Method for determining underground cavern supporting opportunity

Abstract

According to the method for determining the supporting opportunity of the underground cavern, the supporting opportunity of rock masses at all positions of the surrounding rock in the aging deformation process is given based on the fracture closure degree, and for the cavern surrounding rock in the low confining pressure compression state, the fracture closure degree C3 in the radial stress direction serves as the aging supporting basis of the surrounding rock; the change of the fracture closure C3 along with time has three conditions, and by arranging the anchor cables and the anchor rods, additional radial pressure stress can be provided, and radial stress released by surrounding rock can be compensated, so that the difference value between tangential stress and the radial stress is reduced, the fracture closure is kept in a non-destructive stage, and safe and smooth construction of the underground cavern is ensured. The measures are beneficial to maintaining the stability of the surrounding rock, the support design can be optimized, and the engineering efficiency and safety are improved.

Description

Technical Field

[0001] The present invention relates to the field of geotechnical engineering, and particularly to a method for determining the support timing of underground chambers. Background Art

[0002] The buried depth of the underground powerhouse of a hydropower station is large. After the excavation of the chamber, a part of the initial in-situ stress near the chamber wall will be immediately released, while the remaining stress needs a certain amount of time to be completely released. This indicates that the surrounding rock deformation will not be completed in a short time or even instantaneously, but requires a relatively long time to converge. Therefore, for the support of the surrounding rock, it is necessary to determine the appropriate support timing. The so-called appropriate support means allowing the surrounding rock to exert its maximum self-bearing capacity: during the time-dependent deformation after excavation, before the surrounding rock fails, the strain of the surrounding rock reaches at most the ultimate strain before failure. At present, the stress release coefficient is commonly used as an evaluation for determining the initial support at the appropriate time, which mainly considers the time effect of stress release. Many studies have pointed out that when the stress release reaches 100%, the stress at the crown is the minimum, while the stress at the side wall is the maximum. Therefore, it is considered that when the stress release of the surrounding rock reaches 70 - 80%, the stress at the crown of the surrounding rock gradually dissipates but no tensile stress is generated, and the stress near the arch seat has not reached the compressive strength of the surrounding rock. At this time, it is the appropriate support timing. However, in actual engineering, after the excavation of the underground chamber, the stress states of the surrounding rock at different locations are different, and the stress release amount and release time effect are also different, making it difficult to give a quantitative evaluation. Summary of the Invention

[0003] The main object of the present invention is to provide a method for determining the support timing of underground chambers to solve the problems in the above background art.

[0004] To solve the above technical problems, the technical solution adopted by the present invention is: based on the crack closure degree, the support timing of each rock mass of the surrounding rock during the time-dependent deformation is given. For the surrounding rock of the chamber in the low confining pressure compression state, the crack closure degree C3 in the radial stress direction is used as the basis for the time-dependent support of the surrounding rock. There are three situations for the change of the crack closure degree C3 with time, that is, the time-dependent support method for the chamber after excavation is:

[0005] S1. Consider the surrounding rock with the crack closure degree C3 of the rock mass < x as damaged and immediately carry out support;

[0006] S2. For the surrounding rock with x < C3 < 0, allow the tangential and radial stresses of the rock mass to be further released, determine the inflection point of C3 from the change curve of the C3 value, and regard it as the optimal support timing;

[0007] S3. Combining with the actual engineering support method, divide the surrounding rock into three areas: the crown, the upstream side wall, and the downstream side wall. If there is a rock mass in a certain area that needs to be supported during a certain level of excavation, then the surrounding rock of this area at this level will be fully supported.

[0008] Preferably, the maximum principal stress in the surrounding rock is the tangential stress which is parallel to the cave wall, and the minimum principal stress is the radial stress which is perpendicular to the cave wall;

[0009] Rock mass failure is dominated by crack expansion in the radial stress direction. For the surrounding rock of the excavated cavern, after the ground stress is released, the surrounding rock of the cavern wall is regarded as being in a uniaxial compression state without confining stress or a triaxial compression state under low confining stress;

[0010] In conventional triaxial tests, as the confining pressure increases, the failure mode of rock gradually changes from elastic softening to elastic-plastic hardening, so the increase in confining pressure is used as the basis for rock failure;

[0011] Under the action of time, the tangential and radial stresses of the cave wall will gradually release. The corresponding anchor cables and anchor rods provide radial compressive stress for the surrounding rock, compensating for the radial pressure released by the surrounding rock itself, so that the difference between the tangential stress and the radial stress is reduced compared with the unsupported condition, thereby making the fracture closure degree C3 of the rock mass in a non-destructive stage.

