Stepped supporting system based on expanded arch feet and construction method

Abstract

The invention provides a stepped support system based on expanded arch spring.The stepped support system comprises two primary support devices arranged in a mirror image mode, each primary support device comprises an upper support section and two lower support sections connected with the upper support section, the upper support section is of an arc-shaped structure, and the lower support sections are of a vertical structure; the two expanded arch springing sections are arranged in one-to-one correspondence with the two primary supporting devices, each expanded arch springing section is connected with the upper supporting section and the lower supporting section which are located on the same side, and the end, connected with the upper supporting section, of each expanded arch springing section is gradually expanded from the end, connected with the lower supporting section, of the expanded arch springing section; the upper supporting section, the lower supporting section and the expanded arch foot section are each provided with a grating net frame. The technical problems that in the prior art, the geological requirement is high, and stress is concentrated are solved.

Description

Technical Field

[0001] The invention relates to the technical field of tunnel construction, and in particular to a step-type support system based on expanding an arch foot and a construction method. Background Art

[0002] At present, the construction method of most large-section station main tunnels is mainly based on the double-side wall pilot method, a small number of stations are constructed by the arch cover method, and very few stations with good geological conditions are constructed by the step method. The double-side wall pilot method has been developed for many years, and its technical theory is relatively mature. It has been widely used in the construction of various large-span tunnels. However, due to the complicated construction steps and construction difficulties, the double-side wall pilot method has gradually lost its original advantages. Compared with the double-side wall pilot method, the arch cover method has higher efficiency and lower cost, and can adapt well to the geological conditions of "soft on top and hard on the bottom". Compared with other construction methods, the arch cover method has a shorter development time, and the relevant theories and technologies are not mature enough. It also has higher requirements for geological conditions, and its general adaptability still needs further research. Although the step method is mature in technology, it is occasionally used for large-span dark excavation station tunnels, because it has too high requirements for geological conditions and is not generally practical. At the same time, the stress is concentrated at a certain point, which is easy to cause damage and is not convenient for the uniform stress of the overall structure. Summary of the invention

[0003] In view of the deficiencies in the prior art, the present invention provides a stepped support system and a construction method based on expanding the arch foot, which solves the technical problems of high geological requirements and concentrated stress in the prior art.

[0004] According to an embodiment of the present invention, a stepped support system based on enlarging an arch foot comprises:

[0005] Two initial support devices are arranged in a mirror image, each of the initial support devices comprises an upper support segment and two connected lower support segments, the upper support segment is an arc-shaped structure, and the lower support segment is a vertical structure;

[0006] Two enlarged arch foot sections are arranged in one-to-one correspondence with the two initial support devices, each of the enlarged arch foot sections connects the upper support section and the lower support section on the same side, and the end of the enlarged arch foot section connected to the upper support section gradually expands toward the end of the enlarged arch foot section connected to the lower support section;

[0007] The upper support section, the lower support section and the expanded arch foot section all have a grid frame, and the grid frame includes: a plurality of main stress-bearing steel bars, a plurality of auxiliary steel bars and a plurality of stand-up steel bars, the plurality of main stress-bearing steel bars are arranged at intervals, the plurality of auxiliary steel bars are arranged at intervals between two adjacent main stress-bearing steel bars, and any two adjacent auxiliary steel bars and two adjacent main stress-bearing steel bars together form a closed fixed cavity, each of the stand-up steel bars is arranged along the diagonal line of the fixed cavity, and two adjacent stand-up steel bars are symmetrically arranged;

[0008] Among them, one end of the auxiliary steel bar and one end of two adjacent vertical steel bars converge at the same fixed point, and the fixed point is provided with a connecting piece for connecting the auxiliary steel bar and the two vertical steel bars into a whole that bears force together and is fixed to the same main force-bearing steel bar.

[0009] Preferably, the enlarged arch foot section is an arc-shaped structure and is arranged cocentrically with the upper supporting section.

[0010] Preferably, the shape of the main stress-bearing steel bars is consistent with the shape of the corresponding upper supporting segment, the lower supporting segment or the enlarged arch foot segment.

[0011] Preferably, the end of the expanded arch foot section connected to the upper support section is defined as the upper end, and the end of the expanded arch foot section connected to the lower support section is defined as the lower end, and the lengths of the plurality of erected steel bars gradually increase from the upper end of the expanded arch foot section to the lower end.

