CN111502715A - Method for comprehensively constructing fractured rock mass in oversized underground cavern crossing stage - Google Patents
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Abstract
The invention relates to a stage comprehensive construction method for constructing a super-large span underground cavern in a fissure development rock mass, which comprises the following steps: the method comprises the following steps: collecting and analyzing the construction condition data of the oversized underground cavern before construction; step two: the construction initial stage overall initial design stage comprises initial ground stress evaluation, section initial support structure design and pilot tunnel design; step three: performing pilot tunnel and support construction and acquiring actual geological parameters of a cavern; step four: a detailed analysis design stage in the construction process; step five: a construction monitoring feedback stage; step six: and (5) constructing a secondary lining, finishing the construction and summarizing the construction. The stage comprehensive construction method provided by the invention adopts a comprehensive stability analysis method of three stages of initial supporting structure design and pilot tunnel design, pilot tunnel construction actual measurement detailed design and construction monitoring feedback, so that the safety and the reasonability of an oversized cross-cavern construction scheme are ensured, and a solid foundation is provided for construction safety and smooth service of the cavern.
Description
Technical Field
The invention belongs to the technical field of design and construction of tunnels and underground engineering, and particularly relates to a stage comprehensive design and construction method for building a super-large span underground cavern in a rock mass with developed cracks.
Background
The development and utilization of underground space are necessary requirements of Chinese economic development and urbanization, and more underground super-span underground spaces such as stations, tunnels and other projects are built. Meanwhile, oversized cross-caverns such as underground oil reservoirs, hydropower house plants, underground national defense facilities and the like are also in strong demand, and the span of the caverns can reach 50-80 m magnitude. Typical super-large-span caverns built at home and abroad comprise underground stadiums, subway stations, high-speed railway stations and large-span tunnels. The length of a typical engineering Norwegian Gegavick city ice rink main hall is 91m, and the span is 61 m; the excavation width of the big subway station is 21.3m, the cross section area is 336.5m2(ii) a The excavation width of the section at the bifurcation part of the English method strait tunnel reaches 21.2m, the excavation height reaches 15.4m, and the excavation sectional area is 252.2m2(ii) a The single-hole excavation span of the transition line sections at two ends of the great wall station of the Kyoto-Changtao and the octagon reaches 32.7 m. Occasions adopting ultra-large span tunnels have more and more trends.
Compared with the exuberant engineering practice, the research related to the design theory and the construction method of the oversized cross-cavern is lacked. The design methods generally adopted by tunnel engineering include an engineering comparison method, a standard design method and an analytical calculation method. The design and construction of the new Olympic method is widely adopted in the current tunnel engineering practice, namely, the design of supporting parameters is carried out by adopting engineering analogy, and then the adjustment is carried out according to the measurement result of the site construction. For a super-large cross-cavern, a standard design method is obviously inappropriate, and no appropriate standard can be applied; the principle is proper by adopting a new Austrian design combining engineering experience and measurement results, but the design stage adopts which parameters to support and has great randomness and risk, which also brings great hidden danger to the engineering construction process; by adopting an analytical calculation method, a load structure calculation method commonly used in the engineering industry cannot determine a definite load for a super-large span structure, and the stress mode of the load structure calculation method also does not accord with the theoretical hypothesis of a load structure method.
Generally, tunnel engineering does not pay much attention to calculation, but for huge caverns with a span of more than 50m and even larger, only relying on experience and field measurement is not enough, and a scheme with enough safety factor must be designed before construction begins, because the consequences of dangerous accidents such as collapse and instability are unacceptable. Because the ultra-large span cavern has larger width and less height change, an arch structure with a flat shape has to be made, and the following mechanical characteristics are formed: the stress redistribution after excavation becomes unfavorable; the stress concentration at the bottom foot is too large, and larger foundation bearing capacity is required; the vault is unstable, and the large loose ground pressure is generated; the burial depth is relatively small, and the arching effect cannot be well played; the bearing capacity of the supporting structure is relatively small. Especially for a cave with an ultra-large span excavated in a crack development rock mass, the risk of overall instability and local collapse is higher, a comprehensive design method is adopted, the stability of the excavation process of the cave is analyzed, a stable scheme is formulated, and the safety of the construction process and the stability during the use are ensured.