[0012] Preferably, the axial and radial strains are calculated as follows:

[0013] ε v =ε1+2ε3 (1)

[0014] Among them, ε1 and ε3 represent the axial strain and radial strain respectively, ε v is the volumetric strain.

[0015] Preferably, the threshold stress of the rock includes the fracture closure stress σ cc , crack initiation stress σ ci , crack damage stress σ cd and the peak stress σ p , according to the four threshold stresses, the crack evolution of rocks is divided into the crack closure stage, elastic stage, stable crack growth stage, unstable crack growth stage and post-crack peak stage.

[0016] Preferably, rock will produce strain under the action of external load, and the strain can be divided into elastic strain and fracture strain. The volume strain is composed of elastic strain εe v and fracture strain εc v, which can be expressed as:

[0017]

[0018] When the deviatoric stress reaches σ cc When the natural microcracks in the rock are completely closed, the deviatoric stress reaches σ ci After that, the crack continues to expand, so the deviatoric stress is cc and σ ciBetween , the cracks in the rock do not expand, so no crack strain is generated; then, the σ in the rock strain curve can be cc and σ ci The slope of the straight line between the two is regarded as the elastic modulus E of the rock sample;

[0019] Introducing a dimensionless parameter R d , which is the ratio of deviatoric stress to peak strength, is used to analyze test data and can be expressed as:

[0020]

[0021] where σ1-σ3 is the deviatoric stress; σ p is the peak intensity, i.e. R d =1 when the deviatoric stress σ1-σ3;

[0022] The crack closure stress, crack initiation stress, crack damage stress, and the ratio of peak stress to peak strength are expressed as Rcc d, Rci d, Rcd d, and Rp d, respectively.

[0023] Preferably, the axial and radial elastic strains εe1 and εe3 can be expressed as:

[0024]

[0025] Where, μ is Poisson’s ratio;

[0026] The axial crack strain εc1 and radial crack strain εc3 can be expressed as:

[0027]

[0028] Where, εe 1 and εe 3 are the axial and radial elastic strains, εc 1 and εc 3 are the axial and radial crack strains;

[0029] The axial and radial elastic strains are calculated by formula (4), and the deviatoric stress and axial and radial crack strain values ​​under different confining pressures can be calculated by formula (5);

[0030] The axial and radial crack strains at Rcc d, Rci d, Rcd d and Rp d are calculated, that is, ε cc 1,ε cc 3,ε ci 1,ε ci 3,ε cd 1,ε cd 3,ε cp 1 and ε cp 3.

[0031] Preferably, the fracture strain has an exponential relationship with the confining pressure. The confining pressure can affect the axial fracture strain of the rock, and its influencing ability gradually weakens with the increase of the confining pressure. During the fracture closure stage of the rock, the fractures gradually close with the increase of the axial stress. To facilitate the evaluation of the fracture closure degree, the axial and radial fracture closure degrees C1 and C3 are introduced:

[0032]

[0033] Where, ΔR d is the increment of R d During each stage before the peak, ΔR d >0, and during the stage after the peak, ΔR d <0;

[0034] During the stable fracture growth stage of the rock (Rcc d < R d < Rcd d), the fracture evolution is slow, and the fracture closure degree C3 gradually decreases to 0;

[0035] While during the unstable fracture growth stage (Rcd d < R d < 1), the fracture propagation speed gradually increases, and the fracture closure degree C3 rapidly decreases with the increase of R d . When R d = 1, C3 = x, and the rock begins to fail;

[0036] Entering the softening stage, R d decreases, and C3 continues to decrease until the rock is completely disintegrated.

[0037] Preferably, the optimal support timing can be calculated as follows:

[0038]

[0039] In the above formula: the optimal support timing T x (unit: days), the surrounding rock deformation convergence time T c , the strength-stress ratio k σ , the strain margin K, the ratio r of the first principal stress σ1 of the surrounding rock after excavation to the uniaxial compressive strength σ c , the support confining pressure P, and the time-dependent deformation load coefficient α.

