A method for pre-reinforcing a large-span ultra-shallow underground cavern

Abstract

This invention discloses a method for pre-reinforcement of large-span, ultra-shallow underground caverns, relating to the field of power plant engineering. The method includes the following steps: a. pouring side-mounted concrete and a slope-mounted platform along the slope from the bottom to the top of the cavern; b. installing steel pipe piles on both sides of the cavern wall, inserting reinforcing bars into the piles, and then grouting; performing pre-consolidation grouting vertically downwards on both sides and the top of the cavern; c. using the pre-consolidation grouting holes as anchor holes, arranging anchor bundles and prestressed anchors from top to bottom; d. arranging a first interlocking anchor above the cavern top, and a second interlocking anchor on both sides of the cavern face; arranging the first interlocking anchor and the cavern face anchor at the slope-mounted concrete. Compared with existing pre-reinforcement technologies, this invention improves the integrity and stability of the surrounding rock mass of large-span, ultra-shallow underground caverns, reduces the deformation of the surrounding rock after cavern excavation, has extremely high safety, and significantly reduces the amount of support work during cavern excavation, making it highly practical.

Description

Technical Field

[0001] This invention relates to the field of power plant engineering, and in particular to a method for pre-reinforcement of large-span, ultra-shallow underground caverns. Background Technology

[0002] Underground caverns are crucial components of power plant construction. Constrained by factors such as the layout of the power plant hub, site access conditions, dam area topography, and reservoir water level, many large caverns in these projects must be located on slopes with poor geological conditions. This results in shallow burial depths, poor surrounding rock conditions, and significant construction difficulties at the cavern entrance. The entrance section is a vital part of the underground cavern and a critical point for its excavation. For large-span, shallowly buried underground caverns located in areas with poor geological conditions, the rock mass at the entrance section has poor arching properties, making it prone to collapses and large deformations during excavation. Furthermore, the entrance section typically has a thick overburden layer with extremely poor stability, making it susceptible to slope instability and other geological disasters after excavation disturbance. The secondary stress distribution after excavation can also lead to surrounding rock instability and cavern collapse. Pre-reinforcement of shallow underground caverns before excavation not only enhances the stability of the surrounding rock, reduces disturbance to the surrounding rock during excavation, and lowers construction risks, but also reduces the workload of support during excavation. Current methods mainly employ anchor bolts, steel pipe piles, and surface grouting to pre-reinforce the shallow cavern entrance section. However, the entrance section of ultra-shallow underground caverns is even shallower, with poorer slope stability. For pre-reinforcement of large-span ultra-shallow underground caverns, existing reinforcement technologies are not very practical, and a systematic pre-reinforcement scheme has not yet been developed. Therefore, finding an effective pre-reinforcement technology for large-span shallow underground caverns to facilitate underground cavern construction in power plant projects has both theoretical significance and practical value. Summary of the Invention

[0003] The purpose of this invention is to solve the problem that existing underground cavern pre-reinforcement schemes are not very practical for large-span, ultra-shallow underground caverns.

[0004] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a method for pre-reinforcement of large-span ultra-shallow underground caverns, comprising the following steps:

[0005] Step a: Pour slope-supporting concrete along the slope outside the underground cavern from the bottom elevation to the top of the underground cavern. The slope-supporting concrete includes side slope-supporting concrete and slope-supporting platform. The side slope-supporting concrete is inclined, and the slope-supporting platform is horizontal.

[0006] Step b: Install steel pipe piles on both sides of the underground cavern wall, insert steel bars into the steel pipe piles and then grout; perform pre-consolidation grouting vertically downwards on both sides of the underground cavern and from the top of the underground cavern to the ground surface.

[0007] Step c: Use the pre-consolidated grouting holes formed after pre-consolidation grouting as anchor bar holes, and arrange anchor bar bundles from the top of the sloping concrete on both sides of the tunnel wall downwards, and arrange prestressed anchor bars from the top of the sloping concrete on the tunnel top downwards.

[0008] Step d: Arrange the first locking anchor rod in an arc shape above the tunnel top along the tunnel entrance direction, and arrange the second locking anchor rod on both sides of the tunnel face along the tunnel entrance direction; at the slope concrete, arrange the first locking anchor rod and tunnel face anchor rod alternately along the tunnel entrance direction.

