Stability analysis method of pile foundation cave roof in deep karst sites

Abstract

The present invention relates to a method for analyzing the stability of pile-foundation karst cave roofs in deep-covered karst sites, comprising the following steps: S1, collecting and analyzing geological data in the project area to understand the macroscopic development characteristics of the karst in the site; S2, conducting site exploration using an optimized transient electromagnetic method; S3, conducting site seismic reflection exploration; S4, delineating the karst development range based on karst anomaly locations determined by comparison between the optimized transient electromagnetic method and the seismic reflection method; S5, based on the pile foundation design plan, verifying the stability of the roof under the shear bearing capacity of the roof under the limit equilibrium condition for karst caves or soil caves with potential collapse in the roof rock mass below the pile end, and for karst caves in limestone layers with thick roof rock layers; S6, verifying the safe thickness of the roof for fractured rock masses with developed fissures and thick, relatively strong, and intact rock masses; and S7, determining the safe thickness of the cave roof. The present invention effectively controls pile length, reduces project costs, and shortens construction period.

Description

Technical Field

[0001] The present invention relates to the field of geotechnical engineering investigation and design, and more particularly to a method for analyzing the stability of pile foundation cave roofs in deep-covered karst sites. Background Art

[0002] Accurately surveying and evaluating the stability of deep-cover karst is a challenge within the industry. Traditional survey techniques struggle to accurately reveal deep karst characteristics. To ensure structural stability, a stability assessment of the cave roof beneath the pile foundation is necessary. To avoid loading the karst roof, pile foundations often employ a penetrating pile layout. However, for deep-cover karst, penetrating pile layout significantly increases pile length, project costs, and construction duration. Accurately identifying the cave characteristics of deep-cover karst areas and verifying the stability of the cave roof for non-penetrating pile foundations are essential for high-quality pile foundation design. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a method for analyzing the stability of pile foundation cave roofs in deep-covered karst sites, which provides a basis for foundation stability while meeting the foundation bearing capacity and settlement requirements, effectively controls pile length, reduces project costs, and shortens construction period.

[0004] The technical solution adopted by the present invention to solve the technical problem is to construct a method for analyzing the stability of the pile foundation cave roof in a deep karst site, comprising the following steps:

[0005] S1. Collect and analyze geological data in the project area to understand the macroscopic development characteristics of karst in the site;

[0006] S2. Conduct site exploration using optimized transient electromagnetic method;

[0007] S3. Conduct site seismic reflection exploration;

[0008] S4. Delineating the karst development range based on the karst abnormality locations determined by comparing the optimized transient electromagnetic method and the seismic reflection method;

[0009] S5. Based on the pile foundation design, for severely weathered, cracked, or potentially collapsing cavities or soil caves in the top rock mass below the pile end, the collapse-fill method formula is used for verification. For cavities in limestone layers with relatively intact, high-strength, and thick top rock layers, the top plate stability is verified based on the shear bearing capacity of the top plate under the limit equilibrium condition.

[0010] S6. For roofs with fractured rock mass and high-strength, relatively intact thick rock mass, MADISC GTS NX software is used to perform finite element numerical simulation analysis and calculation to verify the safe thickness of the roof;

[0011] S7. Determine the safe thickness of the cave roof.

[0012] According to the above scheme, the specific content of the optimized transient electromagnetic method is as follows: the survey line is arranged along the direction of the karst development zone, the collection point spacing is 2-5m, and the data is interpreted using software with low-pass filtering function and transient relaxation inversion method. According to the size of the inversion depth, a reasonable constraint coefficient and inversion coefficient are selected, the constraint coefficient is 0.8-1.2, and the inversion coefficient is 0.1-03. The two-dimensional electrical structure profile is obtained through inversion, and the location of the corrosion cave is delineated according to the low-resistance abnormal area.

[0013] According to the above plan, the specific contents of the site seismic reflection exploration are as follows: arranging survey lines along the direction of the karst development zone, using a 120KG hammer, a 2.5m drop height, 12 times of coverage, and an asymmetric regular observation system; performing spherical diffusion energy compensation on the original records, and based on the multi-channel spectrum analysis method, through the FK transformation, performing two-dimensional frequency-wavenumber analysis to determine the frequency-wavenumber characteristics of the effective wave and the interference wave, and filtering the low-frequency and low-speed interference waves according to the frequency-wavenumber characteristics of the effective wave and the interference wave.