Therefore, the stress characteristic of the ultra-large span cavern is considered, the unfavorable geological condition of crack development is combined, in order to ensure that the construction risk of the large span cavern is controllable and collapse accidents are avoided, a comprehensive construction method of the fractured rock mass ultra-large span cavern stage is provided, which is designed by overall stability analysis and predesigned, constructed by detailed process stability analysis and construction monitoring feedback, the high-efficiency and safe construction of the ultra-large span cavern is ensured, and the stable service of the ultra-large span cavern and the tunnel is ensured.
Disclosure of Invention
In order to ensure that the construction risk of the fractured rock mass over-large underground cavern is controllable and avoid the risks of integral instability and local collapse, the invention adopts a total stability analysis before construction to pre-design a total supporting structure, a pilot tunnel structure and a pilot tunnel support; analyzing the detailed stability in the construction process, and correcting support parameters to ensure the construction stability of the cavern; and a comprehensive design construction method for real-time feedback of construction and timely parameter correction to ensure construction safety.
The technical scheme adopted by the invention for solving the technical problems is as follows: a method for comprehensively constructing a fractured rock mass in an oversized underground cavern crossing stage comprises the following steps:
the method comprises the following steps: collecting and analyzing the construction condition data of the oversized underground cavern before construction;
step two: the construction initial stage overall initial design stage comprises initial ground stress evaluation, section initial support structure design and pilot tunnel design;
step three: performing pilot tunnel and support construction and acquiring actual geological parameters of a cavern;
step four: in the construction process, a detailed analysis design stage is adopted, and the stability of the excavated and fractured blocks is judged by adopting a discrete unit method mainly in combination with actual geological parameters in the third step; when the stability meets the condition, continuing construction, and when the stability does not meet the standard, correcting the support parameters by calculating until the stability requirement is met;
step five: in the construction monitoring feedback stage, different pilot tunnel excavations are gradually communicated to form an underground cavern, deformation and supporting structure stress measurement are carried out in the site excavation supporting of the oversized cavern, whether the construction meets the stability requirement or not is judged according to the measurement result, when the stability meets the condition, the construction is continued, and when the stability does not meet the standard, the supporting parameters and the excavation step pitch are corrected again through calculation until the stability requirement is met;
step six: and after the underground cavern is formed, constructing a secondary lining, finishing the construction and summarizing the construction.
Further, in the second step, firstly, the influence of the initial ground stress of the site on the size and the direction is evaluated, the trend of the cavern is determined according to the main stress direction of the initial ground stress, the trend of the cavern is parallel to the main stress direction or forms an included angle of 45 degrees with the main stress direction, and the classification of the modified surrounding rock is verified according to the ratio of the initial ground stress to the surrounding rock strength;
secondly, predefining a supporting structure and parameters of the oversized underground cavern by adopting an engineering analogy method, checking by combining finite element analysis calculation, supporting by adopting shotcrete combined with an anchor rod and a prestressed anchor cable in primary supporting structure design, adopting a reinforced concrete structure in secondary lining initial design, adopting a stratum-structure model as a calculation model, calculating the internal force and deformation of the supporting structure and the stratum by adopting a finite element method on the premise of meeting deformation coordination conditions, and checking the stability and the size of the supporting structure;
thirdly, a design mode of partitioned excavation is adopted for the ultra-large span tunnel, 2-3 pilot tunnels are transversely arranged in the ultra-large span tunnel, different pilot tunnels are arranged in parallel, the distance between every two adjacent pilot tunnels is larger than 1 time of pilot tunnel excavation span, the pilot tunnel excavation span is not larger than 12m, the pilot tunnel excavation span is small, the engineering experience is high, and the design is carried out by adopting a standard design method.
