Large-span shallow-buried bias tunnel double-side-wall pilot tunnel construction method

Abstract

The invention discloses a large-span shallow-buried bias tunnel double-side-wall pilot tunnel construction method, which comprises the following steps of: dividing a tunnel section into a plurality of pilot tunnel parts, sequentially excavating each pilot tunnel part according to the sequence and the excavation length, and simultaneously carrying out primary support and temporary support construction after each pilot tunnel part is excavated, so that ground surface settlement and primary support deformation are effectively avoided; the safety and the self-stability of tunnel construction are improved; after the initial support of the tunnel is closed to form a ring, the inverted arch secondary lining and the filling layer are constructed and reach the preset strength, the temporary support is dismantled section by section for the pilot tunnel section which is more than 50m away from the tunnel face, so that the influence of surrounding rock deformation can be reduced, the risk of cracking of the inverted arch secondary lining is reduced, and the stability of the initial support and the surrounding rock structure is improved; and the safety of dismantling the temporary support is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of tunnel engineering, and particularly to a construction method for a double-side drift method in a large-span shallow-buried and eccentrically loaded tunnel. Background Art

[0002] With the rapid development of the national economy and the increasing demand for people's transportation, the density of the highway network has been continuously improved, and the number of highway lanes has also increased accordingly. Highway construction has gradually shifted from plain areas to mountainous and hilly areas, and more and more tunnels with large spans and poor surrounding rock geological conditions have emerged. Due to the complex terrain and geology and the characteristics of ultra-shallow burial depth of the tunnel, there are significant differences in the tunnel support structure and stress conditions compared with conventional deep-buried tunnels. The double-side drift method is applied to large-span shallow-buried tunnels due to its advantages such as controlling ground settlement and reducing construction safety risks. Most of the construction research on the double-side drift method focuses on the finite element simulation analysis of the mechanical properties of the support structure and the optimization of the excavation sequence. There is little research on the removal of the temporary support in the double-side drift method. The removal of the temporary support is a crucial link in the construction of the double-side drift method, which is closely related to the overall stress of the double-side drift method, construction safety, and construction efficiency.

[0003] In the removal of the temporary support in the double-side drift method, the Chinese invention patent with the publication number of CN 113700510 B discloses a "method for removing the temporary support in the double-side drift method", which discloses a method for removing the temporary support in a skip-joint manner. However, skip-joint removal requires precise construction sequence and on-site coordination. If the construction sequence or interval control is improper, it will lead to uneven distribution of the temporary support and stress concentration in local areas. In soft surrounding rock or high-stress areas, it will exceed the bearing capacity of the primary support and increase the risk of tunnel collapse.

[0004] For large-span shallow-buried and eccentrically loaded tunnels, due to the existence of asymmetric loads, it is necessary to strictly control the removal timing and sequence of the temporary support. Moreover, the entrance of the large-span shallow-buried and eccentrically loaded tunnel is a weak part, and its safety risk is higher than that of the tunnel body. If the excavation construction of the double-side drift method and the removal timing of the temporary support are not well controlled, it will affect the construction safety and efficiency of the tunnel. For example, if the removal time of the temporary support is too early, it will affect the construction safety of the tunnel; if the removal time of the temporary support is too late, it will interfere with the subsequent construction and affect the construction efficiency. The removal of the temporary support is not only closely related to the construction progress of the tunnel, but also leads to stress redistribution of the surrounding rock and transformation of the support structure stress system. If the removal plan is inappropriate, it will cause excessive deformation of the tunnel primary support, and in severe cases, it will even trigger engineering accidents such as collapses.

[0005] In the construction of a large-span shallow-buried and bias-pressure tunnel by the double-side drift method, it is a key issue in the construction of the double-side drift of the large-span shallow-buried and bias-pressure tunnel to optimize the construction procedures of excavation and the removal of temporary supports, grasp the removal timing and methods of temporary supports, and safely, reasonably and quickly remove the temporary supports to ensure the construction safety and improve the construction efficiency of the double-side drift of the large-span shallow-buried and bias-pressure tunnel. Summary of the Invention

[0006] One of the purposes of the present invention is at least to provide a construction method for the double-side drift of a large-span shallow-buried and bias-pressure tunnel, which can optimize the construction procedures of excavation and the removal of temporary supports, ensure the construction safety of the large-span shallow-buried and bias-pressure tunnel, and improve the construction efficiency, aiming at how to overcome the problems existing in the above-mentioned prior art.

[0007] In order to achieve the above purpose, the technical solutions adopted by the present invention include the following aspects.

