CN112832775B - Turning underground excavation construction method for square underground well - Google Patents
Turning underground excavation construction method for square underground well Download PDFInfo
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- 230000007704 transition Effects 0.000 claims abstract description 23
- 238000000034 method Methods 0.000 claims abstract description 14
- 238000007789 sealing Methods 0.000 claims abstract description 10
- 230000005641 tunneling Effects 0.000 claims abstract description 5
- 238000002360 preparation method Methods 0.000 claims abstract description 4
- 238000000407 epitaxy Methods 0.000 claims abstract description 3
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- 239000000463 material Substances 0.000 claims description 17
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- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21D—SHAFTS; TUNNELS; GALLERIES; LARGE UNDERGROUND CHAMBERS
- E21D9/00—Tunnels or galleries, with or without linings; Methods or apparatus for making thereof; Layout of tunnels or galleries
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- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21D—SHAFTS; TUNNELS; GALLERIES; LARGE UNDERGROUND CHAMBERS
- E21D11/00—Lining tunnels, galleries or other underground cavities, e.g. large underground chambers; Linings therefor; Making such linings in situ, e.g. by assembling
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- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21D—SHAFTS; TUNNELS; GALLERIES; LARGE UNDERGROUND CHAMBERS
- E21D11/00—Lining tunnels, galleries or other underground cavities, e.g. large underground chambers; Linings therefor; Making such linings in situ, e.g. by assembling
- E21D11/04—Lining with building materials
- E21D11/10—Lining with building materials with concrete cast in situ; Shuttering also lost shutterings, e.g. made of blocks, of metal plates or other equipment adapted therefor
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- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21D—SHAFTS; TUNNELS; GALLERIES; LARGE UNDERGROUND CHAMBERS
- E21D9/00—Tunnels or galleries, with or without linings; Methods or apparatus for making thereof; Layout of tunnels or galleries
- E21D9/14—Layout of tunnels or galleries; Constructional features of tunnels or galleries, not otherwise provided for, e.g. portals, day-light attenuation at tunnel openings
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/20—Hydro energy
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Abstract
The application relates to a turning-back underground excavation construction method of a square underground well, which comprises the following specific steps: firstly, construction preparation; secondly, tunneling a tunnel; thirdly, excavating a construction cavity: when the tunnel reaches the construction area of the square underground well, the excavation of a construction cavity is started; the construction cavity comprises a gradual change section and a main cavity, wherein the gradual change section is communicated with the underground excavated tunnel, and the section of the gradual change section is gradually increased; step four, tunnel epitaxy, turning back and backfilling: extending the end, butted with the transition section, of the tunnel towards the interior of the transition section to form an extension section, sealing the space between the end, far away from the tunnel, of the extension section and the top of the transition section to form a backfill cavity, and finally filling backfill into the backfill cavity; fifthly, constructing the square blind well; sixthly, filling concrete: and pouring concrete into the space between the formed square blind well and the wall of the main chamber. This application has and guarantees the normal development of construction under the square blind shaft construction of heating power cell can't carry out open cut method.
Description
Technical Field
The application relates to the field of building construction, in particular to a turning-back underground excavation construction method for a square underground well.
Background
The central heating has obvious superiority in the aspects of saving energy, protecting environment, managing communities, improving urban life quality, reducing living cost and the like, is an important mode of urban heating at present, and the central heating projects built and proposed are increased gradually. In China, especially in big cities, because the ground space is precious and the underground space is gradually developed for various purposes, the centralized arrangement of thermal pipelines in the construction of thermal tunnels is urgent.
Thermal cells (valve wells) are valve wells for underground pipelines and underground pipes (e.g., tap water, oil, natural gas, etc.). In order to conveniently open and close part of pipe network operation or overhaul operation, a pit (or well) similar to a small room is arranged, a valve and the like are arranged in the pit, so that the pipeline can be conveniently and regularly checked, cleaned and dredged, and a junction for preventing the pipeline from being blocked is provided.