[0014] According to the above plan, the specific contents of delineating the karst development range are as follows: arrange drilling and combine tube wave testing to conduct detailed investigation and verification, analyze the karst development characteristics of the site, find out the location, roof thickness and integrity, shape, size, filling condition, and hydraulic connection between karst water and upper water body of the cave within the influence range of the pile foundation, take rock and soil samples for physical and mechanical tests, and obtain pile foundation design and verification parameters.

[0015] According to the above scheme, the formula used for the collapse filling method is: H = H0 / (K-1)

[0016] Among them, H is the required collapse height, H0 is the maximum height of the cave before collapse, K is the rock looseness coefficient,

[0017] The formula used to verify the stability of the top plate under shear bearing capacity is:

[0018] H=P / SL

[0019] Among them, H is the required roof rock thickness, P is the total load on the roof, S is the calculated shear strength of the rock mass, and L is the plane circumference of the cave.

[0020] According to the above plan, the specific contents are as follows:

[0021] S601, creating a three-dimensional geological model and a pile foundation model of the site based on the geological information and the pile foundation design information, and performing meshing of the rock and soil layers and the pile foundation model;

[0022] S602. Use the Mohr-Coulomb constitutive model for geotechnical materials to assign material properties to each geotechnical layer, and use the elastic model to assign material properties to the pile foundation;

[0023] S603, add loads, including deadweight load, water pressure and pile foundation design load;

[0024] S604, perform three-dimensional stability analysis and calculation to solve the top plate of the pile foundation cave;

[0025] S605. Perform result analysis and post-processing to verify the safety thickness of the top plate.

[0026] According to the above plan, the specific contents are as follows: Compare the results of the collapse and filling method-finite element numerical simulation analysis and the results of the shear bearing capacity verification of the roof-finite element numerical simulation analysis, analyze the rationality of the empirical parameters taken for the collapse and filling method and the shear bearing capacity verification of the roof and the rationality of the finite element analysis boundary conditions, and comprehensively determine the safe thickness of the cave roof based on the load size of the pile foundation, the integrity and nature of the cave roof, the depth of the roof and the law of groundwater activity below.

[0027] The implementation of the method for analyzing the stability of the pile foundation cave roof in a deep-covered karst site of the present invention has the following beneficial effects:

[0028] 1. In view of the difficulties in exploration in deep-covered karst areas, the many interference factors in geophysical exploration, and the poor accuracy, this invention adopts optimized transient electromagnetic method and seismic reflection method to increase the effective detection depth of geophysical exploration and improve the accuracy of detection. Combined with geological drilling and tube wave detection for verification, it can determine the location, size, filling condition of the cave in detail and obtain the rock and soil parameters of the roof;

[0029] 2. Based on the understanding of karst development characteristics, the present invention adopts theoretical verification and numerical simulation to study the stability of the top plate of non-penetrating karst pile foundation and conducts comparative analysis to obtain more reliable calculation results. This provides a reliable basis for the design of pile foundations that do not penetrate caves in deep-covered karst sites, saves pile length, and has greater economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The present invention will be further described below with reference to the accompanying drawings and embodiments, in which:

[0031] Figure 1 The present invention is a flow chart of a method for analyzing the stability of pile foundation cave roofs in deep-covered karst sites. DETAILED DESCRIPTION

[0032] In order to have a clearer understanding of the technical features, purposes and effects of the present invention, specific embodiments of the present invention are now described in detail with reference to the accompanying drawings.

[0033] like Figure 1 As shown, in an embodiment of the method for analyzing the stability of the top plate of a pile foundation cave in a deep covered karst site of the present invention, the following steps are included:

[0034] S1. Collect and analyze geological data in the project area to understand the macroscopic development characteristics of karst in the site;

[0035] S2. Conduct site transient electromagnetic exploration, arrange survey lines along the direction of the karst development zone, and collect data at a spacing of 3m. Use software with low-pass filtering function and transient relaxation inversion method to interpret data. Select reasonable constraint coefficient and inversion coefficient according to the inversion depth. The constraint coefficient can be 1 and the inversion coefficient can be 0.2. Obtain a two-dimensional electrical structure profile through inversion, and delineate the location of the corrosion cave according to the low-resistance anomaly area.