[0040] Preferably, the anchor cable pre-tightening coefficient can be calculated by the following formula:

[0041]

[0042] Where: the anchor cable load-sharing coefficient κ = 0.5, the released stress σ n, the design tonnage of anchor cable is Ns, the spacing between anchor cables is a×b, the time-effect load coefficient is α, the installation time is t (days), and the stable convergence time Tc is 90 days (β=-0.05117), 180 days (β=-0.02558), and 365 days (β=-0.01260), respectively.

[0043] The present invention provides a method for determining the timing of underground cavern support. By conducting systematic tests and calculations based on the rock mass properties of the underground cavern, the degree of crack closure in the radial stress direction is used as the basis for the time-dependent support of the surrounding rock. There are three situations in which the degree of crack closure changes with time. The cavern surrounding rock is treated according to different situations. Under the action of time, the tangential and radial stresses of the cavern wall are gradually released. The setting of anchor cables and anchor rods provides radial compressive stress for the surrounding rock, compensates for the radial stress released by the surrounding rock itself, reduces the difference between the tangential stress and the radial stress compared to the unsupported working condition, and thereby makes the degree of crack closure of the rock mass in a non-destructive stage, thereby ensuring the smooth progress of the underground cavern operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The present invention will be further described below in conjunction with the accompanying drawings and embodiments:

[0045] Figure 1 is the stress-strain curve of the granite rock sample;

[0046] Figure 2 is the crack closure degree and R under confining pressure of 1 MPa d the relationship between;

[0047] Figure 3 It is the time-dependent variation curve of C3;

[0048] Figure 4 It is the threshold stress of granite sample with confining pressure of 10MPa. DETAILED DESCRIPTION

[0049] Example 1

[0050] like Figures 1 to 4 As shown in the figure, the method for determining the timing of underground cavern support is used. In this example, the rock mass is granite. Based on the degree of crack closure, the timing of support for various parts of the surrounding rock mass during the aging deformation process is given. The maximum principal stress (tangential stress) is generally parallel to the cavern wall, while the minimum principal stress (radial stress) is perpendicular to the cavern wall. Rock mass failure is dominated by crack expansion in the direction of radial stress. In actual engineering, where there are obvious compressive damages such as spalling, flaking, and splitting, the tangential stress is the largest, while the radial stress is very small. For the surrounding rock of the excavated cavern, after the ground stress is partially released, the surrounding rock of the cavern wall can be regarded as being in a uniaxial compression state without confining stress or a triaxial compression state under low confining stress. By Figure 1It can be seen that there is a close relationship between the stress-strain and the confining pressure in the conventional triaxial test: as the confining pressure of the rock increases, the failure mode of the rock gradually changes from elastic softening to elastic-plastic hardening, indicating that the confining pressure can be used as a basis for rock failure. In addition, according to Figure 2 , for the rock with a confining pressure of 1 MPa, during the stable crack growth stage (Rcc d < R d < Rd d), the crack evolution is slow, and the crack closure C3 gradually decreases to 0; while during the unstable crack growth stage (Rcd d < Rd < 1), the crack propagation speed gradually increases, and the crack closure C3 decreases rapidly with the increase of R d . When R d = 1, C3 = -17.71, and the rock begins to fail; entering the softening stage, R d decreases, and C3 continues to decrease to -28.0, and the rock is completely disintegrated. Under the action of time effect on the rock mass, the tangential and radial stresses of the tunnel wall will gradually be released. The installation of anchor cables and bolts provides radial compressive stress for the surrounding rock, compensating for the radial stress released by the surrounding rock itself, reducing the difference between the tangential stress and the radial stress compared with the un-supported condition, and thus keeping the crack closure C3 of the rock mass in the non-failure stage.

[0051] Therefore, for the surrounding rock of the tunnel chamber regarded as in the low-confining-pressure compression state, the crack closure C3 in the radial stress direction can be used as the basis for the time-effect support of the surrounding rock. There are three cases of the change of the crack closure C3 with time, as Figure 3 shown. Therefore, the time-effect support method for the tunnel chamber after excavation is as follows:

[0052] (1) Regarding the surrounding rock with the crack closure C3 < -17.0 of the rock mass as damaged, immediate support is required;

[0053] (2) For the surrounding rock with -17 < C3 < 0, allowing the tangential and radial stresses of the rock mass to be further released; determining the inflection point of C3 from the change curve of the C3 value and regarding it as the optimal support time;

[0054] (3) Combining with the actual engineering support method, dividing the surrounding rock into three areas: the crown, the upstream side wall, and the downstream side wall. If there is rock mass in a certain area that needs support during a certain level of excavation, then the surrounding rock of this area at this level will be fully supported.