In view of the above-mentioned related technologies, the inventor found that the depth of various pipeline facilities buried underground in many construction areas is shallow, and if the traditional open cut method is adopted for construction, the development of underground small chamber construction is difficult due to the fact that pipelines are various in types, difficult to move and long in period.
Disclosure of Invention
In order to guarantee the normal development of construction under the condition that the open cut method can not be carried out to carry out the square blind shaft construction of heating power cell, the application provides a turn-back subsurface excavation construction method of a square blind shaft.
The application provides a turn-back subsurface excavation construction method of a square underground well, which adopts the following technical scheme:
a turning-back underground excavation construction method for a square underground well comprises the following specific steps:
first step, construction preparation: determining a constructed vertical shaft communicated with a square blind shaft to be constructed according to a design drawing, and determining a construction direction;
step two, underground excavation of the tunnel: according to the determined construction direction, carrying out underground excavation construction on the constructed vertical shaft to the position of the square underground well to be constructed;
thirdly, excavating a construction cavity: when the tunnel reaches the construction area of the square blind shaft, excavating a construction cavity; the construction cavity comprises a gradual change section which is communicated with the underground excavation tunnel and the section of which is gradually increased from one end communicated with the tunnel to one end far away from the tunnel, and a main cavity for constructing the square underground well inside;
step four, tunnel epitaxy, turn back and backfill: extending the end, butted with the transition section, of the tunnel towards the interior of the transition section to form an extension section, sealing the end, far away from the tunnel, of the extension section and the top of the transition section to form a backfill cavity between the upper part of the extension section and the top of the transition section, and finally filling backfill materials into the backfill cavity;
and fifthly, constructing the square blind well: pouring a square blind well in the main cavity through template engineering, and removing the template after the square blind well is formed;
sixthly, filling concrete: and pouring concrete into the space between the formed square blind well and the wall of the main chamber.
Through adopting above-mentioned technical scheme, through extending the tunnel to backfill between the top between the epitaxial extension section in tunnel and the gradual change section, thereby the one end that makes main cavity and tunnel meet forms vertical wall, thereby has guaranteed the atress of structure, thereby can form the required square blind shaft of design in the inside of main cavity.
Optionally, in the third step and the construction cavity excavating step, the transition section and the main cavity are excavated by a CD method.
By adopting the technical scheme, the method can be suitable for the underground engineering construction with poor and unstable strata and strict ground settlement requirements.
Optionally, the second step and the tunnel excavation specifically include the following steps:
firstly, opening a tunnel door for construction; three vertical shaft grids are connected above the ingate before the ingate construction; vertical connecting ribs of vertical shafts on two sides of the ingate are encrypted; the tunnel grids at the ingate are connected with three trusses; distributing and breaking primary support concrete of a vertical shaft at the upper part of the ingate, constructing grids at the upper part of the ingate, then, entering a hole at the upper part, excavating and supporting, and continuously spraying concrete by two grids at a hole opening;
secondly, excavating earthwork; excavating by adopting a step method;
thirdly, installing a grid arch frame and a reinforcing mesh;
fourthly, wet spraying concrete;
fifthly, backfilling and grouting after backing;
sixthly, constructing a tunnel secondary lining;
and step seven, grouting after backing.
Through adopting above-mentioned technical scheme, can guarantee the structural strength in undercut tunnel.
Optionally, in the step of wet spraying the concrete, the sprayed concrete is sprayed from top to bottom, firstly, the wall corner is sprayed, then, the wall top is sprayed, firstly, the arch foot is sprayed, then, the arch top is sprayed, the sprayed material beam moves in a spiral rotating track, half a circle is pressed for one circle, and the sprayed material beam is longitudinally sprayed in a snake shape.
Through adopting above-mentioned technical scheme, when the damp shotcrete, can guarantee the even degree of the injection of concrete.
