Construction Method For Entering Shallow-buried Multi-arch Tunnel Under Water-rich Geological Conditions

Abstract

A construction method for a shallow-buried multi-arch tunnel under water-rich geological conditions includes the following steps: cleaning a grouting ground surface; marking a grouting reinforcement scope; performing survey setting-out; drawing a cross-section diagram to scale; calculating out coordinates and angles of anchor points that need to be set; marking drilling positions; nailing small wooden piles at the drilling positions for identifying; determining a setting depth of the anchor rods according to ground elevation; drilling holes, cleaning bottom of hole, grouting, performing construction preparation, performing long pipe shed construction at the entrance after an earth-rock of a tunnel entrance and an open cut tunnel is excavated to flush with a springing line of the tunnel. A down-the-hole drill is used for drilling in construction. Long pipe shed grouting is designed based on solidifying a soil mass in limited scope around a consolidation pipe shed.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This patent application claims the benefit and priority of Chinese Patent Application No. 202010465066.3, entitled "Construction Method for Multi-arch Tunnel Entrance under Shallow-buried and Water-rich Geological Conditions" filed on May 28, 2020, the disclosure of which is incorporated by reference herein in its entirety as part of the present application.

BACKGROUND OF THE INVENTION

Field of the Invention

[0002] The present disclosure relates to the technical field of tunnel engineering, in particular, to a construction method for a multi-arch tunnel entrance under shallow-buried and water-rich geological conditions.

Description of the Related Art

[0003] Shallow-buried tunnel is a tunnel engineering under specific conditions. The buried depth is less than twice the diameter of the tunnel. Its construction is not only restricted by geological factors of overburden, but also affected by ground environment. The shallow-buried tunnel does not simply mean that the thickness of a stratum over the top of the tunnel is small, but also should be comprehensively determined in combination with engineering geological conditions of the overburden, including the factors of structural characteristics of surrounding rock, the influence degree of weathering, fracture, and fault, the structural strength, the loose condition, and underground water, etc.

[0004] The tunnel site area of a certain tunnel belongs to low mountains and hills landforms. The structural trace is mainly manifested by the development of joints and fissures, so the geological conditions are poor. The surrounding rocks of fully weathered mixed granites at the entrance and the exit of the tunnel are at Grade V. The fully weathered mixed granites at the entrance of the tunnel is thick and loose in structure, and easily collapses, so that the stability of a rock-soil mass of an face upward slope at the entrance of the tunnel is extremely poor, which is extremely easy to cause shallow engineering landslide or large collapse during construction and excavation, and the quaternary eluvial slope layer at the upper part of the exit of the tunnel is thick and is loose in structure. A strongly weathered rock mass is broken under the influence of joints, fissures and weathering, and the broken rock mass is easy to be soften in case of water, which results in rapid reduction of strength of the rock mass. It is easy to cause large collapse of shallow engineering during construction and excavation. It is necessary to remove residual slope sediments from the ground surface to perform measures of spraying slurry and stone pitching on the surface of strongly weathered rock when the tunnel is constructed. Therefore, higher requirements are put forward for construction methods.

[0005] A shallow-buried tunnel entrance is always a crucial link in tunnel construction. During the tunnel entrance, after tunnel excavation, the ground surface settlement and the stability of the surrounding rock in the tunnel are generally controlled by using large pipe shed pre-grouting. However, it is very difficult to control ground surface settlement and the stability of the surrounding rock in the tunnel effectively by only using pipe-shed support when the geological conditions at the tunnel entrance are complex, the weathering of the surrounding rock is sever, and a tunnel entrance is affected by adverse factors, such as shallow burial and rich water.

BRIEF SUMMARY OF THE INVENTION

[0006] In view of the above-mentioned situations, in order to overcome the defects of the prior art, some embodiments provides a construction method for multi-arch tunnel entrance under shallow-buried and water-rich geological conditions, which solves the problems that it is very difficult to control ground surface settlement and the stability of surrounding rock in a tunnel effectively by using pipe-shed support when a tunnel entrance is affected by adverse factors, such as shallow burial and rich water, due to complex geological conditions at the tunnel entrance and severe weathering of the surrounding rock.

