Method for designing large cavern group of hydropower station under extremely high crustal stress condition

Abstract

The invention provides a large cavern group design method for a hydropower station under an extremely high crustal stress condition. The method comprises the following steps: S1, analyzing a three-dimensional initial crustal stress field of a factory; s2, rock mass mechanical characteristic analysis; s3, underground powerhouse cavern group surrounding rock stability analysis; s4, carrying out design analysis on an anchor cable pre-tightening coefficient and a supporting opportunity; s5, performing micro-seismic early warning analysis on deformation and fracture of the surrounding rock of the cavern group of the underground powerhouse; and S6, providing underground powerhouse rockburst criteria in combination with measured rockburst data: providing underground powerhouse rockburst occurrence and evolution mechanisms based on rock mass characteristics, rockburst characteristics, micro-seismic data and monitoring results, analyzing surrounding rock stress release, relaxation and deformation characteristics, and evaluating surrounding rock stability. The design safety and reliability of the large cavern group of the hydropower station are improved, the construction risk and cost are remarkably reduced, and meanwhile the requirement for environmental protection is met. The improvement measures act together to ensure smooth implementation and long-term stable operation of an engineering project.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of hydraulic engineering, in particular to a large cavern group design method under extremely high stress conditions of a hydropower station. BACKGROUND

[0002] It is difficult to measure and predict the ground stress in a high stress environment, which may lead to inaccurate assessment of the actual ground stress condition in the design phase. This will affect key decisions such as cavern group layout and support structure design. Under extremely high stress conditions, the rock mass may exhibit splitting, shear failure and other phenomena, increasing the difficulty of controlling the stability of the rock mass. More accurate geological exploration data and advanced numerical simulation techniques are needed to predict these risks and take effective engineering measures to address them. Due to the extremely high ground stress, conventional support methods may not be sufficient to ensure the safety and stability of the cavern, and more complex support systems need to be used. SUMMARY

[0003] The main purpose of the present application is to provide a large cavern group design method under extremely high stress conditions of a hydropower station, which solves the problems in the background art.

[0004] To solve the above technical problems, the technical solution adopted by the present application is as follows: S1, analysis of the initial three-dimensional ground stress field of the plant area; S2, analysis of the mechanical properties of the rock mass; S3, analysis of the stability of the surrounding rock of the underground powerhouse cavern group; S4, analysis of the design of the anchor pre-tightening coefficient and support timing; S5, analysis of the deformation and fracture microseismic early warning of the surrounding rock of the underground powerhouse cavern group; S6, combining the measured data of rock burst to propose a rock burst criterion for the underground powerhouse: based on the characteristics of the rock mass, the characteristics of the rock burst, the microseismic data and the monitoring results, the occurrence and evolution mechanism of the rock burst of the underground powerhouse is proposed, the stress release, relaxation and deformation characteristics of the surrounding rock are analyzed, and the stability of the surrounding rock is evaluated.

[0005] Preferably, the specific steps of step S1 are as follows: The measured values of the initial ground stress of the left bank plant area are analyzed, and the initial ground stress measurement points suitable for regression are selected; The actual topography and geological conditions of the left bank plant area are simulated, appropriate geomechanical parameters are selected, and based on the selected plant area initial ground stress measurement points and their values, a three-dimensional initial ground stress field regression analysis model is established, and the initial ground stress field of the plant area is regressed; The rationality of the regressed initial ground stress field of the plant area is analyzed and demonstrated in combination with the actual topography and geological conditions of the hub area.

[0006] Preferably, the three-dimensional grid model of the underground powerhouse cavern group comprises installation rooms, main and auxiliary powerhouse caverns, main transformer rooms, tailwater surge chambers, and main caverns of the powerhouse area such as water diversion tunnels, busbar tunnels, and tailwater tunnels, and rock masses and faults are simulated by using three-dimensional 8-node 6-facet element and its degenerated element; During three-dimensional modeling, geological features such as rock stratum interfaces, terrain, and faults are strictly simulated according to geological profiles, and the influence of slope terrain and geological conditions on underground engineering is fully reflected. Meanwhile, the actual fault thickness, occurrence variation, and pinchout characteristics are considered in the fault surface of the model, so that the numerical calculation realizes engineering simulation.

[0007] Preferably, the specific steps of step S2 are as follows: Physical and mechanical experiments are carried out to analyze the mechanical properties of the surrounding rock of the underground cavern of the hydropower station under loading and unloading; A constitutive model of the deformation and fracture characteristics of the rock mass of the underground powerhouse cavern group is established; The influence of construction blasting damage and excavation unloading on the mechanical properties of the rock mass is proposed; Based on the results of the physical and mechanical experiments, a criterion for rockburst tendency is proposed.

[0008] Preferably, the evolution law of the original fissures in the underground powerhouse granite is analyzed by using conventional triaxial compression tests, and the original fissures in the rock are divided into axial fissures and radial fissures, that is, the axial fissure strain and the radial fissure strain are caused under the action of stress, and the deviatoric stress-peak strength ratio R d is introduced to analyze the test data; A dimensionless parameter R d , that is, the ratio of deviatoric stress to peak strength, is introduced to analyze the test data, which can be expressed as: (1); where σ1-σ3 is the deviatoric stress; σ p is the peak strength, that is, the deviatoric stress σ1-σ3 when R d =1.

