Method for determining safe distance of secondary lining construction of first hole in small-spacing tunnel blasting excavation

Abstract

This invention discloses a method for determining the safe distance for secondary lining construction of the pilot tunnel in small-clearance tunnel blasting excavation, which involves judging the stability of the surrounding rock deformation and deformation rate, and determining the safe allowable distance R. i Given the range of values, design an orthogonal test scheme for on-site vibration velocity monitoring, and calculate the safe distance R for each scheme. i First, based on the safe allowable distance Ri, monitoring points for blasting vibration velocity are set up, and on-site vibration monitoring is conducted. A unique K value and α value are obtained by fitting parameters such as the measured vibration velocity and explosive charge, and the calculated safe allowable distance R is then calculated. j Determine the conservative safety allowable distance R b Does it meet the requirements? If it does, then it is the final safe allowable distance R. z This scheme can determine the safe distance between the blasting source of the subsequent tunnel and the secondary lining of the preceding tunnel, thereby reducing the impact of vibrations generated by the blasting excavation of the subsequent tunnel on the secondary lining of the preceding tunnel.

Description

Technical Field

[0001] This invention belongs to the field of tunnel construction technology and relates to a method for determining the safe distance for the construction of the secondary lining of the pilot tunnel in small-clearance tunnel blasting excavation. Background Technology

[0002] Tunnels are engineering structures excavated underground or within mountains, widely used in transportation, water conservancy, energy, and other fields. During tunnel construction, to ensure safety and stability, support structures are typically employed, including initial support and secondary lining. In the blasting excavation of tunnels with small clearances, due to space constraints or other factors, small-scale blasting excavation is necessary near the existing tunnel structure. The vibrations generated by the blasting can affect the secondary lining, potentially leading to cracking, spalling, and reduced service life. Special care must be taken during construction.

[0003] During tunnel excavation, secondary lining is usually constructed after the initial support is completed, and only after ensuring that the initial support has met the design requirements and has sufficient strength and stability. The technical specifications for highway construction stipulate that secondary lining can be carried out when the following conditions are met: (1) the displacement rate of each test item has converged significantly and the surrounding rock is basically stable; (2) the displacement of each item has reached 80% to 90% of the expected total displacement; (3) the displacement rate of the perimeter is less than 0.1 to 0.2 mm / d, or the settlement rate of the arch is less than 0.07 to 0.15 mm / d, that is, the secondary lining can be carried out after the total deformation and deformation rate of the surrounding rock reach a certain value. The impact of blasting vibration on the timing of tunnel secondary lining construction is not considered. However, when blasting excavation of small-clearance tunnels, due to the limited construction space, if the blasting source of the subsequent tunnel is close to the secondary lining of the preceding tunnel, the vibration generated by the blasting of the subsequent tunnel will have a significant impact on the preceding tunnel, especially on the secondary lining of the preceding tunnel. This may lead to cracks and spalling in the secondary lining, thereby affecting the performance and service life of the secondary lining of the preceding tunnel.

[0004] In summary, there is an urgent need to invent a method for determining the safe distance for the secondary lining of the pilot tunnel during blasting excavation of tunnels with small clearance, to determine the safe distance between the blasting source of the subsequent tunnel and the secondary lining of the pilot tunnel, and to reduce the impact of vibrations generated by the blasting excavation of the subsequent tunnel on the secondary lining of the pilot tunnel. Summary of the Invention

[0005] The purpose of this invention is to provide a method for determining the safe distance for the secondary lining of the tunnel under blasting excavation in small-clearance tunnels. This method determines the safe distance between the blasting source of the subsequent tunnel and the secondary lining of the tunnel under blasting, thereby reducing the impact of vibrations generated by the blasting excavation of the subsequent tunnel on the secondary lining of the tunnel under blasting.

[0006] The technical solution adopted in this invention is a method for determining the safe distance for the secondary lining construction of the pilot tunnel in small-clearance tunnel blasting excavation, comprising the following steps:

[0007] Step 1: Determine the stability of the surrounding rock deformation and deformation rate, determine the orthogonal test scheme for in-situ vibration velocity monitoring, and obtain the safe allowable distance through the in-situ vibration velocity monitoring orthogonal test. {R i ＝ R i1 R i2 R i3…… R in } ;

[0008] Step 2: Based on the safe allowable distance obtained in Step 1 R i Design and implement a field vibration velocity monitoring scheme, and calculate the safe allowable distance R. j ;

[0009] Step 3: The safe allowable distance R obtained from Step 2 j Calculate the conservative safety allowable distance R b Determine the conservative safety allowable distance R b Does it meet the requirements? If it does, then it is the final safe allowable distance R. z If the requirements are not met, continue calculating the conservative safety allowable distance R. b Until the final safe allowable distance R is obtained, the requirements are met. z .

