A high-stress surrounding rock tunnel construction method based on a pre-prevention and post-resistance support principle

Abstract

The application discloses a high-stress surrounding rock tunnel construction method based on a pre-prevention and post-resistance supporting principle, and steps are as follows: determining a small-hole-diameter excavation radius r 0, excavating a tunnel, calculating a surrounding rock loose circle radius R s and determining a length of a prestressed anchor rod or anchor cable L , drilling an anchor hole on a small-hole-diameter excavation profile, placing the prestressed anchor rod or anchor cable in the anchor hole and installing the prestressed anchor rod or anchor cable into a surrounding rock loose circle and original rock behind a design excavation profile, continuing to excavate the tunnel to the design excavation profile line, fixing the prestressed anchor rod or anchor cable on the design excavation profile by using an anchor device, and reinforcing the surrounding rock again by using a supporting measure. The prestressed anchor rod or anchor cable is advanced and anchored in the surrounding rock loose circle and original rock behind the design excavation profile, a radial prestress is provided, and a pressure arch is formed, so that the surrounding rock in a range from the design profile to the surrounding rock loose circle is in a three-dimensional stress state, the prevention capacity of the advanced support is fully exerted, and the strength of the surrounding rock caused by excavation unloading is prevented from being instantaneously weakened.

Description

Technical Field

[0001] This invention belongs to the field of disaster prevention and control in deep tunnels, and specifically relates to a construction method for high-stress surrounding rock tunnels based on the principle of prevention before resistance. Background Technology

[0002] Large deformation of surrounding rock and rockburst are two of the most typical geological hazards during the excavation of tunnels in high-stress surrounding rock. Large deformation is characterized by the rapid deformation rate, large deformation amount, and long deformation duration of weak surrounding rock, while rockburst refers to the ejection, cracking, or collapse of hard surrounding rock. These two geological hazards often cause serious consequences such as damage to the surrounding rock structure, destruction of the support system, and casualties. They both occur during tunnel construction, which mainly includes two steps: excavation and support. Among them, these two geological hazards are induced during the excavation process, and their occurrence characteristics are influenced by the support methods. Therefore, adopting reasonable construction methods is crucial for preventing and controlling large deformation of soft rock and rockburst hazards in hard rock.

[0003] Currently, commonly used construction methods in engineering include the full-face method, the bench method, and the central pilot tunnel method. These methods all employ a "first excavate to the tunnel outline—then actively support" approach. Before tunnel excavation, the deep rock mass is in a three-dimensional stable stress state and is not prone to geological disasters. Excavation is an unloading process. When excavation reaches the tunnel outline, the radial stress of the surrounding rock near the free surface is essentially unloaded to zero, while the circumferential stress increases. The stress state changes from three-dimensional to two-dimensional, reducing the strength of the surrounding rock and making it susceptible to plastic failure or plastic flow in soft rock and brittle failure in hard rock. In severe cases, large deformations and rockburst disasters may occur. At this point, when active support is used, such as prestressed anchor bolts or anchor cables, the stress state of the surrounding rock changes from two-dimensional to three-dimensional, thereby increasing the strength of the surrounding rock and forming a unified whole with the support structure. This creates a pressure arch effect around the tunnel outline to bear the pressure of the surrounding rock. As can be seen from the above construction process, the commonly used support method is based on the principle of "yielding first, then resisting." "Yielding first" refers to excavating first to allow the surrounding rock to undergo unloading, while "resisting later" refers to providing support after excavation and unloading to resist deformation or brittle failure of the surrounding rock. This principle has several drawbacks: the unloading effect during excavation causes radial stress unloading and circumferential stress concentration within the surrounding rock, leading to a decrease in rock strength; a relatively long period is required before support can be completed, while the ground stress adjusts very drastically immediately after excavation. Before support is completed, soft rock exhibits a large deformation rate and amount, while hard surrounding rock faces a significant risk of rockburst; due to the reduced strength of the surrounding rock after excavation, the pressure arch effect formed by the support structure and support body may be insufficient to withstand high ground pressure stress, resulting in poor effectiveness in subsequent disaster prevention. These drawbacks make the current construction method based on the "yielding first, then resisting" principle less effective in preventing large deformations and rockburst disasters in high-stress surrounding rock tunnels.