[0007] The technical solution for solving the problems is a construction method for an entrance of a multi-arch tunnel under shallow-buried and water-rich geological conditions. The construction method includes the following steps:

[0008] a cleaning step for cleaning a ground surface to be grouted, including removing debris that obstruct grouting from the ground surface of a slopes to be grouted, without destroying vegetation on the ground surface of the slopes as far as possible, so as to maintain natural appearance of a mountain;

[0009] a marking step for marking a grouting reinforcement range, including: performing analysis and calculation to obtain the grouting reinforcement range at the entrance of the tunnel that is characterized by: [0010] a longitudinal length of the tunnel of 18 m; [0011] a width extending leftward from a center line of a left main hole of the tunnel is 5 m; [0012] a width extending rightward from a center line of a right main hole of the tunnel is 5 m; and [0013] a depth of 0.5 m distanced from a top of arches of the tunnel, where a length of a pipe shed is 16 m;

[0014] a surveying and setting out step, configured for performing surveying and setting out at corresponding positions on the ground surface of a top of the tunnel according to a circumferential position spacing 100 cm and a longitudinal position spacing 60 cm of radial anchor rods at the entrance of the tunnel in a design drawing, where a cross-section diagram is drawn to scale, coordinates and angles of the positions at which the anchor rods are to be driven are calculated, the positions are marked; small wooden piles are nailed at the positions for identifying; and a depth of a portion of the anchor rods to be driven into is determined according to ground elevation;

[0015] a drilling holes step, configured for drilling at circulation break by using an engineering drilling rig, where the drilling is performed at the positions by using a down-the-hole drill, a drilling angle is adjusted before drilling, and the drilling angle is checked frequently during drilling;

[0016] a sweeping the holes step, configured for sweeping the holes, where grouting is performed in a manner of sweeping each hole twice and grounding the hole sectionally so as to ensure a effect of the grouting, after the hole is drilled, the hole is swept once by way of withdrawing a drill pipe to clean the hole, then the drill pipe is inserted to the bottom of the hole to sweeping the hole once again;

[0017] a grouting step, configured for grouting the ground surface, where several representative field tests are performed on performance of slurries with various mix proportions so as to select an optimal mix proportion suitable for the grouting of the tunnel, and setting time, tensile strength, and flexural strength are compared mainly among the tests;

[0018] a construction preparation step, configured for constructing drainage systems at the entrance after the grouting is completed, where the intercepting ditches at tops of the slopes are arranged not less than 5 m away from tops of the slopes including side slops and front slopes, slope ratios of the intercepting ditches are set according to terrain, and a drainage slope ratio is not less than 3%, so as to avoid sedimentation;

[0019] a cover arch construction step configured for constructing the cover arch, where a long pipe shed construction at the entrance after earth rocks at the entrance and an open cut tunnel is excavated to flush with a springing line of the tunnel, the cover arch adopts a 2 m long C25 concrete cover arch for orientation of the pipe shed, a base of the cover arch is placed on a stable foundation, a concrete guide wall of the cover arch is construed outside a profile of the open cut tunnel; four sections of I16 I-steel beam are arranged in the guide wall, the I-steel beam sections are bent into arcs according to design sizes, and adjacent sections are connected by using bolts to form a semicircle steel frame; orifice pipes of -.sctn. 114 are mounted and fixed at the steel frame after the I-steel beam sections are erected and fixed at the entrance; an internal framework and an external framework are erected for pouring concrete, the cover arch is 60 cm thick;