[0009] Furthermore, when there is an unloading zone around the underground cavern, prestressed anchor cables are installed around the underground cavern and pass through the unloading zone.

[0010] Furthermore, the surface of the side slope concrete is reinforced with a steel mesh; however, the surface of the slope platform is not reinforced with a steel mesh.

[0011] Furthermore, side anchor bars are arranged vertically inward along the side slope concrete surface at a distance from the opening, and the side anchor bars are welded to the steel mesh arranged on the surface of the side slope concrete.

[0012] Furthermore, when the slope of the front side outside the tunnel face is relatively steep, the surface of the slope is sprayed with concrete, reinforced with steel mesh, and short anchor rods are installed; additionally, anchor piles are installed to improve stability.

[0013] Compared with the prior art, the beneficial effects of the present invention are:

[0014] 1. In this invention, the pre-reinforcement of the entrance section of a large-span, ultra-shallow underground cavern improves the integrity and stability of the surrounding rock mass, effectively reduces the deformation of the surrounding rock after cavern excavation, has extremely high safety, and significantly reduces the amount of support work during cavern excavation, making it highly practical.

[0015] 2. In this invention, the entrance section of a large-span, ultra-shallow underground cavern is pre-reinforced to ensure the slope stability of the entrance section. Attached Figure Description

[0016] Figure 1 Schematic diagram of pre-reinforcement of shallow buried section at the entrance of underground cavern (I).

[0017] Figure 2 Schematic diagram (II) for pre-reinforcement of shallow buried section at the entrance of underground cavern.

[0018] In the diagram: 1. Underground cavern; 2. Existing highway; 3. Side slope concrete; 4. Drainage ditch; 5. Pre-consolidated grouting hole; 6. Anchor bar bundle; 7. Prestressed anchor bar; 8. Steel pipe pile; 9. Prestressed anchor cable; 10. Relaxation unloading lower limit; 11. Strong unloading lower limit; 12. Original slope line; 13. First interlocking anchor rod; 14. Second interlocking anchor rod; 15. Anchor bar pile; 16. Fault; 17. Slope platform; 18. Shotcrete and steel mesh on the outer slope of the cavern face; 19. Short anchor rod; 20. Highway slope frame beam reinforcement; 21. Cavern face anchor rod; 22. Side anchor bar bundle. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, so as to provide a better understanding of the concept of the present invention, the technical problem solved, the technical features constituting the technical solution, and the technical effects brought about.

[0020] Combination Figures 1-2 A method for pre-reinforcement of large-span, ultra-shallow underground caverns includes the following steps:

[0021] Step a: Pour slope-supporting concrete along the slope outside underground cavern 1 from the bottom elevation to the top of underground cavern 1. The slope-supporting concrete includes side slope-supporting concrete 3 and slope-supporting platform 17. The side slope-supporting concrete 3 is inclined, and the slope-supporting platform 17 is horizontal.

[0022] Step b: Install steel pipe piles 8 on both sides of the underground cavern 1, insert steel bars into the steel pipe piles 8 and then inject grout; perform pre-consolidation grouting vertically downward on both sides of the underground cavern 1 and from the top of the underground cavern 1 to the ground surface.

[0023] Step c: Using the pre-consolidated grouting holes formed after pre-consolidation grouting as anchor bar holes, anchor bar bundles 6 are arranged from the top of the slope concrete on both sides of the tunnel wall downwards, and prestressed anchor bars 7 are arranged from the top of the slope concrete on the tunnel top downwards.

[0024] Step d: Arrange the first locking anchor 13 in an arc shape above the tunnel top along the tunnel entrance direction, and arrange the second locking anchor 14 on both sides of the tunnel face along the tunnel entrance direction; arrange the first locking anchor 13 and tunnel face anchor 21 alternately along the tunnel entrance direction at the slope concrete.

[0025] Step a: Pour the side slope concrete 3 and the slope platform 17;

[0026] like Figure 1As shown, C25 side slope concrete 3 is poured along the terrain from the elevation of the tunnel floor to the existing highway elevation, with a thickness of 1.5m. Furthermore, a steel mesh with a specification of φ20@20x20cm is arranged on the surface of the side slope concrete 3, which can be adjusted according to the actual situation. A 0.5m thick layer of C25 concrete is poured at the horizontal section from the outer side of the highway to the slope, serving as a slope-supporting platform 17. Further, no steel mesh is provided on the surface of the slope-supporting platform 17.