[0008] A construction method for the double-side drift of a large-span shallow-buried and bias-pressure tunnel includes the following steps: Step 1, construction preparation, and conduct advanced geological prediction on the preset construction area; Step 2, construct the advanced support small pipes of the left drift part 1 and grout; Step 3, excavate the left drift part 1 with a footage of the spacing of one arch frame, and construct the primary support 1, temporary vertical strut 1 and temporary cross strut 1; Step 4, after lagging behind the left drift part 1 by 3m, excavate the left drift part 2 with a footage of the spacing of one arch frame, construct the primary support 2 and temporary vertical strut 2, reinforce the middle rock pillar in the left drift part 2, and close the invert primary support 1; Step 5, construct the advanced support small pipes of the right drift part 3 and grout; Step 6, after lagging behind the left drift part 1 by 15m, excavate the right drift part 3 with a footage of the spacing of one arch frame, construct the primary support 3, temporary vertical strut 3 and temporary cross strut 2; Step 7, after lagging behind the right drift part 3 by 3m, excavate the right drift part 4 with a footage of the spacing of one arch frame, construct the primary support 4 and temporary vertical strut 4, and close the invert primary support 2; Step 8, construct the advanced support small pipes of the middle drift part 5 and grout; after lagging behind the right drift part 3 by 3 - 5m, excavate the middle drift part 5 with a footage of the spacing of one arch frame, and construct the arch primary support; Step 9, after lagging behind the middle drift part 5 by 30 - 50m, excavate the middle drift part 6 and the middle drift part 7 with a footage of the spacing of one arch frame, construct the invert primary support 3 to make the primary support closed into a ring, and then construct the invert secondary lining and fill the invert filling layer; Step 10: After the tunnel monitoring and measurement deformation is stable, for the pilot drift section more than 50 m away from the heading face, gradually remove the corresponding temporary cross braces and temporary vertical braces filled with invert filling layer section by section, hang waterproof board and non-woven geotextile section by section, bind the secondary lining steel bars, and cast the secondary lining concrete by formwork.

[0009] Preferably, during the construction of the temporary vertical brace 1 and the temporary vertical brace 3, advanced horizontal grouting small pipes are constructed along the height directions of the temporary vertical brace 1 and the temporary vertical brace 3 respectively; the advanced horizontal grouting small pipes are arranged from the tops of the temporary vertical brace 1 and the temporary vertical brace 3 to the bottoms, extend downward from the preset fifth part of the middle pilot drift to the sixth part of the middle pilot drift, and extend downward a certain distance from the sixth part of the middle pilot drift.

[0010] Preferably, during the excavation of the sixth part and the seventh part of the middle pilot drift in Step 9, the seventh part of the middle pilot drift is excavated after the sixth part of the middle pilot drift is excavated for 3 - 5 m. After the excavation of the sixth part of the middle pilot drift, a temporary cross brace 3 is set at the bottom of the sixth part of the middle pilot drift.

[0011] Preferably, in Step 10, when the temporary cross braces and temporary vertical braces are removed for the first time, a 5 - m long removal section is selected along the tunnel axis as the test section. After the monitoring and measurement data of the tunnel meet the requirements after the temporary cross braces and temporary vertical braces in the test section are removed, the length of each subsequent formal removal section is 7.5 m; when the temporary cross braces and temporary vertical braces in the test section and the formal removal section are removed, it is carried out in two steps. In the first step, 3.5 m is removed, and in the second step, the remaining part is removed. During the removal of the temporary cross braces and temporary vertical braces in each step, the tunnel is monitored and measured.

[0012] Preferably, during the removal of the steel frames of the temporary vertical braces, the steel frames are first trial - removed in the way of removing 1 every 3. One first trial - removed steel frame is selected every 3 steel frames, and a 2 - 3 - cm cut is made at the top of the first trial - removed steel frame. Observe the monitoring and measurement data of the tunnel. If the monitoring and measurement data meet the requirements, the steel frames are trial - removed in the way of removing 1 every 1. If the requirements are not met, reinforcement measures are taken for the top of the first trial - removed steel frame, and the monitoring and measurement continue until the monitoring and measurement data meet the requirements; during the trial - removal of the steel frames in the way of removing 1 every 1, one second trial - removed steel frame is selected every 1 steel frame, and the top of the second trial - removed steel frame is completely cut. Observe the monitoring and measurement data of the tunnel. If the monitoring and measurement data meet the requirements, the second trial - removed steel frame is removed; if the monitoring and measurement data do not meet the requirements, the second trial - removed steel frame is reinforced in the same way as in the case of removing 1 every 3.

[0013] Preferably, the monitoring and measurement data include one or more of crown settlement, horizontal convergence, ground surface convergence, and ground surface displacement.

[0014] Preferably, the settlement rate of the crown settlement does not exceed 0.1 mm / d, and the convergence rate of the horizontal convergence does not exceed 0.2 mm / d.

[0015] Preferably, during the demolition process of the temporary vertical struts and temporary horizontal struts, the temporary horizontal struts are demolished first and then the temporary vertical struts; during the demolition of the temporary horizontal struts, the temporary horizontal strut one and the temporary horizontal strut two are demolished successively; when there is a temporary horizontal strut three between the middle pilot tunnel part six and the middle pilot tunnel part seven, the temporary horizontal strut three is demolished first, and then the temporary horizontal strut one and the temporary horizontal strut two are demolished successively.

[0016] Preferably, both the temporary horizontal struts and the temporary vertical struts include steel frames, shotcrete and steel meshes, and the steel frames, shotcrete and steel meshes are integrally connected into a side wall structure.

