A construction method for shallow-buried tunnels with a covering layer

Abstract

The present invention discloses a construction method for a shallow-buried tunnel with a covering layer, which includes the following steps: a: forming a solid hard shell in the range of 3-5 m above the tunnel top, and improving the surrounding soil of the tunnel top into a consolidated body with corresponding strength; b: organizing the support construction according to the New Austrian Tunneling Method principle; c: adopting the "bench method" for tunnel excavation to form a shape; d: using the New Austrian Tunneling Method to support the surrounding rock of the upper bench, forming a composite support system of advanced bolt trusses, shotcrete with wire mesh and steel-concrete; e: the lower bench and the key soil of the upper bench are excavated by blasting, and the space between the key soil and the steel support is used to buffer the blasting seismic wave; this construction method for the shallow-buried tunnel with a covering layer avoids the settlement deformation directly caused by excavation, the flexible deformation of the support system, and the overall deformation and displacement of the structure caused by the foundation settlement. The tunnel construction is safe and controllable, and the occurrence of tunnel collapse is prevented.

Description

Technical Field

[0001] The invention relates to the technical field of tunnel engineering, and in particular to a construction method for a shallow buried tunnel in a covering layer. Background Art

[0002] The rocks in the shallow buried section of the tunnel are mostly miscellaneous fill, fully weathered or strongly weathered soil, and residual soil. After the tunnel soil is excavated, the soil retained around the tunnel excavation contour line will produce stress redistribution. The shear force incision angle of the soil and the coagulation strength of the cementing body are low. Under the action of leaking fissure water, the block is very easy to collapse and fall off after losing support, making the tunnel excavation quite dangerous and difficult in construction technology. If the tunnel support process connection cannot be followed up in time or the support strength is weak, it will cause the tunnel roof collapse or surface subsidence, resulting in the inability to carry out normal construction. Summary of the invention

[0003] The purpose of the present invention is to solve the above-mentioned problems and provide a method for constructing a shallow buried tunnel in a covering layer, which avoids the settlement deformation directly caused by excavation, the flexible deformation of the support system, and the overall deformation and displacement of the structure caused by the sinking of its foundation. The tunnel construction is safe and controllable, and the occurrence of tunnel collapse is prevented.

[0004] To achieve the above-mentioned purpose, the technical solution of the present invention is: a method for constructing a shallow buried tunnel in the covering layer, comprising the following steps: a: forming a solidified hard shell within 3-5m of the top of the tunnel, and improving the surrounding rock and soil at the top of the tunnel into a consolidated body with corresponding strength, so as to meet the self-stabilizing ability of the hard shell and the advance anchor to jointly bear the upper loose body load after the tunnel excavation; b: organizing the support construction through the principle of the New Austrian Tunneling Method, forming an advance anchor scaffolding, mesh sprayed concrete and steel concrete composite support system to jointly bear the load above the top of the tunnel; c: the tunnel excavation adopts the "step method" excavation and forming, in which the core soil is reserved for the upper step, and the space for erecting steel supports is formed by trenching; d: the surrounding rock of the upper step is supported by the New Austrian Tunneling Method, forming an advance anchor scaffolding, mesh sprayed concrete and steel concrete composite support system; e: the core soil of the lower step and the upper step is excavated by blasting, and the space between the core soil and the steel support is used to buffer the blasting seismic waves.

[0005] Preferably, in step a, the hard shell is reinforced by rotary jet grouting of the covering layer, wherein the spacing between the rotary jet grouting reinforcements of the covering layer is 1.0m, 10 rows of holes are arranged perpendicular to the axis of the tunnel, and the consolidation range is arranged on both sides of the tunnel excavation range, extending 1.0m on each side and 2.0m along the axis of the tunnel; the high-pressure jet grouting arch depth does not exceed 20cm, and when encountering rocks before reaching the top of the tunnel, it penetrates 50cm into the rock layer; the high-pressure jet grouting drilling diameter is not less than 110mm; and the jet grouting uses 425 ordinary Portland cement with a water-cement ratio of 1:1.

