A new tunneling construction method suitable for soft or broken rock strata

Abstract

The application discloses a novel tunneling construction method suitable for soft or broken rock strata and belongs to the field of tunnel engineering. The method comprises the steps of setting a grouting advanced guide pipe to form a pipe shed, and when an advanced guide chamber and a U-shaped bench are constructed, radial grouting anchor rods are constructed along with the exposure of a roof and a shoulder, bidirectional advanced support of the tunnel roof is realized, preliminary support is then implemented by cooperating with net hanging and anchor jet, and finally, a permanent support layer is constructed, so that the operation safety of the tunnel construction in the soft or broken rock strata is ensured. The application provides a bidirectional grouting process of a novel self-drilling grouting anchor rod, simplifies the drilling and grouting process, and the U-shaped bench method has a smaller exposed area at one time and is safer than the traditional bench method, so that the novel method is an ideal construction method for the soft or broken rock strata.

Description

Technical Field

[0001] This invention relates to the field of tunnel engineering, and in particular to a novel tunnel excavation method applicable to weak or fractured rock strata. Background Technology

[0002] In tunnel engineering projects, weak rock strata and severely fractured rock strata are frequently encountered. Ensuring the safety of tunnel excavation and achieving reliable support is a challenging engineering problem.

[0003] To address this challenge, the engineering community has developed a series of construction techniques. Common techniques in the drill-and-blast method include the bench method, the central diaphragm wall method, the sidewall method, and the advanced pipe roof grouting technique. While these techniques, when used individually, can solve some problems, they often fail to effectively address the safety challenges of tunneling in extremely soft or fractured Class V rock formations. For example, the advanced pipe roof grouting technique, due to uneven grouting within the rock strata and a thin grout layer, can still lead to collapse accidents when the exposed roof area is too large. While the bench method and similar techniques can effectively reduce the exposed roof area, they still cannot completely eliminate the risk of roof collapse when used alone in extremely soft or fractured Class V rock formations.

[0004] To address the challenges of safe tunneling in five types of rock formations that are weak or fractured, new technologies are needed. Summary of the Invention

[0005] The purpose of this invention is to provide a novel tunneling construction method suitable for weak or fractured rock strata. It introduces self-drilling anchor bolt technology and, based on the combined use of the step method and advanced pipe roof grouting technology, further grouts the roof and shoulders by drilling holes, providing advanced grouting in two directions: parallel to the tunnel profile (axial) and perpendicular to the tunnel profile (radial). This provides advanced protection for safe tunneling in five types of weak or fractured rock strata, thus achieving safe tunneling.

[0006] Based on the above concept, the technical solution adopted by this invention is as follows:

[0007] A novel tunneling construction method suitable for weak or fractured rock strata includes:

[0008] Step 1: Construction of the guide wall foundation and reinforced concrete guide wall. The construction of the reinforced concrete guide wall includes the processes of formwork erection, reinforcement erection, concrete pouring, and demolition. When erecting the reinforcement, a pre-embedded plastic pipe is used to form an advanced guide hole.

[0009] Step 2: Drill holes using the pre-guide hole, construct the first section of the pre-guide, and grout.

[0010] Step 3: Construct the pilot tunnel using the drill-and-blast method with a drilling-and-blast cycle. When the pilot tunnel is excavated to a distance of 1.5-2m from the end of the pilot guide pipe, construct the next section of the pilot guide pipe and grout it. While excavating the pilot tunnel, drill holes and grout the newly excavated pilot tunnel roof and shoulders. Construct a preliminary support layer for the roof of this section of the pilot tunnel. Drill holes in the shoulders of the pilot tunnel extend into the rock of the tunnel roof. The preliminary support layer of the pilot tunnel roof is constructed to a distance of 0.5-1m from the tunnel face.

[0011] Step 4: Excavate the next U-shaped step. The top plate of this U-shaped step is the area covered by the tunnel outline of the shoulder borehole of the pilot tunnel. Drill and grout the shoulder borehole formed by this U-shaped step, and construct the preliminary support layer for the top plate formed by this U-shaped step.

[0012] Step 5: Excavate the U-shaped steps between the second and lowest levels. The top plate of each U-shaped step is the area covered by the tunnel outline of the shoulder borehole of the previous U-shaped step. Drill and grout the shoulder boreholes formed by each U-shaped step. Construct the preliminary support layer for the top plate and shoulder formed by each U-shaped step. The shoulder boreholes of each U-shaped step extend into the rock of the tunnel roof.

[0013] Step 6: Excavate the lowest U-shaped step, drill holes and grout the tunnel footing of the lower U-shaped step; and construct the preliminary support layer of the footing; when the lowest U-shaped step is the next or second-to-last U-shaped step, this step shall be the construction step.

[0014] Step 7: Construct a permanent support layer across the entire tunnel section after the preliminary support layer has been constructed; construct one section at a time in each cycle, advancing from the tunnel entrance inwards.

[0015] Step 8: Repeat steps 3, 4, 5, 6 and 7 until tunnel excavation and support are completed.

[0016] Preferably, in steps two and three, a self-drilling anchor is used to construct the pre-drilling guide hole. After drilling is completed, the self-drilling anchor is not pulled out and is used as a pre-drilling guide, and grouting is carried out using the cavity of the self-drilling anchor.

[0017] Preferably, self-drilling anchors are used for drilling holes in the top slab of the pilot tunnel. After drilling, the self-drilling anchors are not pulled out and extend outwards, with the extension length equal to the thickness of the initial support layer. Grouting is then performed using the cavity of the self-drilling anchors, and these anchors are called top slab grouting anchors. Self-drilling anchors are also used for drilling holes in the shoulders and toes of the pilot tunnel and U-shaped steps. After drilling, the self-drilling anchors are not pulled out, and grouting is then performed using the cavity of the self-drilling anchors, and these anchors are called shoulder grouting anchors or toe grouting anchors. After the lower step is constructed, the lower part of the shoulder grouting anchors and toe grouting anchors extending beyond the tunnel outline is retained with a length equal to the thickness of the tunnel's initial support layer, and the remaining part is sheared off.

[0018] Preferably, the pilot tunnel and the lower U-shaped steps are constructed using the drill-and-blast method, with the drilling direction parallel to the tunnel length direction.

[0019] Preferably, the upper step of the U-shaped staircase is 1-4m ahead of the lower step.

[0020] Preferably, the construction procedure for the preliminary support layer of the pre-guide tunnel roof slab is as follows: first, the steel mesh of the preliminary support layer is tied to the lower end of the grouting anchor rods of the roof slab; then, the I-beam arch frame is installed and fixed at both ends of the I-beam arch frame with locking anchor rods; and finally, concrete is sprayed. The construction procedure for the preliminary support layer of the U-shaped step roof slab is as follows: the steel mesh of the preliminary support layer is tied to the lower end of the grouting anchor rods of the roof slab and spliced ​​with the existing preliminary support layer steel mesh of the roof slab to form a whole; then, the I-beam arch frame is installed and spliced ​​with the existing preliminary support layer steel mesh of the roof slab. The I-beams are spliced ​​together to form a whole, and the lower end of the I-beam arch frame is fixed with anchor bolts. Finally, sprayed concrete is applied. The construction procedure for the preliminary support layer of the tunnel siding formed by the lowest U-step is as follows: the steel mesh of the preliminary support layer is tied to the outside part of the grouting anchor bolt hole of the siding and spliced ​​with the existing steel mesh of the preliminary support layer of the roof slab to form a whole. Then, the I-beam arch frame is installed and spliced ​​with the existing I-beam steel frame of the tunnel roof slab to form a whole. The lower end of the I-beam arch frame is fixed with anchor bolts. Finally, sprayed concrete is applied.

