Method for the optimal determination of the axis of a subterranean cavern

Abstract

The application provides a kind of underground cavern axis preferred method, it is related to water conservancy and hydropower engineering field, by the regression and inversion calculation of geostress field, the initial stress condition range of surrounding rock before cavern excavation is obtained, and indoor true triaxial loading test is carried out to simulate, the cracking stress, damage stress, peak stress and other parameters under different stress states and failure evolution characteristics are obtained, so as to obtain the damage risk coefficient range of each failure mode, based on the damage risk coefficient range of each failure mode, the finite element analysis is carried out to each cavern axis scheme, the spatial range of the failure mode of each cavern axis scheme is obtained, according to the failure mode and corresponding spatial range, the corresponding cavern axis scheme is screened out, so as to obtain the corresponding underground cavern axis, solve the problem that how to select underground cavern axis in the prior art, the application is suitable for underground cavern axis selection.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of water conservancy and hydropower engineering, in particular to a large underground cavern axis optimization method. BACKGROUND

[0002] Underground engineering is developing towards "large buried depth, high stress, super scale" and other directions, and the construction environment of underground caverns is becoming more complex. How to efficiently and safely realize the construction of underground engineering in the future has attracted widespread attention from the society.

[0003] The most critical problem affecting the safe and efficient construction of underground caverns is the control of the stability of the surrounding rock, which will directly affect the safety of personnel and equipment, the project schedule and investment, and determine the success or danger of the project.

[0004] The selection of the axis of the underground cavern is one of the most core technical problems of the stability control of the surrounding rock. Especially for deep-buried underground caverns, the release of huge energy and the large deformation of rock mass towards the free space during the excavation of the cavern may lead to large-scale brittle failure phenomena such as rock burst, spalling and unloading relaxation, which may have a serious impact on the project. The mechanical response of the surrounding rock to the excavation blasting is significantly different for different axis schemes, so the optimization of the axis of the underground cavern is particularly important.

[0005] How to scientifically and reasonably select the axis of the cavern to improve the stability of the surrounding rock has always been a key problem in the design of water conservancy and hydropower engineering. For this reason, many domestic universities, research institutions and design units have carried out a large number of researches and formed relevant theoretical achievements and industry standards. In the current "Code for Design of Hydropower Station Buildings", the initial maximum principal stress direction of the rock mass is usually taken as the main basis for the layout of the cavern, and it is clearly stated that the angle between the main axis of the cavern and the direction should not be greater than 30°. This method is based on the initial in-situ stress measured on site and emphasizes the dominant role of the initial maximum principal stress, which to some extent guarantees the overall stability of the surrounding rock of the cavern. However, under complex geological conditions and deep burial conditions, the stress redistribution phenomenon caused by excavation disturbance of the surrounding rock is significant, especially in brittle rock mass, which is prone to induce severe local damage. Therefore, the existing specification does not fully consider the three-dimensional evolution characteristics of the secondary stress field after excavation and its influence on the failure mode, and there is still some optimization space.

[0006] In recent years, some scholars and research institutions have gradually realized the important role of the spatial three-dimensional characteristics of ground stress in the layout of underground caverns. For example, the Chinese invention patent application No. 201611016626.7 proposes a layout method based on the direction of the maximum horizontal principal stress, further expanding the traditional design idea. However, for future underground caverns with high stress and large depth, the brittle failure induced by excavation is more sensitive. Simply using the initial stress direction as the layout reference cannot quantitatively evaluate the damage effect and comprehensively assess and compare the substantial impact of different schemes on the stability of surrounding rock. Especially in the working condition dominated by brittle failure, how to introduce more targeted evaluation indicators to quantitatively analyze and evaluate the differences in damage degree of different schemes, and thus achieve more effective optimization, is currently a blank. SUMMARY

[0007] The technical problem solved by the present application: The present application provides an underground cavern axis optimization method, which solves the problem of how to select the underground cavern axis in the prior art.

[0008] The technical solution adopted by the present application to solve the above technical problem: The underground cavern axis optimization method comprises the following steps:

[0009] S1. According to the measured points on site, the regression and inversion calculation of the ground stress field is carried out to obtain the stress condition range of the surrounding rock before the excavation of the cavern;

[0010] S2. According to the stress condition range of the surrounding rock, indoor true triaxial loading test is carried out to obtain the cracking stress, damage stress and peak stress of the surrounding rock, and the damage risk coefficient is calculated according to the cracking stress, damage stress and peak stress;

[0011] S3. The damage risk coefficients under different stress conditions are compared to obtain the damage risk coefficient range of each damage mode;

[0012] S4. Based on the damage risk coefficient range of each damage mode, finite element analysis is carried out on each cavern axis scheme to obtain the damage mode and corresponding spatial range of each cavern axis scheme;

[0013] S5. According to the damage mode and corresponding spatial range, the corresponding cavern axis scheme is selected to obtain the corresponding underground cavern axis.