[0017] Preferably, during the demolition process of the temporary horizontal struts and the temporary vertical struts, the shotcrete is removed by means of pneumatic picks. When removing the shotcrete of the temporary vertical struts, it is carried out successively for each ring of arch frames from top to bottom; when removing the shotcrete of the temporary horizontal struts, the shotcrete of the corresponding temporary horizontal struts is removed respectively in the left pilot tunnel part one, the middle pilot tunnel part six and the right pilot tunnel part three, or scaffolds are erected respectively in the left pilot tunnel part two, the middle pilot tunnel part seven and the right pilot tunnel part four, and then the corresponding shotcrete is removed with the help of the scaffolds and pneumatic picks.

[0018] In summary, due to the adoption of the above technical solutions, the present invention has at least the following beneficial effects: By dividing the tunnel section into multiple pilot tunnel parts and excavating each pilot tunnel part in sequence and according to the excavation length, and carrying out the initial support and temporary support construction simultaneously after each pilot tunnel part is excavated, it can effectively avoid surface settlement and deformation of the initial support, and improve the safety and self-stability of tunnel construction; after the initial support of the tunnel is closed into a ring, the construction of the inverted arch secondary lining and the filling layer is completed and reaches the preset strength, for the pilot tunnel section more than 50 m away from the tunnel face, gradually removing the temporary support can reduce the influence of surrounding rock deformation, reduce the risk of cracking of the inverted arch secondary lining, improve the stability of the initial support and the surrounding rock structure, and improve the safety during the removal of the temporary support.

[0019] During the removal of the temporary support, a test section is selected for removal. After the monitoring and measurement data after the removal of the temporary support in the test section meet the requirements, the removal of each formal removal section is carried out in a distributed manner, which can ensure the safety during the removal of the temporary support. The length of each formal removal section is not greater than 7.5 m, which can avoid sudden changes in the forces on the surrounding rock and the initial support caused by too long removal length, and is beneficial to ensuring the stability of the surrounding rock and the initial support.

[0020] During the removal of the temporary support, first, the steel frames are tentatively removed in the way of removing one every three. Cut 2 - 3 cm at the connection part between the first tentatively removed steel frame and the primary support, and observe the monitoring and measurement data of the tunnel. If the monitoring and measurement data meet the requirements, then the steel frames are tentatively removed in the way of removing one every one. Cut all the connection parts between the second tentatively removed steel frame and the primary support, and observe the monitoring and measurement data of the tunnel. If the monitoring and measurement data meet the requirements, remove the second tentatively removed steel frame; during the removal of the temporary support, some steel frames can still play a supporting role, which is beneficial to avoiding sudden changes in the forces on the primary support and the surrounding rock, and further improving the safety of tunnel construction. Brief Description of the Drawings

[0021] Figure 1 is the construction flow chart of the double - side drift method for a large - span shallow - buried and eccentrically - loaded tunnel in an exemplary embodiment of the present invention.

[0022] Figure 2 is the cross - sectional view of the double - side drift method construction in an exemplary embodiment of the present invention.

[0023] Figure 3 is the longitudinal - sectional view of the double - side drift method construction in an exemplary embodiment of the present invention.

[0024] Figure 4 is the schematic diagram of the removal sequence of the steel frames of the temporary support in an exemplary embodiment of the present invention.

[0025] Identifications in the figures: 1 - First part of the left drift, 2 - Second part of the left drift, 3 - Third part of the right drift, 4 - Fourth part of the right drift, 5 - Fifth part of the middle drift, 6 - Sixth part of the middle drift, 7 - Seventh part of the middle drift, 8 - Inverted arch filling layer, 9 - First temporary cross - brace, 10 - Second temporary cross - brace, 11 - Third temporary cross - brace, 12 - First temporary vertical brace, 13 - Second temporary vertical brace, 14 - Third temporary vertical brace, 15 - Fourth temporary vertical brace, 16 - First primary support, 17 - Second primary support, 18 - First inverted arch primary support, 19 - Third primary support, 20 - Fourth primary support, 21 - Second inverted arch primary support, 22 - Primary support of the arch part, 23 - Third inverted arch primary support, 24 - Advance support small - diameter pipe, 25 - Advance horizontal grouting small - diameter pipe, 26 - Lock - foot anchor pipe, 27 - Grouting bolt, 28 - Second lining of the inverted arch, 29 - Second lining of the arch and wall, 30 - Face, 31 - Grouting small - diameter pipe of the arch part. Detailed Description of the Embodiment

[0026] The present invention will be further described in detail below in conjunction with the drawings and embodiments, so as to make the purpose, technical solution and advantages of the present invention clearer. It should be understood that the specific embodiments described here are only used to explain the present invention and are not used to limit the present invention.

[0027] The large-span shallow-buried and eccentrically loaded tunnel of the exemplary embodiment of the present invention is a two-way eight-lane separated small clear distance tunnel with a cavity clearance of 18.25×5.0 m. The starting and ending mileage of the tunnel: the left tunnel is from ZK8+300 to ZK8+560, with a total length of 260 m and a maximum buried depth of 58.18 m; the right tunnel is from YK8+300 to YK8+560, with a total length of 260 m and a maximum buried depth of 52.49 m. The right tunnel has a shallow-buried and eccentrically loaded section and adopts an S-M type open cut tunnel lining structure, and the lining form is a composite lining form.