Optionally, when the step of earth excavation is carried out, before passing through a rain pipe and a sewage pipe, manually adopting a luoyang shovel to try to dig a phi 200 exploration hole in the center of an upper arch along the tunneling direction, wherein the exploration hole depth is ensured to be 1-1.5 m before the first arch; and if the underground water flows out, stopping excavating, immediately sealing the tunnel face, analyzing reasons and taking corresponding measures.
By adopting the technical scheme, the construction safety can be ensured.
Optionally, when the earthwork excavation step is carried out, the stability of the stratum of the face is closely noticed, and concrete with the thickness of 100 mm-200 mm needs to be sprayed for sealing.
Through adopting above-mentioned technical scheme, when the unstable condition appears in the face, seal the face through the concrete injection to fix the face, prevent the condition that collapses.
Optionally, before the second step and the underground excavation of the tunnel, firstly, a pipe shed support with a small advanced pipe is adopted to perform pre-grouting reinforcement on the soil body of the section to be excavated of the vertical shaft.
Through adopting above-mentioned technical scheme, consolidate the broken end face of digging of shaft to when digging, can reduce the condition that the section appears collapsing.
Optionally, a support structure is first applied to the inside of the tunnel's extension before backfill is filled into the backfill cavities.
By adopting the technical scheme, when backfill materials are filled into the backfill cavities, the support structure supports the extension sections of the tunnel, so that the extension sections are protected.
In summary, the present application includes at least one of the following beneficial technical effects:
1. the tunnel is extended, and the top between the extension section and the transition section of the tunnel extension is backfilled, so that a vertical wall surface is formed at one end, connected with the tunnel, of the main chamber, the stress of the structure is guaranteed, and a square blind well required by design can be formed in the main chamber;
2. when backfill materials are filled into the backfill cavity, the support structure supports the extension section of the tunnel, and therefore the extension section is protected.
Drawings
FIG. 1 is a flow chart of a method for turning back and excavating construction of a square underground well according to an embodiment of the application;
fig. 2 is a schematic structural view of a tunnel and a construction cavity according to an embodiment of the present application.
Description of reference numerals: 1. a tunnel; 11. an extension section; 2. a construction cavity; 21. a transition section; 22. a main chamber.
Detailed Description
The present application is described in further detail below with reference to figures 1-2.
The embodiment of the application discloses a turning-back underground excavation construction method for a square underground well. Referring to fig. 1 and 2, the construction method of turning back and underground excavation of underground small chamber comprises the following steps:
first step, preparation of construction
And determining a constructed vertical shaft communicated with the square blind shaft to be constructed according to a design drawing, and determining the construction direction.
Second step, tunneling the tunnel
And (4) carrying out underground excavation construction of the tunnel 1 between the well-constructed vertical shaft and the position of the square underground shaft to be constructed according to the determined construction direction.
The general principle of tunnel construction is 'advancing, tight grouting, short excavation, strong support, quick sealing and duty measurement'. Before the construction of the underground excavation tunnel, firstly, an advanced small conduit pipe shed support is adopted to perform pre-grouting reinforcement on the soil body of the section to be excavated of the vertical shaft.
(1) Construction of door with hole
Three vertical shaft grids are connected above the horsehead door before horsehead door construction.
And (4) encrypting vertical connecting ribs of vertical shafts on two sides of the ingate.
According to the monitoring condition, a temporary support is additionally arranged at the ingate of the horse head when necessary.
When the temporary support and the angle brace are inconvenient to be additionally arranged at the position of the ingate, the inner support is applied at the position of the ingate, the plate brace can be arranged, 2-3I-shaped steels are additionally arranged at the position of the ingate according to the specific conditions of each shaft, the plate brace is erected at the position of the opposite brace and the angle brace, and the monitoring and measuring of the stress condition of the bracing at the position of the reinforced ingate are realized.
And the tunnel grating at the ingate is connected with three trusses.