[0006] Preferably, the construction method of the hard shell specifically includes the following steps: a1: level the site according to the construction requirements and excavate the mud trench; a2: measure and lay out the lines, make positioning marks for each hole at the construction site, and ensure that the drilling deviation is no more than 5 cm; a3: the clay stratum adopts the original soil slurry wall drilling process; the gravel stratum adopts the bentonite mud wall drilling process; a4: after the drilling is completed, the hole position, hole depth and bottom formation of the hole are reviewed and accepted; a5: before the jet grouting operation, first conduct a test spraying inspection on the assembled spraying tool for the unobstructed and pressure-bearing conditions of the water, gas and slurry pipes. When the water pressure reaches 1.5 times the design pressure, the pipeline is leak-free and then a test spraying is conducted for 15 minutes before the test spraying inspection is ended; a6: after the test spraying inspection is completed, the spraying tool is rotated and lowered to the designed hole depth; a7: During the initial grouting, just spray without lifting, and spray quietly for 3 to 5 minutes. When the concentration of the slurry returned at the hole mouth reaches or approaches 1.5g / cm3, spray the grout from bottom to top according to the lifting speed and rotation speed required by the parameters to reach the high spraying design elevation in the hole; a8: When the grouting is interrupted, the grouting pipe should be lowered into the sprayed section again, and the overlap length of the spraying should not be less than 100mm; a9: When the designed pile top height is reached or overflow occurs on the ground, stop the rotary spraying of the current pile, pull out the rotary spraying pipe and clean the pipeline; a10: Repeat steps a3-a9 and move on to the next hole operation.

[0007] Preferably, in step d, the upper step support includes the following steps: d1: use the top of the steel support to drill holes for the steel scaffolding, and use a hand-held pneumatic drill to send the steel bars into the holes; d2: weld the tail of the steel bars of the steel scaffolding to the steel support, so that one end of the steel scaffolding penetrates into the surrounding rock and soil in front of the face of the yoke, and the other end is welded to the steel support to form a simply supported beam to support the surrounding rock and soil on the upper part of the arch; d3: arrange peripheral holes and collapse holes along the excavation contour line to complete the splitting and loosening of the cavern excavation rock; d4: remove the loose blocks on the surface of the surrounding rock; d5: use a single spraying protection on the arch shoulder and the surface of the arch ring, the thickness of the plain sprayed concrete is 3-4cm, and then the slag is discharged; d6: use a pneumatic draft to chisel out the protruding rock surface, and the steel support is installed in place; d7: erect the steel support, hang the mesh on the side wall and the top arch to spray concrete, and close the steel support to form a supporting structure with alternating beams and slabs; d8: Repeat steps d1-d8, working at an advance of 1.0m per cycle, and advance the lower step by one cycle; d9: After excavating the upper step for 2 cycles, excavate the lower step.

[0008] Preferably, the lower step support specifically includes the following steps: e1: The locking foot anchor drilling of the upper step steel support leg is constructed by grouting first and then inserting the anchor. The borehole diameter is ø50mm, and cement slurry is used for grouting with a water-cement ratio of 0.38-0.42. The grouting pipe is gradually lifted from the bottom of the hole to the hole mouth until the hole mouth returns to grout. After the hole mouth returns to grout, the anchor is sent to the bottom of the hole with the assistance of a hand pneumatic drill, and then the hole mouth is firmly fixed with a steel wedge; e2: The tail of the locking foot anchor is firmly welded to the steel support column leg; e3: A horizontal blasting hole is used for drilling, the borehole diameter is ø40mm, and the bottom hole has a downward external insertion angle of 30°; e4: The blasting hole is segmented from top to bottom, and then the surrounding holes and the last bottom hole are blasted in sequence, and the slag is loaded and transported out of the hole; e5: A small cannon is used in conjunction with a pneumatic draft to strip off the protective layer; e6: The steel support column legs are installed, and the locking foot anchor is arranged for fixing; e7: Hang a mesh and spray concrete on the side walls to form a support system with alternating beams and slabs; e8: Repeat steps e1-e7, with the lower step lagging behind the upper step by 1-2 cycles to form a flow operation follow-up.