[0009] Preferably, based on the crack closure degree, a unified stress-fissure strain evolution constitutive model is formed by using pseudo-viscosity and plastic elements to represent the relationship between the axial and radial fissure strains and R d at each fissure development stage, which describes the axial and radial fissure evolution processes, and reveals the shear compression phenomenon of the rock in the crack closure process and the shear expansion phenomenon in the crack expansion process; The strain of the rock includes elastic strain and fissure strain, so the axial fissure strain and the radial fissure strain can be expressed as: (2); Where ε1 and ε3 represent axial strain and radial strain, respectively. and It is the elastic strain in both the axial and radial directions. and These are the axial and radial crack strains.

[0010] Preferably, the specific steps of step S3 are as follows: A comprehensive analysis and thorough demonstration of the overall stability characteristics of the surrounding rock of the underground powerhouse cavern group and the support effect of various support measures were conducted, and the three-dimensional overall stability characteristics of the surrounding rock of the underground powerhouse cavern group were analyzed and evaluated. Using the elastoplastic theory of rock mass, this study compares and analyzes the deformation, stress, and distribution of the plastic zone of the underground powerhouse cavern group before and after support, as well as the internal forces of the support structure; evaluates the overall and local stability of the surrounding rock and the effectiveness of the support structure, and proposes optimization suggestions for the support structure. Based on the theories of energy dissipation and stress-fracture strain evolution, and considering the confining pressure effect of the surrounding rock strength, this study compares and analyzes the boundaries between the strongly relaxed zone, the weakly relaxed zone, and the original rock zone around the underground powerhouse cavern group before and after support. It evaluates the overall and local stability of the surrounding rock and the effectiveness of the support structure, and proposes optimization suggestions for the support structure.

[0011] Preferably, the specific steps of step S4 are as follows: Based on the stress and measured displacement distribution of the surrounding rock during phased excavation, the preload coefficients of anchor cables at different locations in the three major tunnel groups are given. The optimal support timing for anchorage support measures in different parts of the three major cavern groups is given.

[0012] Preferably, the optimal support timing can be calculated as follows: and (3); In the above formula: Optimal support timing (Unit: days) Surrounding rock deformation convergence time Strength-stress ratio strain margin The first principal stress of the surrounding rock after excavation With uniaxial compressive strength ratio Support and confining pressure Time-dependent deformation load coefficient Poisson's ratio , bulk density γ, coefficient ; The anchor cable preload coefficient can be calculated using the following formula: (4); Wherein: anchor cable load factor =0.5, stress relief , anchor cable design tonnage Ns, anchor cable inter-row spacing , the time-dependent load factor is respectively α, the installation time t (days), the stable convergence time Tc is respectively 90 days (, 180 days (, 365 days (. ). . .

[0013] Preferably, the specific steps of step S5 are as follows: The stress variation law of the surrounding rock of the cavern during construction: the microseismic information, the conventional monitoring information, the geological structure, the excavation process, the conventional monitoring and the numerical simulation results of the deformation process of the surrounding rock of the underground powerhouse are combined to reveal the basic law of stress field accumulation, release and transfer of the surrounding rock of the cavern and under the construction unloading action.

[0014] Based on the multi-parameter evaluation of the underground cavern excavation damage area such as microseismic information, numerical simulation and conventional monitoring, comparative analysis is carried out; The deformation feedback analysis and grading early warning of the surrounding rock of the underground powerhouse based on multi-parameter information.

[0015] The present application provides a large cavern group design method under extremely high stress conditions of a hydropower station, 1. By screening the measured data, numerical simulation and rationality verification, the understanding of the actual ground stress state of the construction area is improved, and a reliable foundation is provided for subsequent design. The geological characteristics such as rock layer interface, terrain and fault are strictly simulated, the influence of slope terrain and geological conditions on underground engineering is fully reflected, and the accuracy and reliability of the model are improved.

[0016] 2. By carrying out indoor physical and mechanical experiments, analyzing the loading and unloading characteristics of the surrounding rock, and establishing a more accurate constitutive model, the behavior of the rock in the construction process can be better predicted. The rock burst tendency criterion based on experimental results is proposed, which can identify potential rock burst risks in advance, so as to take effective preventive measures.

[0017] 3. The deformation, stress distribution and other information of the cavern before and after supporting are compared and analyzed by using the elastoplastic theory, the overall and local stability of the surrounding rock is evaluated, and the safety and effectiveness of the design scheme are ensured. Based on the energy dissipation and stress-crack strain evolution theory, the optimization suggestions of the supporting structure are put forward, which enhances the effect of the supporting measures and reduces the construction risk.

[0018] 4. According to the stress and measured displacement distribution law of the surrounding rock in the staged excavation, the best anchoring measures and supporting time of different parts are given, which ensures the maximization of the anchoring effect. The optimal supporting time is determined through specific calculation formula, which avoids the safety hazards caused by early or late supporting, and improves the construction efficiency.