[0028] Reference Figure 2 , in the single-hole excavation of the large-span shallow-buried and eccentrically loaded tunnel of the present invention, the tunnel section pilot tunnels from left to right are the left pilot tunnel (including the upper and lower arranged left pilot tunnel part 1 and left pilot tunnel part 2), the middle pilot tunnel (including the upper, middle and lower arranged middle pilot tunnel part 5, middle pilot tunnel part 6 and middle pilot tunnel part 7) and the right pilot tunnel (including the upper and lower arranged right pilot tunnel part 3 and right pilot tunnel part 4); a temporary cross brace 9 is arranged between the left pilot tunnel part 1 and the left pilot tunnel part 2, a temporary cross brace 10 is arranged between the right pilot tunnel part 3 and the right pilot tunnel part 4, and a temporary cross brace 11 is arranged between the middle pilot tunnel part 6 and the middle pilot tunnel part 7. The temporary cross brace 9, the temporary cross brace 10 and the temporary cross brace 11 are in the same plane. A temporary vertical brace 12 is arranged between the left pilot tunnel part 1 and the middle pilot tunnel (middle pilot tunnel part 5 and middle pilot tunnel part 6), and a temporary vertical brace 13 is arranged between the left pilot tunnel part 2 and the middle pilot tunnel (middle pilot tunnel part 7). The temporary vertical brace 12 is respectively connected to the primary support 16 and the temporary vertical brace 13, and the temporary vertical brace 13 is also connected to the invert primary support 18; a temporary vertical brace 14 is arranged between the right pilot tunnel part 3 and the middle pilot tunnel (middle pilot tunnel part 5 and middle pilot tunnel part 6), and a temporary vertical brace 15 is arranged between the right pilot tunnel part 4 and the middle pilot tunnel (middle pilot tunnel part 7). The temporary vertical brace 14 is respectively connected to the primary support 3 19 and the temporary vertical brace 15, and the temporary vertical brace 15 is also connected to the invert primary support 2 21; the temporary cross brace 9 is respectively connected to the primary support 16 and the temporary vertical brace 12, the temporary cross brace 10 is respectively connected to the temporary vertical brace 14 and the primary support 3 19, and the temporary cross brace 11 is respectively connected to the temporary vertical brace 12 and the temporary vertical brace 14. Each primary support is arranged circumferentially along the tunnel to form a closed loop.

[0029] The surrounding rock of the large-span shallow-buried and eccentrically loaded tunnel of the exemplary embodiment of the present invention is grade Ⅴ. The tunnel is constructed by mechanical excavation method, and the principle of "advance pipe jacking, strict grouting, less disturbance, short advance, strong support, early closure and frequent measurement" is followed during construction to avoid large disturbance to the surrounding rock. The tunnel entrance section is reinforced by double-layer pipe roofs, and the pipe roofs adopt Φ108 mm grouting steel pipes with a single length of 40 m.

[0030] Reference Figures 1 to 3 , the double-side drift construction method of the large-span shallow-buried and eccentrically loaded tunnel of the exemplary embodiment of the present invention includes the following steps: Step 1. Construction preparation: conduct advanced geological forecasting for the preset construction area; Step 2. Conduct advanced support and grouting for the small duct 24 in the left pilot drift section 1; Step 3. Excavate the left pilot drift section 1 with an advance of one arch frame spacing, and construct the primary support 16, temporary vertical strut 12 and temporary horizontal strut 9 for the initial stage; Step 4. After the left pilot drift section 1 has advanced 3m, excavate the left pilot drift section 2 with an advance of one arch frame spacing, construct the primary support 17 for the second stage and temporary vertical strut 13, reinforce the rock pillar in the left pilot drift section 2, and seal the inverted arch primary support 18; Step 5. Conduct advanced support and grouting for the small duct 24 in the right pilot drift section 3; Step 6. After the left pilot drift section 1 has advanced 15m, excavate the right pilot drift section 3 with an advance of one arch frame spacing, construct the primary support 19 for the third stage, temporary vertical strut 14 and temporary horizontal strut 10; Step 7. After the right pilot drift section 3 has advanced 3m, excavate the right pilot drift section 4 with an advance of one arch frame spacing, construct the primary support 20 for the fourth stage and temporary vertical strut 15, and seal the inverted arch primary support 21 for the second stage; Step 8. Conduct advanced support and grouting for the small duct in the middle pilot drift section 5; after the right pilot drift section 3 has advanced 3 - 5m, excavate the middle pilot drift section 5 with an advance of one arch frame spacing, and construct the arch primary support 22; Step 9. After the middle pilot drift section 5 has advanced 30 - 50m, excavate the middle pilot drift section 6 and middle pilot drift section 7 with an advance of one arch frame spacing, construct the inverted arch primary support 23 for the third stage and the inverted arch secondary lining 28, and fill the inverted arch filling layer 8; after constructing the inverted arch primary support 23 for the third stage, the primary support (primary support 1, primary support 2, inverted arch primary support 1, inverted arch primary support 2, inverted arch primary support 3, primary support 3, primary support 4 and arch primary support) is integrally closed to form a ring; Step 10. After the tunnel monitoring and measurement shows stable deformation, for the pilot drift section more than 50m and greater than 30m away from the heading face, gradually remove the corresponding temporary horizontal struts and temporary vertical struts filled with the inverted arch filling layer 8 section by section, gradually lay the waterproof board and non - woven geotextile, tie the secondary lining steel bars, and cast the secondary lining concrete by formwork.