[0009] Compared with the prior art, the shallow buried tunnel construction method disclosed by the present invention has the following beneficial effects: 1. By adopting rotary jet grouting for the shallow buried and saturated water covering layer, rotary jet consolidation piles with mutual overlap are formed at the top of the tunnel. The stability of the rotary jet consolidation piles themselves and the clamping effect on the soil between the piles ensure that after the shallow buried tunnel is excavated and formed, the surrounding rock of the cave has a strong self-stabilizing ability, so that the tunnel construction can be carried out smoothly; 2. After the 3-5m thick rotary jet consolidation body is formed at the top of the tunnel, the stratum structure is improved, and the springs and sand are blocked. The pores of the gravel prevented surface water from entering the tunnel, ensuring that the surface ecological environment was not affected during the tunnel excavation; 3. Steel supports, steel scaffolding, and mesh sprayed concrete were used to form a support structure with alternating beams and slabs, which not only strengthened the rigidity of the sprayed structure, but also suppressed excessive deformation of the surrounding rock; 4. The cover layer was improved and the step method was used to excavate shallow tunnels, avoiding settlement deformation directly caused by excavation, flexible deformation of the support system, and overall deformation and displacement of the structure caused by the sinking of its foundation. The tunnel construction was safe and controllable, preventing the occurrence of tunnel collapse. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 It is a schematic cross-sectional diagram of excavation support for a shallow buried tunnel in the overburden layer according to the present invention.

[0011] Figure 2 It is a schematic diagram of the longitudinal section of the excavation support of a shallow buried tunnel in the overburden layer of the present invention.

[0012] In the figure: 1. Consolidation body of the tunnel roof covering layer; 2. Steel support for the upper half of the tunnel; 3. Advanced steel bar scaffolding; 4. Steel support connecting steel plate; 5. Upper step excavation area I; 6. Lower step excavation area II; 7. Steel support pads; 8. Locking anchor rods; 9. Lower step steel support legs; 10. Net sprayed concrete. DETAILED DESCRIPTION

[0013] The present invention will now be described in further detail with reference to the accompanying drawings. The accompanying drawings are simplified schematic diagrams, which only illustrate the basic structure of the present invention in a schematic manner, and thus only show the components related to the present invention.

[0014] Please refer to Figure 1-2 , a construction method for a shallow buried tunnel with a covering layer, comprising the following steps: a: forming a solidified hard shell 1 within 3-5m of the top of the tunnel, improving the surrounding rock soil at the top of the tunnel into a consolidated body with corresponding strength, satisfying the self-stabilizing ability of the hard shell and the advance anchor to jointly bear the upper loose body load after the tunnel excavation, and creating time and space conditions for the erection of steel supports and mesh spraying concrete. b: organizing support construction through the principle of the New Austrian Tunneling Method, forming a composite support system of advance anchor scaffolding, mesh spraying concrete and steel concrete to jointly bear the load above the top of the tunnel. On the other hand, after the upper covering layer of the tunnel is consolidated, it has a good anti-seepage effect, avoiding the safety risks and environmental impacts of the surface depressions and paddy fields caused by surface collapse, which may flood the entire tunnel. c: The tunnel excavation adopts the "step method" excavation and shaping, in which the core soil is reserved for the upper step, and the space for erecting steel supports is formed by trenching; on the premise of erecting the steel supports in the upper half of the tunnel, the core soil reduces the height of the working face, which can prevent the instability of the face, while reducing the amount of mechanical excavation in the upper half of the tunnel, and the core soil platform can be used to facilitate the installation of steel supports. d: The New Austrian Tunneling Method is used to support the surrounding rock of the upper step to form a composite support system of advanced anchor scaffolding, mesh sprayed concrete and steel concrete; e: The core soil of the lower and upper steps is excavated by blasting, and the space between the core soil and the steel support is used to buffer the blasting seismic waves, prevent the blasting vibration in the tunnel from reaching the surface, and reduce the amount of mechanical excavation in the tunnel and the cost of tunnel excavation.