[0019] 5. Integrating microseismic information, conventional monitoring data, geological structure, and excavation process information, the overall monitoring of the stress variation law of the surrounding rock of the cavern during the construction period is realized. The deformation feedback analysis and hierarchical early warning of the underground powerhouse surrounding rock based on multi-parameter information can timely discover and handle potential risks, ensuring the construction safety.

[0020] 6. Combining various advanced technical means, the challenges faced by the construction of the hydropower station under high stress environment are effectively solved, and the safety and economy of the project are improved. Through precise design and construction management, the impact on the surrounding ecological environment is reduced, and the balance between engineering construction and ecological protection is achieved. BRIEF DESCRIPTION OF DRAWINGS

[0021] The application will be further described below in combination with the drawings and examples: Figure 1 is the three-dimensional finite element model of the cavern group (before supporting) of the application; Figure 2 is the three-dimensional finite element model of the cavern group (after supporting) of the application; Figure 3 is the relationship diagram of the three caverns and the main faults of the application; Figure 4 is the granite rock sample of the application; Figure 5 is the stress-strain curve of the granite rock sample of the application; DETAILED DESCRIPTION Example 1 As shown in Figures 1-5 , the design method of large cavern group under extremely high stress conditions of hydropower station, the research on three-dimensional initial ground stress field of plant area; The measured values of the initial ground stress of the left bank plant area are analyzed, and the initial ground stress measurement points suitable for regression are selected; The actual topography and geological conditions (including various structural surfaces, etc.) of the left bank plant area are simulated, appropriate geomechanical parameters are selected, and a reasonable three-dimensional initial ground stress field regression analysis model is established according to the selected plant area measured initial ground stress points and their values, and the initial ground stress field of the plant area is regressed. Combined with the actual topography and geological conditions of the hub area, the rationality of the regressed initial ground stress field of the plant area is analyzed and demonstrated.

[0022] As shown in Figures 1-3As shown, the three-dimensional grid model of the underground powerhouse cavern group includes installation rooms, main and auxiliary powerhouse tunnels, main transformer rooms, tailwater surge chambers, and water diversion tunnels, busbar tunnels, tailwater tunnels, and other main caverns in the plant area. Rock mass and faults are simulated by 3D 8-node 6-surface element and its degenerate element. The overall three-dimensional model has a total of 91479 entity element nodes, 181231 entity elements, 51803 anchor cable and anchor rod element nodes, and 26089 anchor cable and anchor rod elements.

[0023] During three-dimensional modeling, the geological interface, terrain, faults, and other geological features are strictly simulated according to the geological profiles provided by the design institute, fully reflecting the influence of slope terrain and geological conditions on underground engineering. At the same time, the actual fault thickness, occurrence variation, and pinchout characteristics are considered in the fault plane of the model, making the numerical calculation realize engineering simulation and as far as possible reflect the actual situation of the project.

[0024] Rock mass mechanical property analysis: Conduct indoor physical and mechanical experiments to analyze the mechanical properties of the surrounding rock of the underground cavern of the hydropower station under loading and unloading; Establish a constitutive model for the deformation and fracture characteristics of the rock mass of the underground powerhouse cavern group; Propose the influence of construction blasting damage and excavation unloading on the mechanical properties of rock mass; Based on the results of indoor physical and mechanical experiments, propose a criterion for rockburst tendency.

[0025] Moor-Coulomb yield criterion is used to study the influence of underground powerhouse excavation deformation and failure zone; Table 1: Physical and mechanical parameters of underground powerhouse rock mass

[0026] The rock samples used in this test are taken from the granite of the hydropower station, which is mainly composed of microcline, feldspar, quartz, biotite, and muscovite. According to the test procedures published by the International Society for Rock Mechanics (ISRM), the rock is cut into φ50x100 mm cylindrical standard specimens, as shown in Figure 4 The conditions that need to be met for the processing and production of rock samples are: (1) the diameter of the rock sample should be controlled between 48-52 mm; (2) the height-to-diameter ratio of the rock sample should be controlled between 1.9-2.1; (3) the parallelism error of the two end faces of the rock sample should be less than 5x10-2 mm.

[0027] After the rock sample is produced, the rock sample with obvious flaws and cracks on the surface is removed, and the size that does not meet the requirements is polished or repaired until it meets the requirements. A total of 48 rock samples are selected for testing, of which 28 are used for conventional triaxial compression tests, 16 are used for creep tests, and the rest are used as backups.

[0028] The test loading equipment adopts MTS815 Flex Test GT rock triaxial testing machine. Before triaxial compression and creep test, the instrument needs to be calibrated. After starting the equipment, the axial and radial loads are applied simultaneously. For the loading system, when the indicated value is stable, the load is maintained for more than 30 seconds, and the load change range is not more than 0.2% of the full scale during this period. The extensometer should continuously record the deformation of the rock sample during deformation measurement, and the absolute error of the recorded value should be within ±2 μm, and the relative error should be within ±0.5%.

