A comprehensive design method for a soft upper and hard lower stratum tunnel

Abstract

The application provides a comprehensive design method for a soft upper and hard lower stratum tunneling, and relates to the field of tunnel engineering. By constructing a whole-process closed-loop system of "exploration-analysis-optimization-determination", three core parameters of "burial depth-section size-supporting force" are included in a unified analysis framework; experimental test, simulation and cost data are used to replace the experience dominant mode, and the optimal supporting force is optimized and locked through quantitative modeling of "supporting force-deformation-cost"; accurate exploration and stratified test are carried out for complex strata, and the design universality is improved by combining the safety factor scaling model numerical simulation; the critical safety parameters are scientifically determined, the risks such as collapse and rock burst are reduced, the resource allocation is optimized, the excessive or insufficient design is avoided, and the safety and economic benefits are improved.

Description

Technical Field

[0001] This invention relates to the field of tunnel engineering, and in particular to a comprehensive design method for tunnel excavation in soft upper and hard lower strata. Background Technology

[0002] In the field of urban underground engineering construction, the open-cut method is often unsuitable due to numerous constraints caused by factors such as urban surface buildings and traffic, resulting in significant ground disturbance and impact on the surrounding environment. In contrast, the tunneling method has advantages such as less impact on surface traffic and the surrounding environment, and relatively lower requirements for construction sites, making it a commonly used construction design method in urban underground engineering.

[0003] When underground engineering construction traverses strata with a soft upper layer and a hard lower layer, different strata distributions and burial depths have varying impacts. If the tunnel is entirely within a soft stratum, the insufficient bearing capacity of the stratum itself can lead to significant deformation of the surrounding rock after excavation, potentially causing large-scale collapses. This not only threatens the safety of construction workers but also increases the cost and difficulty of support work. If the tunnel is sufficiently deep, traversing entirely hard rock strata, while the strata's stability is relatively good, excavating hard rock requires more blasting operations, generating significant vibrations that may affect surrounding geological structures and buildings, while also increasing excavation time and costs. When a tunnel traverses half soft and half hard strata, this heterogeneity leads to extremely complex stress on the tunnel structure. The soft stratum is prone to deformation, while the hard stratum is relatively stable. This difference causes uneven settlement and stress concentration, posing significant challenges to tunnel design and construction. Therefore, under these complex geological conditions, there exists a critical point for the tunnel's safety distance and support force, which needs to be precisely determined to ensure both project safety and economic efficiency. However, current design methods have significant limitations when dealing with tunnel engineering in this type of soft-over-hard stratum, mainly due to the following drawbacks: 1) Isolated parameter analysis, lacking systematicity: Existing traditional methods mostly rely on geological parameters to determine the support type alone, or select the cross-section size based solely on experience, without establishing the logical relationship between "burial depth-cross section-support force" and "deformation-cost".

[0004] 2) It generally relies on empirical formulas or single numerical simulations, making it difficult to accurately balance the control of surrounding rock stability and economy, especially in determining the optimal combination of critical burial depth, cross-sectional dimensions and support force.

[0005] 3) Insufficient adaptability of stratigraphic boundaries: The location of the interface between soft upper and hard lower strata directly affects the stress mode of the tunnel. However, traditional methods lack sufficient accuracy in the exploration of interface parameters. Numerical simulations often adopt homogenization assumptions and ignore the stress concentration effect at the interface, resulting in a large initial deviation between the burial depth and the cross-sectional dimensions.

[0006] 4) High safety risks: Due to the complex characteristics of the "soft upper layer and hard lower layer" strata, traditional methods are difficult to accurately control the critical safe burial depth. As a result, during tunnel construction, if the burial depth is unreasonable, the soft strata are prone to collapse, and the hard strata may cause rock bursts, which seriously threaten personnel safety and the stability of the engineering structure.

[0007] 5) Difficulty in controlling costs: Isolated parameter analysis and lack of systematicity make it difficult to effectively correlate "burial depth-section-support force" with "deformation-cost". This often leads to over-design, adding unnecessary support materials, resulting in a significant increase in construction costs; or insufficient design, requiring secondary construction to make up for safety hazards later, which also increases costs.