[0031] In the aforesaid steps, the initial support forming a closed loop improves the stability of the surrounding rock of the tunnel, reduces the influence of vertical loads on the surrounding rock, and is conducive to preventing tunnel deformation and collapse. Before removing the temporary cross struts and temporary vertical struts, ensure that the distance from the removal part of the pilot drift section to the tunnel face is greater than 50 m and the inverted arch filling layer 8 at the removal part has been constructed and reached the preset strength, which can avoid the influence of premature removal on the tunnel force system and prevent the tunnel from collapsing during excavation. After the inverted arch filling layer 8 is constructed and reaches the preset strength, the inverted arch filling layer 8 and the secondary lining 28 of the inverted arch jointly form a complete closed structure, improving the overall stiffness and bearing capacity, and reducing the influence of surrounding rock deformation. At this time, when removing the temporary supports (temporary vertical struts and / or temporary cross struts), the load transfer is more uniform, the structural stability is higher, and the risk of cracking of the secondary lining of the inverted arch is reduced. Compared with the method of directly removing the temporary supports after the construction of the secondary lining of the inverted arch, removing the temporary supports after the secondary lining of the inverted arch and the inverted arch filling layer are constructed and reach the preset strength improves the stability of the surrounding rock, improves the safety during the removal of the temporary supports, and can also avoid the cracking of the secondary lining of the inverted arch and avoid the maintenance construction of the secondary lining of the inverted arch.

[0032] In the excavation construction of each pilot drift section in the aforesaid steps, the initial support and temporary supports (temporary vertical struts and / or temporary cross struts) corresponding to each pilot drift are constructed simultaneously. Due to the weak self-stability of the surrounding rock, the simultaneous construction of the initial support and temporary supports corresponding to each pilot drift can effectively avoid ground settlement and deformation of the initial support, and improve the safety and self-stability of tunnel construction.

[0033] In the advance support small ducts 24 of the aforesaid construction, the specifications of the advance support small ducts 24 are Φ42*4 mm and the length is 4.5 m. The circumferential * longitudinal spacing of the advance support small ducts 24 is set according to the support requirements, and the advance support small ducts 24 are arranged in a plum blossom shape. When constructing the advance support small ducts 24 of the left pilot drift part 1 and the right pilot drift part 3, the circumferential arrangement length area of the advance support small ducts 24 is determined according to the surrounding rock stability requirements of the excavation construction. Exemplarily, the circumferential arrangement length area of the advance support small ducts 24 is not less than 3 / 4 of the tunnel sidewall perimeter of the corresponding pilot drift.

[0034] In the steps of the above-mentioned initial support during construction, along the circumferential direction of the tunnel cross-section, at the joint of adjacent segments of the circular steel arch of the initial support, there are lock-foot anchor pipes 26. The lock-foot anchor pipes 26 are connected to the steel arch and extend into the surrounding rock of the tunnel. The lock-foot anchor pipes 26 can effectively transfer the bending moment, horizontal force, and vertical force transmitted by the steel arch into the surrounding rock, enhancing the support effect of the initial support and the stability and safety of the tunnel; the lock-foot anchor pipes 26 can also be arranged at positions such as the crown, arch feet, and arch waist of the steel arch. The specific arrangement positions and quantities are determined according to construction requirements to further enhance the stability of the steel arch and the initial support; at each setting position of each lock-foot anchor pipe 26, there are two lock-foot anchor pipes 26. The lock-foot anchor pipes 26 have a diameter of 42 mm and a length of 4 m. The lock-foot anchor pipes 26 are driven into the surrounding rock at an inclination angle of 20 degrees and welded to the steel arch to form a stable triangular structure.

[0035] During the construction of the initial support one 16, the initial support three 19, and the arch initial support 22, the arch grouting small pipes 31 are constructed circumferentially into the surrounding rock. The arch grouting small pipes 31 are 4.5 m long, with a circumferential * longitudinal spacing of 1.2 * 0.5 m, and are arranged in a plum blossom pattern. The circumferential arrangement length of the arch grouting small pipes 31 is not greater than the circumferential arrangement length of the advanced support small pipes 24; during the construction of the initial support one 16, the initial support two 17, the initial support three 19, and the initial support four 20, the grouting anchor bolts 27 are constructed circumferentially into the surrounding rock. In the initial support one 16 and the initial support three 19, the grouting anchor bolts 27 extend to the edge of the arch grouting small pipes 31 to further enhance the stability of the initial support; the grouting anchor bolts 27 have a specification of Φ25 * 5 mm and a length of 4.5 m, with a circumferential * longitudinal spacing of 1.2 * 0.5 m, and are arranged in a plum blossom pattern.

[0036] The temporary vertical struts (temporary vertical strut one, temporary vertical strut two, temporary vertical strut three, temporary vertical strut four) and the temporary horizontal struts (temporary horizontal strut one, temporary horizontal strut two, temporary horizontal strut three) both include steel frames, shotcrete, and steel meshes. The steel frames, shotcrete, and steel meshes are integrally connected into a sidewall structure; the steel frames are assembled with I22 steel I-beams, and their curvature is determined according to the shape of the sidewall of the drift after excavation; the steel bars of the steel mesh have a diameter of 8 mm and a spacing of 20 * 20 cm.