[0021] When constructing the pre-concrete tunnel and the top slab of the U-shaped step with shotcrete, the ends of the I-beam arch frame should not be covered with shotcrete.

[0022] Preferably, when installing the steel mesh of the preliminary support layer of the top slab of the pre-guide tunnel and the U-shaped step top slab, the construction method is as follows: drill holes in the top slab using self-drilling anchors. After drilling, the self-drilling anchors are not pulled out and extend outwards, with the extension length equal to the thickness of the preliminary support layer. Then, grouting is performed using the cavity of the self-drilling anchors. Finally, the steel mesh of the preliminary support layer is tied to the lower end of the grouting anchors.

[0023] Preferably, the splicing method between the I-beam arch frame and the existing I-beam is as follows: each I-beam is provided with end plates at both ends, on which screw holes are arranged. During splicing, the screw holes of the two end plates are aligned one by one, the screws are passed through and the nuts are tightened.

[0024] Preferably, the lower end of each section of the I-beam arch frame is fixed with locking foot anchor rods. The fixing method is as follows: the top of the two locking foot anchor rods is welded with a crossbar to tightly clamp the I-beam. The upper section of the locking foot anchor rod and the crossbar are welded to the surface of the end plate.

[0025] The anchor bolts are drilled using a self-drilling anchor bolt. After drilling is completed, the self-drilling anchor bolt is not pulled out. Grouting is then performed using the cavity of the self-drilling anchor bolt.

[0026] Preferably, the alternative to step seven is: starting from the tunnel entrance, first construct a waterproof layer on the entire cross section of the tunnel, and then construct permanent support.

[0027] The beneficial effects of this invention are:

[0028] I. This invention provides a pipe roof pre-grouting process, which, in conjunction with pre-guide tunnel construction, ensures that the small-area roof slab of the pre-guide tunnel is protected by the pre-grouted pipe roof during construction, guaranteeing the safety of the pre-guide tunnel construction. Before the construction of the lower U-shaped step, its roof slab has already been bidirectionally supported by pre-grouted pipe roof grouting and pre-grouted anchor bolts on the shoulders of the pre-guide tunnel or the upper step. Combined with the protection provided by the pipe roof itself, this ensures the safety of the newly excavated roof slab of the lower step before initial support, providing a safe and reliable technology for excavating large-section tunnels in weak or fractured surrounding rock.

[0029] Second, this invention uses self-drilling anchor bolts for drilling. The self-drilling anchor bolts serve as horizontal advance pipe roofs, as vertical hanging nets and anchor bolts for the roof, and also as grouting pipes. This not only makes grouting faster than traditional drilling and reduces the exposure time of the roof, but also provides a safe roof and sidewalls for tunneling operations by reinforcing the surrounding rock with cement grout.

[0030] Third, this invention makes full use of the advantage that it is easy to drill holes in soft or broken rocks, and extensively uses drilling grouting anchoring to make full use of the rock's own strength to achieve low-cost support.

[0031] Fourth, the U-shaped step method used in this invention has a smaller exposed area of ​​the top plate each time and is pre-supported in both directions, which greatly improves safety compared to the traditional step method. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the longitudinal section of the tunnel along its length.

[0033] Figure 2 This is a cross-sectional schematic diagram of the pilot tunnel at position AA;

[0034] Figure 3 This is a cross-sectional schematic diagram of the pilot tunnel at the BB position;

[0035] Figure 4 This is a cross-sectional schematic diagram of the pilot tunnel and the next step at position CC;

[0036] Figure 5 This is a cross-sectional view of the tunnel at the DD location;

[0037] Figure 6 This is a cross-sectional view of the tunnel at the EE location;

[0038] Figure 7 Schematic diagram of the guide wall at the tunnel entrance;

[0039] Figure 8 This is a schematic diagram of the arrangement of blast holes at the face of the advanced pilot tunnel;

[0040] Figure 9 This is a schematic diagram of the blast hole layout at the next level of the working face;

[0041] Figure 10 Schematic diagram of the blast hole layout on the working face of the third step down (also the lowest step);

[0042] Figure 11 This is a magnified schematic diagram of a section of the splicing point of the I-beam arch frame.

[0043] In the diagram: 1. Guide wall; 2. Advanced guide hole; 3. Advanced guide pipe; 4. Advanced guide tunnel; 401. Face blast hole; 402. Hole; 5. Top slab grouting anchor; 6. Mesh grouting anchor; 7. Shoulder grouting anchor; 8. Locking foot anchor; 9. Horizontal bar; 10. Guide wall foundation; 11. Next step; 1101. Next step face blast hole; 12. Next second step; 13. Bottom step; 1301. Bottom step face blast hole; 14. Preliminary support layer; 1401. Preliminary support layer steel mesh; 1402. I-beam arch; 1402-1. End plate; 1402-2. Screw rod; 1402-3. Nut; 15. Waterproof layer; 16. Permanent support layer; 1601. Permanent support layer steel mesh; 17. Shoulder grouting anchor. Detailed Implementation

[0044] Embodiments of the present invention are described in detail below. Examples of these embodiments are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0045] In the description of this invention, unless otherwise explicitly specified and limited, the terms "connected," "linked," and "fixed" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0046] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0047] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0048] See Figures 1 to 11 The novel tunneling construction method for soft or fractured rock strata provided in this invention includes:

[0049] Step 1: Construct the foundation 10 and reinforced concrete guide wall 1. The construction of reinforced concrete guide wall 1 includes the processes of formwork erection, reinforcement erection, concrete pouring, and demolition. When erecting reinforcement, a pre-embedded plastic pipe is used to form an advanced guide hole 2.

[0050] Step 2: Drill holes using the pre-guide hole 2, construct the first section of the pre-guide 3, and grout it;

[0051] Step 3: Construct the pilot tunnel 4 using the drill-and-blast method with a drilling and blasting cycle. When the pilot tunnel 4 is excavated to a distance of 1.5-2m from the end of the pilot guide 3, construct the next section of the pilot guide 3 and grout it. While excavating the pilot tunnel 4, drill holes and grout the top and shoulders of the newly excavated pilot tunnel 4. Construct the preliminary support layer 14 for the top of this section of the pilot tunnel 4. The shoulder holes of the pilot tunnel 4 are drilled deep into the rock of the tunnel roof. The preliminary support layer 14 of the top of the pilot tunnel 4 is constructed to a distance of 0.5-1m from the working face of the pilot tunnel 4.

[0052] Step 4: Excavate the next U-shaped step. The top plate of this U-shaped step is the area covered by the tunnel outline of the shoulder borehole of the pilot tunnel 4. Drill and grout the shoulder borehole formed by this U-shaped step, and construct the preliminary support layer 14 for the top plate formed by this U-shaped step.