[0029] According to the underground powerhouse ground stress measuring point data, the ground stress of the plant area is mainly tectonic stress, and the minimum principal stress is 5-11 MPa. After excavation, the stress in the cavern is redistributed, and the minimum principal stress range is 0-10 MPa. In order to study the mechanical properties of the surrounding rock after the excavation of the cavern, the confining pressure of the triaxial compression test is selected as 1, 3, 5 and 10 MPa; in addition, confining pressures of 20 MPa, 30 MPa and 40 MPa are selected for testing to study the mechanical properties of the rock under high ground stress.

[0030] After the rock sample is installed on the test equipment, the test is carried out according to the following test procedure: The confining pressure σ3 and axial load σ1 of each rock sample are simultaneously loaded to the confining pressure value at a loading rate of 0.05 MPa / s, and then the rock sample is kept stable.

[0031] The axial load is continuously increased at a loading rate of 0.5 MPa / s, while the confining pressure is kept constant, until the rock sample fails.

[0032] During the entire test process, the instantaneous axial load, axial and radial displacement of the sample and the corresponding confining pressure are recorded.

[0033] The stress-strain curve of the granite rock sample in the test is shown in Figure 1 ε1 and ε3 represent the axial strain and radial strain respectively, and the positive strain value represents compression of the rock sample, and the negative represents expansion. The volume strain ε v can be calculated from ε1 and ε3: (5); wherein ε1 and ε3 represent the axial strain and radial strain respectively, and ε v is the volume strain.

[0034] From Figure 1 it can be seen that the peak stress σ p increases with the increase of the confining pressure σ3, and when the confining pressure is 1, 3, 5, 10, 20, 30 and 40 MPa, σ p is 114.72, 139.65, 157.22, 182.97, 216.30, 345.73 and 374.63 MPa respectively.

[0035] In order to determine the elastic modulus and Poisson's ratio of granite under different confining pressures, four threshold stresses of granite, i.e. crack closure stress σ cc , crack initiation stress σ ci , crack damage stress σ cd and peak stress σ p , should be determined first. These four threshold stresses can divide the crack evolution of rock 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-peak stage (stage V), as shown in Figure 5 .

[0036] Rock will produce strain under the action of external load, and the strain can be divided into elastic strain and crack strain. The volumetric strain can be composed of elastic volumetric strain and crack volumetric strain , which can be expressed as: (6); When the deviatoric stress reaches σ cc , the natural micro-cracks in the rock are completely closed, and the cracks continue to expand after the deviatoric stress reaches σ ci . Therefore, there is no crack expansion and no crack strain between σ cc and σ ci . Then, the slope value of the straight line between σ cc and σ ci in the rock strain curve can be regarded as the elastic modulus E of the rock sample. Under σ cc = 10 MPa, the elastic modulus E of the rock sample is 54.29 GPa.

[0037] According to Hooke's law, the axial and radial elastic strains and can be expressed as: (7); where µ is the Poisson's ratio.

[0038] Let B = / , then:

[0039] According to equation (7), the Poisson's ratio μ of the elastic stage is 0.1846 when σ cc = 10 MPa. Then, the crack volumetric strain can be expressed as:

[0040] Since σ cc and σ ciNo crack strain occurs in this interval, so the crack volume strain curve is a horizontal line in this interval. The threshold stress, elastic modulus and Poisson's ratio at other confining pressures can also be calculated using the above method, and the results are shown in Table 2.

[0041] Table 2: Mechanical parameters of granite samples at different confining pressures

[0042] 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 d , i.e. the ratio of deviatoric stress to peak strength σ p , is introduced to analyze the test data, which can be expressed as: (1); where σ1-σ3 is the deviatoric stress; σ p is the peak strength, i.e. the deviatoric stress σ1-σ3 when R d =1; The crack closure stress, crack opening stress, crack damage stress, and the ratio of peak stress to peak strength are denoted as , , and , respectively.

[0043] The axial and radial elastic strains and can be expressed as: (8); where µ is the Poisson's ratio; The axial crack strain and the radial crack strain can be expressed as: (2); where ε1 and ε3 represent the axial and radial strains, respectively, and are the axial and radial elastic strains.

[0044] The axial and radial elastic strains are calculated by equation (7), and then the deviatoric stress and the axial and radial crack strains at different confining pressures can be calculated by equation (2); The axial and radial crack strains at , , and are calculated, i.e. , , , , , , and .

[0045] The fissure strain has an exponential relationship with the confining pressure. The confining pressure can affect the axial fissure strain of the rock, and its influencing ability gradually weakens with the increase of the confining pressure. During the fissure closure stage of the rock, the fissures gradually close with the increase of the axial stress. To facilitate the evaluation of the fissure closure degree, the axial and radial fissure closure degrees C1 and C3 are introduced: (9); where R d is a dimensionless parameter, which is the ratio of the deviator stress σ1 - σ3 to the peak strength σ p , and 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, is the ratio of the fissure closure stress to the peak strength, and are the axial and radial fissure strains at

[0046] During the stable growth stage of the rock fissures ( < R d ​​​​​​​​​​​​​​​​​​​​​​​​​Based on the theories of energy dissipation and stress-fracture strain evolution, and considering the confining pressure effect of surrounding rock strength, this paper compares and analyzes the boundaries between the strongly relaxed zone, weakly relaxed zone, and original rock zone around the underground powerhouse cavern group before and after support. The overall and local stability of the surrounding rock and the effectiveness of the support structure are evaluated, and optimization suggestions for the support structure are proposed.