[0020] a driving step is configured for driving steel pipes into a surrounding rock at an angle of one degree greater than a angle of longitudinal slope of the tunnel along a periphery of the tunnel, where the down-the-hole drill is adopted to drilling and the circumferential spacing of the steel pipes is 40 cm, a vertical axis direction of the down-the-hole drill is controlled accurately to ensure correct hole direction at the orifice, and one steel pipe is driven once one hole is drilled;

[0021] a long pipe shed grouting step configured for performing long pipe shed grouting according to a design of a soil mass in a limited range around a pipe shed being consolidated, where a diffusion radius of the slurry is not less than 0.5 m; grouting is performed sectionally; single fluid grouting is adopted for grouting; a test for single fluid grouting is performed before grouting; a grouting ending standard is that a slurry amount grouted into a single pipe reaches a predetermined grouting amount; when the slurry amount grouted does not reach the predetermined grouting amount after the grouting pressure has reached a predetermined final pressure for 10 minutes, the grouting is ended.

[0022] The present disclosure is novel in concept, ingenious in structure, and high in practicability. The present disclosure solves the problems that it is very difficult to control the ground surface settlement and the stability of the surrounding rock in a tunnel effectively by using pipe-shed support when the tunnel entrance is affected by adverse factors, such as shallow burial and rich water, due to complex geological conditions at the tunnel entrance and severe weathering of the surrounding rock.

[0023] Additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The aspects of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0024] The accompanying drawings, which are incorporated in and constitute part of this specification, illustrate embodiments of the invention and together with the description, serve to explain the principles of the invention. The embodiments illustrated herein are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown, wherein:

[0025] FIG. 1 is an arrangement diagram of a cross section of a perforated steel pipe of the present disclosure.

[0026] FIG. 2 is a diagram of an anchor rod arrangement position of the present disclosure.

[0027] FIG. 3 is a flowchart of performing a grouting process under a pressure from the ground surface by longitudinal anchor rods according to the present disclosure.

[0028] FIG. 4 is a construction flowchart of a long pipe shed of the present disclosure

DETAILED DESCRIPTION OF THE INVENTION

[0029] Specific embodiments of present disclosure will be further described below in detail with reference to the accompanying drawings.

[0030] The present disclosure includes the following construction steps.

[0031] In step S1, a ground surface to be grouted is cleaned. Specifically, debris on a surface of slope that obstructs the grouting is removed. During cleaning, vegetation on the surface should not be destroyed as far as possible, so as to maintain the natural appearance of the mountain.

[0032] In step S2, a grouting reinforcement range is marked. Specifically, through analysis and calculation, a grouting layout range at a tunnel entrance is defined as follows: a longitudinal length of the tunnel is 18 m, where a length of a pipe shed is 16 m; a width extending leftward from a center line of a left main hole of the tunnel is 5 m; a width extending rightward from a center line of a right main hole of the tunnel is 5 m; and a depth is about 0.5 m from a top of each arch of the tunnel;

[0033] In step S3, surveying and setting out are performed at corresponding positions on the ground surface of a top of the tunnel according to position spacing 100 o660 cm of radial anchor rods in the tunnel at an entrance section as shown in a design drawing. A cross-section diagram is drawn to scale to calculate coordinates of points at which anchor rods are to be driven and angles for driving the anchor rods at respective points. Drilling positions are marked on the ground surface, and small wooden piles are nailed at the drilling positions for identifying. Driven depths of the anchor rods are determined according to a ground elevation.

[0034] In step S4, holes are drilled without flushing medium by using an engineering drilling rig. Specifically, holes are drilled at grouting points by using a down-the-hole drill. Before drilling, a drilling angle is adjusted, and during drilling, the drilling angle is checked frequently, so as to prevent hole deviation.

[0035] In step S5, the holes are swept. Specifically, ground surface grouting is performed in a manner of sweeping holes twice and grounding sectionally in order to ensure a grouting effect. After the hole is drilled, a hole is swept once in a manner of withdrawing a drill pipe to clean the hole; and after the hole is cleaned, the drill pipe is inserted to the bottom of the hole, to sweeping the hole once again.