[0008] Therefore, a comprehensive design method for tunnel excavation in soft upper and hard lower strata is proposed to solve the above problems. Summary of the Invention

[0009] The purpose of this invention is to provide a comprehensive design method for tunnels excavated in soft-over-hard strata. By constructing a closed-loop system encompassing "exploration-analysis-optimization-finalization," it integrates the three core parameters of "burial depth, cross-sectional dimensions, and support force" into a unified analytical framework, overcoming the problems of isolated parameters and logical breaks in traditional methods. Furthermore, it uses experimental testing, simulation, and cost data as core support to replace the experience-driven design model. Through quantitative modeling and unitized optimization of "support force-deformation-cost," it scientifically identifies the optimal support force that is "deformation-controllable and cost-lowest." This method is applicable to situations such as soft-over-hard strata. Due to the complex geological characteristics, relying on precise geological boundary surveys, layer parameter testing, and numerical simulation of safety factor scaling models, the design avoids homogenization assumption errors, improves the universality and standardization of the design, scientifically determines the critical safe burial depth, rationally plans cross-sectional dimensions and support forces, effectively reduces the risks of soft strata collapse and hard strata rock bursts, and strengthens the safety defense line for tunnel construction and operation. It also establishes the correlation logic between core parameters and "deformation-cost", avoids over-design and under-design under the premise of ensuring safety, reduces material waste and secondary construction costs, and maximizes the economic benefits of the project.

[0010] To achieve the above objectives, the present invention provides a comprehensive design method for tunneling in soft upper and hard lower strata, comprising the following steps: S1: Identify geological exploration and stratigraphic interfaces, arrange boreholes and collect rock and soil cores, record the core properties in detail, and accurately identify the geological boundary between soft soil and hard rock layers; based on the obtained soft and hard strata rock and soil core samples, conduct indoor physical and mechanical tests to determine the physical properties and strength parameters of each stratum; S2: A three-dimensional numerical model of the tunnel-stratum system reflecting the soft upper and hard lower stratum structure is established using numerical simulation software. The tunnel burial depth and cross-sectional height-to-width ratio are used as variable factors. Multiple simulation schemes are designed using orthogonal experimental design method. The stratum strength parameters obtained in S1 are scaled according to the tunnel engineering grade and safety requirements. A series of numerical calculations are then performed. S3: Set stability evaluation indicators, obtain the corresponding safety factors based on the series of numerical calculations in S2, or conduct a systematic analysis of the numerical simulation results in S2 based on the model convergence degree, and determine the optimal critical safe burial depth based on the safety factors corresponding to the obtained multiple simulation schemes. Select the optimal critical safe burial depth and the reasonable cross-sectional dimensions corresponding to the optimal cross-sectional height-to-width ratio ; S4: The optimal critical safe burial depth determined in S3 and the reasonable cross-sectional dimensions corresponding to the optimal cross-sectional height-to-width ratio In the corresponding three-dimensional numerical model, a uniformly distributed pressure equivalent to the support force is applied at the tunnel excavation boundary. The numerical simulation is carried out by changing the magnitude of the support force, and the maximum deformation of the surrounding rock and the area of ​​the surrounding rock failure are recorded. Combined with engineering experience, cost data and construction organization analysis, the support cost estimation function and the support construction cycle estimation function required for different support forces are established. S5: The surrounding rock deformation, surrounding rock failure area, support cost, and support construction period under different support forces in S4 are standardized to form standardized deformation, failure area, cost, and period indicators, respectively. The relationship curves of the four standardized indicators with support force are plotted on the same coordinate system to determine the optimal support force that balances safety and economy. and the range of values; S6: Integrate the optimal critical safe burial depth determined in S3 and the reasonable cross-sectional dimensions corresponding to the optimal cross-sectional height-to-width ratio and the optimal support force obtained in S5 This leads to a comprehensive tunnel design scheme applicable to soft upper and hard lower strata, which serves as the output of core design parameters.

[0011] Preferably, in S1, based on the tunnel depth and geological conditions, rotary drilling is used within the tunnel planning area to conduct borehole exploration using a hydraulic core drilling rig to accurately determine the interface between the soft soil layer and the hard rock layer; the calculated density, elastic modulus, Poisson's ratio, cohesive strength, and frictional strength are obtained through indoor physical and mechanical tests; the indoor physical and mechanical tests specifically include ring cutter test, uniaxial compression test, and triaxial compression test.

[0012] Preferably, in S2, the soil layer boundary location obtained in S1 is combined with the scaled parameters and numerical simulation software to establish a three-dimensional tunnel-stratum model. Determine the strength scaling safety factor based on the tunnel's grade and importance. Strength parameter scaling method for formation strength parameter cohesion Scaling the safety factor according to the corresponding strength The specific formula for scaling cohesive strength is as follows: ; Determine the strength scaling safety factor based on the tunnel's grade and importance requirements. The strength parameter scaling method is used to scale the formation strength parameter friction strength obtained in S1. Scaling the safety factor according to the corresponding strength The specific formula for scaling friction intensity is as follows: ; The scaled cohesive strength and frictional strength were incorporated into various orthogonal experimental schemes. A series of numerical simulations were conducted by varying the tunnel depth and tunnel cross-sectional aspect ratio to analyze the surrounding rock stress, displacement, and plastic zone development. The strength scaling safety factor was recorded. .