[0014] Further, the stress condition range of the surrounding rock is the initial stress field, including the direction and numerical range of the maximum principal stress, the direction and numerical range of the intermediate principal stress and the direction and numerical range of the minimum principal stress.

[0015] Further, in S2, the indoor true triaxial loading test comprises the following steps:

[0016] The indoor true triaxial loading test comprises the following steps:

[0017] S21, selecting a rock sample that is complete, hard, and free of damage or defects, and making the rock sample into a test piece with a size of 50mm*50mm*100mm, and the flatness of the surface of the test piece is controlled to be within ±0.02mm;

[0018] S22, applying stress to the test piece from three directions, the three directions being a minimum principal stress direction, an intermediate principal stress direction, and a maximum principal stress direction respectively, and the stress is loaded to the value of the minimum principal stress at a speed of 0.5MPa / s;

[0019] S23, keeping the value of the stress in the minimum principal stress direction unchanged, and continuing to apply stress in the intermediate principal stress direction and the maximum principal stress direction until the value of the intermediate principal stress is reached;

[0020] S24, keeping the value of the stress in the intermediate principal stress direction unchanged, and continuing to apply stress in the maximum principal stress direction until the test piece is stretched and damaged, and recording the cracking stress, the damage stress, and the peak stress, and calculating the damage risk coefficient according to the cracking stress, the damage stress, and the peak stress;

[0021] S25, changing the values of the minimum principal stress and the intermediate principal stress, and repeating S21 to S24 to obtain the damage risk coefficient of the surrounding rock under different stress conditions.

[0022] Further, the damage modes include shear failure, tensile-shear composite failure, and tensile failure.

[0023] Further, in S2, the calculation formula of the damage risk coefficient is wherein, A represents the damage risk coefficient, σc represents the cracking stress, σd represents the damage stress, σf represents the peak stress.

[0024] Further, in S3, the damage risk coefficient range of each damage mode includes: the damage risk coefficient range of shear failure is A < 0.15; the damage risk coefficient range of tensile-shear composite failure is 0.15 ≤ A ≤ 0.5; and the damage risk coefficient range of tensile failure is A > 0.5.

[0025] Further, in S4, the finite element analysis of each cavern axis scheme is performed by using the FLAC3D software, and in the simulation of the excavation process, the damage risk coefficient is updated in real time, so as to obtain the damage mode and the corresponding spatial range of each cavern axis scheme.

[0026] Furthermore, in S5, based on the failure mode and the corresponding spatial range, the corresponding cavern axis scheme is selected to obtain the corresponding underground cavern axis, including: selecting the cavern axis scheme with the smallest spatial range corresponding to tensile failure as the underground cavern axis.

[0027] The beneficial effects of this invention are as follows: This invention provides a method for optimizing the axis of an underground cavern. By regressing and inverting the geostress field, the initial stress condition range of the surrounding rock before cavern excavation is obtained. A true triaxial loading test is conducted indoors to simulate the stress, obtaining parameters such as initiation stress, damage stress, and peak stress under different stress states, as well as the failure evolution characteristics. This yields the range of failure risk coefficients for each failure mode. Based on the range of failure risk coefficients for each failure mode, finite element analysis is performed on each cavern axis scheme to obtain the spatial range of failure modes for each cavern axis scheme. Based on the failure mode and the corresponding spatial range, the appropriate cavern axis scheme is selected, thus obtaining the corresponding underground cavern axis. This solves the problem in the prior art of not knowing how to select the axis of an underground cavern. Attached Figure Description

[0028] Figure 1 This is a flowchart illustrating a method for optimizing the axis of an underground cavern provided by the present invention. Detailed Implementation

[0029] This invention addresses the challenge of selecting a better axis scheme for existing underground caverns by providing a method for optimizing the axis of underground caverns, such as... Figure 1 As shown, it includes the following steps:

[0030] S1. Based on the measured points on site, perform regression and inversion calculations of the geostress field to obtain the stress condition range of the surrounding rock before the excavation of the tunnel.

[0031] Specifically, the stress condition range of the surrounding rock is the initial stress field, including the direction and numerical range of the maximum principal stress, the direction and numerical range of the intermediate principal stress, and the direction and numerical range of the minimum principal stress.

[0032] S2. Based on the stress conditions of the surrounding rock, conduct indoor true triaxial loading tests to obtain the initiation stress, damage stress, and peak stress of the surrounding rock, and calculate the failure risk coefficient based on the initiation stress, damage stress, and peak stress.

[0033] Specifically, the formula for calculating the damage risk factor is as follows: ,in, Indicates the risk factor of damage. Indicates the initiation stress. Indicates damage stress, Indicates peak stress. This indicates the energy storage capacity during the resilience phase. This indicates the energy storage capacity as it expands from microscopic defects to macroscopic degradation.