[0053] Step 5: Excavate the U-shaped steps between the second and lowest levels. The top plate of each U-shaped step is the area covered by the tunnel outline of the shoulder borehole of the upper U-shaped step. Drill and grout the shoulder boreholes formed by each U-shaped step. Construct the preliminary support layer 14 for the top plate and shoulder formed by each U-shaped step. The shoulder boreholes of each U-shaped step extend into the rock of the tunnel roof.

[0054] Step 6: Excavate the lowest U-shaped step, drill holes and grout the tunnel footing of the lower U-shaped step; and construct the preliminary support layer 14 for the footing; when the lowest U-shaped step is the next or second-to-last U-shaped step, this step shall be the construction step.

[0055] Step 7: Construct permanent support layer 16 across the entire tunnel section above the already constructed preliminary support layer 14; construct one section at a time in each cycle, advancing from the tunnel entrance inwards.

[0056] Step 8: Repeat steps 3, 4, 5, 6 and 7 until tunnel excavation and support are completed.

[0057] Preferably, in steps two and three, a self-drilling anchor is used to construct the pre-guide hole 2. After drilling is completed, the self-drilling anchor is not pulled out and is used as the pre-guide 3, and grouting is carried out using the cavity of the self-drilling anchor.

[0058] Preferably, self-drilling anchors are used to drill holes in the top slab of the pilot tunnel 4. After drilling, the self-drilling anchors are not pulled out and extend outwards, with the extension length equal to the thickness of the preliminary support layer 14. Grouting is then performed using the cavity of the self-drilling anchors, and these anchors are called the top slab grouting anchors 5. Self-drilling anchors are also used to drill holes in the pilot tunnel 4 and the shoulder and foot of the U-shaped step. After drilling, the self-drilling anchors are not pulled out and grouting is then performed using the cavity of the self-drilling anchors, and these anchors are called the shoulder grouting anchors 7 or the foot grouting anchors 17. After the lower step is constructed, the lower part of the shoulder grouting anchors 7 and the foot grouting anchors 17 extending beyond the tunnel outline is retained with a length equal to the thickness of the tunnel's preliminary support layer 14, and the remaining parts are sheared off.

[0059] Preferably, the pilot tunnel 4 and the lower U-shaped step are constructed using the drill-and-blast method, with the drilling direction parallel to the tunnel length direction.

[0060] Preferably, the upper step of the U-shaped staircase is 111-4m ahead of the lower step.

[0061] Preferably, the construction procedure for the preliminary support layer 14 of the top slab of the pilot tunnel 4 is as follows: first, the preliminary support layer steel mesh 1401 is tied to the lower end of the grouting anchor rod 5 of the top slab; then, the I-beam arch frame 1402 is installed and fixed at both ends of the I-beam arch frame 1402 with locking anchor rods 8; and then, concrete is sprayed. The construction procedure for the preliminary support layer 14 of the U-shaped step top slab is as follows: the preliminary support layer steel mesh 1401 is tied to the lower end of the grouting anchor rod 5 of the top slab and spliced ​​with the existing preliminary support layer steel mesh 1401 of the top slab to form a whole; then, the I-beam arch frame 1402 is installed and spliced ​​with the existing preliminary support layer steel mesh 1401 of the top slab to form a whole. The existing roof slab I-beams are spliced ​​together to form a whole. The lower end of the I-beam arch frame 1402 is fixed with locking anchor rods 8, and finally shotcrete is applied. The construction procedure for the preliminary support layer 14 of the tunnel foot formed by the lowest U-step is as follows: the preliminary support layer steel mesh 1401 is tied to the outside part of the foot grouting anchor rod 17 hole and spliced ​​with the existing roof slab preliminary support layer steel mesh 1401 to form a whole. Then, the I-beam arch frame 1402 is installed and spliced ​​with the existing tunnel roof slab I-beam steel frame to form a whole. The lower end of the I-beam arch frame 1402 is fixed with locking anchor rods 8, and finally shotcrete is applied.

[0062] During the sprayed concrete construction of the pilot tunnel 4 and the top slab of the U-shaped step, the two ends of the I-beam arch frame 1402 are not covered with sprayed concrete.

[0063] Preferably, when installing the pre-support layer steel mesh 1401 of the top slab of the pre-guide tunnel 4 and the U-shaped step top slab, the mesh-hanging grouting anchor rod 6 is constructed. The construction method is as follows: a self-drilling anchor rod is used to drill a hole in the top slab. After drilling is completed, the self-drilling anchor rod is not pulled out and extends outward. The extension length is equal to the thickness of the pre-support layer 14. Then, grouting is performed using the cavity of the self-drilling anchor rod. Finally, the pre-support layer steel mesh 1401 is tied to the lower end of the mesh-hanging grouting anchor rod 6.

[0064] Preferably, the splicing method of the I-beam arch frame 1402 and the existing I-beam is as follows: each I-beam is provided with end plates 1402-1 at both ends, and screw holes 1402-2 are arranged on them. When splicing, the screw holes 1402-2 of the two end plates 1402-1 are aligned one by one, the screws 1402-2 are passed through and the nuts 1402-3 are tightened.

[0065] Preferably, the lower end of each section of the I-beam arch frame 1402 is fixed with locking foot anchor rods 8. The fixing method is as follows: the top ends of the two locking foot anchor rods 8 are welded with crossbars 9 to tightly clamp the I-beam. The upper section of the locking foot anchor rods 8 and the crossbars 9 are welded to the surface of the end plate 1402-1.

[0066] The locking anchor 8 uses a self-drilling anchor. After drilling is completed, the self-drilling anchor is not pulled out. Grouting is then performed using the cavity of the self-drilling anchor.

[0067] Preferably, the alternative to step seven is: starting from the tunnel entrance, first construct a waterproof layer 15 on the entire cross section of the tunnel, and then construct permanent support.

[0068] Example 1

[0069] A tunnel is located in a region of fractured sandstone, classified as Class V. The tunnel has a five-centered circular structure with an inverted arch, with a total cross-sectional dimension of 8.65m high and 11m wide. Drill-and-blast method is used for excavation. The pilot tunnel 4 has a cross-sectional dimension of 3m high and 3m wide, with its roof profile being the middle section of the tunnel roof profile. The next U-shaped step is 7m wide and 3m high (i.e., the distance from the bottom of the step to the bottom of the pilot tunnel 4). The next second U-shaped step (the lowest step) is 11m wide and 2.65m high. The pilot tunnel 4 has an average drilling depth of 2m per cycle and a tunneling advance of about 1.5m. The next level and the next level U-shaped steps have an average drilling depth of 1.6m per cycle and a tunneling advance of about 1.5m. The blasting borehole diameter is 40mm, the grouting borehole diameter is 50mm, and all grouting anchors are made of steel pipes with an outer diameter of 42mm and a wall thickness of 4mm. Cement grout is used as the grouting material.

[0070] The tunnel construction includes the following steps:

[0071] 1) Construction of the guide wall 11 foundation and reinforced concrete guide wall 11, wherein the construction of the reinforced concrete guide wall 11 includes the processes of formwork erection, reinforcement erection, concrete pouring, and demolition. During reinforcement erection, advanced guide pipe holes 2 are formed by pre-embedding plastic pipes; the foundation of the guide wall 11 is 0.5m deep and 0.5m thick, and is poured with C20 concrete; the reinforced concrete guide wall 11 is 0.3m thick and 0.4m high. The advanced guide pipe holes 2 are spaced 300mm apart and are formed by pre-embedding PVC plastic pipes with an inner diameter of 65mm and a thickness of 5.25mm.

