A design method for the bearing capacity of pile foundations in karst areas with dissolution and fragmentation zones

Abstract

The present invention discloses a design method for the bearing capacity of pile foundations in karst area dissolution and fracture zones. On the one hand, it fully considers the long-term influencing factors of the dissolution and fracture zones above the fluctuating water level influence range, within the fluctuating water level influence range or at the normal water level range on the end bearing capacity of the pile foundation structure and the side resistance of the pile circumference of the pile foundation structure, effectively improving the accuracy of the bearing capacity of pile foundation structures with long service life and strict deformation requirements, and ensuring the use safety of pile foundation structures in the special geomorphic environment of karst area dissolution and fracture zones. On the other hand, through the simulation environmental test treatment of the core samples taken from the dissolution and fracture zones, the porosity change at the end of the service life is obtained, and the influence degree on the end bearing capacity of the pile foundation and the side resistance of the pile circumference is deduced. This test method is simple to operate, solves the problems of difficult testing and high cost of the pile end bearing capacity and the side resistance of the pile circumference of the test samples, and is convenient for popularization and application.

Description

Technical Field

[0001] The present invention relates to the field of design methods for the bearing capacity of pile foundation structures, and particularly to a design method for the bearing capacity of pile foundations in karst areas with dissolution and fracture zones. Background Art

[0002] Karst landforms are widely distributed in China. The soluble rocks (mainly calcium carbonate) in the foundation of karst areas gradually dissolve in water under the repeated action of groundwater and CO 2 to generate H 2 CO 3 and undergo dissolution and weathering. The soluble rocks gradually dissolve into different forms such as soil-like and fragmented, and form foundation conditions such as karst cavities and dissolution and fracture zones. Among them, the dissolution and fracture zone is a common foundation form of soluble rocks in karst areas. The rock foundation in the dissolution and fracture zone still has a certain rock structure, but the dissolution channels in the fracture zone are completely connected, providing convenient conditions for the infiltration, connection, and further acceleration of dissolution of groundwater. Different from karst caves, the dissolution and fracture zone generally does not cause karst collapse disasters, but under the continuous action of groundwater, its structural strength, frictional resistance, and foundation bearing capacity are in a continuous decreasing process. According to the environmental conditions of groundwater and CO 2 in the foundation of the dissolution and fracture zone, there are certain differences in its deterioration degree: for example, the groundwater and CO 2 in the groundwater fluctuation range are more fully contacted, and the deterioration of the fracture zone is more obvious; above the groundwater fluctuation range, the CO 2 content is high but the infiltration water volume is small and the contact time is short, and the deterioration degree of the fracture zone is lower; in the normal water level range, the groundwater is rich but the CO 2 content is low, but the duration is long, and the deterioration degree of the fracture zone is higher.

[0003] When a pile foundation structure used for force bearing is set in the foundation of the dissolution and fracture zone, if the service life of the pile foundation structure is short or the deformation control standard of the pile foundation structure is low, the influence of the dissolution and fracture zone under the action of groundwater and CO 2 is small and can be ignored; however, when the service life of the pile foundation structure is longer, the importance degree is higher, and the deformation control is more strict, then the influencing factors of the dissolution and fracture zone under the action of groundwater must be fully considered. For example, for high-speed railways with a service life of one hundred years and a deformation control standard of millimeter level, the long-term deterioration influence of the dissolution and fracture zone should be considered. At present, in engineering practice, when a pile foundation structure is set in the foundation of the dissolution and fracture zone, it is usually designed as a friction pile structure, without considering the influence of the continuous decrease of the structural strength, ultimate side resistance, and foundation bearing capacity of the dissolution and fracture zone, and even less considering the factors of different degrees of groundwater action. Summary of the Invention

[0004] The object of the present invention is to provide a design method for the bearing capacity of pile foundations in the karst corrosion and fracture zone, aiming at the problem that the influence of groundwater and CO 2 is ignored in the design process of the bearing capacity when the pile foundation structure as a force-bearing structure is arranged in the karst corrosion and fracture zone. This method fully considers the long-term influence factors of the karst corrosion and fracture zone above the influence range of the fluctuating water level, within the influence range of the fluctuating water level or in the constant water level range on the end bearing capacity of the pile foundation structure and the side resistance of the pile circumference of the pile foundation structure, effectively improving the accuracy of the bearing capacity of the pile foundation structure with a long service life and strict deformation requirements, and ensuring the use safety of the pile foundation structure in the special geomorphic environment of the karst corrosion and fracture zone.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A design method for the bearing capacity of pile foundations in the karst corrosion and fracture zone, comprising the following steps:

[0007] Step 1: Determine the position of the constant water level line and the position of the water level fluctuation influence line in the karst corrosion and fracture zone of the surveyed engineering area. Denote the karst corrosion and fracture zone area above the water level fluctuation influence line as the first area, the water level fluctuation influence area as the second area, and the karst corrosion and fracture zone area below the constant water level line as the third area. The height of the first area is L 1 , and the height of the second area is L 2 ;