[0037] In the above-mentioned steps, during the construction of the temporary vertical strut 12 and the temporary vertical strut 14, the advanced horizontal grouting small pipes 25 are constructed along the height directions of the temporary vertical strut 12 and the temporary vertical strut 14 respectively. Along the tunnel axis direction, the advanced horizontal grouting small pipes 25 are overlapped by 1 - 2 m to form a continuous stress system; the advanced horizontal grouting small pipes 25 are arranged from the tops to the bottoms of the temporary vertical strut 12 and the temporary vertical strut 14, extend downward from the preset middle pilot drift 5 to the middle pilot drift 6, and extend downward a certain distance (not exceeding 1 / 2 of the height of the middle pilot drift 6) from the middle pilot drift 6. The specifications of the advanced horizontal grouting small pipes 25 are Φ42*4mm and the length is 3.5 m, and the circumferential spacing of the advanced horizontal grouting small pipes 25 is 60 mm.

[0038] In the above-mentioned step eight, during the excavation of the middle pilot drift 5, the interface between the middle pilot drift 5 and the middle pilot drift 6 is trapezoidal and wavy. The distance d between the top surface of the trapezoid at the bottom of the middle pilot drift 5 and the tunnel arch is not greater than 3.6 m. Controlling its excavation height can reduce the disturbance to the surrounding rock and improve the stability of the surrounding rock. During the excavation of the middle pilot drift 6 and the middle pilot drift 7 in step nine, the middle pilot drift 7 can be excavated after the middle pilot drift 6 has been excavated for 3 - 5 m. After the middle pilot drift 6 is excavated, a temporary cross strut 11 is set at the bottom of the middle pilot drift 6 to form a closed structure and improve the stability of the support.

[0039] In the above-mentioned step ten, when monitoring the tunnel, the monitoring points are arranged according to relevant requirements to obtain the initial data of the monitoring. During the removal of the temporary support, the items such as the crown settlement are observed according to the specified observation frequency (densifying the observation frequency if necessary) and the monitoring results are fed back to ensure the safety during the removal of the temporary support. Further, the monitoring data includes one or more of the crown settlement, horizontal convergence, surface convergence, and surface displacement. The temporary support can be removed only when the monitoring data reaches the stable condition. Among them, the standard for the crown settlement to reach the stable condition is that the settlement rate does not exceed 0.1 mm / d, and the standard for the horizontal convergence to reach the stable condition is that the convergence rate does not exceed 0.2 mm / d.

[0040] The process of removing the temporary support includes: erecting the scaffold and safety net or using the trolley as the working platform, breaking the shotcrete, removing the temporary support (temporary cross strut or temporary vertical strut), treating the surface attachments, and patching and spraying the concrete to level the pit at the removal position.

[0041] In the above process, the shotcrete is removed by pneumatic pick. When removing the shotcrete of the temporary vertical struts, it is carried out successively for each arch frame from top to bottom; when removing the shotcrete of the temporary horizontal struts, the shotcrete of the corresponding temporary horizontal struts can be removed respectively within the first part of the left pilot tunnel, the sixth part of the middle pilot tunnel, and the third part of the right pilot tunnel, or scaffolds can be erected respectively within the second part of the left pilot tunnel, the seventh part of the middle pilot tunnel, and the fourth part of the right pilot tunnel, and then the corresponding shotcrete can be removed with the help of the scaffolds and pneumatic picks; during the removal process of the shotcrete, ensure the effective connection of the connecting steel bars to the steel frame, so that the temporary support forms an integral body to prevent the sudden instability of the temporary support during the removal process; after the concrete is removed, remove the steel mesh of the temporary support, and the steel mesh is cut by electric welding.

[0042] The temporary support includes temporary horizontal struts and temporary vertical struts. During the removal process, first remove the temporary horizontal struts and then the temporary vertical struts. When removing the temporary horizontal struts, use the above-mentioned method to remove the temporary horizontal strut 9 and the temporary horizontal strut 10 successively (the removal order of the two can also be swapped). When there is a temporary horizontal strut 11 between the sixth part 6 and the seventh part 7 of the middle pilot tunnel, first remove the temporary horizontal strut 11, and then remove the temporary horizontal strut 9 and the temporary horizontal strut 10 successively, in case the simultaneous removal causes excessive changes in the monitoring and measurement data, affecting the stability and safety of the initial support and surrounding rock of the tunnel; first remove the temporary horizontal strut 11, and the temporary horizontal strut 9 and the temporary horizontal strut 10 can still support the tunnel, improving the safety of the tunnel.

[0043] When removing the temporary support for the first time, select a 5m-long removal section along the tunnel axis as the test section. After the monitoring and measurement data after removing the temporary support in the test section meet the requirements, the length of each subsequent formal removal section is 7.5m. When removing the temporary support in the test section and the formal removal section, it is carried out in two steps. The first step is to remove 3.5m, and the second step is to remove the remaining part. During the removal process of the temporary support in each step, strengthen the monitoring and measurement of the tunnel; the formal removal section is removed in the same way as the test section. The stress of the tunnel surrounding rock and the initial support will be redistributed due to the removal of the temporary support. Selecting a 5m-long removal section as the test section can effectively avoid the occurrence of instability phenomena caused by sudden stress changes. When removing the formal removal section, distributed removal is more conducive to the stability of the surrounding rock and the initial support.