[0048] Study on anchor cable preload coefficient and support timing: Based on the stress and measured displacement distribution of the surrounding rock during phased excavation, the preload coefficients of anchor cables at different locations in the three major tunnel groups are given. The optimal support timing for anchorage support measures in different parts of the three major cavern groups is given.

[0049] Based on the theory of time-dependent deformation, the calculation formulas for the optimal support time and prestressed anchorage preload coefficient of the surrounding rock were derived. The optimal support timing can be calculated as follows: and (3); In the above formula: Optimal support timing (Unit: days) Surrounding rock deformation convergence time Strength-stress ratio strain margin The first principal stress of the surrounding rock after excavation With uniaxial compressive strength ratio Support and confining pressure Time-dependent deformation load coefficient Poisson's ratio , bulk density γ, coefficient ; The anchor cable preload coefficient can be calculated using the following formula: (4); Wherein: anchor cable load factor =0.5, stress relief Anchor cable design tonnage (Ns), anchor cable spacing The time-dependent load factor is taken as α, the installation time is t (days), and the stable convergence time is Tc, which are 90 days. ), 180 days ), 365 days ).

[0050] Study on early warning of deformation and fracture of surrounding rock in underground powerhouse cavern complex: Stress variation law of surrounding rock in tunnel during construction: By combining microseismic information and conventional monitoring information on the deformation process of surrounding rock in underground powerhouse with geological structure, excavation procedures, conventional monitoring and numerical simulation results, the basic laws of stress accumulation, release and transfer of surrounding rock in tunnel under construction unloading are revealed. Based on microseismic information, numerical simulation and conventional monitoring, the damage zone of underground caverns is evaluated, and comparative analysis is carried out. Based on multi-parameter information, the deformation feedback analysis and hierarchical early warning of surrounding rock of underground powerhouse are carried out.

[0051] Based on the measured data of rock burst, the rock burst criterion of underground powerhouse is proposed. Based on the characteristics of rock mass, rock burst characteristics, microseismic data and monitoring results, the mechanism of rock burst occurrence and evolution is proposed, the stress release, relaxation and deformation characteristics of surrounding rock are analyzed, and the stability of surrounding rock is evaluated.

[0052] Example 2 Further illustrated in combination with Example 1, the indoor test of surrounding rock, stress-crack strain constitutive relation, creep deformation model analysis, and ground stress inversion are carried out for the supporting time and surrounding rock deformation early warning of large underground cavern group under extremely high ground stress conditions of hydropower station; based on the monitoring data of each level of excavation, the stress and strain of surrounding rock, damage zone distribution, internal force of anchor rod and anchor cable, and optimal supporting time are researched, and the engineering suggestions are proposed. The main conclusions include: Test and constitutive relation: The conventional triaxial compression test is carried out on the granite of underground powerhouse of hydropower station, and the ratio R d of deviatoric stress and peak strength is introduced to analyze the test data, and the expression of five-stage crack closure C is proposed. The stress-strain relationship in the five stages of crack closure, elasticity, stable crack growth, unstable crack growth and post-peak stage is quantitatively described. The crack closure degree and evolution process of rock cracks in the five stages can be quantitatively described by crack closure C.

[0053] The crack closure stress, crack initiation stress, crack damage stress and peak strength have a positive linear relationship with confining pressure. Based on the correlation evolution relationship between crack closure C and R d , a unified stress-crack strain evolution constitutive model is proposed by introducing pseudo-viscosity, plasticity and elasticity elements. This model can describe the evolution process of axial and radial cracks at the same time, overcome the shortcomings of previous models, and especially this model can conveniently describe the mechanical behavior in the post-peak stage, and reveal the law that the radial deformation is obviously larger than the axial deformation in the damage stage.

[0054] Through the indoor creep compression test, an improved Nishihara creep constitutive model is proposed, which can accurately describe the deformation of rock in the attenuation and steady-state creep stages. Based on the R d of stress and peak strength, the unified expressions of instantaneous strain and viscoelastic creep strain under various confining pressures are derived. In addition, the instantaneous elastic modulus EM M , Maxwell viscosity coefficient η K , Kelvin modulus E K and viscosity coefficient η dThe peak strength, confining pressure, peak strength and creep time are calculated, and a method of using R d and steady-state creep strain rate to define the long-term strength of rock.