[0013] Preferably, the stability evaluation index in S3 is set as follows: soft soil displacement ≤ maximum allowable soft soil displacement, hard rock displacement ≤ maximum allowable hard rock displacement, and the area of ​​surrounding rock failure does not exceed the allowable failure area threshold.

[0014] Preferably, the following steps are specifically performed in S4: S41: Maintain the optimal critical safety burial depth Optimal cross-sectional dimensions corresponding to the optimal cross-sectional height-to-width ratio In the corresponding three-dimensional numerical calculation model, different support forces are simulated by applying radial pressure to the surface of the tunnel cross-section profile. Influence; S42: Different support forces Under the given value, for each Numerical simulation of the entire excavation and support process was conducted, and the deformation of key points in the surrounding rock was recorded. and the total area of ​​the plastic zone or failure zone of the surrounding rock. Statistics and establishment of different support forces With the deformation of the surrounding rock Area of ​​surrounding rock damage The correspondence between them; S43: Determine different support forces based on engineering experience, quota surveys, or supplier quotations. Corresponding support costs Based on construction methods, equipment configuration, and work process organization, different support forces are determined. Corresponding support construction period .

[0015] Preferably, the deformation of the surrounding rock obtained in S42 is used in S5. The normalization process is performed to obtain the normalized deformation index. , (0≤ ), Different support forces A decreasing function; ; In the formula, This indicates the surrounding rock deformation value corresponding to the current support force; Indicates the maximum deformation without support; This represents the minimum deformation under the strongest support. The surrounding rock deformation obtained in S42 The normalization process is performed to obtain the normalized deformation index. (0≤ ), Different support forces A decreasing function; ; In the formula, This indicates the area of ​​surrounding rock failure corresponding to the current support force; Indicates the maximum damaged area without support; Indicates the minimum damage area under the strongest support; Support costs obtained from S43 The cost index is obtained by unitization. , (0≤ ≤1), Different support forces An increasing function; ; In the formula, This indicates the cost corresponding to the current support force; This represents the cost corresponding to the minimum support force; This indicates the cost corresponding to the maximum support force; Support costs obtained from S43 After normalization, we obtain the normalized periodic index. , (0≤ ≤1), Different support forces An increasing function; ; In the formula, T represents the construction period corresponding to the current support force; This indicates the construction period corresponding to the minimum support force. This indicates the construction period corresponding to the maximum support force; In the same coordinate system, with the support force as the horizontal axis and the unitized index as the vertical axis, the unitized deformation index is plotted. Unitized damage area index Unit cost indicators Unitized periodicity indicators Depending on the support force Changing curve Identify the equilibrium region where the four curves intersect. The support force corresponding to this region is the optimal support force that satisfies the comprehensive optimization of deformation control, failure range control, cost control, and schedule control. And the optimal interval.

[0016] Therefore, the present invention adopts the above-mentioned comprehensive design method for tunnel excavation in soft upper and hard lower strata, and the technical effects are as follows: (1) This invention constructs a closed-loop system of “exploration-analysis-optimization-finalization” from geological boundary exploration to parameter testing, numerical simulation and cost-deformation coupling optimization. It incorporates the three core parameters of “burial depth-section size-support force” into a unified analysis framework, which solves the problems of isolated parameters and logical breaks in traditional methods.

[0017] (2) This invention uses experimental test data, simulation data, and cost data as core support to replace the traditional design mode that relies on individual engineer experience, so that the design scheme has clear data basis and quantitative verification standards. At the same time, through quantitative modeling and unitized optimization of "support force-deformation-cost", it breaks through the limitations of experience-based values ​​and scientifically locks the optimal support force that is "controllable in deformation and lowest in cost".

[0018] (3) This invention addresses the mechanical differences between soft upper and hard lower strata by conducting precise geological boundary surveys and layer parameter tests to avoid errors in the homogenization assumption; combined with the numerical simulation of the safety factor scaling model, it can be adapted to various complex strata types such as soil-rock and soft rock-hard rock.

[0019] (4) By accurately determining the location of the stratum boundary, obtaining detailed stratum parameters and conducting comprehensive numerical simulation, this invention can scientifically determine the critical safe burial depth, reasonable cross-sectional size and support force, effectively reduce the risks of soft stratum collapse and hard stratum rock burst, lay a solid foundation for the safety of tunnel construction and operation, and significantly reduce the probability of safety accidents.