[0044] During the removal of the steel frame of the temporary vertical strut, first try to remove the steel frame in the way of removing 1 every 3. Select 1 first trial-removal steel frame every 3 steel frames, and cut 2 - 3cm at the top of the first trial-removal steel frame (the connection part between the steel frame and the initial support), and observe the monitoring and measurement data of the tunnel. If the monitoring and measurement data meet the requirements, then try to remove the steel frame in the way of removing 1 every 1 (refer to Figure 4, in the figure, the gray filling represents both the first trial demolition steel frame and the second trial demolition steel frame, and the square filling represents the second trial demolition steel frame). If the requirements are not met, strengthening measures shall be taken for the top of the first trial demolition steel frame, such as welding the cut seam to restore the integrity of the temporary vertical support structure, or welding and strengthening members on both sides of the cut seam to enhance the integrity of the temporary vertical support structure, and continue to conduct monitoring and measurement until the monitoring and measurement data meet the requirements; during the process of trial demolishing the steel frame in the way of demolishing one every other one, select one second trial demolition steel frame every other one steel frame (the first trial demolition steel frame is within the selection range of the second trial demolition steel frame), cut all the tops of the second trial demolition steel frames, observe the monitoring and measurement data of the tunnel. If the monitoring and measurement data meet the requirements, demolish the second trial demolition steel frame; if the monitoring and measurement data do not meet the requirements, strengthen the second trial demolition steel frame in the same way as in demolishing one every other three.

[0045] The demolition method of the steel frame of the temporary cross brace is roughly the same as that of the steel frame of the temporary vertical brace. First, trial demolish the steel frame in the way of demolishing one every other three, select one first trial demolition steel frame every other three steel frames, cut 2 - 3 cm at the connection part between the first trial demolition steel frame and the initial support, observe the monitoring and measurement data of the tunnel. If the monitoring and measurement data meet the requirements, then trial demolish the steel frame in the way of demolishing one every other one. If the requirements are not met, take strengthening measures for the first trial demolition steel frame, such as welding the cut seam to restore the integrity of the temporary cross brace structure, or welding and strengthening members on both sides of the cut seam to enhance the integrity of the temporary cross brace structure, and continue to conduct monitoring and measurement until the monitoring and measurement data meet the requirements. The difference is that when trial demolishing the temporary cross brace in the way of demolishing one every other three, cut 2 - 3 cm at any connection part between the first trial demolition steel frame and the temporary vertical brace.

[0046] As described above, it is only a detailed description of the specific implementation manner of the present invention, rather than a limitation to the present invention. Various substitutions, variations and improvements made by those skilled in the relevant technical fields without departing from the principle and scope of the present invention shall be included within the protection scope of the present invention.

Claims

1. A construction method for a double-side drift method of a large-span shallow-buried and eccentrically loaded tunnel, characterized in that, It includes the following steps: Step 1, construction preparation, and conduct advanced geological prediction for the preset construction area; Step 2, construct the advanced support small pipes in the first part of the left pilot tunnel and grout; Step 3, excavate the first part of the left pilot tunnel, with the footage being the spacing of one arch frame, and construct the primary support 1, temporary vertical strut 1, and temporary horizontal strut 1; Step 4, after lagging behind the first part of the left pilot tunnel by 3m, excavate the second part of the left pilot tunnel, with the footage being the spacing of one arch frame, construct the primary support 2 and temporary vertical strut 2, reinforce the middle rock pillar in the second part of the left pilot tunnel, and close the inverted arch primary support 1; Step 5, construct the advanced support small pipes in the third part of the right pilot tunnel and grout; Step 6, after lagging behind the first part of the left pilot tunnel by 15m, excavate the third part of the right pilot tunnel, with the footage being the spacing of one arch frame, construct the primary support 3, temporary vertical strut 3, and temporary horizontal strut 2; Step 7, after lagging behind the third part of the right pilot tunnel by 3m, excavate the fourth part of the right pilot tunnel, with the footage being the spacing of one arch frame, construct the primary support 4 and temporary vertical strut 4, and close the inverted arch primary support 2; Step 8, construct the advanced support small pipes in the fifth part of the middle pilot tunnel and grout; after lagging behind the third part of the right pilot tunnel by 3 - 5m, excavate the fifth part of the middle pilot tunnel, with the footage being the spacing of one arch frame, and construct the arch primary support; Step 9, after lagging behind the fifth part of the middle pilot tunnel by 30 - 50m, excavate the sixth part and the seventh part of the middle pilot tunnel, with the footage being the spacing of one arch frame, construct the inverted arch primary support 3 to make the primary support form a closed loop, and then construct the inverted arch secondary lining and fill the inverted arch filling layer; Step 10, after the deformation of the tunnel monitoring and measurement is stable, for the pilot tunnel section where the distance from the tunnel face is greater than 50m, gradually remove the corresponding temporary horizontal struts and temporary vertical struts filled with the inverted arch filling layer section by section, gradually lay the waterproof board and non - woven geotextile, bind the secondary lining steel bars, and cast the secondary lining concrete by formwork.