[0055] Stress and deformation analysis of surrounding rock: Initial stress is an important prerequisite for simulating the excavation of underground powerhouses. A method combining lateral stress coefficient and generative adversarial network (GAN) is proposed to invert the initial stress field by paleostratigraphic excavation. It is found that for deep buried engineering, the lateral stress coefficient is inversely proportional to the burial depth, rather than following the commonly used linear relationship. The lateral stress coefficient is determined by the GAN artificial intelligence method, in which the elements containing rock mass and faults are regarded as transversely isotropic equivalent elements, thus reducing the difficulty of numerical model modeling and computing time. The stress field is obtained by paleostratigraphic excavation, simulating the geological effects of surface erosion and weathering, and inverting the present-day stress field. The inversion results can reflect the actual stress field distribution, and can provide reasonable guidance for the simulation of underground cavern excavation and stability analysis.

[0056] Deformation and stress of surrounding rock during excavation: The overall deformation trend of each level of excavation cavity is convergence. After the 9th level of excavation is completed, the three large caverns are excavated, and the deformation after support is reduced to varying degrees compared with that before support. The upstream deformation of the main powerhouse of the installation bay and the 1st to 4th unit sections is greater than that of the downstream, while the downstream side wall displacement of the auxiliary powerhouse is greater than that of the upstream due to the influence of the phonolite vein. Due to the lag of the installation of the multi-point displacement meter, the displacement of the multi-point displacement meter of each unit section is close to the calculated value, and the calculated deformation law is similar to the measured deformation law, indicating that the calculated parameters are consistent with the actual situation.

[0057] The stress distribution law before and after support has little difference. Stress concentration occurs at the two arch ends of the main powerhouse, the rock anchor beam, the two arch ends of the main transformer room, the arch end of the tail regulation room, and the bottom corner of the cavern side wall. The first principal stress extreme value of the main powerhouse, main transformer room and tail regulation room is increased to 55.0, 45.0 and 40.0 MPa, respectively. Excavation has a significant impact on the stress field around the excavation area, but the stress distribution characteristics remain almost unchanged after a certain distance from the cavern wall.

[0058] The plastic zone distribution under each un-supported and supported working condition is basically consistent, but the plastic zone depth and connectivity are reduced, and the plastic zone is reduced after support. The damage zone, strong relaxation zone, weak relaxation zone and excavation disturbance zone of non-fault rock mass divided by crack closure C3 can be used as a reference for support design.

[0059] Distribution of plastic failure zone of surrounding rock: The depth of the shallow plastic zone in the installation bay section is only 2.23 m, which is smaller than that before the support.

[0060] There is no plastic zone in the top arch of the main transformer room of each unit, and only a few plastic damage points appear in the upstream sidewall. The depth of the plastic damage zone in the downstream sidewall is about 4 m, and there is no plastic zone between the main powerhouse and the main transformer room, which is basically in an elastic working state.

[0061] The depth of the plastic damage zone in the upstream sidewall of the tail regulating room is about 4.0-9.0 m, and the depth of the plastic damage zone in the downstream sidewall is about 3.0-5.0 m. The plastic zone in the upstream of the tail regulating room of the 4# unit connects with the plastic zone in the main transformer room through f1 plastic zone, which can easily form a sliding block cutting combination in the top arch of the tail regulating room, affecting the stability of the top arch.

[0062] Compared with the unsupported working condition, the support measures have a significant effect on inhibiting the deformation and plastic zone development of the surrounding rock. After the support, the plastic volume is reduced by 5.78% compared to before the support. After the excavation is completed, the deformation and stress state of the surrounding rock are normal, the plastic zone development is under control, and the overall stability of the surrounding rock is within a reasonable range.

[0063] The surrounding rock after excavation is considered to be in a uniaxial compression state under no confining pressure stress or in a triaxial compression state under low confining pressure stress. Based on the variation law of crack closure degree and apparent deformation modulus during the whole failure process of the surrounding rock, a 5-zone (damage zone, strong relaxation zone, weak relaxation zone, stress disturbance zone, and original rock zone) partition criterion for the relaxation failure of the surrounding rock is proposed. Based on the stress-crack strain evolution constitutive model, the excavation simulation of the underground powerhouse with and without support is carried out, and the crack damage of the rock mass in the plastic state is quantitatively analyzed using crack closure degree. Compared with the traditional plastic zone distribution map, the crack closure degree more intuitively and quantitatively describes the damage degree of the rock mass.

[0064] Distribution of internal forces of surrounding rock anchor rods and cables: After the 9th stage of excavation is completed, the extreme value of the internal force of the ordinary anchor rod and the prestressed anchor rod generally appears near the downstream abutment and the top arch, and the internal force of the anchor rod and cable is within the normal working range.

[0065] Anchor rods or cables can provide additional cohesion increment to the surrounding rock. Through the statistical method of least squares fitting, four empirical formulas of the support strength of anchor rods and cables, the strength stress ratio, and the excavation span B are proposed.

[0066] For both anchor cable support and anchor rod support, there are intervals with fast increasing rate of support strength: for anchor rod support, when the strength stress ratio is greater than 0.5, the underground powerhouse surrounding rock is in high to extremely high stress state, and the required support strength increases rapidly; for anchor cable support, when the strength stress ratio is greater than 0.6, the required support strength increases significantly.