[0034] The indoor true triaxial loading test includes the following steps:

[0035] S21. Select intact, hard rock samples without damage or defects, and prepare them into specimens of 50mm×50mm×100mm, with the surface flatness of the specimens controlled within ±0.02 mm.

[0036] S22. Apply stress to the specimen from three directions, namely the direction of minimum principal stress, the direction of intermediate principal stress, and the direction of maximum principal stress, and apply the stress at a rate of 0.5 MPa / s until the value of minimum principal stress is reached.

[0037] S23. Keep the stress value in the direction of minimum principal stress unchanged, and continue to apply stress in the direction of intermediate principal stress and the direction of maximum principal stress until the stress is applied to the value of intermediate principal stress.

[0038] S24. Keep the stress value in the middle principal stress direction unchanged, and continue to apply stress in the direction of maximum principal stress until the specimen fails tensilely. Record the initiation stress, damage stress and peak stress, and calculate the failure risk coefficient based on the initiation stress, damage stress and peak stress.

[0039] Specifically, the initiation stress is the critical stress value in the elastic stress stage of the specimen, the damage stress is the critical value for the expansion of micro-defects inside the specimen to macro-deterioration, and the peak stress is the ultimate bearing capacity of the specimen.

[0040] S25. Change the values ​​of the minimum principal stress and the intermediate principal stress, and repeat S21 to S24 to obtain the failure risk coefficient of the surrounding rock under different stress conditions.

[0041] S3. Compare the failure risk coefficients under different stress conditions to obtain the range of failure risk coefficients for each failure mode.

[0042] Specifically, the failure risk coefficients under different stress conditions are normalized for easier comparison. The failure modes include shear failure, tensile-shear failure, and tensile failure. Tensile failure is also known as brittle failure. When tensile failure occurs, the indoor true triaxial loading test ends.

[0043] The minimum principal stress of a certain surrounding rock is 5 MPa. Through indoor true triaxial loading tests, the failure risk coefficient range of each failure mode was obtained by comparing different intermediate principal stresses. Based on the failure modes, it can be seen that the failure risk coefficient range of shear failure is A < 0.15; the failure risk coefficient range of tensile-shear combined failure is 0.15 ≤ A ≤ 0.5; and the failure risk coefficient range of tensile failure is A > 0.5.

[0044] S4. Based on the aforementioned failure risk coefficient threshold, perform finite element analysis on each cavern axis scheme to obtain the spatial range of failure modes for each cavern axis scheme.

[0045] Specifically, FLAC3D software was used to perform finite element analysis on the axis schemes of each cavern. During the simulated excavation process, the failure risk coefficient was updated in real time to obtain the failure mode and corresponding spatial range of each cavern axis scheme.

[0046] Specifically, it includes:

[0047] S31: Establish a three-dimensional finite element model: Based on the FLAC3D software platform, construct a three-dimensional numerical model of the underground cavern that conforms to the actual engineering situation. The model should include the division of the surrounding rock area, the spatial layout of the cavern, boundary constraints, and mechanical parameter settings. The surrounding rock is given strength parameters and constitutive models consistent with true triaxial tests, and the cavern excavation is simulated using the static unloading method.

[0048] S32: Setting up multiple cavern axis schemes and stress boundary conditions: For different cavern axis layout schemes to be compared, corresponding model versions are established, and initial triaxial geostress conditions are applied based on field measurements and inversion results. The initial stress includes the numerical range and spatial direction of the maximum principal stress, intermediate principal stress, and minimum principal stress to ensure that the simulation conditions are consistent with the actual measurement environment.

[0049] S33: Introducing the calculation program corresponding to the formula for the failure risk coefficient: The formula for calculating the failure risk coefficient is embedded into the numerical model through the FISH script. During the simulation, the principal stress information of each calculation unit is extracted in real time, its failure risk coefficient is calculated, and it is stored in the zone.extra() variable; combined with the failure risk coefficient range of each failure mode, the failure mode of each unit is determined.

[0050] S34: Simulate tunnel excavation and extract failure results: Simulate the tunnel excavation process according to each axis scheme, and record the failure mode and corresponding spatial range. Classify and statistically analyze the failure modes under different schemes for scheme comparison.

[0051] S5. Based on the failure mode and the corresponding spatial range, select the corresponding cavern axis scheme to obtain the corresponding underground cavern axis.

[0052] Specifically, the cavern axis scheme with the smallest spatial range corresponding to tensile failure is selected to obtain the corresponding underground cavern axis. Alternatively, the spatial range corresponding to tensile failure is taken as the main factor, and the spatial range corresponding to tensile-shear combined failure is taken as the secondary factor. A comprehensive consideration is made to select the corresponding cavern axis scheme to obtain the corresponding underground cavern axis.