[0036] S3. Conduct field seismic reflection exploration, arrange survey lines along the direction of the karst development zone, use a 120KG hammer and a 2.5m drop height to enhance the excitation energy and suppress noise interference; increase the usual 6 coverage times to 12 times to improve the signal-to-noise ratio of the acquired records; improve the conventional equal-offset regular observation system to an asymmetric regular observation system to improve the overall signal-to-noise ratio of the records and meet the requirements of the observation system for strata with different dips; based on the spherical diffusion theory, perform spherical diffusion energy compensation on the original records to effectively improve the deep and long-distance reflection energy; based on the multi-channel spectrum analysis method, use the FK transform to analyze the two-dimensional frequency-wavenumber to determine the frequency-wavenumber characteristics of the effective wave and the interference wave, and filter the low-frequency and low-speed interference waves according to their characteristics; the above measures improve the quality and accuracy of data interpretation.

[0037] S4. Based on the abnormal karst locations identified by the transient electromagnetic method and the seismic reflection method, the karst development range is delineated, and drilling is arranged in combination with tube wave testing for detailed investigation and verification. The karst development characteristics of the site are analyzed, and the location, roof thickness and integrity, shape, size, filling condition, hydraulic connection between karst water and upper water bodies of the caves within the influence range of the pile foundation are determined; rock and soil samples are taken for physical and mechanical tests to obtain pile foundation design and verification parameters.

[0038] S5. Based on the pile foundation design, for caves or earth caves with severely weathered roof rock mass below the pile end, developed cracks, and potential collapse, the collapse and filling method (H = H0 / (K-1)) is used for verification. Here, H is the required collapse height (m), H0 is the maximum height of the cave before collapse (m), and K is the rock looseness (extension coefficient). For moderately weathered limestone, K is 1.2, for strongly weathered limestone, 1.1, and for clay, 1.05. For caves located in limestone layers with relatively intact, high-strength, and thick roof rock, the roof stability is verified based on the shear bearing capacity of the roof under the limit equilibrium condition using the formula H = P / SL. Here, H is the required roof rock thickness (m), P is the total load on the roof (kN), S is the calculated shear strength of the rock mass (kPa), and L is the planar perimeter of the cave (m).

[0039] S6. For roofs consisting of fractured rock masses with developed fissures and thick, relatively intact rock masses with high strength, finite element numerical simulation analyses were conducted, using madis GTS NX software and similar software. The main contents included: 1) creating a three-dimensional geological model of the site and a pile foundation model based on geological information and pile foundation design information, and meshing the rock and soil layers and pile foundation models; 2) assigning material properties to each rock and soil layer using the Mohr-Coulomb constitutive model for geotechnical materials, and assigning material properties to the pile foundation using the elastic model; 3) adding loads, including self-weight load, water pressure, and pile foundation design load; 4) performing a three-dimensional stability analysis and calculation of the pile foundation cave roof; and 5) performing result analysis and post-processing to verify the safe thickness of the roof.

[0040] S7. Compare the results of the finite element numerical simulation analysis of the collapse and filling method with the results of the finite element numerical simulation analysis of the shear bearing capacity of the roof. Analyze the rationality of the empirical parameters used in the collapse and filling method and the shear bearing capacity of the roof, as well as the rationality of the boundary conditions of the finite element analysis. Determine the safe thickness of the cave roof based on the load on the pile foundation, the integrity and nature of the cave roof, the depth of the roof, and the law of groundwater activity below.

[0041] The embodiments of the present invention are described above in conjunction with the accompanying drawings, but the present invention is not limited to the above-mentioned specific implementation methods. The above-mentioned specific implementation methods are merely illustrative and not restrictive. Under the guidance of the present invention, ordinary technicians in this field can also make many forms without departing from the scope of protection of the present invention and the claims, all of which are protected by the present invention.