[0004] To address the shortcomings of construction methods based on the "yield first, then resist" principle, there is an urgent need to propose a new principle to guide construction. Therefore, this invention proposes a construction method for high-stress surrounding rock tunnels based on the principle of "prevention first, then resistance," so as to fully, quickly, and effectively utilize the advanced protective function of the support structure and the bearing capacity of the surrounding rock. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a construction method for high-stress surrounding rock tunnels based on the principle of first-prevention and then-resistance support, aiming to solve problems such as the current construction method where the support lags behind the excavation and unloading of the surrounding rock, and the insufficient bearing capacity of the pressure arch effect formed by the support structure and the surrounding rock.

[0006] The technical solution adopted in this invention is as follows:

[0007] Construction method for high-stress surrounding rock tunnels based on the principle of first preventing and then resisting support.

[0008] Step 1: Determine the initial small-diameter excavation radius r0; this small-diameter excavation radius r0 is smaller than the radius r of the designed excavation outline. d The radius R of the loosened zone of the surrounding rock when excavating with a small-diameter excavation radius r0 is calculated. This radius R does not exceed the radius r of the designed excavation outline. d Excavate the tunnel with a defined small-diameter excavation radius r0;

[0009] Step 2: Calculate the radius R of the loosened zone of the surrounding rock when excavating along the tunnel design excavation outline. s Determine the length L of the prestressed anchor rod or anchor cable to be used. Start drilling anchor holes at the excavation outline after the small-diameter excavation in step 1. The anchor holes pass through the loosened zone outline of the surrounding rock during the first small-diameter excavation and the designed excavation outline in sequence. The end of the anchor hole is located in the surrounding rock behind the loosened zone outline of the surrounding rock when excavating with the designed excavation outline.

[0010] Step 3: Place the prestressed anchor rod or anchor cable in the anchor hole and install it into the loose ring and original rock behind the designed excavation profile; and anchor the prestressed anchor rod or anchor cable in the anchor hole through the anchoring-tensioning-anchoring method;

[0011] Step 4: Continue excavating towards the designed excavation outline using the initial small-diameter excavation radius r0 from Step 1, until the tunnel is finally excavated to the designed excavation outline.

[0012] Step 5: Use anchorages to fix the prestressed anchor rods or anchor cables that were anchored behind the designed excavation outline in Step 2 onto the designed excavation outline surface.

[0013] Step 6: Reinforce the surrounding rock again using support measures.

[0014] The specific method for step 1 is as follows:

[0015] 1) Determine the excavation radius r0 of the small-diameter tunnel to be excavated initially.

[0016] The initial excavation with a diameter smaller than the designed tunnel diameter serves two purposes: firstly, to relieve some of the surrounding rock pressure, and secondly, to create a protective layer of a certain depth in front of the designed excavation profile. This helps prevent the loosening zone, formed by the adjustment of internal stress under excavation load and confining pressure, from extending into the rock mass behind the designed excavation profile. Since the loosening zone is the most dangerous area for large deformations or rock bursts, the initial excavation with a small diameter (assuming an initial excavation radius of r0) must meet the following condition: the radius R of the loosening zone formed after excavation must not exceed the designed excavation radius r. d To meet the above conditions, the initial excavation radius r0 can be determined using empirical methods, numerical analysis methods, and theoretical analysis methods. Empirical methods are determined by engineers based on the specific engineering background and excavation plan. Numerical analysis methods involve inputting the physical and mechanical parameters of the surrounding rock into numerical simulation software, analyzing the evolution of the plastic zone after excavation to determine the loosened zone radius, and then back-calculating the initial excavation radius r0. When there is no relevant engineering experience or numerical analysis conditions available, existing theoretical solutions can be used for calculation. In this regard, Xu Gancheng et al. conducted elastoplastic analysis of the surrounding rock stress under bidirectional isobaric conditions and obtained the theoretical formula for the loosened zone radius as follows:

[0017]

[0018] In the formula: R is the radius of the loosened zone (m), r is the equivalent excavation radius (m), P is the initial original rock stress (MPa), P i The support resistance is given in MPa, and c and φ are the internal friction angle (°) and cohesion (MPa) of the surrounding rock, respectively.