[0072] 2) Drill a hole using the pre-guide hole 2, construct the first section of the pre-guide 3 and grout it; the hole diameter is 50mm; the pre-guide 3 is a steel pipe, 6m long, with an outer diameter of 42mm and a wall thickness of 4mm; the process of drilling first and then inserting the guide pipe is adopted; the upward angle of the pre-guide 3 is 15° (the same below);

[0073] 3) Six drilling and blasting cycles were used to construct the pilot tunnel 4 using the drill and blast method. When the pilot tunnel 4 was excavated to a distance of 1.5m from the end of the pilot guide 3, the next section of the pilot guide 3 was constructed and grouting was performed. While excavating the pilot tunnel 4, holes were drilled and grouting was performed on the top plate and shoulders of the newly excavated pilot tunnel 4. The preliminary support layer 14 was constructed on the top plate of this section of the pilot tunnel 4.

[0074] The pilot tunnel 4 was constructed using the drill-and-blast method, with an average drilling depth of 2m and the drilling direction parallel to the tunnel length. Two rows of six wedge-shaped cut holes were used, located in the middle of the tunnel face. Emulsion explosives with a diameter of 32mm were used.

[0075] The grouting boreholes in the top slab of the pilot tunnel 4 are 2.5m deep and spaced at 600mm intervals; the shoulder boreholes extend 2.5m into the rock of the tunnel roof and are spaced at 600mm intervals along the tunnel outline; the grouting anchor bolts in the top slab 5 are 2.65m long and extend 150mm beyond the hole at the lower end.

[0076] The initial support layer 14 is 150mm thick and uses C30 concrete (the same below). The concrete protective layer of this layer is 50mm thick (the same below). The longitudinal and transverse steel bars of the initial support layer steel mesh 1401 are all made of steel bars with a diameter of 12mm and a mesh size of 250mm×250mm (the same below). The initial support layer steel mesh 1401 is tied to the lower end of the grouting anchor 5 in the top slab.

[0077] During the construction of the initial support layer steel mesh 1401, the mesh-hanging grouting anchor rods 6 are simultaneously constructed. The borehole depth is 1.5m. The mesh-hanging grouting anchor rods 6 are arranged in the middle of the top slab grouting anchor rods 5 and are evenly distributed on the top slab. The mesh-hanging grouting anchor rods 6 are 1.65m long, with the lower end extending 150mm beyond the hole. The initial support layer steel mesh 1401 is tied to the lower end of the mesh-hanging grouting anchor rods 6. Subsequently, the I-beam arch frame 1402 is installed and fixed at both ends of the I-beam arch frame 1402 with two locking anchor rods 8, followed by shotcreting.

[0078] The 1402 I-beam arch frame is of model 180×94×6.5, the 8 anchor bolts are 3.5m long, and the steel bars are made of 22mm outer diameter threaded steel bars. The shotcrete is applied using a wet spraying process and has a strength grade of C20 (all 1402 I-beam arch frames, anchor bolts, and shotcrete are the same across the entire tunnel section, and will not be described again below).

[0079] The preliminary support layer 14 of the top slab of the pilot tunnel 4 was constructed to a distance of 0.7m from the working face of the pilot tunnel 4;

[0080] 4) At a position 4m behind the pilot tunnel 4, excavate the next U-shaped step. The top plate of this U-shaped step is the tunnel outline covered by the shoulder borehole of the pilot tunnel 4. The grouting anchor 5 on the top plate of this step is the same as the shoulder grouting anchor 7 of the pilot tunnel 4. Drill and grout the shoulder borehole formed by this U-shaped step, and construct the preliminary support layer 14 on the top plate formed by this U-shaped step.

[0081] During the excavation of the next U-shaped bench, the blast holes 1101 at the face of the next bench are mainly arranged in rows with the surface of the pilot tunnel 4 as the free surface. The distance from the blast holes in the same row to the bottom plate or sidewall of the pilot tunnel 4 is equal; a small number of top blast holes are arranged along the tunnel outline.

[0082] The top grouting anchor rod 5 of the next U-shaped step (i.e. the shoulder grouting anchor rod 7 of the advance guide tunnel 4) extends 150mm out of the hole, and the rest is cut off;

[0083] The next U-shaped step shoulder borehole extends 2.5m into the tunnel roof rock, with the shoulder boreholes spaced 600mm apart at the tunnel outline.

[0084] The construction procedure for the preliminary support layer 14 of the next U-shaped step top slab is as follows: tie the preliminary support layer steel mesh 1401 to the lower end of the grouting anchor rod 5 of the top slab and splice it with the existing preliminary support layer steel mesh 1401 of the top slab to form a whole; then install the I-beam arch frame 1402 and splice it with the existing I-beam of the top slab to form a whole; use the locking anchor rod 8 at the lower end of the I-beam arch frame 1402; and finally spray concrete.

[0085] When constructing the initial support layer steel mesh 1401, simultaneously construct the mesh-hanging grouting anchor rod 6. The borehole depth is 1.5m. The mesh-hanging grouting anchor rod 6 is arranged in the middle of the top slab grouting anchor rod 5 and is evenly distributed on the top slab. The mesh-hanging grouting anchor rod 6 is 1.65m long, with the lower end extending 150mm beyond the hole, and the lower end is tied to the initial support layer steel mesh 1401.

[0086] 5) Excavate the next level U-shaped step 3m behind the next level U-shaped step (i.e., the lowest level U-shaped step). The top plate of this level U-shaped step is the tunnel outline covered by the shoulder borehole of the previous level U-shaped step. The grouting anchor 5 of the top plate of the next level U-shaped step is the shoulder grouting anchor 7 of the next level U-shaped step 11. Construct the preliminary support layer 14 for the top plate and sidewalls formed by this level U-shaped step until the complete tunnel section is formed.

[0087] During the excavation of the lower two-stage U-shaped bench, the blast holes 401 at the tunnel face are arranged in rows with the surface of the lower bench 11 as the free surface. The distance from the blast holes in the same row to the bottom plate or sidewall of the lower bench 11 is equal. Some blast holes are arranged along the tunnel outline, and hollow holes 402 are set in the middle of multiple blast holes 401 at the tunnel face.

[0088] When constructing the preliminary support layer steel mesh 1401 of the top slab formed by the U-shaped steps, the mesh-hanging grouting anchor rods 6 are constructed simultaneously. The borehole depth is 1.5m. The mesh-hanging grouting anchor rods 6 are arranged in the middle of the top slab grouting anchor rods 5 and are evenly distributed on the top slab. The mesh-hanging grouting anchor rods 6 are 1.65m long and have a lower end extending 150mm beyond the hole. The preliminary support layer steel mesh 1401 is tied to the lower end of the mesh-hanging grouting anchor rods 6. Subsequently, the I-beam arch frame 1402 is installed and fixed at both ends of the I-beam arch frame 1402 with two locking anchor rods 8, and then shotcrete is applied.