[0010] The invention is further characterized by:

[0011] Step 1 includes the following steps:

[0012] Step 1.1: Determine whether the cumulative deformation and deformation rate of the surrounding rock of the pilot tunnel of the small clearance tunnel have reached the values ​​specified in the standard and whether it is basically stable. Initial support has been applied behind the tunnel blasting face of the pilot tunnel of the small clearance tunnel.

[0013] Step 1.2: Determine the lithology of the surrounding rock and determine the range of values ​​for K and α in the Sadovsky formula based on the lithology of the surrounding rock.

[0014] Step 1.3: Design a two-factor, n-level orthogonal test scheme for field vibration velocity monitoring, and determine the safe allowable distance R according to the type of protected object. i .

[0015] Step 1.3 is carried out according to the following steps: using the K value, α value, and amount of explosives used for blasting the tunnel behind the small clearance tunnel obtained in Step 1.2, calculate the safe allowable distance R for each orthogonal test. i To obtain the safe allowable distance R i set {R i =R i1 Ri2 R i3…… R in}, safe allowable distance R i It is the distance between the blasting source of the tunnel following the small clearance tunnel and the arch of the tunnel preceding the small clearance tunnel.

[0016] The ranges of K and α are divided into n values ​​according to the arithmetic sequence, where n∈[2,9] and n is a positive integer.

[0017] Step 2 includes the following steps:

[0018] Step 2.1: Based on the safe allowable distance R obtained in Step 1.3 i The monitoring point for blasting vibration velocity at the arch waist of the tunnel with small clearance was determined, and vibration velocity sensors were installed at the monitoring point to monitor the blasting vibration velocity and obtain the measured vibration velocity value at the point.

[0019] Step 2.2: Based on the actual amount of explosives used, multiple sets of measured vibration values, and the distance between the blasting source and the blasting vibration velocity monitoring point, fit the K and α values, and substitute the K and α values ​​into the Sadovsky formula to calculate the safe allowable distance. R i .

[0020] The blasting vibration velocity monitoring points in step 2.3 include the first vibration velocity monitoring point, the second vibration velocity monitoring point, the third vibration velocity monitoring point, ... the nth vibration velocity monitoring point.

[0021] Conservative safety allowable distance R b The calculation formula is as follows:

[0022] R b =R j *M

[0023] In the formula, M is the safety factor, M∈[1,2].

[0024] Step 3 is performed according to the following steps:

[0025] Step 3.1, Pass through the safe allowable distance R j The conservative safety allowable distance R is calculated. b According to the conservative safety allowable distance R b The monitoring and verification point 23 for blasting vibration velocity at the arch waist of the pilot tunnel with small clearance was determined and blasting vibration velocity was monitored. The vibration monitoring point is located at the arch waist of the pilot tunnel, which is the location with the greatest vibration influence on the same cross section.

[0026] Step 3.2: Determine whether the vibration velocity exceeds the safe allowable vibration velocity for blasting vibration. If it meets the safe allowable vibration velocity for blasting vibration, then the conservative safe allowable distance R is...b That is, the final safe allowable distance R. z ;

[0027] Step 3.3: If the condition is not met, increase the value of M and recalculate the conservative safety allowable distance R. b Continue monitoring of blasting vibration safety until a safe distance R is maintained. b If the vibration velocity at the monitoring point is within the allowable vibration velocity for blasting vibration, the final safe distance that meets the allowable vibration velocity for blasting vibration is the final safe distance R. z .

[0028] The beneficial effects of this invention are: the method for determining the safe distance for the secondary lining construction of the pilot tunnel in the blasting excavation of a small-clearance tunnel provided by this invention, based on the standard stipulation that only the deformation and stability of the surrounding rock should be considered when constructing the secondary lining, further considers the impact of the vibration generated by the subsequent blasting excavation of the small-clearance tunnel on the pilot tunnel, and provides a method for determining the safe distance for the secondary lining construction of the pilot tunnel in the blasting excavation of a small-clearance tunnel, thus ensuring the structural safety and service life of the pilot tunnel lining. Attached Figure Description

[0029] Figure 1 This is a flowchart of the method of the present invention;