[0036] In step S6, grouting is performed. Specifically, in order to select an optimal mix proportion of slurry suitable for the ground surface grouting of the tunnel, several representative field tests are performed on the performance of the slurries with various mix proportions, where the setting times, tensile strengths, and flexural strengths among these grouts are compared mainly.

[0037] In step S7, construction preparation is preformed Specifically, after the ground surface grouting is completed, drainage systems such as an intercepting ditch at the entrance are constructed, where the slope top intercepting ditch is arranged not less than 5 m away from the tops of side slopes and a front slope, a slope ratio of the slope top intercepting ditch is set according to terrain, and drainage slope ratio is not less than 3%, so as to avoid sedimentation.

[0038] In step S8, long pipe shed construction at the tunnel entrance is performed after earth rocks of the entrance and an open cut tunnel is excavated to flush with a springing line of the tunnel, where a 2 m long C25 concrete cover arch is adopted for orientation of the pipe shed, and a base of the cover arch falls on a stable foundation. A concrete guide wall of the cover arch is constructed outside a profile of the open cut tunnel. Four sections of I16 I-steel beam are arranged in the guide wall, the I-steel beam sections are bent into arcs according to design sizes, and any adjacent sections are connected by using bolts to form a semicircle steel frame. Then, orifice pipes with -.sctn. 114 mm are mounted and fixed on the steel frame after the I-steel beam s are erected and fixed at the tunnel entrance, and an internal framework and an external framework are erected for pouring concrete, where the concrete cover arch is 60 cm thick.

[0039] In step S9, holes are drilled by using the down-the-hole drill during construction, the steel pipes are driven into the surrounding rock at an angle greater than an angle of a longitudinal slope of the tunnel by 1 .infin. along the periphery of the tunnel, where the circumferential spacing of the steel pipes is 40 cm, a direction of vertical axis of the down-the-hole drill is controlled accurately to ensure correct hole direction of the orifice, and one steel pipe is pushed in every time one hole is drilled. Further, deviation degree of the hole is checked at any time during drilling, and the deviation is corrected in time upon being checked out.

[0040] In step S10, the long pipe shed grouting is designed based on consolidation of soil mass in a limited range around the pipe shed, where the slurry diffusion radius is not less than 0.5 m, grouting is performed sectionally, and a single fluid grouting is adopted, which can simplify a process and reduce the cost, and is high in consolidation strength. So, a single fluid grouting test should be performed before grouting. The grouting ending standard is that a slurry amount grouted into a single pipe reaches a predetermined grouting amount, and when the slurry inlet amount is still not reached the predetermined grouting amount after the grouting pressure has reached a predetermined final pressure for 10 minutes, the grouting can also be ended. Records should be made seriously in a grouting operation, the operation is analyzed and improved at any time, and primary support and working face conditions are observed, so as to ensure the safety.

[0041] In step S2, materials are prepared, where the anchor rods in the tunnel are -.sctn. 25 mm hollow grouting anchor rods, the anchor rods outside the tunnel are replaced by grouting pipes, and the grouting pipes are -.sctn. 50 mm hot pressed seamless steel pipes with a wall thickness of 3.5 mm. Apertures are drilled in bodies of the steel pipes at a spacing of 15 cm, to form a quincunx, and a diameter of each aperture is 8 mm. The steel pipes are arranged in a quincunx shape at a longitudinal spacing of 1 m and a circumferential spacing of 0.6 m. A reinforcement stiffening hoop of -.sctn. 6 mm is welded at a tail of each steel pipe. A length of each steel pipe is determined according to actual situations on site, a front end of the steel pipe is processed into a cone, four rows of -.sctn. 8 mm grouting apertures are drilled in a periphery of a first pipe section of each grouting pipe within a consolidation range, and no aperture is drilled in a second pipe section of the pipe out of the consolidation range, so that the second pipe sections serve as slurry guide pipes for grouting.