Claims

1. A method for analyzing the stability of pile foundation cave roof in deep karst sites, characterized in that: The steps include: S1. Collect and analyze geological data in the project area to understand the macroscopic development characteristics of karst in the site; S2. Conduct site exploration using optimized transient electromagnetic method; S3. Conduct site seismic reflection exploration; S4. Delineating the karst development range based on the karst abnormality locations determined by comparing the optimized transient electromagnetic method and the seismic reflection method; S5. Based on the pile foundation design, for severely weathered, cracked, or potentially collapsing cavities or soil caves in the top rock mass below the pile end, the collapse-fill method formula is used for verification. For cavities in limestone layers with relatively intact, high-strength, and thick top rock layers, the top plate stability is verified based on the shear bearing capacity of the top plate under the limit equilibrium condition. S6. For roofs with fractured rock mass and high-strength, relatively intact thick rock mass, madisGTS NX software is used to perform finite element numerical simulation analysis and calculation to verify the safe thickness of the roof; S7. Determine the safe thickness of the cave roof; In step S2, the specific content of the optimized transient electromagnetic method is as follows: arranging survey lines along the karst development zone, with a collection point spacing of 2-5m, using software with a low-pass filtering function and a transient relaxation inversion method to interpret the data, selecting a reasonable constraint coefficient and inversion coefficient based on the inversion depth, with the constraint coefficient being 0.8-1.2 and the inversion coefficient being 0.1-0.3, obtaining a two-dimensional electrical structure profile through inversion, and delineating the location of the corrosion cave according to the low-resistance abnormal area; In step S3, the specific contents of the site seismic reflection exploration are as follows: arranging survey lines along the direction of the karst development zone, using a 120 kg hammer, a 2.5 m drop height, 12 coverage times, and an asymmetric regular observation system; performing spherical diffusion energy compensation on the original records, performing a two-dimensional frequency-wavenumber analysis based on a multi-channel spectrum analysis method through FK transformation to determine the frequency-wavenumber characteristics of the effective wave and the interference wave, and filtering the low-frequency and low-speed interference wave according to the frequency-wavenumber characteristics of the effective wave and the interference wave.

2. The method for analyzing the stability of the top plate of a pile foundation cave in a deep karst site according to claim 1 is characterized in that: In step S4, the specific contents of delineating the karst development range are as follows: arranging drilling holes combined with tube wave testing to conduct detailed investigation and verification, analyzing the karst development characteristics of the site, ascertaining the location, roof thickness and integrity, shape, size, filling condition, and hydraulic connection between karst water and upper water bodies of the caves within the influence range of the pile foundation, taking rock and soil samples for physical and mechanical testing, and obtaining pile foundation design and verification parameters.

3. The method for analyzing the stability of the top plate of the pile foundation cave in deep karst sites according to claim 1 is characterized in that: In step S5, the collapse and filling method is verified using the following formula: H=H0 / (K-1) Among them, H is the required collapse height, H0 is the maximum height of the cave before collapse, and K is the rock looseness coefficient; The formula used to verify the stability of the top plate under shear bearing capacity is: H=P / SL Among them, H is the required roof rock thickness, P is the total load on the roof, S is the calculated shear strength of the rock mass, and L is the plane circumference of the cave.

4. The method for analyzing the stability of the top plate of a pile foundation cave in a deep-covered karst site according to claim 1 is characterized in that: In step S6, the specific contents are as follows: S601, creating a three-dimensional geological model and a pile foundation model of the site based on the geological information and the pile foundation design information, and performing meshing of the rock and soil layers and the pile foundation model; S602. Use the Mohr-Coulomb constitutive model for geotechnical materials to assign material properties to each geotechnical layer, and use the elastic model to assign material properties to the pile foundation; S603, add loads, including deadweight load, water pressure and pile foundation design load; S604, perform three-dimensional stability analysis and calculation to solve the top plate of the pile foundation cave; S605. Perform result analysis and post-processing to verify the safety thickness of the top plate.

5. The method for analyzing the stability of the pile foundation cave roof in deep karst sites according to claim 1 is characterized in that: In step S7, the specific contents are as follows: comparing the results of the collapse and filling method-finite element numerical simulation analysis and the results of the shear bearing capacity verification of the roof-finite element numerical simulation analysis, analyzing the rationality of the empirical parameters taken for the collapse and filling method and the shear bearing capacity verification of the roof and the rationality of the finite element analysis boundary conditions, and comprehensively determining the safe thickness of the cave roof based on the load size borne by the pile foundation, the integrity and nature of the cave roof, the buried depth of the roof and the law of groundwater activity below.

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