[0015] In the present invention, in the step a, the hard shell 1 is reinforced by rotary jet grouting of the covering layer, wherein the spacing between the rotary jet grouting reinforcements of the covering layer is 1.0m, 10 rows of holes are arranged perpendicular to the axis of the tunnel, and the consolidation range is arranged on both sides of the tunnel excavation range, extending 1.0m on each side, and extending 2.0m along the axis of the tunnel; the high-pressure jet grouting arch depth does not exceed 20cm, and when encountering rocks before reaching the top of the tunnel, it penetrates 50cm into the rock layer; the high-pressure jet grouting borehole diameter is not less than 110mm; the jet grouting uses 425 ordinary Portland cement with a water-cement ratio of 1:1; the main construction parameters are: air pressure 0.8MPa, water pressure 36MPa, slurry pressure 0.5MPa, high-pressure water jet flow rate 70-80L / min, cement slurry supply 60-80L / min, sand and gravel formation lifting speed 8cm / min, soil layer lifting speed 20cm / min, and rotation speed 10r / min. High-pressure jet grouting is constructed in two sequences, with sequence I holes constructed first and sequence II holes constructed later. The jet grouting adopts the three-tube method, with the final hole spacing of 1.0m and the interval time between adjacent hole constructions being no less than 24 hours.

[0016] The construction method of the hard shell specifically includes the following steps: a1: Level the site and excavate the mud trench according to the construction requirements; a2: Measure and lay out the lines, and make positioning marks for each hole on the construction site to ensure that the drilling deviation is no more than 5cm; a3: The clay stratum adopts the original soil slurry wall drilling process; the gravel stratum adopts the bentonite mud wall drilling process; a4: After the drilling is completed, the hole position, hole depth and bottom stratum of the hole are reviewed and accepted; a5: Before the jet grouting operation, the assembled sprayer is first tested for the unobstructed and pressure-bearing conditions of the water, air and slurry pipes. When the water pressure reaches 1.5 times the design pressure, the pipeline is leak-free and then test sprayed for 15 minutes before the test spray inspection is ended; a6: After the test spray inspection is completed, the sprayer is rotated and lowered to the designed hole depth; a7: During the initial grouting, just spray without lifting, and spray quietly for 3 to 5 minutes. When the concentration of the grout returning from the hole mouth reaches or approaches 1.5g / cm3, spray the grout from bottom to top according to the lifting speed and rotation speed required by the parameters to reach the high spraying design elevation in the hole; a8: When the grouting is interrupted, the grouting pipe should be lowered into the sprayed section again, and the overlap length of the spraying should not be less than 100mm; a9: When the designed pile top height is reached or overflow occurs on the ground, stop the rotary spraying of the current pile, form a net sprayed concrete 10 after solidification, and pull out the rotary spraying pipe and clean the pipeline; a10: Repeat steps a3-a9 and proceed to the next hole operation.