[0089] Drill holes and grout in the tunnel footing formed by the lower second-level U-shaped steps. The hole spacing is 0.5m and the hole depth is 1.5m. After drilling, insert footing grouting anchor rod 17 and grout. Footing grouting anchor rod 17 is 1.65m long and the part extending out of the hole is 150mm long.

[0090] The construction procedure for the preliminary support layer 14 of the tunnel foot formed by the lower second-level U-step is as follows: the preliminary support layer steel mesh 1401 is tied to the outside part of the grouting anchor bolt 17 hole of the foot and spliced ​​with the existing roof preliminary support layer steel mesh 1401 to form a whole; then the I-beam arch frame 1402 is installed and spliced ​​with the existing tunnel roof I-beam to form a whole; the lower end of the I-beam arch frame 1402 is fixed with the locking anchor bolt 8; finally, the concrete is sprayed.

[0091] 6) First, construct the waterproof layer 15 on the entire cross section of the tunnel where the preliminary support layer 14 has been constructed, and then construct the permanent support layer 16; construct one section at a time in each cycle, advancing from the tunnel entrance into the tunnel section by section.

[0092] The permanent support layer 16 is 0.5m thick. The steel bars diameter and mesh size of the two permanent support layer steel mesh 1601 are the same as those of the initial support layer 14. The concrete protective layer of the upper steel mesh is 70mm thick, and the spacing between the two steel meshes is 200mm. The permanent support layer 16 adopts a movable platform, and the process includes the steps of erecting the formwork, erecting the steel bars, pouring concrete, vibrating and curing, and demolding.

[0093] 7) Repeat steps three, four, five, six and seven in a loop until the tunnel excavation and support are completed.

[0094] In all steps of the construction of the pilot tunnel 4, the U-shaped step top plate and the tunnel footing preliminary support layer 14, during the sprayed concrete construction, the two ends of the I-beam arch frame 1402 are not covered with sprayed concrete.

[0095] Example 2

[0096] A certain tunnel is located in a weak shale formation, classified as Class V. The tunnel has a five-centered circular structure with an inverted arch, with a total cross-sectional dimension of 9.4m high and 14.1m wide. Drill-and-blast method is used for excavation. The pilot tunnel 4 has a cross-sectional dimension of 3m high and 3m wide, and its roof profile is the middle section of the tunnel roof profile. The next U-shaped step is 7m wide and 2.4m high (i.e., the distance from the bottom of the step to the bottom of the pilot tunnel 4). The second U-shaped step is 11m wide and 2m high. The third U-shaped step (the lowest step) is 14.1m wide and 2m high. The pilot tunnel 4 has an average drilling depth of 2m per cycle and a tunneling advance of about 1.5m. The next level and the next level U-shaped steps have an average drilling depth of 1.6m per cycle and a tunneling advance of about 1.5m. The blasting boreholes have a diameter of 40mm, and the grouting boreholes have a diameter of 50mm. All grouting holes are drilled using self-drilling anchor bolts, which are not pulled out after drilling and are used for grouting. All grouting holes use steel pipes with an outer diameter of 42mm and a wall thickness of 4mm. Cement grout is used as the grouting material.

[0097] The tunnel construction includes the following steps:

[0098] 1) Construction of the guide wall 11 foundation and reinforced concrete guide wall 11, wherein the construction of the reinforced concrete guide wall 11 includes the processes of formwork erection, reinforcement erection, concrete pouring, and demolition. During reinforcement erection, advanced guide pipe holes 2 are formed by pre-embedding plastic pipes; the foundation of the guide wall 11 is 0.6m deep and 0.6m thick, and is poured with C20 concrete; the reinforced concrete guide wall 11 is 0.4m thick and 0.5m high. The advanced guide pipe holes 2 are spaced 250mm apart and are formed by pre-embedding PVC plastic pipes with an inner diameter of 65mm and a thickness of 5.25mm.

[0099] 2) Drill using the pre-guide hole 2, construct the first section of the pre-guide 3 and grout it; the borehole diameter is 50mm; the pre-guide 3 is a self-drilling anchor rod, 6m long, 42mm outer diameter, and 4mm wall thickness; the pre-guide 3 has an upward angle of 15° (the same below);

[0100] 3) The pilot tunnel 4 was constructed using the drill-and-blast method with 48 drill-and-blast cycles. When the pilot tunnel 4 was excavated to a distance of 2m from the end of the pilot guide 3, the next section of the pilot guide 3 was constructed and grouting was performed. While excavating the pilot tunnel 4, holes were drilled and grouting was performed on the top plate and shoulders of the newly excavated pilot tunnel 4. The preliminary support layer 14 was constructed on the top plate of this section of the pilot tunnel 4.

[0101] The pilot tunnel 4 was constructed using the drill-and-blast method, with an average drilling depth of 2m and the drilling direction parallel to the tunnel length. Two rows of six wedge-shaped cut holes were used, located in the middle of the tunnel face. Emulsion explosives with a diameter of 32mm were used.

[0102] The grouting boreholes in the top slab of the pilot tunnel 4 are 2.5m deep and spaced at 500mm intervals; the shoulder boreholes extend 2.5m into the rock of the tunnel top slab and are spaced at 500mm intervals along the tunnel outline; the grouting anchor bolts in the top slab 5 are 2.65m long and extend 150mm beyond the hole at the lower end.

[0103] The initial support layer 14 is 150mm thick and uses C30 concrete (the same below). The concrete protective layer of this layer is 50mm thick (the same below). The longitudinal and transverse steel bars of the initial support layer steel mesh 1401 are all made of steel bars with a diameter of 12mm and a mesh size of 250mm×250mm (the same below). The initial support layer steel mesh 1401 is tied to the lower end of the grouting anchor 5 in the top slab.

[0104] During the construction of the initial support layer steel mesh 1401, the mesh-hanging grouting anchor rods 6 are simultaneously constructed. The borehole depth is 1.5m. The mesh-hanging grouting anchor rods 6 are arranged in the middle of the top slab grouting anchor rods 5 and are evenly distributed on the top slab. The mesh-hanging grouting anchor rods 6 are 1.65m long, with the lower end extending 150mm beyond the hole. The initial support layer steel mesh 1401 is tied to the lower end of the mesh-hanging grouting anchor rods 6. Subsequently, the I-beam arch frame 1402 is installed and fixed at both ends of the I-beam arch frame 1402 with two locking anchor rods 8, followed by shotcreting.

[0105] The 1402 I-beam arch frame is of model 180×94×6.5, the 8 anchor bolts are 3.5m long, and the steel bars are made of 22mm outer diameter threaded steel bars. The shotcrete is applied using a wet spraying process and has a strength grade of C20 (all 1402 I-beam arch frames, anchor bolts, and shotcrete are the same across the entire tunnel section, and will not be described again below).

[0106] The preliminary support layer 14 of the top slab of the pilot tunnel 4 was constructed to a distance of 0.5m from the working face of the pilot tunnel 4;

[0107] 4) At a position 4m behind the pilot tunnel 4, excavate the next U-shaped step. The top plate of this U-shaped step is the tunnel outline covered by the shoulder borehole of the pilot tunnel 4. The grouting anchor 5 on the top plate of this step is the same as the shoulder grouting anchor 7 of the pilot tunnel 4. Drill and grout the shoulder borehole formed by this U-shaped step, and construct the preliminary support layer 14 on the top plate formed by this U-shaped step.