Preferably, in the design of the primary supporting structure, the strength of the sprayed concrete is C35, the thickness of the sprayed concrete is 350mm, and the thickness of a reinforced concrete structure adopted by the secondary lining is 1500 mm.
Preferably, the anchor rods comprise long anchor rods and short anchor rods, the length of each short anchor rod is 4.5m, the circumferential distance is 1.5-4 m, the longitudinal distance is 1.5-4 m, the length of each long anchor rod is 9m, the circumferential distance is 3-8 m, the longitudinal distance is 3-8 m, the length of each prestressed anchor cable is 35m, the circumferential distance is 3-8 m, and the longitudinal distance is 3-8 m.
Preferably, in the pilot tunnel design, the supporting structure of the pilot tunnel adopts sprayed concrete and local anchor rods for supporting, the strength of the sprayed concrete is C35, the thickness of the sprayed concrete is 150mm, the length of each local anchor rod is 3m, the circumferential distance and the longitudinal distance are determined according to the situation of surrounding rocks on site, system anchor rods are arranged on four-level surrounding rocks and worse surrounding rock sections, the length of each system anchor rod is 3-5 m, the transverse distance is 1-3 m, and the longitudinal distance is 1-3 m.
And further, in the third step, pilot tunnel excavation and supporting structure construction are carried out according to pilot tunnel design, fracture position, trend, width and lithology data actually revealed by excavation are recorded on site, and relevant physical and mechanical parameters are obtained through experiments.
Furthermore, in the fourth step, according to geological parameters recorded in real time in the construction process of the third step, a discrete unit method is adopted to consider the fracture action to divide the rock mass into blocks, the excavation working condition is simulated, and the excavation and structural stability is judged according to the following three indexes;
(1) the strength index of the surrounding rock is as follows: calculating whether the shear stress of the surrounding rock reaches the shear strength by adopting a molar coulomb constitutive model
Wherein τ represents: shear force, in units of: MPa; c represents: the cohesive force of the structural surface is expressed by the unit: MPa; σ represents: normal stress, in units of: MPa;
denotes the internal friction angle of the structural surface in units of: degree; solving the distribution of main stress and shear stress by adopting a discrete unit method through a mechanical balance equation, a displacement equation and boundary conditions, wherein when the maximum shear stress is smaller than the shear strength, the integral stability strength of the surrounding rock of the cavern meets the index requirement;
(2) the bearing capacity index of the supporting structure is as follows: the axial force locking value of the prestressed anchor cable is generally 70% of the design value, the maximum axial force locking value of the anchor cable in the supporting structure is obtained through a discrete element calculation method, and the anchor cable is damaged when the maximum axial force locking value of the anchor cable exceeds the design value of the bearing capacity;
(3) deformation index: the segmented blocks do not generate large deformation, the convergence displacement distribution of the surrounding rock is solved by adopting a discrete unit method through a mechanical balance equation, a displacement equation and boundary conditions, meanwhile, the convergence displacement limit value of the surrounding rock is determined based on the grade of the surrounding rock, the lateral pressure coefficient and engineering practice experience, and when the convergence displacement of the surrounding rock is less than or equal to 45mm, the deformation meets the stability requirement;
and if one of the three conditions is not met, correcting the support parameters until the calculation result meets the three conditions.
Further, in the sixth step, deformation of the primary supporting structure tends to be stable before secondary lining construction, a load structure model is adopted to calculate secondary lining structure reinforcing bars, and the load is taken into consideration of rock mass collapse load with the height of 3-6m besides dead weight and serves as safe storage.
Further, in the first step, the cavern construction condition data comprises construction geology, hydrology, planning requirements and surrounding environment conditions.
The method for comprehensively constructing the underground cavern stage can be suitable for constructing the ultra-large span underground cavern with the span of 30-80m in the fractured rock mass.