[0020] (5) This invention establishes the correlation logic between “burial depth-section-support force” and “deformation-cost” to optimize the design as a whole. Under the premise of ensuring tunnel safety, it avoids over-design and under-design, rationally allocates resources, reduces waste of design materials and secondary construction costs, achieves effective control of project costs, and improves economic benefits. Attached Figure Description

[0021] Figure 1 This is a flowchart of a comprehensive design method for tunnel excavation in soft-over-hard strata according to the present invention. Figure 2 This is a diagram showing the relationship between tunnel burial depth and surrounding rock deformation in Embodiment 2 of the present invention; Figure 3 This is a diagram showing the unitized relationship between the surrounding rock deformation, support cost, area of ​​surrounding rock failure, support construction period, and optimal support force in Embodiment 2 of the present invention. Detailed Implementation

[0022] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0023] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0024] Example 1 This invention provides a comprehensive design method for tunnel excavation in soft-over-hard strata, such as... Figure 1 As shown, it includes the following steps: S1: Identify geological exploration and stratigraphic interfaces, arrange boreholes and collect core samples, record core properties in detail, and accurately identify the geological boundary between soft soil and hard rock layers; based on the obtained soft and hard strata core samples, conduct indoor physical and mechanical tests to determine the physical properties and strength parameters of each stratum; in S1, according to the tunnel depth and stratigraphic conditions, use rotary drilling and hydraulic core drilling rigs to conduct borehole exploration within the tunnel planning area to accurately determine the boundary between soft soil and hard rock layers; indoor physical and mechanical tests include ring cutter tests, uniaxial compression tests, and triaxial compression tests; The core samples of soft soil and hard rock were subjected to ring cutter tests. Soft soil samples were cut using a ring cutter of known volume, weighed, and their density was calculated. For regular rock specimens, directly measure the dimensions to calculate the volume, weigh the specimens, and then calculate the density. ; Uniaxial compression tests were conducted on the obtained soft soil and hard rock core samples. Stress-strain curves were plotted based on the experimental data, and the elastic modulus of soft soil and hard rock was calculated through the elastic stage of the curve. At the same time, Poisson's ratio was calculated based on the ratio of lateral strain to axial strain. Triaxial compression tests were conducted on the obtained soft soil and hard rock core samples. Consolidation undrained tests were performed on the samples using a triaxial apparatus. Based on the Mohr-Coulomb strength theory, Mohr circles were plotted using the failure stress under different confining pressures, and the cohesive strength and frictional strength were obtained by fitting the results.

[0025] S2: A three-dimensional numerical model of the tunnel-stratum system reflecting the soft upper and hard lower stratum structure is established using numerical simulation software. The tunnel burial depth and cross-sectional height-to-width ratio are used as variable factors. Multiple simulation schemes are designed using orthogonal experimental design method. The stratum strength parameters obtained in S1 are scaled according to the tunnel engineering grade and safety requirements. A series of numerical calculations are then performed. By combining the soil layer boundary locations obtained in S1 with the scaled parameters and numerical simulation software, a three-dimensional model of the tunnel-stratum is established to realistically reflect the soft upper and hard lower stratum structure.

[0026] Furthermore, a two-factor, ten-level orthogonal experiment was constructed to conduct a significance analysis on tunnel burial depth and tunnel cross-sectional aspect ratio. Multiple simulation schemes were designed using the orthogonal experimental method. Here, 100 working conditions are listed, with burial depths ranging from 0 to 40 meters in 4-meter increments; the aspect ratio is selected from 0.5 to 2.0, with examples of 0.5, 0.6, 0.7, 0.8, 1.0, 1.1, 1.2, 1.3, 1.5, and 2.0, covering different cross-sectional proportions. The orthogonal experiments are shown in Table 1 below. Table 1

[0027] Determine the strength scaling safety factor based on the tunnel's grade and importance. Strength parameter scaling method for formation strength parameter cohesion According to the corresponding target safety factor The specific formula for scaling cohesive strength is as follows: ; The safety factor is determined based on the tunnel's classification and importance requirements. =2, using the strength parameter scaling method, the formation strength parameter friction strength obtained in S1 is... According to the corresponding target safety factor The specific formula for scaling friction intensity is as follows: ; The scaled cohesive strength and frictional strength were incorporated into various orthogonal experimental schemes. A series of numerical simulations were conducted by varying the tunnel depth and tunnel cross-sectional aspect ratio to analyze the development of surrounding rock stress, displacement, plastic zone, and failure extent. The strength scaling safety factor was recorded. , >1, Level 1 project 2.0-2.5, Level 2 project 1.8-2.0.

[0028] S3: Set stability evaluation indicators, obtain the corresponding safety factors based on the series of numerical calculations in S2, or conduct a systematic analysis of the numerical simulation results in S2 based on the model convergence degree, and determine the optimal critical safe burial depth based on the safety factors corresponding to the obtained multiple simulation schemes. Select the optimal critical safe burial depth and the reasonable cross-sectional dimensions corresponding to the optimal cross-sectional height-to-width ratio ; In S3, the stability evaluation indicators are set as follows: soft soil displacement ≤ 10mm (maximum allowable soft soil displacement), hard rock displacement ≤ 5mm (maximum allowable hard rock displacement), and the area of ​​surrounding rock failure does not exceed the allowable failure area threshold. Displacement limits are based on industry standards such as the "Highway Tunnel Design Code"; maximum principal stress ≤ stratum compressive strength; the plastic zone is not continuous in soft soil, and there is no large-scale plastic zone in hard rock. A stable condition is defined as the surrounding rock displacement being less than the maximum allowable range, the maximum principal stress being within the stratum compressive strength range, and the development of the plastic zone and failure area meeting the requirements. Alternatively, if the entire simulation model fails to converge, the corresponding next-level... This is the tunnel stability coefficient under this working condition.