[0067] Based on the empirical fitting formula, the dimensionless support index concept is proposed. The index can intuitively represent the relative relationship between the designed support strength and the engineering experience support strength. The support index can be used as a quantitative evaluation standard for support strength to guide the design of anchor rod and anchor cable.

[0068] The anchor rod support strength index of underground powerhouse is about 1.17 to 1.24, while the anchor cable support strength index is 1.55 to 1.70. The support index is within the reasonable range of other similar projects, meeting the support design requirements.

[0069] Analysis of surrounding rock reasonable support timing and anchor cable pre-tightening coefficient: According to the time-dependent damage degree of cavern characterized by crack closure and the principle of maximum self-bearing capacity of surrounding rock, a method of determining support timing based on crack closure inflection point is proposed. Based on the improved Nishihara creep constitutive model, time-dependent calculation of underground powerhouse is carried out. It is found that the tangential stress and radial stress of surrounding rock will gradually release under the action of time-dependent, but the release rate of tangential stress is faster than that of radial stress. The support timing of each cavern surrounding rock is suggested based on the crack closure inflection point.

[0070] The optimal support time of each level of excavation is given: the stress concentration degree of the upstream and downstream arch spring of the 1# unit main powerhouse is large, and the optimal support timing is about 12 to 15 days, while the optimal support timing of the crown arch is about 21.0 days. The optimal support timing of the upstream and downstream side walls is about 27 days. The stress concentration of the upstream arch spring of the 1# unit main transformer room is larger than that of the downstream side, and the optimal support timing of the upstream and downstream is about 19 days and 22 days, respectively, while the optimal support timing of the crown arch is about 23 days. The optimal support timing of the upstream and downstream side walls of the main transformer room is about 24 to 21 days. The optimal support time of the upstream arch spring of the 1# unit tail regulation room crown arch is 16 days, the downstream arch spring is 20 days, and the optimal support time of the crown arch middle part is 23 days; the optimal support time of the upstream and downstream side walls is 27 days and 22 days, respectively. The optimal support time of surrounding rock determined based on time-dependent deformation theory is smaller on the upstream side of the crown arch than on the downstream side and the downstream side wall, which is similar to the distribution law of the optimal support timing determined based on crack closure.

[0071] The pre-tightening coefficient of the crown arch of the three large caverns is about 0.92, and the pre-tightening coefficient of the upstream and downstream side walls is about 0.8.

[0072] The above embodiments are only preferred technical solutions of the present application, and should not be regarded as a limitation of the present application. The protection scope of the present application should be the technical solutions recited in the claims, including equivalent replacement solutions of the technical features recited in the claims. That is, equivalent replacement improvements within this scope are also within the protection scope of the present application.

Claims

1. Design method for large cavern groups in hydropower stations under extremely high ground stress conditions. The steps are as follows: S1. Three-dimensional initial geostress field analysis of the plant area; S2. Analysis of rock mass mechanical characteristics; S3. Stability analysis of the surrounding rock of the underground powerhouse cavern group; S4. Design analysis of anchor cable preload coefficient and support timing; S5. Microseismic early warning analysis of deformation and fracturing of surrounding rock in underground powerhouse cavern group; S6. Based on measured rockburst data, propose rockburst criteria for underground powerhouses: Based on rock mass characteristics, rockburst features, microseismic data and monitoring results, propose the generation and evolution mechanism of rockburst in underground powerhouses, analyze the stress release, relaxation and deformation characteristics of surrounding rock, and evaluate the stability of surrounding rock.

2. The design method for a large cavern group under extremely high ground stress conditions in a hydropower station according to claim 1, wherein the specific steps of step S1 are as follows: The measured values ​​of initial geostress in the left bank plant area were analyzed to select the initial geostress measurement points that could be used for regression. Simulate the actual topography and geological conditions of the left bank plant area, select appropriate geomechanical parameters, and establish a three-dimensional initial geostress field inversion regression analysis model based on the selected initial geostress points and their measured values ​​in the plant area. Invert and regress the initial geostress field of the plant area. Based on the actual topography and geological conditions of the hub area, the rationality of the plant area returning to the initial geostress situation is analyzed and demonstrated.

3. The design method for a large cavern group under extremely high ground stress conditions of a hydropower station according to claim 2, wherein the method is as follows: the three-dimensional mesh model of the underground powerhouse cavern group includes the main caverns of the power plant area, such as the installation room, main and auxiliary powerhouse caverns, main transformer room, tailrace surge chamber, water diversion tunnel, busbar tunnel, and tailrace tunnel. The rock mass and faults are simulated by three-dimensional 8-node 6-hedral isoparametric solid elements and their degradation elements. During 3D modeling, geological features such as rock strata interfaces, topography, and faults are strictly simulated based on geological profiles to fully reflect the impact of slope topography and geological conditions on underground engineering. At the same time, the model's fault interruption layers take into account the actual fault thickness, attitude changes, and pinch-out characteristics, enabling numerical calculations to achieve engineering simulation.