Compared with the prior art, the invention has the following advantages and effects:
1. the invention provides a method for systematically constructing a fractured rock mass ultra-large span underground cavern tunnel for the first time, provides a systematic construction scheme for the construction design of an ultra-large span 30-80m underground cavern, and can be directly adopted in future similar ultra-large span cavern and tunnel construction.
2. The invention adopts different analytical calculation theory methods such as an engineering class comparison method, a finite element method, a discrete element method and the like to mutually verify, thereby ensuring the rationality of the design of the supporting structure.
3. The construction method provided by the invention mainly solves the key problem of the safety construction of the oversized cross-cavern: stability of the fractured mass. By adopting the comprehensive stability analysis method of the three stages of initial supporting structure design, pilot tunnel construction actual measurement detailed design and construction monitoring feedback, the safety and the reasonability of the design scheme of the oversized cross-chamber are ensured, and a solid foundation is provided for construction safety and smooth service.
4. The construction method can realize the safe construction operation of the oversized underground cavern in the fractured developing rock mass, ensures that the cavern can be efficiently and safely excavated, and has the advantages of simple construction method process, safe and reasonable supporting structure, universal design method and low construction risk.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to these drawings without creative efforts.
FIG. 1 is a flow chart of the stage comprehensive construction method of the fractured rock mass over-large underground cavern.
Fig. 2 is a schematic view of the design of the cross-section supporting structure of the present invention.
Fig. 3 is a schematic diagram of arrangement of pilot holes in embodiment 1 of the present invention.
Fig. 4 is a schematic diagram of the position of the crack after the pilot tunnel excavation is completed.
FIG. 5 is a cloud of shear stresses after excavation of a cavern of the invention.
FIG. 6 is a cloud diagram of the maximum axial force of the prestressed anchor cable after the excavation of the cavern is completed.
FIG. 7 is a displacement cloud of a wedge formed by fracture cutting.
Description of reference numerals: 1. a cavern; 2. a first pilot hole; 21. a second pilot hole; 22. a third pilot hole;
3. primary support spraying concrete; 4. secondary lining; 5. a long anchor rod; 6. a pre-stressed anchor cable; 7. short anchor rods; 8. the position of the arch springing; 9. the maximum axial force position of the anchor cable; 10. the position of maximum displacement of the bottom of the wedge-shaped block body.
Detailed Description
The present invention will be described in further detail with reference to examples, which are illustrative of the present invention and are not to be construed as being limited thereto.
Example 1: as shown in fig. 1 to 3, a comprehensive construction method for constructing a lower chamber with a span of 60 meters in a fractured rock mass comprises the following steps:
the method comprises the following steps: collecting and analyzing the construction condition data of the oversized underground cavern before construction;
step two: the construction initial stage overall initial design stage comprises initial ground stress evaluation, section initial support structure design and pilot tunnel design;
step three: performing pilot tunnel and support construction and acquiring actual geological parameters of a cavern;
step four: in the construction process, a detailed analytic design stage is adopted, and the stability of excavation and surrounding rock is judged by adopting a discrete unit method mainly in combination with actual geological parameters in the third step; when the stability meets the condition, continuing construction, and when the stability does not meet the standard, correcting the support parameters by calculating until the stability requirement is met;
step five: in the construction monitoring feedback stage, different pilot tunnel excavations are gradually communicated to form an underground cavern, deformation and supporting structure stress measurement are carried out in the site excavation supporting of the oversized cavern, whether the construction meets the stability requirement or not is judged according to the measurement result, when the stability meets the condition, the construction is continued, and when the stability does not meet the standard, the supporting parameters and the excavation step pitch are corrected again through calculation until the stability requirement is met;
step six: and after an underground cavern with the span of 60 meters is formed, constructing a secondary lining, and finishing construction and summarizing construction.