[0012] Preferably, an upper connecting steel plate is provided at the upper end of the enlarged arch foot section, the upper supporting section is fixed to the upper connecting steel plate, and the end area of ​​the upper supporting section is equal to the area of ​​the upper connecting steel plate.

[0013] Preferably, a lower connecting steel plate is provided at the lower end of the enlarged arch foot section, the lower supporting section is fixed to the lower connecting steel plate, and the projected area of ​​the end of the lower supporting section projected on the lower connecting steel plate is less than or equal to 1 / 2 of the area of ​​the lower connecting steel plate.

[0014] Preferably, the connecting member includes a connecting sleeve arranged to embrace the main stress-bearing steel bars, and a connecting block arranged on the connecting sleeve, wherein the connecting block extends in a radial direction of the connecting sleeve and is used to connect the auxiliary steel bars and the frame steel bars.

[0015] Preferably, the connecting sleeve comprises two detachably connected semi-annular seats, and the connecting block is fixed to at least one of the semi-annular seats.

[0016] Preferably, the upper supporting section, the lower supporting section and the enlarged arch foot section are all provided with a plurality of locking foot anchor rods.

[0017] According to an embodiment of the present invention, a construction method based on an expanded arch foot stepped support system includes the following steps:

[0018] S1. Divide the tunnel section into upper step pilot pit, middle step pilot pit and lower step pilot pit from top to bottom;

[0019] S2. Excavate the upper left part of the upper step pilot pit and construct the upper support section and temporary middle partition wall;

[0020] S3, excavate the lower left part of the upper step guide pit and expand the arch foot section;

[0021] S4. Excavate the upper right part of the upper step pilot pit and construct the upper support section;

[0022] S5. Excavate the lower right part of the upper step guide pit and expand the arch foot section;

[0023] S6. Remove the temporary middle partition of the upper step guide pit and excavate the middle core soil of the upper step guide pit;

[0024] S7. Excavate the left side of the middle step pilot pit and construct the lower support section;

[0025] S8. Excavate the right side of the middle step guide pit and construct the lower support section;

[0026] S9, excavate the middle core soil of the middle step pilot pit;

[0027] S10, excavate the left side of the lower step guide pit and construct the lower support section;

[0028] S11. Excavate the right side of the lower step guide pit and construct the lower support section;

[0029] S12, excavate the middle core soil of the lower step pilot pit;

[0030] S13. Lay the arch water layer and cast the tunnel arch secondary lining structure; lay the arch and wall waterproof layer and cast the tunnel arch, wall secondary lining and internal structure.

[0031] Compared with the prior art, the present invention has the following beneficial effects: the upper and lower support sections are respectively connected to the enlarged arch foot section to adapt to the tunnel section structure; and the self-stabilizing ability of the surrounding rock and the spatial constraint of the excavation surface are fully utilized, and the bearing capacity and stability of the initial support of the arch are improved by setting the enlarged arch foot section, so as to transfer the arch load to the surrounding rock at the arch foot; furthermore, the use of the grid frame allows the entire support system to withstand a larger load, and can effectively disperse the load to prevent damage caused by local stress concentration. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 is a schematic structural diagram of a support system in one embodiment of the present invention;

[0033] Figure 2 It is a structural schematic diagram of an upper support section and an enlarged arch foot section in one embodiment of the present invention;

[0034] Figure 3 It is a structural schematic diagram of a connection piece installed at an enlarged arch foot section in one embodiment of the present invention;

[0035] Figure 4 This is a schematic structural diagram of a connecting piece installed at an upper supporting section in one embodiment of the present invention.

[0036] In the figure:

[0037] 1. Upper support section; 2. Expanded arch foot section; 3. Lower support section; 4. Main load-bearing steel bars; 5. Auxiliary steel bars; 6. Frame steel bars; 7. Connectors; 701. Connecting sleeves; 702. Connecting blocks. DETAILED DESCRIPTION

[0038] It should be noted that in the description of the present invention, unless otherwise clearly specified and limited, the terms "set", "install", "connect", etc. should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be an indirect connection through an intermediate medium, or it can be the internal communication of two components. For those skilled in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0039] The following will be combined with the attached Figure 1-4 , the present invention is further described.