2. The construction method of double-side drift for large-span shallow-buried and eccentrically loaded tunnels according to claim 1, wherein During the construction of the temporary vertical strut 1 and the temporary vertical strut 3, construct the advanced horizontal grouting small pipes along the height directions of the temporary vertical strut 1 and the temporary vertical strut 3 respectively; the advanced horizontal grouting small pipes are arranged from the top to the bottom of the temporary vertical strut 1 and the temporary vertical strut 3, extend from the preset fifth part of the middle pilot tunnel down to the sixth part of the middle pilot tunnel, and extend a certain distance downward from the sixth part of the middle pilot tunnel.

3. The construction method of double-sided drift for large-span shallow-buried and eccentrically loaded tunnels according to claim 1, characterized in that, During the excavation of the sixth part and the seventh part of the middle pilot tunnel in Step 9, excavate the seventh part of the middle pilot tunnel after excavating 3 - 5m in the sixth part of the middle pilot tunnel. After excavating the sixth part of the middle pilot tunnel, set a temporary horizontal strut 3 at the bottom of the sixth part of the middle pilot tunnel.

4. The construction method of double-side drift for large-span shallow-buried and unsymmetrical-pressure tunnel according to claim 1, characterized in that In Step 10, when removing the temporary horizontal struts and temporary vertical struts for the first time, select a 5m - long removal section along the tunnel axis as the test section. After the monitoring and measurement data after removing the temporary horizontal struts and temporary vertical struts in the test section meet the requirements, the length of each subsequent formal removal section is 7.5m; when removing the temporary horizontal struts and temporary vertical struts in the test section and the formal removal section, it is carried out in two steps. The first step is to remove 3.5m, and the second step is to remove the remaining part. During the process of removing the temporary horizontal struts and temporary vertical struts in each step, monitor and measure the tunnel.

5. The construction method of double-sided drift for large-span shallow-buried and eccentrically loaded tunnels according to claim 4, characterized in that During the demolition of the steel frames of the temporary vertical struts, first try to demolish the steel frames in the way of demolishing 1 out of every 3. Select 1 first trial demolition steel frame every 3 bays of steel frames. Cut 2 - 3 cm at the top of the first trial demolition steel frame, and observe the monitoring and measurement data of the tunnel. If the monitoring and measurement data meet the requirements, then try to demolish the steel frames in the way of demolishing 1 out of every 1. If the requirements are not met, take reinforcement measures at the top of the first trial demolition steel frame, continue the monitoring and measurement until the monitoring and measurement data meet the requirements; during the process of trying to demolish the steel frames in the way of demolishing 1 out of every 1, select 1 second trial demolition steel frame every 1 bay of steel frames, cut all the way through the top of the second trial demolition steel frame, and observe the monitoring and measurement data of the tunnel. If the monitoring and measurement data meet the requirements, demolish the second trial demolition steel frame; if the monitoring and measurement data do not meet the requirements, reinforce the second trial demolition steel frame in the same way as in the case of demolishing 1 out of every 3.

6. The construction method of double-side drift for large-span shallow-buried and eccentrically loaded tunnels according to claim 5, characterized in that The monitoring and measurement data includes one or more of crown settlement, horizontal convergence, ground surface convergence, and ground surface displacement.

7. The construction method of double-side drift for large-span shallow-buried and bias-pressure tunnels according to claim 6, wherein, The settlement rate of the crown settlement does not exceed 0.1 mm / d, and the convergence rate of the horizontal convergence does not exceed 0.2 mm / d.

8. The construction method of double-side drift for large-span shallow-buried and bias-pressure tunnel according to claim 1, characterized in that During the demolition process of the temporary vertical struts and the temporary horizontal struts, first demolish the temporary horizontal struts and then the temporary vertical struts; during the demolition of the temporary horizontal struts, first demolish the temporary horizontal strut 1 and then the temporary horizontal strut 2; when there is a temporary horizontal strut 3 between the middle pilot tunnel part 6 and the middle pilot tunnel part 7, first demolish the temporary horizontal strut 3, and then demolish the temporary horizontal strut 1 and the temporary horizontal strut 2 successively.

9. The construction method of double-side drift for large-span shallow-buried and unsymmetrical-pressure tunnel according to any one of claims 1 to 8, characterized in that Both the temporary horizontal struts and the temporary vertical struts include steel frames, shotcrete, and steel mesh, and the steel frames, shotcrete, and steel mesh are integrally connected to form a sidewall structure.

10. The construction method of double-side drift for large-span shallow-buried and eccentrically loaded tunnels according to claim 9, characterized in that During the demolition process of the temporary horizontal struts and the temporary vertical struts, the shotcrete is removed by pneumatic pick. When removing the shotcrete of the temporary vertical struts, it is carried out successively for each arch frame from top to bottom; when removing the shotcrete of the temporary horizontal struts, remove the shotcrete of the corresponding temporary horizontal struts respectively in the left pilot tunnel part 1, the middle pilot tunnel part 6, and the right pilot tunnel part 3, or set up scaffolding respectively in the left pilot tunnel part 2, the middle pilot tunnel part 7, and the right pilot tunnel part 4, and then use the scaffolding and pneumatic pick to remove the corresponding shotcrete.

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