In the first step, the construction condition data of the oversized underground cavern is collected and analyzed before construction, and the construction condition data mainly comprises geology, hydrology, planning requirements, surrounding environment conditions and the like. Collecting the planning and functional requirements of the cavern, selecting a proper position and setting a proper section limit; geological exploration is carried out to obtain engineering and hydrogeological data, including parameters such as geological structure, bedding, lithology and underground water; the ambient environmental conditions have been data collected from existing buildings.
In the second step, firstly, the influence of the size and the direction of the initial ground stress of the site is evaluated, the trend of the cavern is determined according to the main stress direction of the initial ground stress, the trend of the cavern is parallel to the main stress direction or forms an included angle of 45 degrees, and the classification of the corrected surrounding rock is verified according to the ratio of the size of the ground stress to the strength of the surrounding rock;
secondly, predefining a supporting structure and supporting parameters of the oversized underground cavern by adopting an engineering analogy method, checking by combining finite element analysis calculation, supporting by adopting shotcrete 3 in combination with an anchor rod and a prestressed anchor cable 6 in a primary supporting structure, adopting a reinforced concrete structure in a secondary lining 4, adopting a stratum-structure model as a calculation model, calculating the internal force and deformation of the supporting structure and the stratum by adopting a finite element method on the premise of meeting deformation coordination conditions, and checking the stability and the size of the supporting structure;
thirdly, a design mode of partitioned excavation is adopted for the ultra-large span tunnel, 2-3 pilot tunnels are transversely arranged in the ultra-large span tunnel, different pilot tunnels are arranged in parallel, the distance between every two adjacent pilot tunnels is 1 time of pilot tunnel excavation span, the pilot tunnel excavation span is not more than 12m, the pilot tunnel excavation span is small, the engineering experience is high, and the design is carried out by adopting a standard design method.
Specifically, as shown in fig. 2, in the second step, the strength of the shotcrete 3 used in the primary support structure is C35, the thickness of the shotcrete is 350mm, the anchor rods include long anchor rods 5 and short anchor rods 7, the length of the short anchor rods 7 is 4.5m, the circumferential distance is 2.4m, the longitudinal distance is 2.4m, the length of the long anchor rods 5 is 9m, the circumferential distance is 4.8m, the longitudinal distance is 4.8m, the length of the prestressed anchor cables 6 is 35m, the circumferential distance is 4.8m, the longitudinal distance is 4.8m, and the reinforced concrete structure adopted in the primary lining 4-stage design is 1500mm thick.
Preferably, in this embodiment 1, the number of the pilot holes is set to 3, which are the first pilot hole 2, the second pilot hole 21, and the third pilot hole 22, the distance between two adjacent pilot holes is 12m, the span of the pilot holes is 12m, and the pilot holes are specifically arranged as shown in fig. 3. In the pilot tunnel design, the supporting construction design adopts spray concrete and local stock to strut, the spray concrete intensity is C35, and thickness is 150mm, and the length of local stock is 3m, and the hoop interval is 2m, and longitudinal spacing is 2m, sets up the system stock to level four country rock and worse country rock section, the length of system stock is 5m, and transverse spacing is 1.5m, and longitudinal spacing is 1.5 m.
Further, in the third step, pilot tunnel excavation and support construction are carried out according to pilot tunnel design, data of fracture positions, trends, widths, lithology and the like actually revealed by excavation are recorded on site, relevant physical mechanical parameters are obtained through experiments, the positions and the trends of the large fractures f1, f2 and f3 are revealed by the site excavation, and rock stratum physical mechanical parameters are obtained through experiments. The schematic position diagram of the crack after the pilot tunnel excavation is completed is shown in fig. 4, wherein the abscissa in the diagram is the longitudinal length of the cavern, and the angle represents the fault structural plane occurrence.
In the specific construction process, the first pilot tunnel 2 and the second pilot tunnel 22 are excavated from two sides to the middle, and the middle pilot tunnel 21 is excavated from the middle to two sides to form the 60-meter-span underground cavern 1.