[0108] During the excavation of the next U-shaped bench, the blast holes 1101 at the face of the next bench are mainly arranged in rows with the surface of the pilot tunnel 4 as the free surface. The distance from the blast holes in the same row to the bottom plate or sidewall of the pilot tunnel 4 is equal; a small number of top blast holes are arranged along the tunnel outline.

[0109] The top grouting anchor rod 5 of the next U-shaped step (i.e. the shoulder grouting anchor rod 7 of the advance guide tunnel 4) extends 150mm out of the hole, and the rest is cut off;

[0110] The next U-shaped step shoulder borehole extends 2.5m into the tunnel roof rock, with the shoulder boreholes spaced 500mm apart at the tunnel outline.

[0111] The construction procedure for the preliminary support layer 14 of the next U-shaped step top slab is as follows: tie the preliminary support layer steel mesh 1401 to the lower end of the grouting anchor rod 5 of the top slab and splice it with the existing preliminary support layer steel mesh 1401 of the top slab to form a whole; then install the I-beam arch frame 1402 and splice it with the existing I-beam of the top slab to form a whole; use the locking anchor rod 8 at the lower end of the I-beam arch frame 1402; and finally spray concrete.

[0112] When constructing the initial support layer steel mesh 1401, simultaneously construct the mesh-hanging grouting anchor rod 6. The borehole depth is 1.5m. The mesh-hanging grouting anchor rod 6 is arranged in the middle of the top slab grouting anchor rod 5 and is evenly distributed on the top slab. The mesh-hanging grouting anchor rod 6 is 1.65m long, with the lower end extending 150mm beyond the hole, and the lower end is tied to the initial support layer steel mesh 1401.

[0113] 5) Excavate the second-lower U-shaped step (i.e., the lowest U-shaped step) 3m behind the next U-shaped step. The top plate of this U-shaped step is the tunnel outline covered by the shoulder borehole of the previous U-shaped step. The grouting anchor 5 of the top plate of the second-lower step is the shoulder grouting anchor 7 of the next step 11. Construct the preliminary support layer 14 (14) for the top plate and sidewalls formed by this U-shaped step. Continue until the complete tunnel section is formed.

[0114] The excavation method for the lower-level U-shaped bench is the same as that for the next-level U-shaped bench. The specifications and construction methods of the top slab grouting anchor bolt 5, the shoulder grouting anchor bolt 7, and the wire mesh grouting anchor bolt 6 are also the same as those for the next-level U-shaped bench. The specifications and construction methods of the preliminary support layer 14, the preliminary support layer steel mesh 1401, and the I-beam arch frame 1402 are also the same as those for the next-level U-shaped bench.

[0115] 6) Excavate the lower third U-shaped step (i.e., the lowest U-shaped step) 3m behind the lower second U-shaped step; the top plate of this U-shaped step is the tunnel outline covered by the borehole of the lower second U-shaped step shoulder; construct the preliminary support layer 14 for the top plate and sidewalls formed by this U-shaped step; until the complete tunnel section is formed;

[0116] During the excavation of the lowest U-shaped bench, the blast holes 1301 at the bottom bench face of the bottom bench 13 are mainly arranged in rows on the surface of the next two benches, with the blast holes in the same row being equidistant from the bottom plate or sidewall of the next two benches; some blast holes are arranged along the tunnel outline.

[0117] When constructing the preliminary support layer steel mesh 1401 of the top slab formed by the U-shaped steps, the mesh-hanging grouting anchor rods 6 are constructed simultaneously. The borehole depth is 1.5m. The mesh-hanging grouting anchor rods 6 are arranged in the middle of the top slab grouting anchor rods 5 and are evenly distributed on the top slab. The mesh-hanging grouting anchor rods 6 are 1.65m long and have a lower end extending 150mm beyond the hole. The preliminary support layer steel mesh 1401 is tied to the lower end of the mesh-hanging grouting anchor rods 6. Subsequently, the I-beam arch frame 1402 is installed and fixed at both ends of the I-beam arch frame 1402 with two locking anchor rods 8, and then shotcrete is applied.

[0118] Drill holes and grout in the tunnel footing formed by the lowest U-shaped step. The hole spacing is 0.5m and the hole depth is 1.5m. After drilling, insert footing grouting anchor rod 17 and grout. Footing grouting anchor rod 17 is 1.65m long and the part extending out of the hole is 150mm long.

[0119] The construction procedure for the preliminary support layer 14 of the tunnel foot formed by the lowest U-step is as follows: the steel mesh 1401 of the preliminary support layer is tied to the outside part of the grouting anchor bolt 17 hole of the foot and spliced ​​with the existing preliminary support layer steel mesh 1401 of the roof slab to form a whole. Then, the I-beam arch frame 1402 is installed and spliced ​​with the existing tunnel roof I-beam to form a whole. The lower end of the I-beam arch frame 1402 is fixed with the locking anchor bolt 8. Finally, the concrete is sprayed.

[0120] 7) First, construct the waterproof layer 15 on the entire cross section of the tunnel where the preliminary support layer 14 has been constructed, and then construct the permanent support layer 16; construct one section at a time in each cycle, advancing from the tunnel entrance inwards.

[0121] The permanent support layer 16 is 0.5m thick. The steel bars diameter and mesh size of the two permanent support layer steel mesh 1601 are the same as those of the initial support layer 14. The concrete cover of the upper steel mesh is 70mm thick. The spacing between the two steel meshes is 200mm.

[0122] 8) Repeat steps three, four, five, six and seven in a loop until the tunnel excavation and support are completed.

[0123] In all steps of the construction of the pilot tunnel 4, the U-shaped step top plate and the tunnel footing preliminary support layer 14, during the sprayed concrete construction, the two ends of the I-beam arch frame 1402 are not covered with sprayed concrete.

[0124] Example 3

[0125] A certain tunnel is surrounded by fractured limestone, classified as Class V. The tunnel has a three-centered circular structure without an invert arch, with a total cross-sectional dimension of 7.09m high and 7.46m wide. Drill-and-blast method is used for excavation. The pilot tunnel 4 has a cross-sectional dimension of 3.49m high and 2.46m wide, and its roof profile is the middle portion of the tunnel roof profile. The next U-shaped step (the lowest step) is 7.46m wide and 3.6m high (i.e., the distance from the bottom of the step to the bottom of the pilot tunnel 4). The pilot tunnel 4 has an average drilling depth of 2m per cycle and a tunneling advance of about 1.5m. The next level and the next level U-shaped steps have an average drilling depth of 1.6m per cycle and a tunneling advance of about 1.5m. The blasting boreholes have a diameter of 40mm. All grouting holes are drilled using self-drilling anchor bolts, which are not pulled out after drilling and are used for grouting. The pilot guide pipe 3 is made of steel pipe with an outer diameter of 42mm and a wall thickness of 4mm. The remaining grouting pipes have an outer diameter of 34mm (drilling borehole diameter of 40mm) and a wall thickness of 2.5mm. Cement grout is used as the grouting material.