[0040] Based on the enlarged arch foot step support system, it includes: two initial support devices and two enlarged arch foot sections 2; the two initial support devices are arranged in a mirror image, each of the initial support devices includes an upper support section 1 and two connected lower support sections 3, the upper support section 1 is an arc-shaped structure, and the lower support section 3 is a vertical structure; the two enlarged arch foot sections 2 are arranged in a one-to-one correspondence with the two initial support devices, each of the enlarged arch foot sections 2 connects the upper support section 1 and the lower support section 3 located on the same side, and the end of the enlarged arch foot section 2 connected to the upper support section 1 is gradually enlarged toward the end thereof connected to the lower support section 3;

[0041] The upper support section 1, the lower support section 3 and the expanded arch foot section 2 all have a grid frame, and the grid frame includes: a plurality of main stress-bearing steel bars 4, a plurality of auxiliary steel bars 5 and a plurality of stand-up steel bars 6, the plurality of main stress-bearing steel bars 4 are arranged at intervals, the plurality of auxiliary steel bars 5 are arranged at intervals between two adjacent main stress-bearing steel bars 4, and any two adjacent auxiliary steel bars 5 and two adjacent main stress-bearing steel bars 4 together form a closed fixed cavity, each of the stand-up steel bars 6 is arranged along the diagonal of the fixed cavity, and the two adjacent stand-up steel bars 6 are symmetrically arranged;

[0042] Among them, one end of the auxiliary steel bar 5 and one end of the two adjacent stand steel bars 6 converge at the same fixed point, and the fixed point is provided with a connecting piece 7, which is used to connect the auxiliary steel bar 5 and the two stand steel bars 6 into a whole that bears force together and is fixed to the same main force-bearing steel bar 4.

[0043] Specifically, in an embodiment of the present invention, two initial support devices are arranged in one-to-one correspondence with two enlarged arch foot sections 2, so that the upper support section 1 and the lower support section 3 of each initial support device are respectively connected to the two ends of the enlarged arch foot section 2 to form a tunnel section, so that the support system can better adapt to various types of tunnel sections and geological conditions. At the same time, by setting the two initial support devices in a mirror image, and each device is equipped with an enlarged arch foot section 2, the stability of the entire support system can be significantly improved. The design of the enlarged arch foot section 2 helps to transfer the upper load to the bottom more evenly, thereby reducing the risk of local overload, that is, the enlarged arch foot section 2 can help disperse the pressure from above and reduce the pressure on a single support point, thereby improving the reliability of the entire system. Of course, the upper support section 1 is set as an arc structure, which can fit the curved surface of the top of the tunnel well, provide uniform support, and avoid deformation or rupture caused by excessive local pressure. At the same time, the arc structure has a high compressive resistance and good stability, can effectively resist the pressure from above, and when subjected to external force, the arc shape can better disperse stress and reduce stress concentration inside the structure. The lower support section 3 is a vertical structure that can better transmit the pressure from above vertically to the bottom, reduce the horizontal thrust, and thus improve the stability of the overall structure; and compared with inclined or curved structures, it is easier to position and install during the construction process, and is also conducive to the subsequent concrete pouring or other reinforcement measures.

[0044] In addition, the end of the expanded arch foot section 2 connected to the upper support section 1 is gradually expanded toward the end connected to the lower support section 3, which can better bear the load transmitted from the upper part and distribute these loads more evenly to the bottom. The use of a progressive expansion design can effectively avoid stress concentration, reduce the risk of local damage, increase the contact area with the surrounding rock and soil, and help improve the stability of the entire support system. The larger bottom area can provide a better support foundation and reduce the possibility of sinking or sliding. At the same time, by gradually increasing the cross-section, the expansion of the arch foot section 2 can improve the stress distribution, so that the upper pressure is diffused along a wider area instead of being concentrated on a few points, and its expansion degree can be adjusted according to specific geological conditions to adapt to weak surrounding rocks or unstable geological environments, ensuring that the overall performance of the support system is not affected, so as to make full use of the self-stabilizing ability of the surrounding rock and the spatial constraint of the excavation surface to improve its practicality.

[0045] In the embodiment of the present invention, the upper support section 1, the lower support section 3 and the expanded arch foot section 2 are all provided with a grid frame; wherein the upper support section 1 and the lower support section 3 are both provided with a double-layer main force steel bar 4 structure (such as Figure 2 As shown, in this embodiment, the above support section 1 is used for explanation), and the expanded arch foot section 2 adopts a three-layer main force-bearing steel bar 4 structure.

