure 1 , a method for determining the external water pressure of a shield tunnel, comprising the steps of: S1. Determine the static water level line in the longitudinal section of the shield tunnel and the surrounding rock classification of the entire shield tunnel. Specifically, adopt comprehensive exploration methods including geological mapping, drilling and geophysical exploration, etc. According to the water level measured in the detailed exploration hole during the detailed exploration period, determine the preliminary static water level line in the longitudinal section of the shield tunnel, and determine the surrounding rock classification of the entire shield tunnel based on the detailed exploration results. The surrounding rock is divided into Class I surrounding rock, Class II surrounding rock and Class III surrounding rock according to type.

[0032] S2. Determine the positions of long-term observation holes in different surrounding rock classifications according to the static water level line and set up long-term observation holes. Specifically, long-term observation holes are set up on Class I surrounding rock, Class II surrounding rock and Class III surrounding rock. The long-term observation holes include static water level long-term observation holes and external water level long-term observation holes. Static water level long-term observation holes and external water level long-term observation holes are set up at Class I surrounding rock, Class II surrounding rock and Class III surrounding rock respectively. The static water level long-term observation holes and external water level long-term observation holes set up at the same typical surrounding rock are spaced at a set interval and are located in the same seepage field of the same hydrogeological unit. The long-term observation holes should be arranged at the highest point of the static water level of the surrounding rock section as far as possible. Due to the mountainous terrain and plant conditions, long-term observation holes can often only be arranged at higher water level locations, but not at the highest static water level locations. The setting method of the static water level long-term observation hole is: The hole is drilled with a diameter of 109 mm and a depth of 2 m below the bottom of the shield tunnel. The hole is washed and pumped according to the requirements of the hydrogeological test, and a steel casing is installed to avoid blockage caused by the falling of the hole wall. The casing water level gauge is inserted in the hole to form a Figure 5 The static water level long-term observation hole 4 shown in the figure extends to 2m below the bottom of the shield tunnel. A steel casing 41 is arranged inside the static water level long-term observation hole 4, and a second water level gauge 42 is arranged in the middle of the steel casing 41; The setting method of the external water level long-term observation hole is: Drilling holes with a diameter of 400-600mm and a depth of about 2m above the top of the shield tunnel. The larger the diameter of this section, the more advantageous it is, but the longer the construction period and the higher the cost, so it must be selected according to needs; Then use a drill bit with a diameter of 109 mm to continue drilling to the final hole 2 m below the bottom of the shield tunnel; Wash the borehole and pump water (or lift water) according to the requirements of the hydrogeological test; Configure the filter pipe and waterproof steel casing, with a flange joint in the middle, an expansion water stop tape at the bottom of the flange, and the expansion water stop tape wrapped around the upper 2m section. The flange diameter is slightly smaller than the upper aperture to ensure that it can be smoothly installed at the diameter change. The water riser above the flange uses a conventional steel casing, and the filter pipe below the flange has a pipe diameter of 98mm. The filter pipe has a 20cm spacing between the filter holes, and the adjacent rows are staggered, with a hole diameter of 1cm. Pour cement slurry from the top of the hole to the outside of the riser until the cement slurry overflows the hole, then re-pour; When the hole depth is large and the center of the small diameter hole at the diameter change point deviates greatly from the center of the large diameter hole, the filter pipe below the half plate may not be installed into the lower hole section. In this case, the lower filter pipe can be removed because the fine particles that seep out of the bedrock are generally small and will sink to the bottom sedimentation section, which has little effect on the water level measurement, forming a Figure 4The second type of external water level long-term observation hole 3 shown in the figure has a watertight pipe 31 arranged in the middle thereof. The watertight pipe 31 adopts a steel casing. The lower end of the watertight pipe 31 is connected with a filter tube 33 through a flange 33. The upper and lower sides of the flange 33 are covered with expansion water-stop tapes 34. A second cement slurry layer is arranged in the gap between the watertight pipe 31 and the hole wall. A first water level gauge 36 is arranged in the middle of the watertight pipe 31.