[0030] Figure 2 This is a schematic diagram of the overall layout of the blasting vibration velocity monitoring points in the 2-factor, 3-level orthogonal test scheme of Embodiment 1 of the present invention;

[0031] Figure 3 This is a schematic diagram of the layout of the blasting vibration velocity monitoring and verification points according to Embodiment 1 of the present invention;

[0032] In the diagram, 1. Pre-tunnel with small clearance; 2. Subsequent tunnel with small clearance; 3. Surrounding rock; 4. Blasting source; 5. First vibration velocity monitoring point; 6. Second vibration velocity monitoring point; 7. Third vibration velocity monitoring point; 8. Fourth vibration velocity monitoring point; 9. Fifth vibration velocity monitoring point; 10. Sixth vibration velocity monitoring point; 11. Seventh vibration velocity monitoring point; 12. Eighth vibration velocity monitoring point; 13. Ninth vibration velocity monitoring point; 14. Safe allowable distance R. i1 15. Safe allowable distance R i2 16. Safe allowable distance R i3 17. Safe allowable distance R i4 18. Safe allowable distance R i5 19. Safe allowable distance R i6 20. Safe allowable distance R i7 21. Safe allowable distance R i8 22. Safe allowable distance R i923. Verification points for monitoring blasting vibration velocity; 24. Conservative safety allowable distance R. b . Detailed Implementation

[0033] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0034] The present invention relates to a method for determining the safe distance for the construction of the secondary lining of the pilot tunnel in small-clearance tunnel blasting excavation, the process of which is as follows: Figure 1 As shown, it includes the following steps:

[0035] Step 1: Determine the stability of the deformation amount and deformation rate of the surrounding rock 3, determine the orthogonal test scheme for on-site vibration velocity monitoring, and obtain the safe allowable distance through the on-site vibration velocity monitoring orthogonal test. {R i =R i1 R i2 R i3…… R in } ;

[0036] Step 2: Based on the safe allowable distance obtained in Step 1 R i Design and implement a field vibration velocity monitoring scheme, and calculate the safe allowable distance R. j ;

[0037] Step 3: The safe allowable distance R obtained from Step 2 j Calculate the conservative safety allowable distance R b Determine the conservative safety allowable distance R b Does it meet the requirements? If it does, then it is the final safe allowable distance R. z If the requirements are not met, continue calculating the conservative safety allowable distance R. b Until the final safe allowable distance R is obtained, the requirements are met. z .

[0038] Step 1 includes the following steps:

[0039] Step 1.1: Determine whether the cumulative deformation and deformation rate of the surrounding rock 3 of the pilot tunnel 1 of the small clearance tunnel have reached the value specified in the standard and whether it is basically stable. Initial support has been applied behind the tunnel blasting face of the pilot tunnel 1 of the small clearance tunnel.

[0040] Step 1.2: Determine the lithology of the surrounding rock 3. Based on the lithology of the surrounding rock 3, determine the range of values ​​for K and α in the Sadovsky formula. The lithology is divided into three types: hard, medium hard, and soft rock. Refer to Table 1 for the parameter value range.

[0041] Table 1

[0042] Lithology K α hard 50～150 1.3～1.5 medium hard 150～250 1.5～1.8 soft rock 200～350 1.8～2.0

[0043] Step 1.3: Design a two-factor, n-level orthogonal test scheme for field vibration velocity monitoring, and determine the safe allowable distance R according to the type of protected object. i ,

[0044] Step 1.3 is carried out according to the following steps: using the K value, α value, and the amount of explosives used for blasting the tunnel 2 behind the small clearance tunnel obtained in Step 1.2, calculate the safe allowable distance R for each orthogonal test. i To obtain the safe allowable distance R i set {R i =R i1 R i2 R i3…… R in}, safe allowable distance R i The distance between the blasting source 4 of the subsequent tunnel 2 and the arch waist of the preceding tunnel 1 is calculated as follows:

[0045] After obtaining the K and α values ​​of the orthogonal experiment according to the arithmetic sequence, and determining the amount of explosives used for blasting in the secondary tunnel after the small clearance tunnel, the amount of explosives used is calculated according to the weight provided by the on-site construction party. The safe permissible vibration velocity for each orthogonal experiment in the orthogonal table corresponding to the K and α values ​​is calculated using the Sadovsky formula:

[0046]

[0047] In the formula:

[0048] R—Safe allowable distance for blasting vibration, in meters (m);

[0049] Q—Quantity of explosives; for simultaneous blasting, it is the total amount of explosives; for delayed blasting, it is the maximum amount of explosives in a single segment; unit: kg.