Furthermore, in the fourth step, combining geological parameters recorded in the third step, adopting 3DEC discrete unit method calculation software to carry out calculation analysis, taking the fracture action into consideration to divide the rock mass into blocks, simulating the excavation working condition, and judging the excavation and the structural stability of the fractured blocks according to the following three indexes;
(1) the strength index of the surrounding rock is as follows: calculating whether the shear stress of the surrounding rock reaches the shear strength by adopting a molar coulomb constitutive model
Wherein τ represents: shear force, in units of: MPa; c represents: the cohesive force of the structural surface is expressed by the unit: MPa; σ represents: normal stress, in units of: MPa;
denotes the internal friction angle of the structural surface in units of: degree; the laboratory test determines that c is 1.5MPa for the surrounding rock of the cavern,
solving the distribution of main stress and shear stress by adopting a discrete unit method through a mechanical equilibrium equation, a displacement equation and boundary conditions, wherein the maximum main stress of the surrounding rock appears at an arch springing position of 8, sigma is 7.5MPa, and the shear strength of the surrounding rock is 7.5MPa
And in the figure 5, the maximum shear stress of the surrounding rock at the position 8 of the deepest arch foot of the color is 5.98MPa, and when the maximum shear stress is smaller than the shear strength, the integral stable strength of the surrounding rock of the cavern meets the index requirement.
(2) The bearing capacity index of the supporting structure is as follows: the axial force locking value of the prestressed anchor cable is generally 70% of the design value, the cavern adopts a system anchor cable with the bearing capacity design value of 1500kN, and the axial force locking value of the anchor cable is 70% of the design value, namely the prestress is 1000 kN. After the excavation and support of the grotto are completed through discrete element calculation, the maximum axial force (shown as a position 9 in the figure 6) of the middle lower part of the anchor cable at the deepest color in the figure 6 is 1226kN and is smaller than the designed bearing capacity value, and the requirement of the bearing capacity index of the integral stable support structure is met.
(3) Deformation index: the segmented blocks do not generate large deformation, the surrounding rock convergence displacement distribution is solved by adopting a discrete unit method through a mechanical balance equation, a displacement equation and boundary conditions, meanwhile, the surrounding rock convergence displacement limit value is determined based on the surrounding rock grade, the side pressure coefficient and engineering practice experience, and when the surrounding rock convergence displacement is less than or equal to 45mm, the deformation meets the stability requirement. In the figure 7, the maximum displacement of the bottom of the wedge-shaped block at the deepest color position is 22.7mm (shown as a position 10 in the figure 7), the displacement value is basically close to that of the adjacent surrounding rock, no order of magnitude difference exists, and the requirement of local stable deformation index is met.
And if one of the three conditions is not met, correcting the support parameters until the calculation result meets the three conditions.
Further, in the construction process of pilot tunnel excavation and supporting structures, the deformation of the supporting structures before secondary lining construction tends to be stable, a load structure model is adopted to calculate the reinforcing bars of the secondary lining structure, and the load is taken into consideration of the rock mass collapse load with the height of 3-6m as safety reserve besides the dead weight.
The construction method provided by the embodiment 1 of the invention can ensure that the construction scheme is safe, reasonable, economical and feasible by performing multistage interactive verification on the collection of construction early-stage data, the pre-design of a construction early-stage supporting structure and a pilot tunnel, the excavation of the pilot tunnel, the stability analysis after supporting construction, the construction monitoring feedback and the parameter correction, and the auxiliary construction verification by adopting multiple calculation means of finite elements and discrete elements.
The construction method in the embodiment 1 of the invention is suitable for building underground caverns and tunnels with ultra-large span of fractured rock masses, and the shape and the size of the fracture surface can be determined and adjusted according to functional requirements and stress requirements.