[0033] S3. Obtain the static water level line and external water level pressure water level line in the flood season in different surrounding rock classifications through long-term observation holes. Specifically, the water level observation time is more than one hydrological year. The static water level in the flood season is observed through the static water level long-term observation hole, and then the static water level line in the flood season is fitted through multiple static water levels in the flood season; the external water pressure in the flood season is observed through the external water level long-term observation hole, and then the external water level pressure water level line in the flood season is fitted through multiple external water pressures in the flood season, and the following is drawn: Figure 2 The static water level line and external water level pressure water level line during the flood season are shown.

[0034] S4. Calculate the reduction coefficients of surrounding rocks of different grades according to the static water level line in the flood season and the external water level pressure water level line in the flood season. Specifically, first, calculate the hydrostatic pressure head H in the first, second and third types of surrounding rocks. e and external water pressure head H w , use a water level meter to measure the static water level depth, subtract the shield tunnel axis depth, and obtain the static water pressure head H e , use a water level meter to measure the external water level depth, subtract the shield tunnel axis depth, and obtain the external water pressure head H w , and thus calculate the hydrostatic pressure head H in a type of surrounding rock e 1 and external water pressure head H w 1. Hydrostatic pressure head H in the second type of surrounding rock e 2 and external water pressure head H w 2 and the hydrostatic pressure head H in the three types of surrounding rock e 3 and external water pressure head H w 3; Then, the external water pressure reduction factor β is established e The calculation formula is: Among them, β e : external water pressure reduction coefficient; H w : The pressure head actually acting on the outer surface of the lining structure after being affected by seepage, i.e. the external water pressure head, m; H e : The acting head from the groundwater level to the tunnel center is the hydrostatic pressure head, m; Finally, according to the formula, we get: External water pressure reduction factor of a type of surrounding rock External water pressure reduction factor for the second type of surrounding rock External water pressure reduction factors for three types of surrounding rock

[0035] S5. Calculate the external water pressure at different surrounding rock classifications according to the reduction coefficients of surrounding rocks of different classifications. Specifically, determine the surrounding rock classification where the external water pressure head needs to be calculated, obtain the hydrostatic pressure head at the location where the external water pressure head needs to be calculated according to the hydrostatic level, and calculate the external water pressure head at this location according to the following formula: H w =H e ×β e Among them, β e : external water pressure reduction coefficient; H w : The pressure head actually acting on the outer surface of the lining structure after being affected by seepage, i.e. the external water pressure head, m; H e : The acting head from the groundwater level to the tunnel center is the hydrostatic pressure head, m; When calculating the external water pressure head at a certain position α of the second type of surrounding rock, the external water pressure reduction coefficient β of the first type of surrounding rock is taken e 2. Obtain the hydrostatic pressure head H at position ɑ according to the hydrostatic line e ɑ, then the external water pressure head H at position ɑ w ɑ=H e ɑ×β e 2.

[0036] Embodiment 2: The difference between this embodiment and the first embodiment is that the arrangement of the long-term water level observation holes inside and outside the embodiment is as follows: The drilling hole is usually 109mm in diameter and about 2m deep below the bottom of the shield tunnel; Wash the borehole and pump water (or lift water) according to the requirements of the hydrogeological test; Calculate the fine sand body according to the pore diameter, and first put in a 4m long section of fine sand; Two water pressure gauges were placed, and the two water pressure gauges were offset by about 2.2m; Fine sand of 6.2m length is put in. So far, the total length of fine sand filled in the bottom of the hole is 10.2m, the bottom surface is at the bottom of the hole, and the top surface is 10.2m above the bottom of the hole, that is, 2m above the top of the shield tunnel; Let it stand for a while until the fine sand settles and stabilizes; The cement slurry is poured in from the bottom of the hole, and gradually poured upwards until the cement slurry overflows the hole mouth, and the entire hole section above the sand is poured into concrete; Formed as Figure 3The first type of external water level long-term observation hole 2 shown in the figure has a hole depth of about 2m below the bottom of the shield tunnel, a fine sand layer 22 is arranged at the bottom of the hole, the thickness of the fine sand layer 22 is 10.2m, a first cement slurry layer 23 is arranged above the fine sand layer 22, and two water pressure gauges are arranged at high and low positions in the middle of the fine sand layer 22.