[0027] Furthermore, when an unloading zone exists around underground cavern 1, prestressed anchor cables 9 are installed around underground cavern 1, and the prestressed anchor cables 9 pass through the unloading zone. Due to the steep slope and the presence of an unloading zone around the cavern, to prevent the formation of large, locally unstable blocks, two rows of prestressed anchor cables 9 are used for reinforced support. That is, two rows of 150T pressure-dispersing prestressed anchor cables 9 are installed at the top of the cavern, with a spacing of 3-4m between rows. If a fault exists around the cavern, the anchor cable length should penetrate the fault, with an anchorage length of 10m. The specific length can be adjusted according to the actual situation. If there are already locally unstable blocks around the cavern, 1-2 rows of 150T anchor cables should be used for reinforced support.

[0028] like Figure 1 As shown, after the side slope concrete 3 is poured, further, side anchor bar bundles 22 are arranged vertically inward along the slope surface of the side slope concrete 3 at a distance from the opening, and the side anchor bar bundles 22 are welded to the steel mesh arranged on the surface of the side slope concrete 3; the anchor bar bundle parameters are 3φ28, length L=12m, and spacing 2.0m.

[0029] Step b: Steel pipe piles 8 are installed on the sidewalls of underground cavern 1, and pre-consolidation grouting is carried out around underground cavern 1;

[0030] like Figure 1 As shown, after the side slope concrete 3 and slope platform 17 are poured, in order to ensure the stability of the sidewall of the underground cavern 1, a row of steel pipe piles 8 are installed on each side of the cavern wall, 30cm away from the sidewall and extending 1m into the bottom of the bottom slab. The steel pipe piles 8 are made of φ127 steel pipes, with a spacing of 1.0m. The length of the steel pipe piles 8 on the left side of the cavern opening is 55m, and the length of the steel pipe piles 8 on the right side of the cavern opening is 35m. After the steel pipe piles 8 are constructed, 3 φ28 steel bar bundles are inserted into the steel pipes, and then pressureless cement grouting is carried out until the steel pipe piles 8 are fully grouted.

[0031] Based on the grouting test at the tunnel entrance, it is proposed to adopt a top-down segmented pre-consolidation grouting method with a row spacing of 2m. The grouting method will be cyclical, and the construction will be carried out using a two-stage densification method (Section I and Section II). The initial pressure of the Section I holes is 0.1MPa, and the target pressure is 0.2MPa; the initial pressure of the Section II holes is 0.2MPa, and the target pressure is 0.5MPa. Drilling can only begin when the slope-facing concrete strength reaches 50% or more. Using the slope-facing platform 17 and the side slope-facing concrete 3 as the cover weight, pre-consolidation grouting will be carried out vertically downward within a 10m range on both sides of the tunnel body, with rows spaced 2m apart and staggered. The hole depth is approximately 30-50m, and the grouting range is from the elevation of the slope-facing concrete to the elevation of the tunnel floor.

[0032] Step c: Arrange anchor bar bundles 6 on both sides of the opening and arrange prestressed anchor bars 7 on the top of the opening;

[0033] like Figure 1 As shown, after the pre-consolidation grouting is completed, the slope-adhering platform 17 and the side slope-adhering concrete 3 serve as the cover weight. Using the pre-consolidation grouting holes as anchor bar holes, anchor bar bundles 6 and prestressed anchor bars 7 are arranged vertically downward from the height of the slope-adhering concrete within a 10m range on both sides of the tunnel opening. The anchor bar bundle 6 has parameters of 3φ20 and a length of 12-20m. The prestressed anchor bars 7 are arranged on the tunnel roof to connect the tunnel roof arch and the slope-adhering concrete into a whole. The prestressed anchor bars 7 have parameters of φ32 and the length is adjusted according to the layout position. The prestress is 150KN.