[0029] S4: The optimal critical safe burial depth determined in S3 and the reasonable cross-sectional dimensions corresponding to the optimal cross-sectional height-to-width ratio In the corresponding three-dimensional numerical model, a uniformly distributed pressure equivalent to the support force is applied at the tunnel excavation boundary. Numerical simulations are conducted by varying the magnitude of the support force, recording the maximum deformation of the surrounding rock. Based on engineering experience and cost data, a support cost estimation function for different support forces is established. A positive correlation exists between support cost and support force. =a+b ,in a Based on basic cost, b This represents the cost coefficient corresponding to a unit increase in support force; there is a positive correlation between the support construction period and the support force. ,in Basic construction period, The construction period coefficient is the unit increment of support force.

[0030] The specific steps to be performed in S4 are as follows: S41: Maintain the optimal critical safety burial depth. Optimal cross-sectional dimensions corresponding to the optimal cross-sectional height-to-width ratio In the corresponding three-dimensional numerical calculation model, different support forces are simulated by applying radial pressure to the surface of the tunnel cross-section profile. Influence; S42: Simulate when When the pressure is 0, 50, 100, 150, 200, 250, 300, 350, 400, 450 kPa, for each Numerical simulation of the entire excavation and support process was conducted, and the deformation of key points in the surrounding rock was recorded. The areas of the plastic zone and failure zone of the surrounding rock were extracted and statistically analyzed. Establish different support forces With the deformation of the surrounding rock Area of ​​surrounding rock damage The correspondence between them; S43: Estimate or calculate different support forces based on engineering experience, quota surveys, or supplier quotations. Corresponding support costs Based on construction methods, equipment configuration, and work process organization, determine the support construction period corresponding to different support forces Pi. .

[0031] S5: The surrounding rock deformation, surrounding rock failure area, support cost, and support construction period under different support forces in S4 are standardized to form standardized deformation, failure area, cost, and period indicators, respectively. The relationship curves of these four standardized indicators with support force are plotted on the same coordinate system to determine the optimal support force that achieves synergy among the four indicators and balances safety, economy, and timeliness. and the range of values; The surrounding rock deformation obtained in S42 in S5 The normalization process is performed to obtain the normalized deformation index. , (0≤ ), Different support forces A decreasing function; ; In the formula, This indicates the surrounding rock deformation value corresponding to the current support force; Indicates the maximum deformation without support; This represents the minimum deformation under the strongest support. The surrounding rock deformation obtained in S42 The normalization process is performed to obtain the normalized deformation index. (0≤ ), Different support forces A decreasing function; ; In the formula, This indicates the area of ​​surrounding rock failure corresponding to the current support force; Indicates the maximum damaged area without support; Indicates the minimum damage area under the strongest support; Support costs obtained from S43 The cost index is obtained by unitization. , (0≤ ≤1), Different support forces An increasing function; ; In the formula, This indicates the cost corresponding to the current support force; This represents the cost corresponding to the minimum support force; This indicates the cost corresponding to the maximum support force; Support costs obtained from S43 After normalization, we obtain the normalized periodic index. , (0≤ ≤1), Different support forces An increasing function; ; In the formula, T represents the construction period corresponding to the current support force; This indicates the construction period corresponding to the minimum support force. This indicates the construction period corresponding to the maximum support force; In the same coordinate system, with the support force as the horizontal axis and the unitized index as the vertical axis, the unitized deformation index is plotted. Unitized damage area index Unit cost indicators Unitized periodicity indicators Depending on the support force Changing curve Identify the equilibrium region where the four curves intersect. The support force corresponding to this region is the optimal support force that satisfies the comprehensive optimization of deformation control, failure range control, cost control, and schedule control. And the optimal interval.

[0032] S6: Integrate the optimal critical safe burial depth determined in S3 Optimal cross-sectional dimensions corresponding to the optimal cross-sectional height-to-width ratio and the optimal support force obtained in S5 This leads to a comprehensive tunnel design scheme applicable to soft upper and hard lower strata, which serves as the output of core design parameters.