[0019] It should be noted that the equivalent excavation radius r in formula (1) is the radius of a circular tunnel, and for other tunnel cross-sectional shapes, it is the minimum circumscribed circle radius of the excavation profile. Formula (1) can be directly applied to the case where the lateral pressure coefficient λ = 1 (bidirectional isobaric state). For the case where the lateral pressure coefficient λ is not 1 (bidirectional unequal pressure state), the bidirectional unequal pressure state can be adjusted to the bidirectional isobaric state using formula (2):

[0020]

[0021] In the formula, λ is the lateral pressure coefficient (the ratio of the initial horizontal stress to the vertical stress), and P' is the equivalent surrounding rock pressure. When applying formula (1) under bidirectional unequal pressure, P is converted to P' for solution (at this time, the initial original rock stress P calculated by substituting into formula (2) is the larger value among the initial horizontal and vertical stresses).

[0022] When the condition "the radius of the loosened zone R does not exceed the design excavation radius r" is met...d "Sometimes:"

[0023]

[0024] From formula (3), the initial excavation radius r0 should satisfy:

[0025]

[0026] Therefore, the maximum upper limit of the initial excavation radius r0 can be determined according to formula (4). In addition, since subsequent support construction of the surrounding rock is required, the excavation radius needs to meet the space requirements of on-site construction, so the lower limit of the initial excavation radius r0 can be determined accordingly. The final value of r0 can be selected within the upper and lower limits according to the actual situation.

[0027] 2) Excavate the tunnel according to the initial small-diameter excavation radius r0 determined in step 1).

[0028] The specific method for step 2 is as follows:

[0029] (1) Calculate the radius R of the loosened zone when excavation reaches the design outline. s

[0030] The radius R of the loosened zone of the surrounding rock when excavation reaches the design profile surface is calculated using theoretical analysis. s The calculation formula is:

[0031]

[0032] In the formula: r d The design excavation radius (m).

[0033] The radius R of the loosened zone when excavating to the designed profile surface can be estimated using formula (5). s .

[0034] (2) Based on the actual engineering design requirements, determine the length L of the prestressed anchor bolt or anchor cable to be used, and ensure that the length of the prestressed anchor bolt or anchor cable exceeds the loosening zone depth H after the designed excavation profile. The loosening zone depth is: H = R s -r d Furthermore, the end anchorage section of the prestressed anchor rod or anchor cable is located after the loosening zone.

[0035] (3) Start drilling anchor holes at the excavation outline after the first small-diameter excavation. The anchor holes pass through the loosened zone outline of the surrounding rock during the first small-diameter excavation and the designed excavation outline in sequence. The end of the anchor hole is located in the surrounding rock behind the loosened zone outline of the surrounding rock during the excavation with the designed excavation outline. The drilling depth is equal to the length L of the prestressed anchor rod or anchor cable determined in step (2).

[0036] The specific method for step 3 is as follows:

[0037] (1) The prestressed anchor rod or anchor cable adopts the two-end anchoring type, that is, the end of the anchor rod or anchor cable at the bottom of the borehole is anchored and the tail of a certain length near the excavation outline is also anchored. The specific steps are as follows:

[0038] 1) Insert the prestressed anchor rod or anchor cable into the anchor rod or anchor cable borehole;

[0039] 2) Fill the end anchoring section of the prestressed anchor rod or anchor cable with anchoring agent to bond the anchor rod or anchor cable to the surrounding rock. At this time, the tail anchoring section of the prestressed anchor rod or anchor cable is located in the surrounding rock behind the designed excavation outline.

[0040] 3) Use a tensioning device to tension the prestressed anchor rod or anchor cable to the designed prestress value, and then fill the tail anchoring section with anchoring agent to bond the prestressed anchor rod or anchor to the surrounding rock; leave a section of prestressed anchor rod or anchor cable in the tail anchoring section;

[0041] 4) Remove the tensioning device.

[0042] This invention proposes a construction method for high-stress surrounding rock tunnels based on the principle of first-prevention and then-resistance support. The specific implementation process of this invention is described in [link to invention]. Figure 1 .