[0126] The tunnel construction includes the following steps:

[0127] 1) Construction of the guide wall 11 foundation and reinforced concrete guide wall 11, wherein the construction of the reinforced concrete guide wall 11 includes the processes of formwork erection, reinforcement erection, concrete pouring, and demolition. During reinforcement erection, advanced guide pipe holes 2 are formed by pre-embedding plastic pipes; the foundation of the guide wall 11 is 0.4m deep and 0.4m thick, and is poured with C20 concrete; the reinforced concrete guide wall 11 is 0.3m thick and 0.4m high. The advanced guide pipe holes 2 are spaced 300mm apart and are formed by pre-embedding PVC plastic pipes with an inner diameter of 65mm and a thickness of 5.25mm.

[0128] 2) Drill using the pre-guide hole 2, construct the first section of the pre-guide 3 and grout it; the drill hole diameter is 50mm; the pre-guide 3 is a self-drilling anchor rod, 6m long; the upward angle of the pre-guide 3 is 15° (the same below);

[0129] 3) Six drilling and blasting cycles were used to construct the pilot tunnel 4 using the drill and blast method. When the pilot tunnel 4 was excavated to a distance of 1.5m from the end of the pilot guide 3, the next section of the pilot guide 3 was constructed and grouting was performed. While excavating the pilot tunnel 4, holes were drilled and grouting was performed on the top plate and shoulders of the newly excavated pilot tunnel 4. The preliminary support layer 14 was constructed on the top plate of this section of the pilot tunnel 4.

[0130] The pilot tunnel 4 was constructed using the drill-and-blast method, with an average drilling depth of 2m and the drilling direction parallel to the tunnel length. Two rows of six wedge-shaped cut holes were used, located in the middle of the tunnel face. Emulsion explosives with a diameter of 32mm were used.

[0131] The grouting boreholes in the top slab of the pilot tunnel 4 are 2m deep and spaced 600mm apart; the shoulder boreholes extend 20m into the rock of the tunnel roof and are spaced 600mm apart at the tunnel outline; the grouting anchor bolts in the top slab 5 are 2.12m long and extend 120mm beyond the hole at the bottom.

[0132] The initial support layer 14 is 120mm thick and uses C30 concrete (the same below). The concrete protective layer of this layer is 50mm thick (the same below). The longitudinal and transverse steel bars of the initial support layer steel mesh 1401 are all made of steel bars with a diameter of 12mm and a mesh size of 250mm×250mm (the same below). The initial support layer steel mesh 1401 is tied to the lower end of the grouting anchor 5 in the top slab.

[0133] During the construction of the initial support layer steel mesh 1401, the mesh-hanging grouting anchor rods 6 are simultaneously constructed. The borehole depth is 1.5m. The mesh-hanging grouting anchor rods 6 are arranged in the middle of the top slab grouting anchor rods 5 and are evenly distributed on the top slab. The mesh-hanging grouting anchor rods 6 are 1.62m long, with the lower end extending 120mm beyond the hole. The initial support layer steel mesh 1401 is tied to the lower end of the mesh-hanging grouting anchor rods 6. Subsequently, the I-beam arch frame 1402 is installed and fixed at both ends of the I-beam arch frame 1402 with two locking anchor rods 8, followed by shotcreting.

[0134] The 1402 I-beam arch frame is of model 160×88×6, the 8 anchor bolts are 3m long and made of 20mm outer diameter threaded steel bars. The shotcrete is wet sprayed and has a strength grade of C20 (all 1402 I-beam arch frames, 8 anchor bolts and shotcrete are the same in the entire tunnel section, and will not be described again below).

[0135] The preliminary support layer 14 of the top plate of the pilot tunnel 4 was constructed to a distance of 1m from the working face of the pilot tunnel 4;

[0136] 4) 3m behind the pilot tunnel 4, excavate the next U-shaped step (i.e., the lowest level). The top plate of this U-shaped step is the tunnel outline covered by the shoulder borehole of the pilot tunnel 4. The grouting anchor 5 of the top plate of this step is the same as the shoulder grouting anchor 7 of the pilot tunnel 4. Construct the preliminary support layer 14 for the top plate and sidewalls formed by this U-shaped step until the complete tunnel section is formed.

[0137] During the excavation of the lowest U-shaped bench, the blast holes 1301 at the bottom bench face of the bottom bench 13 are mainly arranged in rows with the bottom plate and sidewalls of the advance guide tunnel 4 as the free surfaces. The distance from the blast holes in the same row to the bottom plate or sidewalls of the next second bench is equal; some blast holes are arranged along the tunnel outline.

[0138] When constructing the preliminary support layer steel mesh 1401 of the top slab formed by the U-shaped steps, the mesh-hanging grouting anchor rods 6 are constructed simultaneously. The borehole depth is 1.5m. The mesh-hanging grouting anchor rods 6 are arranged in the middle of the top slab grouting anchor rods 5 and are evenly distributed on the top slab. The mesh-hanging grouting anchor rods 6 are 1.62m long and have a lower end that extends 120mm beyond the hole. The preliminary support layer steel mesh 1401 is tied to the lower end of the mesh-hanging grouting anchor rods 6. Subsequently, the I-beam arch frame 1402 is installed and fixed at both ends of the I-beam arch frame 1402 with two locking anchor rods 8. Then, shotcrete is applied.

[0139] Drill holes and grout in the tunnel footing formed by the lowest U-shaped step. The hole spacing is 0.6m and the hole depth is 1.5m. After drilling, insert footing grouting anchor rod 17 and grout. Footing grouting anchor rod 17 is 1.62m long and the part extending out of the hole is 120mm long.

[0140] The construction procedure for the preliminary support layer 14 of the tunnel foot formed by the lowest U-step is as follows: the steel mesh 1401 of the preliminary support layer is tied to the outside part of the grouting anchor bolt 17 hole of the foot and spliced ​​with the existing preliminary support layer steel mesh 1401 of the roof slab to form a whole. Then, the I-beam arch frame 1402 is installed and spliced ​​with the existing tunnel roof I-beam to form a whole. The lower end of the I-beam arch frame 1402 is fixed with the locking anchor bolt 8. Finally, the concrete is sprayed.

[0141] 5) First, construct the waterproof layer 15 on the entire cross section of the tunnel where the preliminary support layer 14 has been constructed, and then construct the permanent support layer 16; construct one section at a time in each cycle, advancing from the tunnel entrance inwards.

[0142] The permanent support layer 16 is 0.4m thick. The steel bars diameter and mesh size of the two permanent support layer steel mesh 1601 are the same as those of the initial support layer 14. The concrete protective layer of the upper steel mesh is 60mm thick. The spacing between the two steel meshes is 200mm.

[0143] 6) Repeat steps three, four, and five in a loop until the tunnel excavation and support are completed.

[0144] In all steps of the construction of the pilot tunnel 4, the U-shaped step top plate and the tunnel footing preliminary support layer 14, during the sprayed concrete construction, the two ends of the I-beam arch frame 1402 are not covered with sprayed concrete.