[0046] Specifically, in the upper support section 1, two main stress-bearing steel bars 4 are arranged at intervals from the inside to the outside, and a plurality of auxiliary steel bars 5 are arranged at intervals between the two main stress-bearing steel bars 4, and each auxiliary steel bar 5 extends along the radial direction of the main stress-bearing steel bars 4; in this way, a closed fixed cavity can be formed between any two adjacent auxiliary steel bars 5 and the two main stress-bearing steel bars 4, and the cross-section of the fixed cavity is approximately rectangular; standoff steel bars 6 are arranged along the diagonal direction of the fixed cavity, and the adjacent standoff steel bars 6 located in the two adjacent fixed cavities are symmetrically arranged with the auxiliary steel bars 5 therebetween as the axis, thereby forming a plurality of triangular stable structures.

[0047] Similarly, for the expansion of the arch foot section 2, the upper support section 1 structure as described above is adopted, and a main stress-bearing steel bar 4 is added on this basis, so that the auxiliary steel bars 5 and the frame steel bars 6 located between the three main stress-bearing steel bars 4 are symmetrically arranged relative to the main stress-bearing steel bar 4 in the middle.

[0048] Therefore, for the upper support section 1, a plurality of fixed points are formed on one main stress-bearing steel bar 4, which is formed by the convergence of one end of the auxiliary steel bar 5 and the ends of two symmetrically arranged stand-up steel bars 6; for the expanded arch foot section 2, a plurality of fixed points are formed on the three main stress-bearing steel bars 4, which are also formed by the ends of the auxiliary steel bars 5 and the ends of the two stand-up steel bars 6. On the one hand, the steel bars can be connected to better resist the action of external forces, prevent the relative movement between the steel bars, and improve the overall rigidity and stability; on the other hand, a connector 7 is set at the above-mentioned fixed point, one end of the connector 7 is fixed to the corresponding main stress-bearing steel bar 4, and the other end is used to connect the auxiliary steel bar 5 and the two stand-up steel bars 6, so that the three form a whole body under common stress, increase the local bearing capacity and rigidity, and disperse the stress, so that the system can reduce the dependence on geological conditions and improve its practicality. That is, it can jointly bear the action of force at the fixed point, so that the stress can be distributed more evenly, and fatigue or fracture of certain parts due to stress concentration can be avoided. In addition, the steel bars are indirectly connected by connecting parts 7 instead of direct welding. High-quality connecting parts 7 can provide a reliable connection effect, ensuring that the steel bars can bear the force together when subjected to external forces and will not easily loosen or break; and have good corrosion resistance, especially in humid or salty environments, to ensure its stability in long-term use.

[0049] In one embodiment, if Figure 1 As shown, the enlarged arch foot section 2 is an arc-shaped structure and is arranged cocentrically with the upper support section 1. The geometric consistency of the entire support system can be ensured, making the structure more harmonious and unified; the cocentric design allows stress to be smoothly transferred from the upper support section 1 to the enlarged arch foot section 2, avoiding stress interruption or mutation caused by different centers, and improving the continuity and stability of the structure.

[0050] In one embodiment, if Figure 1 , Figure 2 As shown, the shape of the main stress-bearing steel bar 4 is consistent with the shape of the corresponding upper support section 1, the lower support section 3 or the expanded arch foot section 2. It can ensure that the steel bars are evenly distributed in the entire support structure, so as to better transfer and disperse the loads from all directions and avoid stress concentration. When the shape of the main stress-bearing steel bar 4 is consistent with the shape of the support section, the steel bar can be better embedded in the support structure to form a complete force system, which enhances the integrity and stability of the structure, makes it more convenient and quick to install the main stress-bearing steel bar 4, reduces the workload of on-site adjustment and correction, and improves construction efficiency. By accurately designing the shape of the main stress-bearing steel bar 4 to match the shape of the support section, unnecessary material use can be reduced, thereby reducing material costs. At the same time, for support sections of different shapes, the corresponding shape of the main stress-bearing steel bar 4 can be customized, so that it can be optimized and adjusted according to actual geological conditions and design requirements to improve the adaptability of the support system. In addition, since the main load-bearing steel bars 4 are consistent in shape with the support section, it can ensure that the steel bars will not deviate or break due to shape mismatch under stress, thereby improving the safety of the entire support system, helping to reduce stress concentration caused by shape mismatch, thereby reducing the early failure of steel bars due to fatigue and extending the service life of the support structure.