[0043] Compared with the prior art, the present invention achieves at least the following beneficial effects:

[0044] (1) The method of the present invention has the advantage of "prevention" for large deformation and rock burst disasters in tunnels. On the one hand, the use of small-diameter excavation can relieve part of the surrounding rock pressure in advance, thereby suppressing the situation where the surrounding rock pressure is too large when excavating to the design outline surface and causing serious damage to the remaining surrounding rock, which can effectively protect the surrounding rock after the design outline surface; on the other hand, the use of prestressed anchor rods or anchor cables with anchors at both ends after small-diameter excavation can provide radial stress to the remaining surrounding rock during the process of excavation to the design outline surface, so that the surrounding rock is in a three-dimensional stress state to suppress the instantaneous weakening and excessive overall weakening of the surrounding rock, thereby preventing large deformation or excessively fast deformation rate in soft rock in the early stage of excavation completion, and preventing hard rock from sudden brittle failure due to insufficient bearing capacity of the surrounding rock;

[0045] (2) The method of the present invention has the advantage of "post-excavation resistance" against large deformation and rock burst disasters in tunnels. On the one hand, before the excavation is completed, the prestressed anchor rods or anchor cables can be bonded to the surrounding rock behind the designed contour surface to form a strong pressure arch effect to resist the pressure of the surrounding rock after excavation; on the other hand, after the excavation is completed, the prestressed anchor rods or anchor cables that have been installed can also play a role in resisting the deformation of the surrounding rock, thus gaining effective time for the construction of the subsequent support system.

[0046] (3) Traditional methods based on the "yield first, then resist" principle often result in significant damage and deformation of the surrounding rock before the support is completed. Therefore, even after the support is completed, the pressure arch effect formed by the prestressed anchor bolts or cables and the surrounding rock is insufficient to withstand the confining pressure. Compared with the two beneficial effects mentioned above, the high-stress surrounding rock tunnel construction method proposed in this invention based on the principle of "prevention first, then resistance" has no similar methods in practice. Compared with the traditional "yield first, then resistance" principle, it has significant advantages in controlling large deformation of the surrounding rock and preventing rockbursts, and has great creativity and novelty. In addition, the construction technology involved in the excavation and support process involved in this invention is currently relatively mature, and the construction can be carried out efficiently on site according to the method of this invention, which has great practicality. Attached Figure Description

[0047] Figure 1 This is a flowchart of an embodiment of the method of the present invention;

[0048] Figure 2 This is a schematic diagram of the contour lines, excavation radius, and loosening zone radius in an embodiment of the present invention;

[0049] Figure 3 This is a schematic diagram of the drilling and prestressed anchor bolt or anchor cable arrangement after the first small-diameter excavation in an embodiment of the present invention;

[0050] Figure 4 This is a diagram showing the arrangement of prestressed anchor bolts or anchor cables and anchorage sections on the right side wall of the tunnel in an embodiment of the present invention, after the design outline.

[0051] Figure 5 This is a diagram showing the arrangement of prestressed anchor bolts or anchor cables along the entire tunnel excavation outline after the first small-diameter excavation in this embodiment of the invention.

[0052] Figure 6 In this embodiment of the invention, a prestressed anchor rod or anchor cable is used to fix the prestressed anchor rod or anchor cable at the excavation outline.

[0053] Figure 7 This is a diagram showing the arrangement of prestressed anchor bolts or anchor cables for the entire outline after excavation to the design outline in an embodiment of the present invention.

[0054] The markings in the attached diagram are explained as follows:

[0055] 1. Excavation outline during the initial small-diameter excavation; 2. Excavation radius r0 during the initial small-diameter excavation; 3. Outline of the loosened zone of the surrounding rock during the initial small-diameter excavation; 4. Radius of the loosened zone of the surrounding rock during the initial small-diameter excavation; 5. Designed excavation outline; 6. Designed excavation radius r d 7. The outline of the loosened zone of the surrounding rock when excavating to the design outline surface; 8. The radius R of the loosened zone of the surrounding rock when excavating to the design outline surface. s9. Anchor bolt or anchor cable borehole; 10. Prestressed anchor bolt or anchor cable; 11. End of prestressed anchor bolt or anchor cable; 12. Tail of prestressed anchor bolt or anchor cable; 13. End anchorage section of prestressed anchor bolt or anchor cable; 14. Tail anchorage section of prestressed anchor bolt or anchor cable; 15. Free section of prestressed anchor bolt or anchor cable; 16. Anchor; 17. Anchor bolt or anchor cable connection device; 18. Anchor bolt or anchor cable tray; 19. Anchor bolt or anchor cable fixing bolt. Detailed Implementation

[0056] The flowchart of the method embodiment of the present invention is shown below. Figure 1 Furthermore, to facilitate the use of the method by those skilled in the art, taking the construction of a circular tunnel as an example, and in conjunction with the accompanying drawings of the embodiments of the present invention, the technical solutions of the embodiments of the present invention will be clearly and completely described. It should be noted that the following embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art are within the scope of protection of the present invention.