[0055] Example 2

[0056] Based on the time-effect deformation theory, the calculation formulas for the optimal support time of the surrounding rock and the pre-tightening coefficient of prestressed anchorage are derived. The optimal support time can be calculated as follows:

[0057]

[0058] In the above formula: the optimal support time T x (unit: days), the surrounding rock deformation convergence time Tc , strength stress ratio k σ , strain margin K, the first principal stress σ1 of the surrounding rock after excavation and the uniaxial compressive strength σ c The ratio r, the support confining pressure P, and the time-dependent deformation load coefficient α.

[0059] The anchor cable preload coefficient can be calculated by the following formula:

[0060]

[0061] Where: Anchor cable load sharing coefficient κ = 0.5, release stress σ n , the design tonnage of anchor cable is Ns, the spacing between anchor cables is a×b, the time-effect load coefficient is α, the installation time is t (days), and the stable convergence time Tc is 90 days (β=-0.05117), 180 days (β=-0 . 02558), 365 days (β=-0.01260).

[0062] Example 3

[0063] The stress-strain curve of the granite sample in the test is as follows: Figure 1 As shown. ε1 and ε3 represent axial strain and radial strain respectively. The positive strain value represents compression of the rock sample, while the negative strain value represents expansion. The volumetric strain εv can be calculated from ε1 and ε3:

[0064] ε v =ε1+2ε3 (1)

[0065] Among them, ε1 and ε3 represent the axial strain and radial strain respectively, ε v is the volumetric strain.

[0066] Depend on Figure 1 It can be seen that the peak stress σ p It increases with the increase of confining pressure σ3. When the confining pressure is 1, 3, 5, 10, 20, 30 and 40 MPa, σ p They are 114.72, 139.65, 157.22, 182.97, 216.30, 345.73 and 374.63 MPa respectively.

[0067] In order to determine the elastic modulus and Poisson's ratio of granite under different confining pressures, it is necessary to first determine the four threshold stresses of granite, namely the crack closure stress σ cc , crack initiation stress σ ci , crack damage stress σ cd and the peak stress σ pThese four threshold stresses can divide the evolution of rock cracks into five stages: crack closure stage (stage I), elastic stage (stage II), stable crack growth stage (stage III), unstable crack growth stage (stage IV), and post-crack peak stage (stage V). Figure 4 shown.

[0068] Rocks will produce strain under the action of external loads, and the strain can be divided into elastic strain and fracture strain. The volume strain can be composed of elastic body strain εe v and fracture body strain εc v, which can be expressed as:

[0069]

[0070] When the deviatoric stress reaches σ cc When the natural microcracks in the rock are completely closed, the deviatoric stress reaches σ ci After that, the crack continues to expand, so the deviatoric stress is cc and σ ci Between , the cracks in the rock do not expand, so no crack strain is generated; then, the σ in the rock strain curve can be cc and σ ci The slope of the straight line between them is regarded as the elastic modulus E of the rock sample. When σ3 = 10 MPa, the elastic modulus E of the rock sample is 54.29 GPa.

[0071] According to Hooke's law, the axial and radial elastic strains εe 1 and εe 3 can be expressed as:

[0072]

[0073] where μ is Poisson's ratio.

[0074] Let B = εe 3 / εe 1, then

[0075]

[0076] According to formula (4), when σ3 = 10 MPa, the Poisson's ratio μ in the elastic stage is 0.1846. Then, the crack volume strain can be expressed as:

[0077]

[0078] Since there is no crack strain between σcc and σci, the crack volume strain curve in this interval is a horizontal line. The above method can also be used to calculate different threshold stresses, elastic moduli and Poisson's ratios under other confining pressures. The results are shown in Table 1 below.

[0079] Table 1: Mechanical parameters of granite specimens under different confining pressures

[0080]

[0081] For triaxial compression tests, the ratio of threshold stress to peak strength is widely used to analyze the mechanical properties of rocks. In this study, a dimensionless parameter R is introduced. d , which is the ratio of the deviatoric stress to the peak strength σp, is used to analyze the test data and can be expressed as:

[0082]

[0083] where σ1-σ3 is the deviatoric stress; σ p is the peak intensity, i.e. R d =1 when the deviatoric stress σ1-σ3;

[0084] The crack closure stress, crack initiation stress, crack damage stress, and the ratio of peak stress to peak strength are expressed as Rcc d, Rci d, Rcd d, and Rp d, respectively.