4. The design method for a large cavern group under extremely high ground stress conditions in a hydropower station according to claim 1, wherein the specific steps of step S2 are as follows: Conduct indoor physical and mechanical experiments to analyze the mechanical properties of the surrounding rock in underground caverns of hydropower stations under loading and unloading conditions; Establish a constitutive model for the deformation and fracture characteristics of the rock mass in the underground power plant cavern complex; The effects of construction blasting damage and excavation unloading on the mechanical properties of rock mass are proposed. Based on the results of indoor physical and mechanical experiments, a criterion for rockburst tendency is proposed.

5. The design method for a large cavern group under extremely high ground stress conditions in a hydropower station according to claim 4, wherein the method is as follows: The evolution law of primary fractures in the granite of the underground powerhouse is analyzed using conventional triaxial compression tests. The primary fractures in the rock are divided into axial fractures and radial fractures, which, under stress, will respectively lead to the generation of axial fracture strain and radial fracture strain. The deviatoric stress-peak strength ratio R is then introduced. d To analyze the experimental data; Introduce a dimensionless parameter R d , which is the ratio of deviatoric stress to peak strength, is used to analyze experimental data and can be expressed as: （1）； Where σ1-σ3 are deviatoric stresses; σ p It is the peak intensity, i.e., R d The deviatoric stress σ1-σ3 when =1.

6. The design method for large cavern groups under extremely high ground stress conditions in hydropower stations according to claim 5, wherein the method is: based on the fracture closure degree, using pseudo-viscous and plastic elements to represent the axial and radial fracture strain and R at each fracture development stage. d The relationship between stress and fracture strain evolution is then established to form a unified constitutive model that describes both axial and radial fracture evolution processes. This model reveals the shear compression phenomenon of rock during fracture closure and the shear propagation phenomenon during fracture propagation. The strain in a rock includes elastic strain and fracture strain, so what is the axial fracture strain? and radial crack strain It can be represented as: （2）； in, ε1 and ε3 represent axial strain and radial strain, respectively. and It is the elastic strain in both the axial and radial directions. and These are the axial and radial crack strains.

7. The design method for large cavern groups under extremely high ground stress conditions in hydropower stations according to claim 1, wherein the specific steps of step S3 are as follows: A comprehensive analysis and thorough demonstration of the overall stability characteristics of the surrounding rock of the underground powerhouse cavern group and the support effect of various support measures were conducted, and the three-dimensional overall stability characteristics of the surrounding rock of the underground powerhouse cavern group were analyzed and evaluated. Using the elastoplastic theory of rock mass, this study compares and analyzes the deformation, stress, and distribution of the plastic zone of the underground powerhouse cavern group before and after support, as well as the internal forces of the support structure; evaluates the overall and local stability of the surrounding rock and the effectiveness of the support structure, and proposes optimization suggestions for the support structure. Based on the theories of energy dissipation and stress-fracture strain evolution, and considering the confining pressure effect of the surrounding rock strength, this study compares and analyzes the boundaries between the strongly relaxed zone, the weakly relaxed zone, and the original rock zone around the underground powerhouse cavern group before and after support. It evaluates the overall and local stability of the surrounding rock and the effectiveness of the support structure, and proposes optimization suggestions for the support structure.

8. The design method for a large cavern group under extremely high ground stress conditions in a hydropower station according to claim 1, wherein the specific steps of step S4 are as follows: Based on the stress and measured displacement distribution of the surrounding rock during phased excavation, the preload coefficients of anchor cables at different locations in the three major tunnel groups are given. The optimal support timing for anchorage support measures in different parts of the three major cavern groups is given.

9. The design method for large cavern groups under extremely high ground stress conditions in hydropower stations according to claim 8, wherein the optimal support timing can be calculated as follows: and (3); In the above formula: Optimal support timing (Unit: days) Surrounding rock deformation convergence time Strength-stress ratio strain margin The first principal stress of the surrounding rock after excavation With uniaxial compressive strength ratio Support and confining pressure Time-dependent deformation load coefficient Poisson's ratio , bulk density γ, coefficient ; The anchor cable preload coefficient can be calculated using the following formula: （4）； in: Anchor cable load distribution factor =0.5, stress relief Anchor cable design tonnage (Ns), anchor cable spacing The time-dependent load factor is taken as α, the installation time is t (days), and the stable convergence time is Tc, which are 90 days. ), 180 days ), 365 days ).

10. The design method for a large cavern group under extremely high ground stress conditions in a hydropower station according to claim 1, wherein the specific steps of step S5 are as follows: Stress variation law of surrounding rock in tunnel during construction: By combining microseismic information and conventional monitoring information on the deformation process of surrounding rock in underground powerhouse with geological structure, excavation procedures, conventional monitoring and numerical simulation results, the basic laws of stress accumulation, release and transfer of surrounding rock in tunnel under construction unloading are revealed. Based on microseismic information, numerical simulation and conventional monitoring, a multi-parameter evaluation of the excavation damage zone of underground caverns was conducted, and a comparative analysis was carried out. Based on multi-parameter information, feedback analysis and graded early warning of surrounding rock deformation in underground powerhouses.

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