[0051] In one embodiment, if Figure 2 As shown, the end of the expanded arch foot section 2 connected to the upper support section 1 is defined as the upper end, and the end connected to the lower support section 3 is defined as the lower end. From the upper end of the expanded arch foot section 2 to the lower end thereof, the lengths of the plurality of stand-up steel bars 6 gradually increase. Specifically, as the expanded arch foot section 2 gradually expands downward, the increasing length of the stand-up steel bars 6 can better adapt to changes in the structure, ensuring that there is sufficient support force at each position, thereby improving the stability of the entire support system. At the same time, by adjusting the length of the stand-up steel bars 6, stress can be more evenly distributed in different parts of the expanded arch foot section 2, avoiding structural damage caused by excessive stress concentration; and, as the length of the stand-up steel bars 6 increases, the lower end of the expanded arch foot section 2 can withstand a greater load, and can more effectively disperse these loads to the support structure at the bottom.

[0052] In one embodiment, if Figure 2As shown, an upper connecting steel plate is provided at the upper end of the enlarged arch foot section 2, the upper support section 1 is fixed to the upper connecting steel plate, and the end area of ​​the upper support section 1 is equal to the area of ​​the upper connecting steel plate. Specifically, in order to connect the upper support section 1 and the enlarged arch foot section 2, this embodiment sets an upper connecting steel plate at the upper end of the enlarged arch foot section 2, and uses the upper connecting steel plate as an intermediate connection for the steel bars of the upper support section 1 to be welded to the upper connecting steel plate; in addition, in order to ensure that the stress transfer between the two is more uniform and to improve the stability of the overall structure, by setting the area of ​​the upper connecting steel plate equal to the end area of ​​the upper support section 1, the contact area can be maximized, so that the stress distribution is more uniform and the stress concentration is reduced.

[0053] In one embodiment, if Figure 2 As shown, the lower end of the enlarged arch foot section 2 is provided with a lower connecting steel plate, the lower support section 3 is fixed to the lower connecting steel plate, and the projection area of ​​the end of the lower support section 3 projected on the lower connecting steel plate is less than or equal to 1 / 2 of the area of ​​the lower connecting steel plate. Specifically, in order to connect the enlarged arch foot section 2 and the lower connecting steel plate, this embodiment provides a lower connecting steel plate at the lower end of the enlarged arch foot section 2, so as to achieve a stable connection between the lower support section 3 and the enlarged arch foot section 2 by welding the steel bars of the lower support section 3 to the lower connecting steel plate. In addition, by designing the projection area of ​​the end of the lower support section 3 to be less than or equal to 1 / 2 of the area of ​​the lower connecting steel plate, stress can be effectively dispersed; this means that the pressure of the lower support section 3 on the lower connecting steel plate will not be concentrated in a small area of ​​the steel plate, but will be dispersed through a larger steel plate area, thereby reducing local stress concentration and improving the service life and safety of the connector 7. At the same time, the area of ​​the lower connecting steel plate is larger than the projected area of ​​the end of the lower support segment 3, so that the steel plate has sufficient margin to bear and disperse the load, improve the stability of the structure, especially in the case of poor geological conditions or uneven settlement, the large area of ​​the steel plate can provide better support. When the end of the lower support segment 3 only occupies a part of the area of ​​the connecting steel plate, the stress transfer path can be improved, so that the stress is more evenly transferred to the surrounding structure, rather than just through a small area, which can reduce the potential damage risk caused by stress concentration. Since the connection area between the end of the lower support segment 3 and the lower connecting steel plate is small, but the steel plate itself is large enough to provide sufficient shear resistance, it can ensure the connection strength while avoiding local shear failure caused by excessive connection area. Of course, when there is a certain margin between the lower support segment 3 and the lower connecting steel plate, construction personnel can more easily align and fix it to ensure installation accuracy; and when the geological conditions of the tunnel or underground project may change, the adaptability of the structure to different geological conditions can be improved, and even if local disturbances are encountered, good structural performance can be maintained.