[0085] The axial and radial elastic strains εe 1 and εe 3 can be expressed as:

[0086]

[0087] Where, μ is Poisson’s ratio;

[0088] The axial crack strain εc1 and radial crack strain εc3 can be expressed as:

[0089]

[0090] Where, εe 1 and εe 3 are the axial and radial elastic strains, εc 1 and εc 3 are the axial and radial crack strains;

[0091] The axial and radial elastic strains are calculated by formula (4), and the deviatoric stress and axial and radial crack strain values ​​under different confining pressures can be calculated by formula (5);

[0092] The axial and radial crack strains at Rcc d, Rci d, Rcd d and Rp d are calculated, that is, εcc 1, ε cc 3,ε ci 1,ε ci 3,ε cd 1,ε cd 3,ε cp 1 and ε cp 3.

[0093] The fracture strain has an exponential relationship with the confining pressure. The confining pressure can affect the axial fracture strain of the rock, and its influencing ability gradually weakens with the increase of the confining pressure. During the fracture closure stage of the rock, the fractures gradually close with the increase of the axial stress. To facilitate the evaluation of the fracture closure degree, the axial and radial fracture closure degrees C1 and C3 are introduced:

[0094]

[0095] where ΔR d is the increment of R d , and ΔR d >0 in each stage before the peak, and ΔR d <0 in the stage after the peak;

[0096] During the stable fracture growth stage of the rock (Rcc d < R d < Rcd d), the fracture evolution is slow, and the fracture closure degree C3 gradually decreases to 0;

[0097] In the unstable fracture growth stage (Rcd d < R d < 1), the fracture propagation speed gradually increases, and the fracture closure degree C3 rapidly decreases with the increase of R d . When R d = 1, C3 = -17.71, and the rock begins to fail;

[0098] Entering the softening stage, R d decreases, and C3 continues to decrease until the rock is completely disintegrated.

[0099] The above embodiments are only the preferred technical solutions of the present invention and should not be regarded as limitations on the present invention. The protection scope of the present invention should be the technical solutions recorded in the claims, including the equivalent replacement solutions of the technical features in the technical solutions recorded in the claims. That is, the equivalent replacement improvements within this scope are also within the protection scope of the present invention.

Claims

1. Method for determining the support timing of underground chambers, the method being: based on the crack closure degree, the support timing of each part of the surrounding rock during the time-dependent deformation process is given. For the surrounding rock of chambers in the low confining pressure compression state, the crack closure degree C3 in the radial stress direction is used as the basis for the time-dependent support of the surrounding rock. There are three situations for the change of the crack closure degree C3 with time, that is, the time-dependent support method for the chamber after excavation is as follows: S1. The surrounding rock with the crack closure degree C3 < x of the rock mass is regarded as damaged and needs to be supported immediately; S2. For the surrounding rock with x < C3 < 0, the tangential and radial stresses of the rock mass are allowed to be further released, and the inflection point of C3 is determined from the change curve of the C3 value and regarded as the optimal support timing; S3. Combining with the actual engineering support method, the surrounding rock is divided into three areas: the crown, the upstream side wall, and the downstream side wall. If there is rock mass in a certain area that needs to be supported during a certain level of excavation, then the surrounding rock of this area at this level is fully supported.

2. The method for determining the timing of underground cavern support according to claim 1 is characterized by: The maximum principal stress in the surrounding rock is the tangential stress parallel to the cave wall, and the minimum principal stress is the radial stress perpendicular to the cave wall; The failure of the rock mass is dominated by the crack propagation in the radial stress direction. For the surrounding rock of the excavated chamber, after the in-situ stress is released, the surrounding rock of the cave wall is regarded as being in the uniaxial compression state without confining pressure stress or the triaxial compression state under low confining pressure stress; In the conventional triaxial test, as the confining pressure of the rock increases, the failure mode of the rock gradually changes from elastic softening to elastic-plastic hardening, so the increase of the confining pressure is used as the basis for rock failure; Under the action of time-dependent effect, the tangential and radial stresses of the cave wall will be gradually released. The corresponding cable bolts and rock bolts provide radial compressive stress for the surrounding rock, compensating for the radial pressure released by the surrounding rock itself, so that the difference between the tangential stress and the radial stress is reduced compared with the non-supported working condition, and further the crack closure degree C3 of the rock mass is in the non-damaged stage.