The embodiment 1 of the invention provides a comprehensive construction method of a fractured rock mass oversized cross-chamber stage, which is characterized by overall stability analysis pre-design, detailed process stability analysis, support parameter correction and construction monitoring feedback, ensures efficient and safe construction of the oversized cross-chamber, and ensures stable service of the oversized cross-chamber and a tunnel.
In addition, it should be noted that the specific embodiments described in the present specification may differ in the shape of the components, the names of the components, and the like. All equivalent or simple changes of the structure, the characteristics and the principle of the invention which are described in the patent conception of the invention are included in the protection scope of the patent of the invention. Various modifications, additions and substitutions for the specific embodiments described may be made by those skilled in the art without departing from the scope of the invention as defined in the accompanying claims.
Claims (10)
1. A method for comprehensively constructing a fractured rock mass in an oversized underground cavern crossing stage is characterized by comprising the following steps:
the method comprises the following steps: collecting and analyzing the construction condition data of the oversized underground cavern before construction;
step two: the construction initial stage overall initial design stage comprises initial ground stress evaluation, section initial support structure design and pilot tunnel design;
step three: performing pilot tunnel and support construction and acquiring actual geological parameters of a cavern;
step four: in the construction process, a detailed analysis design stage is adopted, the stability of the excavated and fractured blocks is judged by adopting a discrete unit method mainly in combination with actual geological parameters in the third step, when the stability meets the conditions, construction is continued, and when the stability does not meet the standard, support parameters are corrected by calculation until the stability requirement is met;
step five: in the construction monitoring feedback stage, different pilot tunnel excavations are gradually communicated to form an underground cavern, deformation and supporting structure stress measurement are carried out in the site excavation supporting of the oversized cavern, whether the construction meets the stability requirement or not is judged according to the measurement result, when the stability meets the condition, the construction is continued, and when the stability does not meet the standard, the supporting parameters and the excavation step pitch are corrected again through calculation until the stability requirement is met;
step six: and after the underground cavern is formed, constructing a secondary lining, finishing the construction and summarizing the construction.
2. The method for comprehensively constructing the fractured rock mass ultra-large span underground cavern stage according to claim 1, wherein in the second step, the influence of the initial ground stress of the site on the size and the direction is evaluated, the trend of the cavern is determined according to the main stress direction of the initial ground stress, the trend of the cavern is parallel to the main stress direction or within a 45-degree included angle, and the classification of the modified surrounding rock is verified according to the ratio of the initial ground stress to the surrounding rock strength;
secondly, predefining a supporting structure and parameters of the oversized underground cavern by adopting an engineering analogy method, checking by combining finite element analysis calculation, supporting by adopting shotcrete combined with an anchor rod and a prestressed anchor cable in primary supporting structure design, adopting a reinforced concrete structure in secondary lining initial design, adopting a stratum-structure model as a calculation model, calculating the internal force and deformation of the supporting structure and the stratum by adopting a finite element method on the premise of meeting deformation coordination conditions, and checking the stability and the size of the supporting structure;
thirdly, a design mode of partitioned excavation is adopted for the ultra-large span tunnel, 2-3 pilot tunnels are transversely arranged in the ultra-large span tunnel, different pilot tunnels are arranged in parallel, the distance between every two adjacent pilot tunnels is larger than 1 time of pilot tunnel excavation span, the pilot tunnel excavation span is not larger than 12m, the pilot tunnel excavation span is small, the engineering experience is high, and the design is carried out by adopting a standard design method.
3. The method for comprehensively constructing a fractured rock mass ultra-large span underground cavern stage according to claim 2, wherein in the design of the primary supporting structure, the strength of sprayed concrete is C35, the thickness of the sprayed concrete is 350mm, and the thickness of a reinforced concrete structure adopted by a secondary lining is 1500 mm.
4. The method for comprehensively constructing a fractured rock mass with an oversized underground cavern in a stage according to claim 2, wherein the anchor rods comprise long anchor rods and short anchor rods, the length of each short anchor rod is 4.5m, the circumferential distance is 1.5-4 m, the longitudinal distance is 1.5-4 m, the length of each long anchor rod is 9m, the circumferential distance is 3-8 m, the longitudinal distance is 3-8 m, the length of each prestressed anchor cable is 35m, the circumferential distance is 3-8 m, and the longitudinal distance is 3-8 m.