[0145] The above embodiments merely illustrate the basic principles and characteristics of the present invention. The present invention is not limited to the above embodiments. Various changes and modifications can be made to the present invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A tunnel excavation method suitable for weak or fractured rock strata, characterized in that, include: Step 1: Construction of the guide wall foundation (10) and reinforced concrete guide wall (1). The construction of the reinforced concrete guide wall (1) includes the processes of erecting formwork, erecting reinforcing bars, pouring concrete, and demolition. When erecting reinforcing bars, a pre-embedded plastic pipe is used to form an advanced guide hole (2). Step 2: Drill holes using the advanced guide hole (2), construct the first section of the advanced guide (3), and grout; Step 3: Construct the pilot tunnel (4) using the drill-and-blast method. When the pilot tunnel (4) is excavated to a distance of 1.5-2m from the end of the pilot guide pipe, construct the next section of the pilot guide pipe (3) and grout it. While excavating the pilot tunnel (4), drill holes and grout the top plate and shoulders of the newly excavated pilot tunnel (4). Construct the preliminary support layer (14) for the top plate of the pilot tunnel (4). The shoulder holes of the pilot tunnel (4) are drilled deep into the rock of the tunnel top plate. The preliminary support layer (14) of the top plate of the pilot tunnel (4) is constructed to a distance of 0.5-1m from the working face of the pilot tunnel (4). Step 4: Excavate the next U-shaped step. The top plate of this U-shaped step is the area covered by the shoulder borehole of the pilot tunnel (4). Drill and grout the shoulder borehole formed by this U-shaped step, and construct the preliminary support layer (14) for the top plate formed by this U-shaped step. Step 5: Excavate the U-shaped steps between the second and the lowest levels. The top plate of each U-shaped step is the area covered by the tunnel outline of the shoulder borehole of the upper U-shaped step. Drill and grout the shoulder boreholes formed by each U-shaped step. Construct the preliminary support layer (14) for the top plate and shoulder formed by each U-shaped step. The shoulder boreholes of each U-shaped step extend into the rock of the tunnel roof. Step 6: Excavate the lowest U-shaped step, drill holes and grout the tunnel footing of the lower U-shaped step; and construct the preliminary support layer of the footing (14); when the lowest U-shaped step is the next or the next two U-shaped steps, this step shall be the construction step. Step 7: Construct a permanent support layer (16) across the entire tunnel section of the already constructed preliminary support layer (14); construct one section at a time in each cycle, advancing from the tunnel entrance inwards. Step 8: Repeat steps 3, 4, 5, 6 and 7 until tunnel excavation and support are completed; Self-drilling anchors are used to drill holes in the top plate of the pilot tunnel (4). After drilling, the self-drilling anchors are not pulled out and are extended outwards. The extension length is equal to the thickness of the initial support layer (14). Then, grouting is carried out using the cavity of the self-drilling anchors. This anchor is the top plate grouting anchor (5). Self-drilling anchors are used to drill holes in the pilot tunnel and the shoulder and foot of the U-shaped step. After drilling, the self-drilling anchors are not pulled out. Then, grouting is carried out using the cavity of the self-drilling anchors. This anchor becomes the shoulder grouting anchor (7) or the foot grouting anchor (17). After the lower step is constructed, the lower part of the shoulder grouting anchor (7) and the foot grouting anchor (17) extending outside the tunnel outline is retained with a length equal to the thickness of the initial support layer (14) of the tunnel. The rest is cut off.

2. The tunnel excavation method applicable to weak or fractured rock strata according to claim 1, characterized in that, In steps two and three, a self-drilling anchor is used to construct the pre-conduit hole (2). After drilling is completed, the self-drilling anchor is not pulled out and is used as a pre-conduit (3). Grouting is carried out using the cavity of the self-drilling anchor.

3. The tunnel excavation method applicable to weak or fractured rock strata according to claim 1, characterized in that, The pilot tunnel (4) and the lower U-shaped steps are constructed using the drill-and-blast method, with the drilling direction parallel to the tunnel length.

4. The tunnel excavation method applicable to weak or fractured rock strata according to claim 1, characterized in that, The upper step of the U-shaped staircase is 1-4m ahead of the lower step.

5. The tunnel excavation method applicable to weak or fractured rock strata according to claim 3, characterized in that, The construction procedure for the preliminary support layer (14) of the top slab of the advanced pilot tunnel (4) is as follows: first, tie the preliminary support layer steel mesh (1401) to the lower end of the grouting anchor rod (5) of the top slab, then install the I-beam arch frame (1402), and fix it at both ends of the I-beam arch frame (1402) with locking foot anchor rods (8), and then spray concrete; the construction procedure for the preliminary support layer (14) of the top slab of the U-shaped step is as follows: tie the preliminary support layer steel mesh (1401) to the lower end of the grouting anchor rod (5) of the top slab and splice it with the existing top slab preliminary support layer steel mesh (1401) into a whole, then install the I-beam arch frame (1402) and splice it with the existing top slab I-beam into a whole, fix it at the lower end of the I-beam arch frame (1402) with locking foot anchor rods (8), and finally spray concrete; The construction procedure for the preliminary support layer (14) of the tunnel foot formed by the lowest U-step is as follows: the steel mesh (1401) of the preliminary support layer is tied to the outside part of the grouting anchor (17) hole of the foot and spliced ​​with the existing preliminary support layer steel mesh (1401) of the roof slab to form a whole. Then, the I-beam arch frame (1402) is installed and spliced ​​with the existing tunnel roof I-beam arch frame (1402) to form a whole. The lower end of the I-beam arch frame is fixed with the locking anchor (8). Finally, the concrete is sprayed. When spraying concrete on the top plate of the pilot tunnel (4) and the U-shaped step, the two ends of the I-beam arch frame (1402) are not covered with sprayed concrete.

6. The tunnel excavation method applicable to weak or fractured rock strata according to claim 5, characterized in that, When installing the pre-guide tunnel (4) and the U-shaped step top plate, the pre-support layer steel mesh (1401) of the top plate is installed. The construction method is as follows: self-drilling anchors are used to drill holes in the top plate. After drilling, the self-drilling anchors are not pulled out and are extended outward. The extension length is equal to the thickness of the pre-support layer (14). Then, grouting is carried out using the cavity of the self-drilling anchors. Finally, the pre-support layer steel mesh (1401) is tied to the lower end of the grouting anchor (6).

7. The tunnel excavation method for weak or fractured rock strata according to claim 5, characterized in that, The splicing method of the I-beam arch frame (1402) and the existing I-beam is as follows: each I-beam is equipped with end plates (1402-1) at both ends, with screw holes arranged on them. When splicing, the screw holes of the two end plates are aligned one by one, the screws (1402-2) are passed through and the nuts (1402-3) are tightened.

8. The tunnel excavation method applicable to weak or fractured rock strata according to claim 5, characterized in that, The lower end of each section of the I-beam arch frame (1402) is fixed with a locking foot anchor rod (8). The fixing method is as follows: the top of the two locking foot anchor rods (8) is welded with a horizontal bar (9) to tightly hug the I-beam. The upper section of the locking foot anchor rod (8) and the horizontal bar (9) are welded to the surface of the end plate (1402-1). The locking anchor (8) uses a self-drilling anchor. After drilling is completed, the self-drilling anchor is not pulled out. Then, grouting is performed using the cavity of the self-drilling anchor.

9. The tunnel excavation method applicable to weak or fractured rock strata according to claim 1, characterized in that, The alternative steps for step seven are: starting from the tunnel entrance, first construct a waterproof layer (15) across the entire tunnel cross section, and then construct a permanent support layer (16).

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