[0034] Step d: Pre-reinforce the opening area;

[0035] like Figure 2As shown, two rows of arc-shaped first anchor bolts 13 are arranged on the upper part of the tunnel roof along the direction of entry. The anchor bolt parameters are 3φ28, the length is 15m, and the row spacing is 4m, arranged alternately. Two rows of second anchor bolts 14 are arranged on both sides of the tunnel face along the direction of entry. The anchor bolt parameters are φ32, the length is 10m, and the row spacing is 1~2m, arranged alternately. The remaining concrete slope of the tunnel face is evenly and alternately arranged with first anchor bolts 13 and tunnel face anchor bolts 21; the anchor bolt parameters of tunnel face anchor bolts 21 are φ28, the length is 9m, and the row spacing is 4m. Furthermore, when the slope of the front side of the tunnel face is large, the surface of the slope is sprayed with concrete, reinforced with steel mesh, and short anchor bolts 19 are arranged at the same time; additionally, anchor piles 15 are arranged to improve stability. Because the slope on the left side of the tunnel face is steep and the presence of a fault could affect stability, the left side of the tunnel face needs to be reinforced. A 15cm thick layer of C25 concrete is sprayed onto the slope surface, and a steel mesh with parameters of φ8@20x20cm is installed. Short anchor bolts (19) with a length of 4m and parameters of φ25 are also installed, spaced 2 meters apart. Two additional sets of anchor piles (15) are installed to further enhance stability. Each set of anchor piles (15) is arranged in two rows and four columns, with anchor pile parameters of 3φ28 and a length L=18m. The spacing between rows within a set is 4m, and the two sets of anchor piles are 10m apart. The spacing between the two sets of anchor piles can be adjusted according to the site's geological conditions.

[0036] The terms "connection" and "fixing" appearing in the description of this invention can refer to fixed connection, processing and forming, welding, or mechanical connection. The specific meaning of the above terms in this invention should be understood according to the specific circumstances.

[0037] In the description of this invention, the terms "center," "upper," "lower," "horizontal," "inner," and "outer," etc., are used only to indicate the orientation or positional relationship for the convenience of describing this invention and to simplify the description, and do not indicate or imply a specific orientation that the device or element referred to must have, and therefore should not be construed as a limitation of this invention.

[0038] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for pre-reinforcement of large-span, ultra-shallow underground caverns, characterized in that, Includes the following steps: Step a: Along the slope outside the underground cavern (1), pour slope-supporting concrete from the bottom elevation to the top of the underground cavern (1). The slope-supporting concrete includes side slope-supporting concrete (3) and slope-supporting platform (17). The side slope-supporting concrete (3) is inclined, and the slope-supporting platform (17) is horizontal. Step b: Install steel pipe piles (8) on both sides of the underground cavern (1), insert steel bars into the steel pipe piles (8) and then grout; perform pre-consolidation grouting vertically downward on both sides of the underground cavern (1) and from the top of the underground cavern (1) to the ground. Step c: Using the pre-consolidated grouting holes formed after pre-consolidation grouting as anchor bar holes, anchor bar bundles are arranged from the top of the slope concrete on both sides of the tunnel wall downwards (6), and prestressed anchor bars are arranged from the top of the slope concrete on the tunnel top downwards (7). Step d: Arrange the first locking anchor (13) in an arc shape above the tunnel top along the tunnel entrance direction, and arrange the second locking anchor (14) on both sides of the tunnel face along the tunnel entrance direction; arrange the first locking anchor (13) and tunnel face anchor (21) alternately along the tunnel entrance direction at the slope concrete. When there is an unloading zone around the underground cavern (1), prestressed anchor cables (9) are installed around the underground cavern (1) and the prestressed anchor cables (9) pass through the unloading zone; When the slope of the front side outside the tunnel face is large, the surface of the slope is sprayed with concrete, steel mesh is hung, and short anchor rods (19) are arranged at the same time; in addition, anchor piles (15) are arranged to improve stability.

2. The method for pre-reinforcement of large-span, ultra-shallow underground caverns according to claim 1, characterized in that: The surface of the side slope concrete (3) is provided with a steel mesh; the surface of the slope platform (17) is not provided with a steel mesh.

3. The method for pre-reinforcement of large-span, ultra-shallow underground caverns according to claim 2, characterized in that: Side anchor bars (22) are arranged vertically inward along the slope of the side slope concrete (3) which is far from the opening, and the side anchor bars (22) are welded to the steel mesh arranged on the surface of the side slope concrete (3).

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