[0050] V—Permissible particle velocity at the location of the protected object, in cm / s;

[0051] K and α are coefficients and attenuation indices related to the terrain and geological conditions between the blasting point and the protected object, and should be determined through field tests.

[0052] The ranges of K and α are divided into n values ​​according to an arithmetic sequence, where n ∈ [2, 9] and n is a positive integer. i takes a positive integer value, and the range of i is determined by the value of n and the number of experiments in the designed orthogonal experiment. When n is 3, i ranges from 1 to 9; when n is 4, i ranges from 1 to 16; when n is 5, i ranges from 1 to 25; and when n is 6, i ranges from 1 to 36.

[0053] Step 2 includes the following steps:

[0054] Step 2.1: Based on the safe allowable distance R obtained in Step 1.3i Step 2.2: Determine the monitoring point for blasting vibration velocity at the arch waist of the pilot tunnel 1 with small clearance, and deploy vibration velocity sensors at the monitoring point to monitor the blasting vibration velocity and obtain the measured vibration velocity value at that point. Based on the actual amount of explosives used, multiple sets of measured vibration values, and the distance between the blasting source 4 and the monitoring point, fit the K and α values. Then, substitute the K and α values ​​into the Sadovsky formula to calculate the safe allowable distance. R i ,

[0055] The blasting vibration velocity monitoring points in step 2.3 include the first vibration velocity monitoring point, the second vibration velocity monitoring point, the third vibration velocity monitoring point, ... the nth vibration velocity monitoring point.

[0056] Conservative safety allowable distance R b The calculation formula is as follows:

[0057] R b =R j *M

[0058] In the formula, M is the safety factor, M∈[1,2].

[0059] Step 3 is performed according to the following steps:

[0060] Step 3.1, Pass through the safe allowable distance R j The conservative safety allowable distance R is calculated. b According to the conservative safety allowable distance R b Determine the monitoring and verification point 23 for blasting vibration velocity at the arch waist of the pilot tunnel 1 with small clearance distance and conduct blasting vibration velocity monitoring;

[0061] Step 3.2: Determine whether the vibration velocity exceeds the safe allowable vibration velocity for blasting vibration. If it meets the safe allowable vibration velocity for blasting vibration, then the conservative safe allowable distance R is... b 24 is the final safe allowable distance R. z ;

[0062] Step 3.3: If the condition is not met, increase the value of M and recalculate the conservative safety allowable distance R. b Continue monitoring of blasting vibration safety until a safe distance R is maintained. b The vibration velocities at the 24 monitoring points were within the permissible vibration velocities for blasting vibration. The final safe distance R is the distance that meets the permissible vibration velocities for blasting vibration. z .

[0063] The advantages of this invention are: it can accurately determine the safe allowable blasting distance for the secondary lining of the pre-tunnel. Based on orthogonal field monitoring tests and theoretical data analysis, this method provides a method for determining the safe distance for the secondary lining of the pre-tunnel during blasting excavation of tunnels with small clearance. It can also transform the safe allowable distance from an empirically determined range to a theoretically determined precise and unique value. This can effectively reduce the impact of vibrations generated by the blasting excavation of the subsequent tunnel with small clearance on the secondary lining of the pre-tunnel, and avoid the impact of the blasting excavation of the subsequent tunnel with small clearance on the structural safety and service life of the pre-tunnel lining.

[0064] Example 1

[0065] Construction site reference Figure 2 , Figure 3 .

[0066] Step 1: After applying initial support behind the tunnel face of the pilot tunnel 1 of the small-clearance tunnel, perform the judgment in Step 1.1 to determine whether the cumulative deformation and deformation rate of the surrounding rock 3 of the pilot tunnel 1 of the small-clearance tunnel have reached the values ​​specified in the standard and whether it is basically stable.

[0067] According to the technical specifications for highway tunnel construction, the following requirements should be met during the construction of the secondary lining:

[0068] (1) The displacement rates of each test item converged significantly, indicating that the surrounding rock was basically stable.

[0069] (2) The total displacement is expected to be 80% to 90% of the total displacement that has already occurred;

[0070] (3) The peripheral displacement rate is less than 0.1 to 0.2 mm / d, or the arch subsidence rate is less than 0.07 to 0.15 mm / d.

[0071] If the values ​​specified in the technical specifications for highway tunnel construction are met and the deformation of the surrounding rock is basically stable, proceed with steps 1.2 and 1.3, conduct field tests and theoretical analyses, and determine the safe allowable distance R obtained from orthogonal experiments. i .