[0054] In one embodiment, if Figure 3 , Figure 4As shown, the connecting member 7 includes a connecting sleeve 701 arranged around the main stress-bearing steel bar 4, and a connecting block 702 arranged on the connecting sleeve 701, and the connecting block 702 extends in the radial direction of the connecting sleeve 701, and is used to connect the auxiliary steel bars 5 and the stand steel bars 6. Specifically, in order to connect the auxiliary steel bars 5 and the stand steel bars 6 that converge at a fixed point, this embodiment defines the connecting piece 7 as a connecting sleeve 701 and a connecting block 702, so that the connecting sleeve 701 can be fixedly sleeved on the corresponding main stress-bearing steel bar 4. Since the connecting block 702 is fixed at the connecting sleeve 701, it extends a welding platform outward to provide a steel bar welding point. Compared with directly welding multiple steel bars to the main stress-bearing steel bar 4, the connecting piece 7 is used to make each steel bar converge to the connecting block 702 for welding. On the one hand, it can reduce the local stress concentration of the main stress-bearing steel bar 4 and improve the overall stability of the structure. The connecting block 702 provides a larger contact area. Through the welding platform, multiple steel bars can be firmly connected together, thereby enhancing the bearing capacity of the connection point; on the other hand, it avoids the positioning difficulties and uneven welding problems that may occur when directly welding multiple steel bars to the main stress-bearing steel bar 4, such as incomplete welding, cracks, etc., thereby reducing the probability of rework due to poor welding and improving work efficiency.

[0055] As for the expanded arch foot section 2, since the expanded arch foot section 2 in this embodiment is provided with three main stress-bearing steel bars 4, a fixed point is formed at each main stress-bearing steel bar 4, so that the connector 7 is fixed at each fixed point, which is used to connect different erection steel bars 6 and auxiliary steel bars 5, so that each steel bar forms a whole under common stress, which can prevent stress from being concentrated at the same place. Similarly, as for the upper support section 1, a connector 7 can be provided on one of the main stress-bearing steel bars 4 (since the connection method of the lower support section 3 is the same as that of the upper support section 1, it will not be described in detail here).

[0056] Further, in one embodiment, if Figure 3 , Figure 4As shown, the connecting sleeve 701 includes two detachably connected semi-annular seats, and at least one of the semi-annular seats is fixed with the connecting block 702. Specifically, in order to fix the connecting sleeve 701 at any position of the main stress-bearing steel bar 4 so as to connect the steel bars at various locations; in this embodiment, the connecting sleeve 701 is set as two detachable semi-annular seats, and the two semi-annular seats are spliced ​​to surround the main stress-bearing steel bar 4 and are locked with screws; at the same time, the connecting block 702 is welded on the surface of at least one semi-annular seat to meet the installation needs of the upper support section 1 or the expanded arch foot section 2. In other words, when the connector 7 acts on the upper support section 1, only one connecting block 702 is welded at a semi-circular seat, so that each steel bar is welded to the connecting block 702; similarly, when the connector 7 acts on the expanded arch foot section 2, for the two main force-bearing steel bars 4 inside and outside the expanded arch foot section 2, only one connecting block 702 is set at the connecting sleeve 701, and for the main force-bearing steel bars 4 in the middle, the two semi-circular seats of the connecting sleeve 701 are both provided with connecting blocks 702 for connecting steel bars in different directions. In this way, by changing the number and connection position of the connecting blocks 702, it can be applied to the installation requirements of different scenarios, thereby improving its versatility.

[0057] Of course, the upper support section 1, the lower support section 3 and the expanded arch foot section 2 are all provided with a number of locking anchor rods (not shown). For example, two long anchor rods are set at the large arch foot of each arch frame to pre-treat the surrounding rock at the arch foot, thereby further improving the bearing capacity of the surrounding rock at the large arch foot. Both shotcrete and anchor rods are simulated by isotropic elastic materials, in which shotcrete is a beam unit, and anchor rods are simulated by implanted truss units to increase the diameter and length of the system anchor rods in this section, thereby improving the bearing capacity and stability of the initial support of the side wall. In this way, anchor rods and shotcrete are used as support means to reinforce the surrounding rock and restrain the relaxation and deformation of the surrounding rock. "Expanded arch foot grid steel frame + shotcrete + system anchor rods" are used as the initial support to form a synergistic force system, and the arch load is transferred to the surrounding rock of the arch foot by expanding the arch foot.