[0057] In this embodiment of the invention, the initial horizontal stress of the circular tunnel section to be excavated is 20 MPa, the initial vertical stress is 30 MPa, the cohesion of the surrounding rock is c = 1.0 MPa, the internal friction angle is φ = 35°, and the radius r of the designed excavation profile is... d =6m. This invention discloses a construction method for high-stress surrounding rock tunnels based on the principle of first-prevention and then-resistance support, comprising the following steps:

[0058] Step 1: Determine the initial small-diameter excavation radius r0; this small-diameter excavation radius r0 is smaller than the radius r of the designed excavation outline. d The radius R of the loosened zone of the surrounding rock when excavating with a small-diameter excavation radius r0 is calculated. This radius R does not exceed the radius r of the designed excavation outline. d Excavate the tunnel with a defined small-diameter excavation radius r0;

[0059] Specifically, to determine the initial excavation diameter r0 of the tunnel section to be excavated in this embodiment of the invention (e.g., Figure 2 As shown), this embodiment uses theoretical analysis to calculate the stress state of the section to be excavated (initial horizontal stress is 15MPa, vertical stress is 20MPa), which is a bidirectional unequal pressure situation. According to formula (2), the lateral pressure coefficient λ=15 / 20=0.75 can be obtained, and the equivalent surrounding rock pressure P'=[(1+0.75) / 2]×20MPa=17.5MPa; from c=1.0MPa, φ=35°, P'=17.5MPa and r d =6m, and since no support is provided at the excavation outline during the initial small-diameter excavation, the support resistance P i=0MPa, so according to formula (4), the radius of the first excavation r0 ≤ 3.73m can be obtained. The upper limit of the radius of the first excavation r0 is 3.73m. Since the on-site construction and subsequent equipment installation require at least 5m (length, width and height) of space, the lower limit of r0 is 5 / 2m = 2.5m. Therefore, the range of the radius of the first excavation r0 is 2.5m ≤ r0 ≤ 3.73m, and finally r0 = 3.5m is selected.

[0060] Step 2: Calculate the radius R of the loosened zone of the surrounding rock when excavating along the tunnel design excavation outline. s Determine the length L of the prestressed anchor rod or anchor cable to be used. Start drilling anchor holes at the excavation outline after the small-diameter excavation in step 1. The anchor holes pass through the loosened zone outline of the surrounding rock during the first small-diameter excavation and the designed excavation outline in sequence. The end of the anchor hole is located in the surrounding rock behind the loosened zone outline of the surrounding rock when excavating with the designed excavation outline.

[0061] Specifically, first estimate the radius R of the loosened zone of the surrounding rock when excavating to the designed outline surface. s (like Figure 2 As shown), when using theoretical analysis, r d =6m, c=1.0MPa, φ=35° and P'=17.5MPa, take the support resistance P i =0MPa, according to formula (5), the radius R of the loosened zone of the surrounding rock when excavating to the design outline surface can be obtained. s =9.65m. In this embodiment, prestressed anchor bolts are selected for support. Therefore, the following description will continue to illustrate the invention using prestressed anchor bolt support. To determine the length of the prestressed anchor bolts, it is necessary to calculate the loosening zone depth H = R after designing the excavation profile. s -r d =9.65m-6m=3.65m, therefore the tail anchoring section of the anchor rod needs to be behind the loosening zone depth, so the length of the prestressed anchor rod is selected as 8m. Among them, the lengths behind and in front of the loosening zone are 4m each, the end anchoring section behind the loosening zone is 2m, and the free section is 2m; the tail anchoring section in front of the loosening zone is 2m, and the free section is 2m. Drill anchor holes at the excavation outline line during the first small-diameter excavation. Drill the anchor holes to the surrounding rock behind the loosening zone outline line when excavating to the design outline surface. The drilling depth is equal to the length L of the prestressed anchor rod or anchor cable determined in step (2) (e.g., Figure 3 (As shown).