[0033] Example 2 like Figures 2-3 As shown, a city's underground integrated utility tunnel project requires traversing a soft upper layer and a hard lower layer. The upper layer consists of Tertiary silty clay, while the lower layer is moderately weathered granite. The tunnel is designed with a rectangular cross-section and will later serve as a transportation route for municipal pipelines, thus requiring a high level of safety and economy. This design scheme is adopted, and the specific steps are as follows: Step 1: Geological exploration and stratigraphic boundary determination; Along the planned tunnel route, 12 exploration boreholes were laid out every 50 meters along the axis, using an XY-50 hydraulic core drilling rig for rotary drilling, reaching a depth of 15 meters below the tunnel floor. Observation of the obtained complete core samples revealed that the boundary between the silty clay layer and the moderately weathered granite layer was stable, with an average burial depth of 15 meters. That is, the layer from the surface to 15 meters is a soft layer, and the layer below 15 meters is a hard layer.

[0034] Step 2: Obtain formation parameters through indoor physical and mechanical tests; Laboratory physical and mechanical tests were conducted on the soft and hard core samples obtained from the borehole. Ring cutter method test: using a volume of 200 The density of the soft soil layer was calculated by weighing the silty clay sample taken from the ring cutter. =1.85g / The density of the hard layer was calculated by directly measuring the size and mass of the hard rock granite specimens. =2.7g / .

[0035] Uniaxial compression test: Using a WAW-1000 electro-hydraulic servo pressure testing machine, the elastic modulus of the soft layer was measured to be E1=65MPa and Poisson's ratio ν1=0.38; the elastic modulus of the hard layer was E2=3500MPa and Poisson's ratio ν2=0.18.

[0036] Triaxial compression test: Consolidation undrained test was conducted using a TSZ-6 fully automatic triaxial apparatus. Based on the Mohr-Coulomb strength theory, the cohesive strength of the soft layer was fitted to be c1 = 22 kPa, and the frictional strength was... 1=16°; hard layer cohesive strength c2=200kPa, frictional strength 2 = 48°.

[0037] Step 3: Safety factor scaling and numerical simulation preparation; Orthogonal experimental design scheme: Based on the actual engineering situation, the tunnel burial depth is selected within the range of 16-36m. Groups are formed in 4m increments, with cross-sectional height-to-width ratios of 0.8, 1.0, 1.2, 1.4, and 1.6. A two-factor, five-level orthogonal experiment is constructed, resulting in 25 orthogonal simulation design schemes, as shown in Table 2 below: Table 2

[0038] This tunnel is an urban underground utility tunnel, classified as a Class I engineering project. A strength scaling safety factor F is determined accordingly. s The value ranges from 2.0 to 2.5. The safety factor F is used. s Taking 2.2 as an example, the strength parameter scaling method is used to scale the formation strength parameters: Cohesion strength scaling: Cohesion strength after scaling of soft layer =22 / 2.2=10kPa; Cohesive strength of the hard layer after scaling =200 / 2.2=90.91kPa.

[0039] Friction intensity scaling: soft layer tan16° / 2.2 = 0.1303, corresponding to =7.4°; hard layer tan48° / 2.2 = 0.5048, corresponding to =26.8°.

[0040] Step 4: Determine the critical burial depth and reasonable cross-sectional dimensions; Using FLAC3D numerical simulation software, a numerical model of the tunnel-stratum system was established according to the scaled stratum parameters. 25 orthogonal simulation test schemes were simulated. The strength scaling safety factor for each scheme was set to 2.0, 2.1, 2.2, 2.3, 2.4 and 2.5 respectively. A series of numerical calculations were performed, and the values ​​of surrounding rock stress, displacement and plastic zone development under different safety factors for each scheme were recorded, as shown in Table 3. Table 3

[0041] With the aspect ratio fixed at 1.0, analyze different burial depth schemes. When the burial depth is 18m, the safety factor F... s =2.2, the simulation results show that the stress distribution of the surrounding rock is uniform, the plastic zone and the failure range only appear locally in the soft layer, the maximum displacement is 8mm, which is less than the allowable displacement of 10mm, the model converges, and the stability requirements are met; when the burial depth is less than 18m, for example, burial depth = 16m, the safety factor F s =2.0, the soft layer plastic zone is penetrated, the failure range exceeds the standard, the displacement reaches 15mm, and the model does not converge. Therefore, the optimal critical safe burial depth needs to be determined. =18m.

[0042] fixed =18m, comparing different height-to-width ratio schemes, when the height-to-width ratio is 1.2, the safety factor F s =2.2, the surrounding rock stress distribution is uniform with no obvious stress concentration, and the failure range is minimal; when the height-to-width ratio is >1.4, the local stress in the hard rock layer exceeds the limit, and there is a risk of rockburst. Considering the functional requirements of the utility tunnel, the tunnel width is determined to be 8m. Based on the optimal height-to-width ratio of 1.2, the tunnel height is calculated to be 9.6m, corresponding to the reasonable cross-sectional dimensions. =8 × 9.6 = 76.8m 2 .