3. The method for determining the timing of underground cavern support according to claim 2 is characterized by: The axial and radial strains are calculated as follows: e v =ε1+2ε3 (1) Among them, ε1 and ε3 represent the axial strain and radial strain respectively, ε v is the volumetric strain.

4. The method for determining the timing of underground cavern support according to claim 3 is characterized by: The threshold stress of rock includes the fracture closure stress σ cc , crack initiation stress σ ci , crack damage stress σ cd and the peak stress σ p , according to the four threshold stresses, the crack evolution of rocks is divided into the crack closure stage, elastic stage, stable crack growth stage, unstable crack growth stage and post-crack peak stage.

5. The method for determining the timing of underground cavern support according to claim 4 is characterized by: The rock will generate strain under the action of external load, and the strain can be divided into elastic strain and crack strain. The volumetric strain is composed of the elastic strain εe v and the crack strain εc v, and can be expressed as: When the deviatoric stress reaches σ cc When the natural microcracks in the rock are completely closed, the deviatoric stress reaches σ ci After that, the crack continues to expand, so the deviatoric stress is cc and σ ci Between , the cracks in the rock do not expand, so no crack strain is generated; then, the σ in the rock strain curve can be cc and σ ci The slope of the straight line between the two is regarded as the elastic modulus E of the rock sample; Introducing a dimensionless parameter R d , which is the ratio of deviatoric stress to peak strength, is used to analyze test data and can be expressed as: where σ1-σ3 is the deviatoric stress; σ p is the peak intensity, i.e. R d =1 when the deviatoric stress σ1-σ3; The ratios of the crack closure stress, the crack initiation stress, the crack damage stress, and the peak stress to the peak strength are respectively expressed as Rcc d, Rci d, Rcd d, and Rp d.

6. The method for determining the timing of underground cavern support according to claim 5, characterized in that: The axial and radial elastic strains εe 1 and εe 3 can be expressed as: where μ is the Poisson's ratio; The axial crack strain εc 1 and the radial crack strain εc 3 can be expressed as: where εe 1 and εe 3 are the axial and radial elastic strains, and εc 1 and εc 3 are the axial and radial crack strains; The axial and radial elastic strains are calculated by formula (4), and then the deviator stress and the axial and radial crack strain values under different confining pressures can be calculated by formula (5); The axial and radial crack strains at Rcc d, Rci d, Rcd d and Rp d are calculated, that is, ε cc 1,ε cc 3,ε ci 1,ε ci 3,ε cd 1,ε cd 3,ε cp 1 and ε cp 3.

7. The method for determining the timing of underground cavern support according to claim 6, characterized in that: The crack strain has an exponential relationship with the confining pressure. The confining pressure can affect the axial crack strain of the rock, and its influencing ability gradually weakens with the increase of the confining pressure. During the crack closure stage of the rock, the cracks gradually close with the increase of the axial stress. In order to facilitate the evaluation of the crack closure degree, the axial and radial crack closure degrees C1 and C3 are introduced: Among them, ΔR d YesR d The increment of each stage before the peak ΔR d >0, post-peak ΔR d <0; When the rock is in the stage of stable crack growth (Rcc d < R d < Rcd d), the crack evolution is slow, and the crack closure degree C3 gradually decreases to 0; In the unstable crack growth stage (Rcd d <R d <1), the crack expansion speed gradually increases, and the crack closure degree C3 increases with R d The increase of R d =1, C3=x, and the rock begins to break; Entering the softening stage, R d decreases, while C3 continues to decrease until the rock completely disintegrates.

8. The method for determining the timing of underground cavern support according to claim 1 is characterized by: The optimal support timing can be calculated as follows: In the above formula: optimal support time T x (Unit: day), surrounding rock deformation convergence time T c , strength stress ratio k σ , strain margin K, the first principal stress σ1 of the surrounding rock after excavation and the uniaxial compressive strength σ c The ratio r, the support confining pressure P, and the time-dependent deformation load coefficient α.

9. The method for determining the timing of underground cavern support according to claim 1, characterized in that: The cable bolt pre-tightening coefficient can be calculated by the following formula: Where: Anchor cable load sharing coefficient κ = 0.5, release stress σ n , anchor cable design tonnage N s , the spacing between anchor cables is a×b, the time-effect load coefficient is α, the installation time is t (days), and the stable convergence time Tc is 90 days (β=-0.05117), 180 days (β=-0.02558), and 365 days (β=-0.01260), respectively.

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