[0037] In this plan, the following matters need to be noted: Two water pressure gauges are placed in each hole to reduce the risk of failure of the water pressure gauge to read the data. When both water pressure gauges are working properly, they can be checked against each other to improve the accuracy of the data.

[0038] Considering that cement particles in the cement slurry will precipitate to a certain extent, the cement consolidation effect in the upper section of the hole may be poor. At this time, re-pouring can be carried out to ensure that the upper cement pouring is complete and to ensure that the bedrock fissure water in the non-fine sand section enters the hole.

[0039] When calculating the external water pressure level, use a sinusoidal readout to measure the data. According to the conversion formula provided by the sinusoidal readout, obtain the water level elevation, subtract the shield tunnel axis elevation, and obtain the external water pressure head.

[0040] In summary, in this application, the external water pressure reduction coefficients in different surrounding rock levels are calculated by setting static water level long-term observation holes and external water level long-term observation holes on different surrounding rock levels, thereby obtaining accurate external water pressure reduction coefficients corresponding to the surrounding rock levels, and then calculating the external water level pressure value at the corresponding position by the external water pressure reduction coefficient, thereby improving the accuracy of the external water level pressure value calculation, improving the reliability of the data, reducing the construction cost, and ensuring the safety of the construction. Therefore, this application effectively overcomes the various shortcomings in the prior art and has a high industrial utilization value.

[0041] The above embodiments are merely illustrative of the principles and effects of the present application and are not intended to limit the present application. Anyone familiar with the technology may modify or change the above embodiments without violating the spirit and scope of the present application. Therefore, all equivalent modifications or changes made by those with ordinary knowledge in the technical field without departing from the spirit and technical ideas disclosed in the present application shall still be included in the protection scope of the present application.

Claims

1. A method for determining the external water pressure of a shield tunnel, characterized in that the steps include: Determine the hydrostatic line in the longitudinal section of the shield tunnel and the surrounding rock classification of the entire shield tunnel; Determine the location of long-term observation holes for different surrounding rock classifications according to the static water level and set up long-term observation holes; Through long-term observation holes, the static water level and external water pressure water level in the flood season of different surrounding rock classifications were obtained; The reduction coefficients of surrounding rocks of different grades are calculated according to the static water level line in the flood season and the external water level pressure water level line in the flood season; The maximum external water pressure at different surrounding rock grades is calculated based on the reduction coefficients of surrounding rocks of different grades.

2. A method for determining the external water pressure of a shield tunnel according to claim 1, characterized in that: The determination of the static water level line in the longitudinal section of the shield tunnel and the surrounding rock classification of the entire shield tunnel specifically includes: using comprehensive exploration means including one or more methods of geological mapping, drilling and geophysical exploration to determine the static water level line in the longitudinal section of the shield tunnel and the surrounding rock classification of the entire shield tunnel.

3. A method for determining the external water pressure of a shield tunnel according to claim 1, characterized in that: The method of obtaining the static water level line and the external water level pressure level line during the flood season by observing through the long-term observation holes specifically includes: the long-term observation holes include static water level long-term observation holes and external water level long-term observation holes, the static water level during the flood season is observed through the static water level long-term observation holes, and then the static water level line during the flood season is fitted through the static water levels of multiple flood seasons; the external water pressure during the flood season is observed through the external water level long-term observation holes, and then the external water level pressure level line during the flood season is fitted through the external water pressures of multiple flood seasons.

4. A method for determining the external water pressure of a shield tunnel according to claim 3, characterized in that: The method for setting the static water level long-term observation holes and the external water level long-term observation holes specifically includes: setting static water level long-term observation holes and external water level long-term observation holes at several typical surrounding rock locations respectively, and the static water level long-term observation holes and the external water level long-term observation holes set at the same typical surrounding rock location are spaced at a set interval and are located in the same seepage field of the same hydrogeological unit.