[0062] Step 3: Place the prestressed anchor rod or anchor cable in the anchor hole and install it into the loose ring and original rock behind the designed excavation profile; and anchor the prestressed anchor rod or anchor cable in the anchor hole through the anchoring-tensioning-anchoring method;

[0063] Specifically, prestressed anchor bolts are placed inside the borehole, and anchoring agent is filled at the end anchoring section to bond the anchor bolt body to the surrounding rock. The anchor bolts are tensioned to the designed prestress value using a tensioning device, and then anchoring agent is placed at the tail anchoring section to bond the anchor bolt body to the surrounding rock (e.g., ...). Figure 4 (As shown). Finally, remove the tensioning device. The above steps are also applied to the anchor bolt installation positions around the excavation perimeter. A schematic diagram of the prestressed anchor bolts after installation is shown below. Figure 5 As shown.

[0064] Step 4: Continue excavating towards the designed excavation outline using the initial small-diameter excavation radius r0 from Step 1, until the tunnel is finally excavated to the designed excavation outline.

[0065] Specifically, the tunnel section to be excavated in this embodiment continues to be excavated until the designed excavation outline is reached. At this point, the radius of the excavated tunnel is equal to the designed excavation outline radius r. d =6m.

[0066] Step 5: Use anchorages to fix the prestressed anchor rods or anchor cables that were anchored behind the designed excavation outline in Step 2 onto the designed excavation outline surface.

[0067] Specifically, the prestressed anchor bolts installed in step 3 above are fixed to the excavation contour surface using anchors. In this embodiment, the following method is adopted: Figure 6 The anchor 16 shown is used to install anchor bolts. Anchor 16 consists of an anchor bolt connecting device 17, a tray 18, and fixing bolts 19. First, the connecting device 17 is connected to the anchor bolt body 15 inside the anchor bolt hole. Then, the tray 18 is fitted onto the connecting device 17, and finally, the bolts 17 are screwed onto the connecting device 17. There are many types of existing anchors, and the production technology is very mature. In practical applications, users can choose anchors that match the anchor bolts or anchor cables according to the actual situation to fix the anchor bolts or anchor cables. Finally, the schematic diagram of the prestressed anchor bolt arrangement along the entire excavation outline is shown below. Figure 7 As shown.

[0068] Step 6: Reinforce the surrounding rock again using support measures.

[0069] Specifically, in order to further reinforce the surrounding rock of the tunnel in this embodiment, after excavation to the designed excavation outline, the following measures were taken to reinforce the surrounding rock: large-diameter, high-prestressed anchor cables were installed around the excavation outline, and steel mesh was fixed to the surface of the surrounding rock and sprayed with plain concrete.

[0070] Thus, the high-stress surrounding rock tunnel construction method based on the principle of first prevention and then resistance support, as described in this embodiment, has been realized in practice.

[0071] It should be noted that the above examples are only used to describe the method of the present invention in detail, and are not intended to limit its scope of application. Those skilled in the art can make modifications and substitutions to the method of the present invention, but this does not mean that it departs from the scope defined by the claims of the present invention.

Claims

1. A construction method for high-stress surrounding rock tunnels based on the principle of first-prevention and then-resistance support, characterized in that, Includes the following steps: Step 1: Determine the initial small-diameter excavation radius r0; this small-diameter excavation radius r0 is smaller than the radius r of the designed excavation outline. d The radius R of the loosened zone of the surrounding rock when excavating with a small-diameter excavation radius r0 is calculated. This radius R does not exceed the radius r of the designed excavation outline. d Excavate the tunnel with a defined small-diameter excavation radius r0; Step 2: Calculate the radius R of the loosened zone of the surrounding rock when excavating along the tunnel design excavation outline. s Determine the length L of the prestressed anchor rod or anchor cable to be used. Start drilling anchor holes at the excavation outline after the small-diameter excavation in step 1. The anchor holes pass through the loosened zone outline of the surrounding rock during the first small-diameter excavation and the designed excavation outline in sequence. The end of the anchor hole is located in the surrounding rock behind the loosened zone outline of the surrounding rock when excavating with the designed excavation outline. Step 3: Place the prestressed anchor rod or anchor cable in the anchor hole and install it into the loose ring and original rock behind the designed excavation profile; and anchor the prestressed anchor rod or anchor cable in the anchor hole through the anchoring-tensioning-anchoring method; Step 4: Continue excavating towards the designed excavation outline using the initial small-diameter excavation radius r0 from Step 1, until the tunnel is finally excavated to the designed excavation outline. Step 5: Use anchorages to fix the prestressed anchor rods or anchor cables that were anchored behind the designed excavation outline in Step 2 onto the designed excavation outline surface. Step 6: Reinforce the surrounding rock again using support measures.