[0058] In an embodiment of the present invention, a construction method based on an expanded arch foot step support system is also proposed, comprising the following steps: S1, dividing the tunnel section from top to bottom into an upper step guide pit, a middle step guide pit and a lower step guide pit; S2, excavating the upper left part of the upper step guide pit, constructing an upper support segment 1 and a temporary middle partition wall; S3, excavating the lower left part of the upper step guide pit, constructing an expanded arch foot segment 2; S4, excavating the upper right part of the upper step guide pit, constructing an upper support segment 1; S5, excavating the lower right part of the upper step guide pit, constructing an expanded arch foot segment 2; S6, removing the temporary middle partition wall of the upper step guide pit; Next door, excavate the middle core soil of the upper step pilot pit; S7, excavate the left side of the middle step pilot pit, and construct the lower support segment 3; S8, excavate the right side of the middle step pilot pit, and construct the lower support segment 3; S9, excavate the middle core soil of the middle step pilot pit; S10, excavate the left side of the lower step pilot pit, and construct the lower support segment 3; S11, excavate the right side of the lower step pilot pit, and construct the lower support segment 3; S12, excavate the middle core soil of the lower step pilot pit; S13, lay the invert water layer, and cast the secondary lining structure of the tunnel invert; lay the arch and wall waterproof layer, and cast the tunnel arch, wall secondary lining and internal structure.

[0059] From the analysis of the initial support internal force, when the upper step is excavated, the maximum value of the initial support internal force appears at the intersection of the arch and the middle partition. After the middle partition is removed, the internal force at the large arch foot of the tunnel gradually increases with the excavation of the tunnel. When the pilot pit on the left side of the middle step is supported, the initial support bending moment reaches the maximum value of 183.31 kN·m, located at the large arch foot on the right side of the tunnel. When the excavation reaches the pilot pit on the right side of the middle step, the forces on the left and right sides of the tunnel tend to be symmetrical, and the bending moment value at the large arch foot decreases by about 50%, and the axial force decreases by about 21%. However, with the further excavation of the tunnel, the surrounding rock stress is further released, and the value of the initial support internal force of the tunnel gradually increases and tends to be stable. The initial support of this project is calculated according to the compression bending member, which can meet the initial support force requirements.

[0060] Therefore, the enlarged arch foot step method has less disturbance to the surrounding rock and smaller deformation of the surrounding rock than the double side wall pilot method. The horizontal clearance convergence value of the side wall after excavation is 188.24% greater than that of the enlarged arch foot step method, and the arch crown settlement is also increased by 67.55%. At the same time, both methods are based on the large-section tunnel excavation of the New Austrian Tunneling Method, and the load is jointly borne by the tunnel primary support structure and the surrounding rock. For the part of the load borne by the primary support, the large arch foot in the enlarged arch foot step method can transfer this part of the load to the surrounding rock at the arch foot, while for the tunnel constructed by the double side wall pilot method, this part of the load can only be borne by the primary support structure. Therefore, when the double side wall pilot method is used for construction, its primary support force is significantly greater than that of the enlarged arch foot step method, and the maximum bending moment of the primary support increases by about 55.36%.

[0061] The arch foot step method cleverly divides the large-section tunnel into several parts by setting up the initial large arch foot and excavating in reasonable steps and stages, making full use of the self-stabilizing ability of the surrounding rock and the spatial constraint of the excavation surface. The setting of the arch foot grid steel frame also improves the bearing capacity and stability of the initial support of the arch. Compared with the traditional large-section construction method, the arch foot step method reduces the complex temporary support system, and the step excavation also provides sufficient construction space for the excavation of large-section tunnels, effectively improving the construction efficiency of the tunnel. At the same time, under the same conditions, the arch foot step method has less disturbance to the surrounding rock and is more conducive to the deformation control of the surrounding rock.