[0072] The lithology of the surrounding rock 3 is determined, and the range of values ​​for K and α in the Sadovsky formula is determined based on the lithology of the surrounding rock 3. As shown in Table 1, the lithology of the rock in Example 1 is hard, the range of K is 50 to 150, and the range of α is 1.3 to 1.5.

[0073] Design a two-factor, three-level orthogonal experimental scheme for field vibration velocity monitoring. Divide the K and α values ​​corresponding to hard rock into three values ​​according to an arithmetic sequence. Then, design L9(3) values ​​based on the K and α values ​​divided according to the arithmetic sequence. 2 Using an orthogonal array, nine field vibration monitoring tests were conducted. Taking the test with monitoring point number 1 as an example, calculate R. iGiven the values, K = 50, α = 1.3, and the safe permissible vibration velocity determined by Table 2 of GB 6722-2014 Blasting Safety Regulations 13.2.2, the protected object is a traffic tunnel, so V = 15 cm / s is taken. Q is determined based on the actual amount of material used in the construction, so Q = 196 kg is taken. The safe permissible distance R is then obtained. i1 The value is 14.67m.

[0074] Step 2: First, obtain the measured vibration velocity at the safe allowable distance from the pilot tunnel. This provides necessary data support for obtaining the accurate K and α values ​​for fitting and for calculating the safe allowable distance. The safe allowable distance is calculated based on the orthogonal test scheme for on-site vibration velocity monitoring. {R i =R i1 R i2 R i3 R i4 R i5 R i6 R i7 R i8 R i9 Nine blasting vibration velocity monitoring points were identified at the arch waist of the first tunnel section of the small-clearance tunnel: point 5 (first vibration velocity monitoring point), point 6 (second vibration velocity monitoring point), point 7 (third vibration velocity monitoring point), point 8 (fourth vibration velocity monitoring point), point 9 (fifth vibration velocity monitoring point), point 10 (sixth vibration velocity monitoring point), point 11 (seventh vibration velocity monitoring point), point 12 (eighth vibration velocity monitoring point), and point 13 (ninth vibration velocity monitoring point). Vibration velocity sensors were deployed at each monitoring point to monitor the blasting vibration velocity and obtain the measured vibration velocity values ​​at the nine points. To ensure the accuracy of the monitoring velocity, the connection between the sensors and the arch waist of the first tunnel section of the small-clearance tunnel must be rigid. Specific connection methods include adhesive, quicklime + water, and quick-setting cement.

[0075] Based on the actual amount of explosives used, multiple sets of measured vibration values, and the distance between the blast source 4 and the monitoring point, K and α values ​​were obtained through mathematical statistics and software such as Python or MathLab. These fitted K and α values ​​were then substituted into the Sadovsky formula to calculate the unique safe allowable distance R. j .

[0076] Step 3: Calculate the safe allowable distance R j Multiplying by a safety factor M, the vibration velocity at the safe allowable distance is ensured to be less than the safe allowable vibration velocity value, and verified through on-site monitoring, thereby determining the final safe allowable distance.

[0077] With M set to 1.2, the first conservative safety allowable distance R is calculated using the formula. bBased on the conservative safety allowable distance Rb, the monitoring and verification point 23 for blasting vibration velocity at the arch waist of the pilot tunnel 1 with small clearance is determined, and blasting vibration velocity is monitored to determine whether the vibration velocity exceeds the safe blasting vibration velocity. If the safe blasting vibration velocity is met, then the conservative safety allowable distance Rb is considered valid. b1 24 is the final safe allowable distance R. z .

[0078] If this condition is not met, the value of M increases, specifically by increments of 1.3, 1.4, and so on, with the maximum value not exceeding 2. A new R is then calculated. b1 R b2 ..., R bn Regarding the subsequent new R... b1 R b2 ..., R bn Continue monitoring of blasting vibration safety until the final conservative safety distance R is reached. bn The vibration velocities at the 24 monitoring points were within the permissible vibration velocities for blasting vibration. The final safe distance R is the distance that meets the permissible vibration velocities for blasting vibration. z The secondary lining construction of the pilot tunnel 1 of the small clearance tunnel is carried out outside the safe allowable distance, and is less affected by blasting vibration, which can ensure the structural safety and service life of the secondary lining.