[0017] In the scheme of the present invention, the upper step excavation zone I 5 adopts the upper half hole steel support 2, the bottom of the upper half hole steel support 2 is provided with a locking foot anchor rod 8, the upper half hole steel support is advanced supported by an advanced steel bar scaffolding 3, the bottom is provided with a steel support pad 7, the steel bar diameter is ø28mm, the steel bar scaffolding is 3.5m deep into the tunnel face, and the tunnel top arch 120 is provided with a steel support pad 7. 0Range layout, control the overlap length of each scaffolding to be no less than 1.50m, the spacing between steel scaffoldings to be controlled at 10-20cm, and the external insertion angle to be controlled at 3-5 0 .After the construction of the advanced steel bar anchor scaffolding is completed, control 3 along the excavation contour line 0 The outer corner drilling depth is about 1m, the drilling diameter is 42-50mm, and the drilling hole spacing is 30cm-40cm. After the side hole drilling is completed, the YG-450 hydraulic splitter hydraulic station supplies 60Mpa pressure oil to the splitting gun. The splitting gun generates dozens of tons of thrust, pushing the wedge to expand to both sides. After the expansion force reaches hundreds of tons, the rock is split and separated along the excavation line, completing the rock breaking operation of the surrounding holes. The mechanical excavation collapse hole is 80cm-100cm away from the surrounding holes, and the drilling spacing is 80cm-100cm. After the drilling is completed, the hydraulic splitter is used to break the rock according to the surrounding hole breaking method. In order to facilitate slag discharge, after the larger block is drilled after rock breaking, a small amount of explosive blasting is used to assist in loosening. The slag is transported out of the hole by a slag scraper with a dump truck.

[0018] The steel support is processed and formed in the processing plant. Each steel support is divided into two sections, and the steel support connecting steel plate 4 is welded in the middle. The steel support connecting steel plate 4 is a 10mm thick steel plate. Each steel support is assembled and bolted during installation. When a collapsed cavity appears in a local tunnel section, a steel support arch is added to support it firmly. When the steel support cannot be installed tightly against the surrounding rock, prefabricated concrete blocks or steel wedges are used, and then sprayed with concrete to fix it. The upper step support specifically includes the following steps: d1: Use the top of the steel support to drill holes for the steel scaffolding, and use a hand-held pneumatic drill to send the steel bars into the holes; d2: Weld the tail of the steel bars of the steel scaffolding to the steel support, so that one end of the steel scaffolding penetrates into the surrounding rock and soil in front of the face of the yoke, and the other end is welded to the steel support to form a simply supported beam to support the surrounding rock and soil on the upper part of the arch; d3: Arrange peripheral holes and collapse holes along the excavation contour to complete the splitting and loosening of the rock for cavern excavation; d4: Clear the loose blocks on the surface of the surrounding rock; d5: Use one-time spraying protection for the arch shoulder and the surface of the arch ring, with a thickness of 3-4cm for plain spraying concrete, and then remove the slag; d6: Use a pneumatic chisel to remove the protruding rock surface, and install the steel support in place; d7: Erect the steel support, hang mesh spraying concrete on the side walls and top arch, and close the steel support to form a support structure with alternating beams and slabs; d8: Repeat steps d1-d8, working at an advance of 1.0m per cycle, and advance the lower step by one cycle; d9: After excavating the upper step for 2 cycles, excavate the lower step.

[0019] The lower step excavation area II 6 adopts the horizontal hole drilling and blasting method, and lags behind the upper half of the cave for 1-2 cycles. In order to ensure the quality of the cavern side wall forming and the safety of the cavern blasting, a 30m protective layer is reserved on the cavern wall excavation contour line. The protective layer excavation adopts segmented stripping, and the lower step steel support leg 9 is lengthened and installed to the foundation surface as the protective layer excavation is completed. Before the excavation of the lower half of the cave, 4 anchor bolts are arranged for each leg of the upper half of the cave steel support to lock. The anchor bolt is ø28mm, 5m long, and penetrates 4.8m into the bedrock. The tail of the anchor bolt is firmly welded to the steel support leg. Specifically, the following steps are included: e1: The locking foot anchor drilling of the upper step steel support leg is constructed by grouting first and then inserting the anchor. The borehole diameter is ø50m, and cement slurry is used for grouting with a water-cement ratio of 0.38-0.42. The grouting pipe is gradually lifted from the bottom of the hole to the hole mouth until the hole mouth returns to grout. After the hole mouth returns to grout, the anchor is sent to the bottom of the hole with the assistance of a manual hand pneumatic drill, and then the hole mouth is firmly fixed with a steel wedge; e2: The tail of the locking foot anchor 8 is firmly welded to the steel support column leg; e3: The horizontal blasting hole is used for drilling, the borehole diameter is ø40mm, and the bottom hole has a downward external insertion angle of 30°; e4: The blasting hole is divided into sections from top to bottom, and then the surrounding holes and the last bottom holes are blasted in sequence, and the slag is loaded and transported out of the hole; e5: A small cannon is used in conjunction with a pneumatic draft to peel off the protective layer; e6: The steel support column legs are installed, and the locking foot anchor is arranged for fixing; e7: Hang a mesh and spray concrete on the side walls to form a support system with alternating beams and slabs; e8: Repeat steps e1-e7, with the lower step lagging behind the upper step by 1-2 cycles to form a flow operation follow-up.