And (3) breaking the preliminary supporting concrete and the steel bar grating of the vertical shaft at the ingate step by step, and constructing the ingate reinforced arched girder. And after the bottom sealing construction of the vertical shaft is finished, breaking the primary support concrete of the vertical shaft according to the tunnel construction steps. Firstly, breaking primary support concrete of a vertical shaft at the upper part of the ingate, applying a grid at the upper part of the ingate, then digging and supporting the upper part of the ingate, continuously spraying concrete on two grids at the opening of the ingate, and ensuring the stress of the opening. The grid was then lined with shotcrete at a design pitch of 500 mm.
And the tunnel earthwork excavation construction is carried out after the reinforcing and grouting are finished. The first lining construction of the tunnel adopts a step method for construction and follows the construction principle of 'short excavation and early support'. And a part of core soil is reserved in the middle of the upper step to support and block the excavation surface and enhance the stability.
Because the soil body self-stability capability of the soil layer where the engineering tunnel is located is poor, the vault part of the tunnel can generate an over-excavation phenomenon in the process of installing the advanced guide pipe in a drilling hole and the process of excavating. According to the soil texture, the stability of the tunnel face stratum is closely paid attention to, and concrete with the thickness of 100 mm-200 mm is sprayed to seal the tunnel face stratum when necessary.
During excavation, core soil with the width of 1.5 meters, the height of 1.0 meter and the longitudinal length of about 2 meters is reserved on an upper step. The distance between the upper step and the lower step is 3-4 m, so that enough supporting force is kept on the soil body of the lower step, and the stability of the soil body of the tunnel face is maintained. During operation, the soil is dug by a shovel manually and transferred to a lower step, the soil on the lower step is transported to the bottom of the shaft by a small cart and lifted to a soil unloading yard by a crane. The tunnel excavation outline should guarantee straight, smooth and straight, and the size should be greater than tunnel section outline 50mm to the installation of preliminary bracing steel bow member.
Before passing through a rain pipe and a sewage pipe, manually adopting a Luoyang shovel to try to draw a phi 200 probing hole in the center of an upper arch along the tunneling direction, wherein the probing hole depth is ensured to be 1-1.5 m before a first arch; and if the underground water flows out, stopping excavating, immediately sealing the tunnel face, analyzing reasons and taking corresponding measures.
The upper arch of the grid steel structure needs to be supported in time, when the straight wall part falls to the bottom, preliminary bracing needs to be carried out in time, the bottom of each arch truss needs to be firmly supported, a foot locking anchor rod is arranged, the foundation cannot be placed on the deficient soil, all connecting plate bolts need to be firmly connected, and interface reinforcing ribs are additionally welded, so that the tunnel structure is sealed into a ring, and the preliminary bracing is integrally formed. Because the existing pipelines along the underground excavation section of the engineering are densely distributed, the ground traffic flow is large, the excavation roof truss distance is strictly controlled in the construction, the circulating distance of each excavation is 500mm, and the grids are encrypted to 400mm within the range of 5m before and after the pipeline is crossed; and (5) strictly prohibiting over excavation in construction.
When the soil quality is poor and the pipeline passes through the road junction, attention needs to be paid to accelerating the closing speed of the grating. The supporting state should be monitored at any time during excavation, operation should be stopped when abnormal conditions are found, the abnormal conditions are reflected to related personnel in time, reasons are found out, a reliable scheme is made, and then construction is carried out. When the excavation needs to be stopped in work, concrete must be sprayed to close the tunnel face.
(3) Installation of grid arch and reinforcing mesh
The grid arch frame and the net-sprayed concrete combined support in the underground excavated loose stratum has instant strength and rigidity, and can control the overlarge deformation of the soil body. And erecting a grid arch frame in time after the soil body excavation surface is formed. Before installation, the false slag and other impurities under the bottom foot should be removed, and the over-excavated part is filled with concrete.