Claims

1. A method for determining the safe distance for secondary lining construction of the pilot tunnel in small-clearance tunnel blasting excavation, characterized in that, Includes the following steps: Step 1: Determine the stability of the surrounding rock deformation and deformation rate, determine the orthogonal test scheme for on-site vibration velocity monitoring, and obtain the safe allowable distance through the on-site vibration velocity monitoring orthogonal test. R i = R i1 ，R i2 ，R i3…… R in Step 1 includes the following steps: Step 1.1: Determine whether the cumulative deformation and deformation rate of the surrounding rock of the pilot tunnel of the small clearance tunnel have reached the values ​​specified in the standard and whether they are basically stable. Initial support has been applied behind the tunnel blasting face of the pilot tunnel of the small clearance tunnel. Step 1.2: Determine the lithology of the surrounding rock. Based on the lithology of the surrounding rock, determine the range of values ​​for K and α in the Sadovsky formula. The ranges of K and α are divided into n values ​​according to an arithmetic sequence. n is a positive integer; Step 1.3: Design a two-factor, n-level orthogonal test scheme for field vibration velocity monitoring, and determine the safe allowable distance based on the type of protected object. R i ; Step 2: Based on the safe allowable distance obtained in Step 1 R i Design and implement a field vibration velocity monitoring scheme, and calculate the safe allowable distance. R j Step 2 includes the following steps: Step 2.1: Based on the safe allowable distance obtained in Step 1.3 R i The monitoring point for blasting vibration velocity at the arch waist of the tunnel with small clearance was determined, and a vibration velocity sensor was installed at the monitoring point to monitor the blasting vibration velocity and obtain the measured vibration velocity value at the point. Step 2.2: Based on the actual amount of explosives used, multiple sets of measured vibration velocity values, and the distance between the blasting source and the blasting vibration velocity monitoring point, fit the K value and α value, and substitute the K value and α value into the Sadovsky formula to calculate the safe allowable distance. R j ; Step 3: The safe allowable distance obtained from Step 2 R j Calculate the conservative safety allowable distance R b Determine the safe and permissible distance. R b Does it meet the requirements? If it does, then it is the final safe allowable distance. R z If the requirements are not met, continue calculating the conservative safety allowable distance. R b Until the requirements are met and the final safe allowable distance is obtained. R z .

2. The method for determining the safe distance for secondary lining construction of the pilot tunnel in small-clearance tunnel blasting excavation according to claim 1, characterized in that, Step 1.3 is implemented according to the following steps: using the K value, α value, and the amount of explosives used for blasting the tunnel behind the small clearance tunnel obtained in step 1.2, calculate the safe allowable distance for each orthogonal experiment. R i To obtain a safe and permissible distance R i gather{ R i = R i1 ，R i2 ，R i3…… R in The safe allowable distance R i It is the distance between the blasting source of the tunnel following the small clearance tunnel and the arch of the tunnel preceding the small clearance tunnel.

3. The method for determining the safe distance for secondary lining construction of the pilot tunnel in small-clearance tunnel blasting excavation according to claim 1, characterized in that, The blasting vibration velocity monitoring points in step 2.1 include the first vibration velocity monitoring point, the second vibration velocity monitoring point, the third vibration velocity monitoring point, ... the nth vibration velocity monitoring point.

4. The method for determining the safe distance for secondary lining construction of the pilot tunnel in small-clearance tunnel blasting excavation according to claim 1, characterized in that, The conservative safety allowable distance R b The calculation formula is as follows: R b = R j *M In the formula, M For safety reasons, .

5. The method for determining the safe distance for secondary lining construction of the pilot tunnel in small-clearance tunnel blasting excavation according to claim 4, characterized in that, Step 3 is implemented according to the following steps: Step 3.1, maintain a safe distance R j Calculate the conservative safety allowable distance R b According to the aforementioned conservative safety allowable distance R b Determine the monitoring and verification points for blasting vibration velocity at the arch waist of the pilot tunnel with small clearance and conduct blasting vibration velocity monitoring; Step 3.2: Determine whether the vibration velocity exceeds the safe allowable vibration velocity for blasting vibration. If it meets the safe allowable vibration velocity for blasting vibration, then the conservative safe allowable distance... R b This is the final safe allowable distance. R z ; Step 3.3: If the condition is not met, then increase the value. M Recalculate the conservative safety allowable distance. R b Continue monitoring of blasting vibration safety until the conservative safety allowable distance is reached. R b If the vibration velocity at the monitoring point is within the permissible vibration velocity for blasting vibration, the final safe distance that meets the permissible vibration velocity for blasting vibration is the final safe distance. R z .

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