[0020] The present invention adopts rotary jet grouting for shallow buried and saturated water covering layer, and forms mutually interlocking and overlapping rotary jet consolidation piles at the top of the tunnel. The stability of the rotary jet consolidation piles and the clamping effect on the soil between the piles ensure that after the shallow buried tunnel is excavated, the surrounding rock of the cavern has a strong self-stabilizing ability, so that the tunnel construction can proceed smoothly; after the 3-5m thick rotary jet consolidation body is formed at the top of the tunnel, the stratum structure is improved, the pores of springs and gravel are blocked, and the surface water is prevented from entering the tunnel, ensuring that the surface ecological environment is not affected during the tunnel excavation. The support structure with alternating beams and slabs is formed by steel support, steel scaffolding, and mesh sprayed concrete, which not only strengthens the rigidity of the spray protection structure, but also suppresses the excessive deformation of the surrounding rock. The improved covering layer and the step method are used to excavate shallow tunnels, which avoids the settlement deformation directly caused by excavation, the flexible deformation of the support system, and the overall deformation and displacement of the structure caused by the sinking of its foundation. The tunnel construction is safe and controllable, and the occurrence of tunnel collapse is prevented.

[0021] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection of the invention.

Claims

1. A method for constructing a shallow buried tunnel in an overburden layer. It is characterized in that The method comprises the following steps: a: forming a solidified hard shell within 3-5m of the top of the tunnel, and improving the surrounding rock and soil at the top of the tunnel into a consolidated body with corresponding strength, so as to meet the self-stabilizing ability of the hard shell and the advance anchor rod to jointly bear the upper loose body load after the tunnel excavation; in the step a, the hard shell is reinforced by rotary jet grouting of the covering layer, wherein the row spacing between the rotary jet reinforcement of the covering layer is 1.0m, 10 rows of holes are arranged vertically to the axis of the tunnel, and the consolidation range is arranged on both sides of the tunnel excavation range, extending 1.0m on each side, and extending 2.0m along the axis of the tunnel; the high-pressure jet grouting arch depth does not exceed 20cm, and when encountering rocks before reaching the top of the tunnel, it penetrates 50cm into the rock layer; the high-pressure jet grouting drilling diameter is not less than 110mm; the jet grouting adopts 425 ordinary silicate cement with a water-cement ratio of 1:1; b: Organize support construction by using the New Austrian Tunneling Method principle to form a composite support system of advanced anchor scaffolding, mesh shotcrete and steel concrete to jointly bear the load above the tunnel top; c: The tunnel excavation adopts the "step method" excavation and shaping, wherein the core soil is reserved for the upper step, and the space for erecting steel supports is formed by trenching; d: The New Austrian Tunneling Method is used to support the surrounding rock of the upper step, forming an advanced anchor scaffolding, mesh shotcrete and steel concrete composite support system; in the step d, the upper step support includes the following steps: d1: Drilling holes for the steel scaffolding using the top of the steel support, and using a hand drill to send the steel bars into the holes; d2: Welding the tail of the steel bar scaffolding to the steel support, so that one end of the steel scaffolding is deeply embedded in the surrounding rock soil in front of the face of the siege, and the other end is welded to the steel support to form a simply supported beam to support the surrounding rock soil on the upper part of the vault; d3: Arrange peripheral holes and caving holes along the excavation contour to complete the splitting and loosening of the rock in the cavern excavation; d4: Remove the loose blocks on the surface of the surrounding rock; d5: Use one-time spraying protection on the surface above the spandrel and the arch ring, with a thickness of 3-4cm of plain sprayed concrete, and then remove the slag; d6: Use wind draft to chisel off the protruding rock surface and install the steel support in place; d7: Set up the steel support, hang mesh on the side wall and top arch to spray concrete, and close the steel support to form a supporting structure with beams and slabs; d8: Repeat steps d1-d7, operate at a feed rate of 1.0m per cycle, and advance the lower step by 1 cycle; d9: After excavating the upper step for 2 cycles, excavate the lower step; e: The core soil of the lower step and the upper step is excavated by blasting, and the space between the core soil and the steel support is used to buffer the blasting seismic waves; in the step e, the lower step support specifically includes the following steps: e1: The upper step steel support leg locking foot anchor drilling is constructed by grouting first and then inserting the anchor, the borehole diameter is ø50mm, and cement slurry is used for grouting with a water-cement ratio of 0.38-0.