5. A method for determining the external water pressure of a shield tunnel according to claim 4, characterized in that: The method of setting up the static water level long-term observation hole specifically includes: drilling a hole with a hole diameter of 109 mm and a hole depth of 2 m below the bottom of the shield tunnel, washing the hole, pumping water and setting a steel casing according to the requirements of the hydrogeological test to avoid the occurrence of blockages on the hole wall causing blockage in the hole.

6. A method for determining the external water pressure of a shield tunnel according to claim 5, characterized in that: The arrangement of the external water level long-term observation hole specifically includes: Drill holes with a diameter of 400-600 mm and a depth of 10-11 m to the top of the shield tunnel; Use a drill bit with a diameter of 100 to 110 mm to drill into the final hole 1 to 3 m below the bottom of the shield tunnel; Carry out hole washing and water pumping according to the requirements of hydrogeological tests; Configure water filter pipes and water risers. The water riser is above the flange and uses conventional steel casing. The filter pipe is below the flange. The pipe diameters are all 90-100mm. The spacing between the filter holes of the filter pipe is 15-25cm. The two adjacent rows are staggered. The diameter of the filter holes of the filter pipe is 0.5-1.5cm. Pour cement slurry from the top of the hole to the outside of the water riser until the cement slurry overflows from the hole mouth.

7. A method for determining the external water pressure of a shield tunnel according to claim 5, characterized in that: The arrangement of the external water level long-term observation hole specifically includes: Drill holes with a diameter of 100 to 110 mm and a depth of 1 to 3 m below the bottom of the shield tunnel; Carry out borehole washing and water pumping according to the requirements of hydrogeological tests; Calculate the fine sand body according to the pore diameter, and first put in a 4m long section of fine sand; Two water pressure gauges are placed, and two water presses are set in an offset manner; Add fine sand and let it stand until the fine sand settles and stabilizes; Cement slurry is poured into the bottom of the hole until it overflows from the hole mouth, and the entire hole section above the sand is poured into concrete.

8. A method for determining the external water pressure of a shield tunnel according to claim 1, characterized in that: The reduction coefficient β of surrounding rocks of different grades is calculated according to the static water level line in the flood season and the external water level pressure water level line in the flood season. e , specifically including: establishing the calculation formula of the external water pressure reduction coefficient βe: Among them, β e : external water pressure reduction coefficient; H w : The pressure head actually acting on the outer surface of the lining structure after being affected by seepage, i.e. the external water pressure head, m; H e : The acting head from the groundwater level to the tunnel center is the hydrostatic pressure head, m; Calculate the external water pressure reduction factor β according to the formula e .

9. A method for determining the external water pressure of a shield tunnel according to claim 8, characterized in that: The hydrostatic pressure head H e The calculation method specifically includes: using a level meter to measure the static water level depth value, subtracting the shield tunnel axis depth value, and obtaining the static water pressure head H e ; The external water pressure head H w The calculation method specifically includes: using a water level meter to measure the external water level depth value, subtracting the shield tunnel axis depth value, and obtaining the external water pressure head H w .

10. A method for determining the external water pressure of a shield tunnel according to claim 9, characterized in that: The method of calculating the external water pressure at different surrounding rock classifications according to the reduction coefficients of surrounding rocks of different classifications specifically includes: determining the surrounding rock classification at the location where the external water pressure head needs to be calculated, obtaining the hydrostatic pressure head at the location where the external water pressure head needs to be calculated according to the hydrostatic level line, and calculating the external water pressure head at the location according to the following formula: H w =H e ×β e Among them, β e : external water pressure reduction coefficient; H w : The pressure head actually acting on the outer surface of the lining structure after being affected by seepage, i.e. the external water pressure head, m; H e : The acting head from the groundwater level to the center of the tunnel is the hydrostatic pressure head, m.

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