Two arch frames are welded by using phi 20mm longitudinal connecting steel bars. The inner layer and the outer layer of the connecting reinforcing steel bars are arranged in a staggered mode, the circumferential distance cannot be larger than 1m, and the lap joint cannot be smaller than 10d. At the same time, the phi 6 @ 100mm x 100mm steel bar net sheets are fully laid along the main reinforcement of steel arch, and the lap joint of the inner and outer double-layer net sheets is not less than 100mm.
During excavation, the advanced guide pipe which is circularly constructed in the previous step is exposed (the advanced guide pipe with good quality is arranged into a regular arc shape), and when the grid steel structure is erected, the grid steel structure is welded with each advanced steel pipe, so that a stable shed frame is formed by the two advanced steel pipes. The steel pipes with individual inward deviation invading clearance need to be cut by gas cutting (or electric welding).
(4) Wet sprayed concrete
The sprayed concrete strength is C20, and the material is mixed by a self-falling type mixer from the well head. The wet spraying method is adopted to spray concrete, and the anchor spraying machine is arranged outside a well mouth.
The mixing proportion of the sprayed concrete mixture is as follows: the theoretical mixing proportion is designed in advance by qualified test units and is adjusted in time according to sampling tests in the construction process. The weight ratio of the cement to the sand is preferably 1-1.
After the jet machine is installed and debugged, firstly injecting water and then ventilating, and cleaning air cylinder and pipeline, at the same time cleaning (blowing) sprayed surface by using high-pressure water (air).
And continuously feeding, keeping the machine barrel full of material, and arranging a 12mm screen at the upper opening of the hopper to prevent the oversize aggregate from entering the machine.
When in spraying, water is injected firstly (the nozzle is downward to avoid water flowing into the pipe), then air is supplied, and then the materials are loaded. The water injection amount is adjusted according to the sprayed surface and the condition of the sprayed concrete, so that the sprayed concrete is easy to adhere, has small rebound and has wet surface gloss.
The injection should be performed in segments and slices. The liquid is ejected one time back and forth by rotating and moving horizontally from the lower part, and then moves upward. The thickness of the first spraying is determined according to the spraying position and the design thickness, the arch part is preferably 5 cm-6 cm, the side wall is 7 cm-10 cm, and the later spraying is carried out after the first spraying is solidified.
During spraying, the material beam is sprayed from bottom to top, namely, the wall foot is firstly sprayed to the wall top, the arch foot is firstly sprayed to the arch top, the dead angle is avoided, the sprayed material beam moves in a spiral rotating track, the material beam is pressed into a half circle for one circle, the material beam is longitudinally sprayed in a snake shape, and the length of the snake-shaped spray is 50mm each time.
The distance from the nozzle opening to the sprayed surface is preferably 0.6 to 1.2 m. The spray beam is preferably directed perpendicularly to the spray surface. The wind pressure and the feeding amount are adjusted according to the conditions of the injection position, the model and the like. Generally, the working wind pressure is 0.16-0.22 Mpa, and the feeding amount is 3-4 m for harvesting.
(5) A back-filling grouting
And (3) after the primary support strength is formed in a closed mode, and the strength reaches more than 70% of the design strength, grouting is carried out after the whole primary support back, grouting is carried out every 4m, and a water stop ring is formed after the primary support back. Specifically, 2 grouting short pipes are preset in the primary arch part every 4m along the longitudinal direction of the tunnel, and are uniformly arranged above the arch raising line. The vertical shaft grouting pipes are arranged in a quincuncial shape of 5m along the longitudinal 2m ring direction. The grouting holes are preset before the primary anchor spraying. A CZJ-30 grouting pump was used. The grouting pressure is 0.1-0.5 MPa. The water section is reinforced by pressure injection cement and water glass slurry, and the mixing ratio is determined by tests. Meanwhile, the water seepage part is plugged by a material without leakage, so that the primary support and the surrounding rock are tightly attached, the deformation of the surrounding rock is prevented, and the waterproof quality is improved. The diffusion radius of grouting behind the primary support is 1.4-1.8m, and the grouting speed is not more than 50L/min.