[0062] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. Based on the enlarged arch foot step support system, it is characterized by: include: Two initial support devices are arranged in a mirror image, each of the initial support devices comprises an upper support segment and two connected lower support segments, the upper support segment is an arc-shaped structure, and the lower support segment is a vertical structure; Two enlarged arch foot sections are arranged in one-to-one correspondence with the two initial support devices, each of the enlarged arch foot sections connects the upper support section and the lower support section on the same side, and the end of the enlarged arch foot section connected to the upper support section gradually expands toward the end of the enlarged arch foot section connected to the lower support section; The upper support section, the lower support section and the expanded arch foot section all have a grid frame, and the grid frame includes: a plurality of main stress-bearing steel bars, a plurality of auxiliary steel bars and a plurality of stand-up steel bars, the plurality of main stress-bearing steel bars are arranged at intervals, the plurality of auxiliary steel bars are arranged at intervals between two adjacent main stress-bearing steel bars, and any two adjacent auxiliary steel bars and two adjacent main stress-bearing steel bars together form a closed fixed cavity, each of the stand-up steel bars is arranged along the diagonal line of the fixed cavity, and two adjacent stand-up steel bars are symmetrically arranged; Among them, one end of the auxiliary steel bar and one end of two adjacent vertical steel bars converge at the same fixed point, and the fixed point is provided with a connecting piece for connecting the auxiliary steel bar and the two vertical steel bars into a whole that bears force together and is fixed to the same main force-bearing steel bar.

2. The step-type support system based on expanding the arch foot according to claim 1 is characterized in that: The enlarged arch foot section is an arc-shaped structure and is arranged cocentrically with the upper support section.

3. The step-type support system based on expanding the arch foot according to claim 2 is characterized in that: The shape of the main stress-bearing steel bars is consistent with the shape of the corresponding upper supporting segment, the lower supporting segment or the enlarged arch foot segment.

4. The step-type support system based on expanding the arch foot according to claim 1 is characterized in that: The end of the expanded arch foot section connected to the upper support section is defined as the upper end, and the end of the expanded arch foot section connected to the lower support section is defined as the lower end. From the upper end of the expanded arch foot section to the lower end, the lengths of the plurality of erected steel bars gradually increase.

5. The step-type support system based on expanding the arch foot according to claim 4 is characterized in that: An upper connecting steel plate is provided at the upper end of the enlarged arch foot section, the upper supporting section is fixed to the upper connecting steel plate, and the end area of ​​the upper supporting section is equal to the area of ​​the upper connecting steel plate.

6. The step-type support system based on expanding the arch foot according to claim 4 or 5, characterized in that: A lower connecting steel plate is provided at the lower end of the enlarged arch foot section, the lower supporting section is fixed to the lower connecting steel plate, and the projection area of ​​the end of the lower supporting section projected on the lower connecting steel plate is less than or equal to 1 / 2 of the area of ​​the lower connecting steel plate.

7. The step-type support system based on expanding the arch foot according to claim 1 is characterized in that: The connecting piece includes a connecting sleeve arranged around the main stress-bearing steel bar, and a connecting block arranged on the connecting sleeve. The connecting block extends in the radial direction of the connecting sleeve and is used to connect the auxiliary steel bars and the frame steel bars.

8. The step-type support system based on expanding the arch foot according to claim 7 is characterized in that: The connecting sleeve comprises two semi-annular seats which are detachably connected, and the connecting block is fixed to at least one of the semi-annular seats.

9. The step-type support system based on expanding the arch foot according to claim 1 is characterized in that: The upper support section, the lower support section and the enlarged arch foot section are all provided with a plurality of locking foot anchor rods.

10. The construction method based on the expanded arch foot stepped support system according to any one of claims 1 to 9 is characterized in that: The method comprises the following steps: S1. Divide the tunnel section into upper step pilot pit, middle step pilot pit and lower step pilot pit from top to bottom; S2. Excavate the upper left part of the upper step pilot pit and construct the upper support section and temporary middle partition wall; S3, excavate the lower left part of the upper step guide pit and expand the arch foot section; S4. Excavate the upper right part of the upper step pilot pit and construct the upper support section; S5. Excavate the lower right part of the upper step pilot pit and expand the arch foot section; S6. Remove the temporary middle partition of the upper step pilot pit and excavate the middle core soil of the upper step pilot pit; S7. Excavate the left side of the middle step pilot pit and construct the lower support section; S8. Excavate the right side of the middle step pilot pit and construct the lower support section; S9, excavate the middle core soil of the middle step pilot pit; S10, excavate the left side of the lower step guide pit and construct the lower support section; S11. Excavate the right side of the lower step guide pit and construct the lower support section; S12, excavate the middle core soil of the lower step pilot pit; S13. Lay the arch water layer and cast the tunnel arch secondary lining structure; lay the arch and wall waterproof layer and cast the tunnel arch, wall secondary lining and internal structure.

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