[0043] The optimal critical safe burial depth was finally determined. =18m, the reasonable cross-sectional dimensions corresponding to the optimal cross-sectional height-to-width ratio. =8 × 9.6 = 76.8m 2 .

[0044] Step 5: Establish the support force-deformation relationship and cost analysis; Keep =18m =76.8m 2 In the case of applying different radial support forces in the numerical model =0, 50, 100, 150, 200, 250, 300, 350, 400, 450 kPa, simulating the entire excavation and support process, and recording the deformation of key points in the surrounding rock. The support cost is calculated by combining the quota and the supplier's quotation. The support construction period is determined in conjunction with the construction organization. The results are shown in Table 4: Table 4

[0045] Step 6: Unitization and determination of optimal support force; The surrounding rock deformation, the area of ​​surrounding rock failure, the support cost, and the support construction period were each unitized. Four unitized index curves were plotted on the same coordinate system. The support force corresponding to the equilibrium region where the four curves intersect is 160 kPa. At this point, deformation is controllable, the failure range meets the standard, the cost is moderate, and the construction period is optimal. The optimal support force P was then determined. =160kPa.

[0046] Step 7: Output core design parameters; Based on the above steps, the core design parameters of the tunnel are finally output: the optimal critical safe burial depth. =18m; Optimal cross-sectional dimensions corresponding to the optimal height-to-width ratio. =76.8m2 The corresponding width is 8m, the height is 9.6m, and the height-to-width ratio is 1.2; the optimal support force P =160kPa.

[0047] Subsequent construction was carried out according to these parameters. During the tunnel excavation, no soft layer collapsed, there was no risk of rock burst in the hard layer, the actual deformation of the surrounding rock was about 9mm, the support cost was controlled at 23,000 yuan / m, and the construction period was shortened by 15% compared with the traditional scheme, which met the dual requirements of safety and economy.

[0048] Therefore, this invention adopts the aforementioned comprehensive design method for tunnel excavation in soft-over-hard strata, constructing a closed-loop system encompassing the entire process of "exploration-analysis-optimization-finalization," and incorporating the three core parameters of "burial depth, cross-sectional dimensions, and support force" into a unified analysis framework. It replaces the experience-driven model with experimental testing, simulation, and cost data, and optimizes and locks in the optimal support force through quantitative modeling of "support force-deformation-cost." For complex strata, it conducts precise exploration and layered testing, and combines numerical simulation with a safety factor scaling model to improve the design's universality. This approach scientifically determines critical safety parameters, reducing risks such as collapse and rock bursts, while also optimizing resource allocation to avoid over- or under-design, achieving a dual improvement in safety and economic benefits.

[0049] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A comprehensive design method for tunnel excavation in soft-over-hard strata, characterized in that, Includes the following steps: S1: Identify geological exploration and stratigraphic interfaces, arrange boreholes and collect rock and soil cores, record the core properties in detail, and accurately identify the geological boundary between soft soil and hard rock layers; based on the obtained soft and hard strata rock and soil core samples, conduct indoor physical and mechanical tests to determine the physical properties and strength parameters of each stratum; S2: A three-dimensional numerical model of the tunnel-stratum system reflecting the soft upper and hard lower stratum structure is established using numerical simulation software. The tunnel burial depth and cross-sectional height-to-width ratio are used as variable factors. Multiple simulation schemes are designed using orthogonal experimental design method. The stratum strength parameters obtained in S1 are scaled according to the tunnel engineering grade and safety requirements. A series of numerical calculations are then performed. S3: Set stability evaluation indicators, obtain the corresponding safety factors based on the series of numerical calculations in S2, or conduct a systematic analysis of the numerical simulation results in S2 based on the model convergence degree, and determine the optimal critical safe burial depth based on the safety factors corresponding to the obtained multiple simulation schemes. Select the optimal critical safe burial depth and the reasonable cross-sectional dimensions corresponding to the optimal cross-sectional height-to-width ratio ; S4: The optimal critical safe burial depth determined in S3 and the reasonable cross-sectional dimensions corresponding to the optimal cross-sectional height-to-width ratio In the corresponding three-dimensional numerical model, a uniformly distributed pressure equivalent to the support force is applied at the tunnel excavation boundary. The numerical simulation is carried out by changing the magnitude of the support force, and the maximum deformation of the surrounding rock and the area of ​​the surrounding rock failure are recorded. Combined with engineering experience, cost data and construction organization analysis, the support cost estimation function and the support construction cycle estimation function required for different support forces are established. S5: The surrounding rock deformation, surrounding rock failure area, support cost, and support construction period under different support forces in S4 are standardized to form standardized deformation, failure area, cost, and period indicators, respectively. The relationship curves of the four standardized indicators with support force are plotted on the same coordinate system to determine the optimal support force that balances safety and economy. and the range of values; S6: Integrate the optimal critical safety burial depth determined in S3 and the reasonable cross-sectional dimensions corresponding to the optimal cross-sectional height-to-width ratio and the optimal support force obtained in S5 This leads to a comprehensive tunnel design scheme applicable to soft upper and hard lower strata, which serves as the output of core design parameters.