2. The construction method for high-stress surrounding rock tunnels based on the principle of first-prevention and then-resistance support, as described in claim 1, is characterized in that... The specific method for step 1 is as follows: 1) Determine the excavation radius r0 of the small-diameter tunnel to be excavated initially. Let the initial excavation radius be r0. When excavating with a small diameter for the first time, the following condition must be met: the radius R of the loosened zone outline formed after excavation does not exceed the designed excavation radius r. d ; To meet the above conditions, the initial excavation radius r0 is determined using theoretical analysis. The radius R of the loosened zone outline is obtained using the following formula: (1) Where: R is the radius of the loosened zone outline, m; r is the equivalent excavation radius, m; P is the initial original rock stress, MPa; P i φ represents the support resistance, MPa; c represents the cohesion of the surrounding rock, MPa; φ represents the internal friction angle of the surrounding rock, °. In the above formula (1), the equivalent excavation radius r is taken as the radius of the circular tunnel. When the lateral pressure coefficient λ is not 1, the bidirectional unequal pressure state is adjusted to a bidirectional isobaric state using formula (2): (2) In the formula, λ is the lateral pressure coefficient and P' is the equivalent surrounding rock pressure. When applying formula (1) under bidirectional unequal pressure conditions, P is converted to P' for solution. When the radius R of the loosening zone outline does not exceed the design excavation radius r d The following formula is established under certain conditions: (3) From formula (3), the initial excavation radius r0 should satisfy: (4) Therefore, the maximum upper limit of the initial excavation radius r0 is determined according to formula (4); 2) Excavate the tunnel according to the initial excavation radius r0 determined in step 1).

3. A construction method for high-stress surrounding rock tunnels based on the principle of first-prevention and then-resistance support, as described in claim 1 or 2, is characterized in that... The specific method for step 2 is as follows: 1) Calculate the radius R of the loosened zone when excavation reaches the design outline. s Calculate the radius R of the loosened zone of the surrounding rock when excavating to the design outline using formula (5). s : (5) In the formula: r d For the equivalent design excavation radius, m; 2) Based on the actual engineering design requirements, determine the length L of the prestressed anchor bolt or cable to be used, ensuring that the length of the prestressed anchor bolt or cable exceeds the loosening zone depth H after the designed excavation profile. The loosening zone depth is: H = R s -r d Furthermore, the end anchorage section of the prestressed anchor rod or anchor cable is located after the loosening zone. 3) Start drilling anchor holes at the excavation outline after the first small-diameter excavation. The anchor holes pass through the loosened zone outline of the surrounding rock during the first small-diameter excavation and the designed excavation outline in sequence. The end of the anchor hole is located in the surrounding rock behind the loosened zone outline of the surrounding rock during the excavation with the designed excavation outline. The drilling depth is equal to the length L of the prestressed anchor rod or anchor cable determined in step (2).

4. A construction method for high-stress surrounding rock tunnels based on the principle of first-prevention and then-resistance support, as described in claim 1 or 3, characterized in that... Prestressed anchor bolts or anchor cables adopt a two-end anchorage type, with the two ends being the end anchorage section and the tail anchorage section, respectively. The end of the anchor rod or cable at the bottom of the borehole is the end anchorage section, and the tail of the anchor rod or cable near the borehole opening is the tail anchorage section; anchorage is carried out within a certain length of the tail of the anchor rod or cable near the excavation outline. The specific steps for anchoring prestressed anchor bolts or cables are as follows: 1) Place the prestressed anchor rod or anchor cable into the anchor hole drilled in step 2; 2) Fill the end anchoring section of the prestressed anchor rod or anchor cable with anchoring agent to bond the anchor rod or anchor cable to the surrounding rock. At this time, the tail anchoring section of the prestressed anchor rod or anchor cable is located in the surrounding rock behind the designed excavation outline. 3) Use a tensioning device to tension the prestressed anchor rod or anchor cable to the designed prestress value, and then fill the tail anchoring section with anchoring agent to bond the prestressed anchor rod or anchor to the surrounding rock; leave a section of prestressed anchor rod or anchor cable in the tail anchoring section; 4) Remove the tensioning device; 5) After excavating to the designed excavation outline in step 4, use anchors to fix and connect the reserved prestressed anchor rods or anchor cables.

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