(6) Tunnel secondary lining construction
And constructing a tunnel secondary lining on the inner side of the tunnel primary lining, wherein the tunnel secondary lining adopts injection molding concrete.
(7) Two backing materials for post-grouting
The close adhesion degree of the waterproof layer and the sprayed concrete is influenced by the flatness of the surface of the sprayed concrete, a certain gap often exists between the waterproof layer and the sprayed concrete, and the arch part cannot be completely filled with the pumped concrete during pouring, so that the arch part of the tunnel can easily form the gap to form a water storage bag and a water seepage channel, and the stress and the water resistance of the tunnel structure are both unfavorable. Therefore, the horizontal construction joints of the arch and the side walls need to be grouted and backfilled. The filling and grouting should be carried out after the strength of the secondary lining concrete reaches over 75 percent of the designed strength (about 14 days for curing).
Thirdly, excavating the construction cavity
The construction chamber 2 comprises a gradual section 21 of increasing section communicating with the tunnel 1 and a main chamber 22 for the construction of the square blind shaft inside. The transition section 21 and the main chamber 22 are excavated by the CD method.
The construction sequence of the transition section 21 and the main chamber 22 is as follows:
(1) Excavating a guide pit on one side, and performing primary supporting and closing on the guide pit in time;
(2) Excavating a pilot tunnel at the other side after an adaptive distance is formed, and constructing a primary support;
(3) Excavating upper core soil, constructing an arch part primary support, and supporting arch springing on primary supports of guide pits on two side walls;
(4) Excavating a lower step, and constructing a primary support at the bottom to close the full section of the primary support;
(5) Dismantling the primary support of the guide pit near the empty part;
(6) And constructing an inner lining.
Fourthly, extending the tunnel and backfilling the tunnel
The inner lining of the tunnel 1 extends towards the inner direction of the transition section or a pipeline communicated with the inside of the tunnel 1 is additionally arranged at the end of the tunnel to be used as the extension section 11 of the tunnel 1 until the inner lining of the tunnel 1 extends to the position which is butted with the position of the main chamber 21. And (3) closing the end, far away from the tunnel 1, of the extending section 11 of the tunnel 1 and the top of the transition section 21 through a formwork, so as to form a backfill cavity between the extending section 11 and the top of the transition section 21, and pouring concrete into the backfill cavity. And (4) removing the template after the concrete is solidified. And after the concrete backfilled in the backfilling cavity is solidified, a supporting structure for supporting the top of the gradual change section is formed, and meanwhile, a side wall which is vertically arranged is formed on one side of the main cavity connected with the gradual change section.
Before pouring concrete into the backfill cavity, a support frame is firstly erected inside the extension section 11, so as to support the top end of the extension section 11.
Fifth step, construction of square blind well
And pouring a square blind well forming a thermal small chamber in the main chamber through template engineering, and communicating the square blind well with the interior of the tunnel.
Sixthly, backfilling concrete;
and sealing the space between the construction cavity and the square blind well by pouring concrete.
The implementation principle of the turning-back underground excavation construction method of the square underground well in the embodiment of the application is as follows: the tunnel is extended to the transition section, and the space between the extension part of the tunnel and the transition section is backfilled, so that the excavated well is a square blind well.
The above embodiments are preferred embodiments of the present application, and the protection scope of the present application is not limited by the above embodiments, so: all equivalent changes made according to the structure, shape and principle of the present application shall be covered by the protection scope of the present application.