[0053] Example:

[0054] Taking a 480m vertically buried underground cavern as an example, the stress conditions of the surrounding rock before the excavation of the underground cavern are as follows: initial maximum principal stress The numerical range of the principal stress is 17.5 MPa to 30 MPa, with a horizontal projection azimuth of N80°W. The numerical range of the intermediate principal stress is 12 MPa to 20 MPa, with a horizontal projection azimuth of N18°W. The numerical range of the minimum principal stress is 7.5 MPa to 15 MPa, with a horizontal projection azimuth of N85°E. Scheme 1 for the cavern axis is N50°W, which is consistent with the initial maximum principal stress. With an included angle of 30°, the second option for the cavern axis is: N35°W, with an included angle of 45° with the initial stress field.

[0055] Through indoor true triaxial loading tests and comparisons, it was found that when the failure risk coefficient is greater than 0.5, there is a risk of brittle failure. Finite element analysis showed that the volume of the brittle failure zone in Scheme 1 is about 52,000 cubic meters, while the volume of the brittle failure zone in Scheme 2 is about 41,000 cubic meters. Therefore, Scheme 2 is the more favorable scheme, and the corresponding underground cavern axis of Scheme 2 was obtained.

Claims

1. A method for optimizing the axis of an underground cavern, characterized in that, Includes the following steps: S1. Based on the measured points on site, perform regression and inversion calculations of the geostress field to obtain the stress condition range of the surrounding rock before the excavation of the tunnel. S2. Based on the stress conditions of the surrounding rock, conduct indoor true triaxial loading tests to obtain the initiation stress, damage stress and peak stress of the surrounding rock, and calculate the failure risk coefficient based on the initiation stress, damage stress and peak stress. The indoor true triaxial loading test includes the following steps: S21. Select intact, hard rock samples without damage or defects, and prepare them into specimens of 50mm×50mm×100mm, with the surface flatness of the specimens controlled within ±0.02 mm. S22. Apply stress to the specimen from three directions, namely the direction of minimum principal stress, the direction of intermediate principal stress, and the direction of maximum principal stress, and apply the stress at a rate of 0.5 MPa / s until the value of minimum principal stress is reached; S23. Keep the stress value in the direction of minimum principal stress unchanged, and continue to apply stress in the direction of intermediate principal stress and the direction of maximum principal stress until the stress is applied to the value of intermediate principal stress. S24. Keeping the stress value in the intermediate principal stress direction unchanged, continue applying stress in the direction of the maximum principal stress until the specimen fails tensilely. Record the initiation stress, damage stress, and peak stress. Calculate the failure risk factor based on the initiation stress, damage stress, and peak stress. The formula for calculating the failure risk factor is: ,in, Indicates the risk factor of damage. Indicates the initiation stress. Indicates damage stress, Indicates peak stress; S25. Change the values ​​of the minimum principal stress and the intermediate principal stress, and repeat S21 to S24 to obtain the failure risk coefficient of the surrounding rock under different stress conditions. S3. Compare the failure risk coefficients under different stress conditions to obtain the range of failure risk coefficients for each failure mode; the failure modes include shear failure, tensile-shear combined failure, and tensile failure. S4. Based on the range of failure risk coefficients for each failure mode, finite element analysis is performed on each tunnel axis scheme to obtain the failure mode and corresponding spatial range of each tunnel axis scheme. S5. Based on the failure mode and the corresponding spatial range, select the corresponding cavern axis scheme to obtain the corresponding underground cavern axis.

2. The method for optimizing the axis of an underground cavern according to claim 1, characterized in that, The stress conditions of the surrounding rock are defined as the initial stress field, which includes the direction and numerical range of the maximum principal stress, the direction and numerical range of the intermediate principal stress, and the direction and numerical range of the minimum principal stress.

3. The method for optimizing the axis of an underground cavern according to claim 1, characterized in that, In S3, the failure risk coefficient ranges for each failure mode are as follows: the failure risk coefficient range for shear failure is A < 0.15; the failure risk coefficient range for tensile-shear combined failure is 0.15 ≤ A ≤ 0.5; and the failure risk coefficient range for tensile failure is A > 0.

5.

4. The method for optimizing the axis of an underground cavern according to claim 3, characterized in that, In S4, FLAC3D software was used to perform finite element analysis on the axis schemes of each cavern. During the simulated excavation process, the failure risk coefficient was updated in real time to obtain the failure mode and corresponding spatial range of each cavern axis scheme.

5. The method for optimizing the axis of an underground cavern according to claim 4, characterized in that, In S5, based on the failure mode and the corresponding spatial range, the corresponding cavern axis scheme is selected to obtain the corresponding underground cavern axis, including: selecting the cavern axis scheme with the smallest spatial range corresponding to tensile failure, thereby obtaining the corresponding underground cavern axis.

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