5. The method for comprehensive construction of the fractured rock mass over-large underground cavern stage according to claim 2, wherein in pilot tunnel design, a pilot tunnel supporting structure is supported by sprayed concrete and local anchor rods, the strength of the sprayed concrete is C35, the thickness of the sprayed concrete is 150mm, the length of the local anchor rods is 3m, the circumferential distance and the longitudinal distance are determined according to the situation of surrounding rocks on site, system anchor rods are arranged on four-level surrounding rocks and surrounding rock sections which are worse, the length of the system anchor rods is 3-5 m, the transverse distance is 1-3 m, and the longitudinal distance is 1-3 m.
6. The method for comprehensively constructing the fractured rock mass over-large underground cavern stage according to the claim 1, wherein in the third step, pilot tunnel excavation and supporting structure construction are carried out according to pilot tunnel design, fracture position, trend, width and lithology data actually disclosed by excavation are recorded on site, and relevant physical and mechanical parameters are obtained through experiments.
7. The method for comprehensively constructing the fractured rock mass over-large underground cavern stage according to claim 1, wherein in the fourth step, according to geological parameters recorded in real time in the third step, a discrete unit method is adopted to consider the fracture action to segment the rock mass into blocks, the excavation working condition is simulated, and the excavation and the stability of the fractured blocks are judged according to the following three indexes:
(1) the strength index of the surrounding rock is as follows: calculating whether the shear stress of the surrounding rock reaches the shear strength by adopting a molar coulomb constitutive model
Wherein τ represents: shear force, in units of: MPa; c represents: the cohesive force of the structural surface is expressed by the unit: MPa; σ represents: normal stress, in units of: MPa;
denotes the internal friction angle of the structural surface in units of: degree; solving the distribution of main stress and shear stress by adopting a discrete unit method through a mechanical balance equation, a displacement equation and boundary conditions, wherein when the maximum shear stress is smaller than the shear strength, the integral stability strength of the surrounding rock of the cavern meets the index requirement;
(2) the bearing capacity index of the supporting structure is as follows: the axial force locking value of the prestressed anchor cable is 70% of the design value, the maximum axial force locking value of the anchor cable in the supporting structure is obtained through a discrete element calculation method, and the anchor cable is damaged when the maximum axial force locking value of the anchor cable exceeds the design value of the bearing capacity;
(3) deformation index: the segmented blocks do not generate large deformation, the convergence displacement distribution of the surrounding rock is solved by adopting a discrete unit method through a mechanical balance equation, a displacement equation and boundary conditions, meanwhile, the convergence displacement limit value of the surrounding rock is determined based on the grade of the surrounding rock, the lateral pressure coefficient and engineering practice experience, and when the convergence displacement of the surrounding rock is less than or equal to 45mm, the deformation meets the stability requirement;
and if one of the three conditions is not met, correcting the support parameters until the calculation result meets the three conditions.
8. The method for comprehensively constructing the fractured rock mass over-large underground cavern stage according to claim 1, wherein in the sixth step, deformation of a primary supporting structure tends to be stable before secondary lining construction, a load structure model is adopted to calculate reinforcement of the secondary lining structure, and the load needs to consider a rock mass collapse load with a height of 3-6m as a safety reserve besides self weight.
9. The method for comprehensively constructing the fractured rock mass ultra-large span underground cavern stage according to claim 1, wherein in the step one, the cavern construction condition data comprise construction geology, hydrology, planning requirements and surrounding environment conditions.
10. The method for comprehensively constructing the fractured rock mass ultra-large span underground cavern stage as claimed in claim 1, wherein the span of the ultra-large span underground cavern constructed in the fractured rock mass is 30-80 m.
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