Claims (8)
1. A turning underground excavation construction method of a square underground well is characterized by comprising the following specific steps:
first step, construction preparation: determining a constructed vertical shaft communicated with a square blind shaft to be constructed according to a design drawing, and determining a construction direction;
step two, underground excavation of the tunnel: according to the determined construction direction, carrying out underground excavation construction on the tunnel (1) at the position of the square underground well to be constructed from the constructed vertical well;
thirdly, excavating a construction cavity: when the tunnel (1) reaches a construction area of the square blind shaft, the construction cavity (2) is excavated; the construction cavity (2) comprises a transition section (21) which is communicated with the underground excavated tunnel (1) and has a section gradually increasing from one end communicated with the tunnel (1) to one end far away from the tunnel (1), and a main cavity (22) for constructing a square underground well inside;
step four, tunnel epitaxy, turning back and backfilling: extending the end, butted with the transition section (21), of the tunnel (1) towards the interior of the transition section (21) to form an extension section (11), closing a space between the end, far away from the tunnel (1), of the extension section (11) and the top of the transition section (21) so as to form a backfill cavity between the upper part of the extension section (11) and the top of the transition section (21), and finally filling backfill into the backfill cavity;
and fifthly, constructing the square blind well: pouring a square blind well in the main chamber (22) through template engineering, and removing the template after the square blind well is formed;
sixthly, filling concrete: and pouring concrete into the space between the formed square blind well and the wall of the main chamber (22).
2. A turn-back subsurface excavation construction method for a square underground well according to claim 1, characterized in that: in the third step, the construction cavity excavation step, the transition section (21) and the main cavity (22) are excavated by the CD method.
3. A turn-back subsurface excavation construction method for a square underground well according to claim 1, characterized in that: the second step of underground excavation of the tunnel comprises the following specific steps:
firstly, opening a tunnel door for construction; three vertical shaft grating connecting frames are arranged above the horsehead door before horsehead door construction; vertical connecting ribs of vertical shafts on two sides of the ingate are encrypted; the tunnel (1) at the ingate is connected with three trusses through grids; distributing and breaking primary support concrete of a vertical shaft at the upper part of the ingate, constructing grids at the upper part of the ingate, then, entering a hole at the upper part, excavating and supporting, and continuously spraying concrete by two grids at a hole opening;
step two, earth excavation; excavating by adopting a step method;
thirdly, installing a grid arch frame and a reinforcing mesh;
fourthly, wet spraying concrete;
fifthly, backfilling and grouting after backing;
sixthly, constructing a tunnel secondary lining;
and step seven, grouting after backing the backing material.
4. A turn-back subsurface excavation construction method for a square underground well according to claim 3, characterized in that: in the step of wet spraying concrete, the sprayed concrete is sprayed from top to bottom, firstly, the wall corner is sprayed, then, the wall top is sprayed, firstly, the arch foot is arched, then, the arch top is sprayed, the sprayed material beam moves in a spiral rotating track, and the sprayed material beam is pressed for half a circle and longitudinally sprayed in a snake shape.
5. A turn-back subsurface excavation construction method for a square underground well according to claim 3, characterized in that: when the earth excavation step is carried out, before passing through a rain pipe and a sewage pipe, manually adopting a Luoyang shovel to try to dig a phi 200 exploration hole in the center of an upper arch along the tunneling direction, wherein the exploration hole depth is ensured to be 1-1.5 m before a first arch; and if the underground water flows out, stopping excavating, immediately sealing the tunnel face, analyzing reasons and taking corresponding measures.
6. A reentry underground excavation construction method for a square underground well according to claim 3, characterized in that: when the earthwork excavation step is carried out, the stability of the stratum of the tunnel face is closely noticed, and concrete with the thickness of 100 mm-200 mm is sprayed for sealing if necessary.
7. A turn-back subsurface excavation construction method for a square underground well according to claim 1, characterized in that: and (3) before the second step of underground excavation of the tunnel, firstly adopting an advanced small conduit pipe shed support to pre-grouting and reinforcing the soil body of the section to be excavated of the vertical shaft.
8. A turn-back subsurface excavation construction method for a square underground well according to claim 1, characterized in that: before filling the backfill into the backfill chamber, a support structure is first applied to the interior of the tunnel (1) extension (11).
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