[0042] In step S6, grouting material adopts single-fluid cement slurry, and cement in the single-fluid cement slurry adopts ordinary portland cement above 42.5R with high activity and a manufacture date no more than 3 months. In order to allow the grouting to be performed without a break, before grouting, a condition of a grouting pump, accessories, raw materials for preparing the slurry, and quality of the raw materials is checked carefully. The water-cement ratio is 0.5 and is adjusted properly according to actual situations.

[0043] In step S6, the grouting is performed sequentially. Specifically, holes are drilled and grouted in a construction sequence from two sides to a center in a transverse direction of the tunnel, and from low to high in a longitudinal direction of the tunnel. A space interval construction method is adopted in the same row in order to prevent slurry from channeling between adjacent construction holes.

[0044] In step S8, specification of the steel pipes of the pipe shed is that: the pipe shed is made by hot rolled seamless steel pipes with an outside diameter -.sctn. of 108 mm and a wall thickness of 6 mm. Front ends of the steel pipes are all in tip cone shape, and -.sctn. 10 stiffening hoops are welded at the tails of the steel pipes. Four rows of -.sctn. 12 mm grouting holes are drilled in the periphery of the pipe walls, drilled holes are spaced apart from each other at an interval of 15 cm and are arranged in quincunx. The steel pipes are driven into the surrounding rock at an angle greater than the angle of the longitudinal slope by 1 .infin.along the periphery of the tunnel. A total length of each perforated steel pipe is 18 m, and a section with a length 4.5 m at the tail end of the perforated steel pipe has no hole drilled therein, a length of the tip cone is 10 cm. Each steel pipe is consisted of multiple pipe segments, lengths of pipe segments is 3 m or 6 m. Both ends of each pipe segment are preprocessed into external threads. A number of joints of pipe segments in the same cross section of the pipe shed does not exceed 50% of the total number of the steel pipes; the first segment of each odd-numbered steel pipe adopts a 3 m perforated steel pipe segment, the first segment of each even-numbered steel pipe adopts a 6 m perforated steel pipe segment, and each of the following segments of the steel pipes adopts a 6 m perforated steel pipe segment. The last segment of each steel pipe driven into the surrounding rock is intercepted according to a length of the cover arch, so that a length of the steel pipe in soil is a designed length 16 m.

[0045] Compared with the prior art, the present disclosure has the following beneficial effects.

[0046] Upon construction personnel performing construction according to the present protection method, in order to make the concrete more compact after the concreted is poured by the pouring operation in step S5, a small amount of concrete is poured while being paved through a scraper, so that the concrete fills an internal framework of a middle partition wall, and the concrete in the middle partition wall is poured to the highest position during a pouring process. Therefore, the top of the middle partition wall is backfilled compactly from a side view after a main hole is excavated in a later stage, and settlement observation data shows that the top of the arch does not have obvious subsidence. The positions where the shoulders of the middle partition wall and the top of the arch are joined are sprayed and sealed by spraying the concrete in step S4 and step S5 to prevent slurry leakage in the grouting process. The slurry is grouted from a low position to a high position under a pressure, exhaust valves of a grouting pipe is opened before pressure grouting, and are closed one by one after thick slurry comes out. The slurry is grouted continuously under a pressure, and the framework in the middle partition wall is filled with concrete, so that the tunnel entrance is solid and firm, its structure is not easy to loose, and a collapse phenomenon is prevented. Further, the structure at the upper part of the tunnel entrance is still stable and compact, so a strongly weathered rock mass does not easily affected by joints, fissures, and weathering, and the phenomena of collapse, crumble, and block falling are not easily caused in an excavation process. In addition, the flying stone caused by blasting is not easy to cause great damage to the middle partition wall while impacting it, which improves the rigidity and the strength of the middle partition wall, without affecting the weight bearing performance of the tunnel, i.e., the middle partition wall of the multi-arch tunnel can be protected effectively, thereby further improving the stability of the multi-arch tunnel.