42. The grouting pipe is gradually lifted from the bottom of the hole to the hole mouth until the hole mouth returns to grout. After the hole mouth returns to grout, the anchor is sent to the bottom of the hole by a manual assisted hand pneumatic drill, and then the hole mouth is fixed firmly with a steel wedge; e2: The tail of the locking foot anchor is welded firmly to the steel support column leg; e3: Horizontal blasting hole drilling is adopted, the borehole diameter is ø40mm, and the bottom hole downward external insertion angle is 30°; e4: According to the blasting hole segmentation from top to bottom, and then the surrounding holes and the last bottom hole blasting sequence, the micro-difference blasting operation is carried out in segmentation, and the slag is transported out of the hole; e5: A small cannon is used in combination with a pneumatic draft to peel off the protective layer; e6: Install the steel support column legs and arrange the locking anchor rods for fixing; e7: Hang the mesh and spray the concrete on the side walls to form a support system with alternating beams and slabs; e8: Repeat steps e1-e7, with the lower step lagging behind the upper step by 1-2 cycles to form a flow operation follow-up.

2. The shallow buried tunnel construction method of the overburden layer according to claim 1, It is characterized in that The construction method of the hard shell specifically includes the following steps: a1: level the site and excavate the mud trench according to the construction requirements; a2: measure and lay out the lines, and make positioning marks for each hole on the construction site to ensure that the drilling deviation is no more than 5 cm; a3: the clay stratum adopts the original soil slurry wall drilling process; the gravel stratum adopts the bentonite mud wall drilling process; a4: after the drilling is completed, the hole position, hole depth and bottom stratum of the hole are reviewed and accepted; a5: before the jet grouting operation, the assembled sprayer is first tested for the unobstructed and pressure-bearing conditions of the water, air and slurry pipes. When the water pressure reaches 1.5 times the design pressure and there is no leakage in the pipeline, test spray for 15 minutes and end the test spray inspection; a6: after the test spray inspection is completed, rotate the sprayer down to the designed hole depth; a7: during the initial spraying, only spray without lifting, spray statically for 3 to 5 minutes, and wait until the slurry concentration at the hole mouth reaches 1.5g / cm 3 When the grouting is done, the grouting is sprayed from bottom to top according to the lifting speed and rotation speed required by the parameters to reach the high-pressure spraying design elevation in the hole; a8: When grouting is interrupted, the grouting pipe should be lowered into the sprayed section again, and the overlap length of the sprinkler should not be less than 100mm; a9: When the designed pile top height is reached or overflow occurs on the ground, stop the rotary spraying of the current pile, pull out the rotary spraying pipe and clean the pipeline; a10: Repeat steps a3-a9 and proceed to the next hole operation.

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