2. The comprehensive design method for tunnel excavation in soft-over-hard strata according to claim 1, characterized in that, Based on the tunnel depth and geological conditions, S1 uses rotary drilling and hydraulic core drilling rigs to conduct borehole exploration within the tunnel planning area to accurately determine the interface between soft soil and hard rock layers. Calculated density, elastic modulus, Poisson's ratio, cohesive strength, and frictional strength are obtained through indoor physical and mechanical tests. These tests specifically include ring cutter tests, uniaxial compression tests, and triaxial compression tests.

3. The comprehensive design method for tunnel excavation in soft-over-hard strata according to claim 2, characterized in that, In S2, the soil layer boundary location obtained in S1 is combined with the scaled parameters and numerical simulation software to establish a three-dimensional model of the tunnel-stratum. Determine the strength scaling safety factor based on the tunnel's grade and importance. Strength parameter scaling method for formation strength parameter cohesion Scaling the safety factor according to the corresponding strength The specific formula for scaling cohesive strength is as follows: ； The safety factor is determined based on the tunnel's classification and importance requirements. The strength parameter scaling method is used to scale the formation strength parameter friction strength obtained in S1. Scaling the safety factor according to the corresponding strength The specific formula for scaling friction intensity is as follows: ； The scaled cohesive strength and frictional strength were incorporated into various orthogonal experimental schemes. A series of numerical simulations were conducted by varying the tunnel depth and tunnel cross-sectional aspect ratio to analyze the surrounding rock stress, displacement, and plastic zone development. The strength scaling safety factor was recorded. .

4. The comprehensive design method for tunnel excavation in soft-over-hard strata according to claim 1, characterized in that, In S3, the stability evaluation index is set as follows: soft soil displacement ≤ maximum allowable soft soil displacement, hard rock displacement ≤ maximum allowable hard rock displacement, and the area of ​​surrounding rock failure does not exceed the allowable failure area threshold.

5. The comprehensive design method for tunnel excavation in soft-over-hard strata according to claim 1, characterized in that, The specific steps to be performed in S4 are as follows: S41: Maintain the optimal critical safety burial depth Optimal cross-sectional dimensions corresponding to the optimal cross-sectional height-to-width ratio In the corresponding three-dimensional numerical calculation model, different support forces are simulated by applying radial pressure to the surface of the tunnel cross-section profile. Influence; S42: Different support forces Under the given value, for each Numerical simulation of the entire excavation and support process was conducted, and the deformation of key points in the surrounding rock was recorded. and the total area of ​​the plastic zone or failure zone of the surrounding rock. Statistics and establishment of different support forces With the deformation of the surrounding rock Area of ​​surrounding rock damage The correspondence between them; S43: Determine different support forces based on engineering experience, quota surveys, or supplier quotations. Corresponding support costs Based on construction methods, equipment configuration, and work process organization, different support forces are determined. Corresponding support construction period .

6. The comprehensive design method for tunnel excavation in soft-over-hard strata according to claim 5, characterized in that, The surrounding rock deformation obtained in S42 in S5 The normalization process is performed to obtain the normalized deformation index. , (0≤ ), Different support forces A decreasing function; ； In the formula, This indicates the surrounding rock deformation value corresponding to the current support force; Indicates the maximum deformation without support; This represents the minimum deformation under the strongest support. The surrounding rock deformation obtained in S42 The normalization process is performed to obtain the normalized deformation index. (0≤ ), Different support forces A decreasing function; ； In the formula, This indicates the area of ​​surrounding rock failure corresponding to the current support force; Indicates the maximum damaged area without support; Indicates the minimum damage area under the strongest support; Support costs obtained from S43 The cost index is obtained by unitization. , (0≤ ≤1), Different support forces An increasing function; ； In the formula, This indicates the cost corresponding to the current support force; This represents the cost corresponding to the minimum support force; This indicates the cost corresponding to the maximum support force; Support costs obtained from S43 After normalization, we obtain the normalized periodic index. , (0≤ ≤1), Different support forces An increasing function; ； In the formula, T represents the construction period corresponding to the current support force; This indicates the construction period corresponding to the minimum support force. This indicates the construction period corresponding to the maximum support force; In the same coordinate system, with the support force as the horizontal axis and the unitized index as the vertical axis, the unitized deformation index is plotted. Unitized damage area index Unit cost indicators Unitized periodicity indicators Depending on the support force The changing curves; identify the equilibrium region where the four curves intersect. The support force corresponding to this region is the optimal support force that satisfies the comprehensive optimization of deformation control, failure range control, cost control, and schedule control. And the optimal interval.

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