[0047] Of note, the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "includes", and/or "including," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

[0048] As well, the corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.

[0049] Having thus described the invention of the present application in detail and by reference to embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims as follows:

Claims

1. A construction method for entering a shallow-buried multi-arch tunnel under water-rich geological condition, comprising: a cleaning step for cleaning a ground surface to be grouted, comprising removing debris that obstruct grouting from the ground surface of a slopes to be grouted, without destroying vegetation on the ground surface of the slopes as far as possible; a marking step for marking a grouting reinforcement range, comprising: performing analysis and calculation to obtain the grouting reinforcement range at the entrance of the tunnel that is characterized by: a longitudinal length of the tunnel of 18 m; a width extending leftward from a center line of a left main hole of the tunnel is 5 m; a width extending rightward from a center line of a right main hole of the tunnel is 5 m; and a depth of 0.5 m distanced from a top of arches of the tunnel, wherein a length of a pipe shed is 16 m; a surveying and setting out step, configured for performing surveying and setting out at corresponding positions on the ground surface of a top of the tunnel according to a circumferential position spacing 100 cm and a longitudinal position spacing 60 cm of radial anchor rods at the entrance of the tunnel in a design drawing, wherein a cross-section diagram is drawn to scale, coordinates and angles of the positions at which the anchor rods are to be driven are calculated, the positions are marked; small wooden piles are nailed at the positions for identifying; and a depth of a portion of the anchor rods to be driven into is determined according to ground elevation; a drilling holes step, configured for drilling at circulation break by using an engineering drilling rig, wherein the drilling is performed at the positions by using a down-the-hole drill, a drilling angle is adjusted before drilling, and the drilling angle is checked frequently during drilling; a sweeping the holes step, configured for sweeping the holes, wherein grouting is performed in a manner of sweeping each hole twice and grounding the hole sectionally so as to ensure a effect of the grouting, after the hole is drilled, the hole is swept once by way of withdrawing a drill pipe to clean the hole, then the drill pipe is inserted to the bottom of the hole to sweeping the hole once again; a grouting step, configured for grouting the ground surface, wherein several representative field tests are performed on performance of slurries with various mix proportions so as to select an optimal mix proportion suitable for the grouting of the tunnel, and setting time, tensile strength, and flexural strength are compared mainly among the tests; a construction preparation step, configured for constructing drainage systems at the entrance after the grouting is completed, wherein the intercepting ditches at tops of the slopes are arranged not less than 5 m away from tops of the slopes comprising side slops and front slopes, slope ratios of the intercepting ditches are set according to terrain, and a drainage slope ratio is not less than 3%, so as to avoid sedimentation; a cover arch construction step configured for constructing the cover arch, wherein a long pipe shed construction at the entrance after earth rocks at the entrance and an open cut tunnel is excavated to flush with a springing line of the tunnel, the cover arch adopts a 2 m long C25 concrete cover arch for orientation of the pipe shed, a base of the cover arch is placed on a stable foundation, a concrete guide wall of the cover arch is construed outside a profile of the open cut tunnel; four sections of I16 I-steel beam are arranged in the guide wall, the I-steel beam sections are bent into arcs according to design sizes, and adjacent sections are connected by using bolts to form a semicircle steel frame; orifice pipes of .PHI.114 are mounted and fixed at the steel frame after the I-steel beam sections are erected and fixed at the entrance; an internal framework and an external framework are erected for pouring concrete, the cover arch is 60 cm thick; a driving step is configured for driving steel pipes into a surrounding rock at an angle of 1.degree. greater than an angle of longitudinal slope of the tunnel along a periphery of the tunnel, wherein the down-the-hole drill is adopted to drilling and the circumferential spacing of the steel pipes is 40 cm, a vertical axis direction of the down-the-hole drill is controlled accurately to ensure correct hole direction at the orifice, and one steel pipe is driven once one hole is drilled; a long pipe shed grouting step configured for performing long pipe shed grouting according to a design of a soil mass in a limited range around a pipe shed being consolidated, wherein a diffusion radius of the slurry is not less than 0.5 m; grouting is performed sectionally; single fluid grouting is adopted for grouting; a test for single fluid grouting is performed before grouting; a grouting ending standard is that a slurry amount grouted into a single pipe reaches a predetermined grouting amount; when the slurry amount grouted does not reach the predetermined grouting amount after the grouting pressure has reached a predetermined final pressure for 10 minutes, the grouting is ended.

2. The construction method according to claim 1, wherein in the marking step, materials are prepared, the anchor rods in the tunnel are .PHI.25 hollow grouting anchor rods; the anchor rods outside the tunnel are replaced by grouting pipes, wherein the grouting pipes are .PHI.50 mm hot pressed seamless steel pipes with the wall a thickness of 3.5 mm; apertures are drilled on bodies of the steel pipes at the spacing of 15 cm to form a quincunx; a diameter of each aperture is 8 mm; the steel pipes are arranged in a quincunx shape at a longitudinal spacing of 1 m and a circumferential spacing of 0.6 m; a .PHI.6 reinforcement stiffening hoops are welded at tails of the steel pipes; a length of the steel pipes is determined according to actual situations on site; front ends of the steel pipes each are processed into a cone; four rows of .PHI.8 mm grouting apertures are drilled in a periphery of a portion of the grouting pipes located in the grouting reinforcement range; no aperture is drilled in other portion of the grouting pipe located out of the grouting reinforcement range, to serve as slurry guide pipe for grouting.

3. The construction method according to claim 1, wherein in the grouting step, a material for grouting adopts single-fluid cement slurry; cement of the single-fluid cement slurry adopts ordinary portland cement above 42.5 R with high activity, and a manufacture date of the cement does not exceed 3 months; in order to allow the grouting to be performed without a break, a condition of a grouting pump, accessories, raw materials for preparing the slurry, and quality of the raw materials is checked before grouting, and a water-cement ratio is 0.5 and is adjusted properly according to actual situations.

4. The construction method according to claim 1, wherein in the grouting step, drilling and grouting are performed in a construction sequence of from both sides to a center in a transverse direction of the tunnel, and from low to high in a longitudinal direction of the tunnel; an interval construction method is adopted in a same row to prevent the slurry from channeling between adjacent construction holes.

5. The construction method according to claim 1, wherein in the cover arch construction step, specification of the steel pipes in the pipe shed is that: the pipe shed is made by adopting hot rolled seamless steel pipes with an outside diameter .PHI. of 108 mm and a wall thickness of 6 mm; front ends of the steel pipes are tip cones; .PHI.10 stiffening hoops are welded at tails of the steel pipes; four rows of .PHI.12 mm grouting apertures are drilled in a periphery of a pipe wall of each steel pipe; drilled apertures are spaced apart at a spacing of 15 cm and are arranged in quincunx; the steel pipes are driven into the surrounding rock at an extrapolation angle of 1.degree. along the periphery of the tunnel; a total length of the steel pipes is 18 m, and a portion with a length 4.5 m at the tail end of each steel pipe has no aperture drilled therein; a length of the tip cone is 10 cm; the steel pipe comprises a plurality of pipe segments, a length of each segment is 3 m or 6 m, both ends of each segment of the steel pipe are preprocessed into external threads; a number of joints of the pipe segments in a same cross section does not exceed 50% of a total number of the steel pipes; a first segment of each odd-numbered steel pipe adopts a 3 m perforated steel pipe segment, and a first segment of each even-numbered steel pipe adopts a 6 m perforated steel pipe; each of segments following the first segment adopts a 6 m perforated steel pipe; a last section of each steel pipe driven into the surrounding rock is intercepted according to a length of the cover arch, so that a length of the steel pipe in soil is a predetermined length 16 m.

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