[0008] Step 2: Denote the depth of the pile foundation extending into the karst corrosion and fracture zone after installation as l, and drill and take cores from the area to be extended; stratify the core samples of the first area according to the porosity and soil content of the karst corrosion and fracture zone. If any, stratify the core samples of the second area, and if any, stratify the core samples of the third area; and test the initial allowable bearing capacity of the karst corrosion and fracture zone foundation at the position where the bottom end of the pile foundation is located after installation, the initial ultimate side resistance of the karst corrosion and fracture zone foundation of each layer where the entire pile body of the pile foundation is located after installation, and the initial porosity of the karst corrosion and fracture zone foundation of each layer where the entire pile body of the pile foundation is located after installation; where σ ni is the initial allowable bearing capacity (kPa) of the i-th layer of karst corrosion and fracture zone foundation;

[0009] f ni is the initial ultimate side resistance (kPa) of the i-th layer of karst corrosion and fracture zone foundation;

[0010] e ni0 is the initial porosity of the i-th layer of karst corrosion and fracture zone foundation;

[0011] i is the number of layers, and i is a positive integer greater than or equal to 1; n takes 1, 2, or 3; n = 1 represents the dissolution and fracture zone within the first region, n = 2 represents the dissolution and fracture zone within the second region, and n = 3 represents the dissolution and fracture zone within the third region;

[0012] Step 3: Simulate the rainfall environment during the service life of the pile foundation. Measure the porosity of the core samples of the foundation of each layer of the dissolution and fracture zone where the entire pile body is located after the pile foundation is installed, and calculate the ultimate side resistance influence coefficient of the foundation of each layer of the dissolution and fracture zone where the entire pile body is located after the pile foundation is installed and the allowable bearing capacity influence coefficient of the foundation of the dissolution and fracture zone at the bottom end position after the pile foundation is installed; the calculation process is shown in Formula 1 and Formula 2;

[0013] Formula 1: λ ni = f' ni / f ni ≈ e ni0 / e ni ;

[0014] Formula 2:

[0015] Among them, λ ni is the ultimate side resistance influence coefficient of the foundation of the i-th layer of the dissolution and fracture zone;

[0016] ζ ni is the allowable bearing capacity influence coefficient of the foundation of the i-th layer of the dissolution and fracture zone;

[0017] f' ni is the ultimate side resistance (kPa) at the end of the service life of the foundation of the i-th layer of the dissolution and fracture zone;

[0018] σ' ni is the allowable bearing capacity (kPa) at the end of the service life of the foundation of the i-th layer of the dissolution and fracture zone;

[0019] e ni is the porosity at the end of the service life of the foundation of the i-th layer of the dissolution and fracture zone;

[0020] k n is the influence coefficient of the bearing porosity of the fracture zone foundation;

[0021] i is the number of layers, and i is a positive integer greater than or equal to 1; n takes 1, 2, or 3; n = 1 represents the dissolution and fracture zone within the first region, n = 2 represents the dissolution and fracture zone within the second region, and n = 3 represents the dissolution and fracture zone within the third region;

[0022] Step 4: Calculate the bearing capacity of the pile foundation; specifically as follows:

[0023] When l ≤ L 1 :

[0024] When L 1 <l ≤ L 1 +L 2 :

[0025]

[0026] When L 1 +L 2 <l:

[0027]

[0028] Where l is the length (m) of the area where the pile foundation extends into the karst dissolution and fractured zone after installation; [P] is the bearing capacity (kN) of a single pile foundation; U is the perimeter (m) of the pile body cross-section; l 1i , l 2i and l 3i are respectively the thicknesses of different layers of the karst dissolution and fractured zone within the first area, the thicknesses of different layers of the karst dissolution and fractured zone within the second area, and the thicknesses of different layers of the karst dissolution and fractured zone within the third area (m); m 0 is the reduction coefficient of the pile tip bearing force; A is the area of the pile tip cross-section (m 2 ).

[0029] The design method of the bearing capacity of the pile foundation structure in the karst area erosion and fracture zone provided by the present application, firstly, the constant water level line and the water fluctuation influence line in the erosion and fracture zone of the project area to be tested, the position of the pile foundation is designed, and the foundation of the installation design position of the pile foundation is drilled and cored, and the initial allowable bearing capacity of the position where the bottom end of the designed installation pile foundation is located and the initial limit lateral resistance of the side where the designed installation pile foundation is located are calculated according to the coring sample; at the same time, in order to improve the test accuracy, the erosion and fracture zone around the pile foundation is layered and calculated according to the conditions of the soil layer. Then, the rainfall environment within the service life of the pile foundation is simulated, and the coring around the pile foundation is subjected to environmental impact treatment, and then the porosity of the coring sample is measured to obtain the influence coefficient of the limit lateral resistance of the pile foundation in each layer of the erosion and fracture zone foundation and the allowable bearing capacity influence coefficient at the position where the bottom end of the pile foundation is located, and then the relationship between the length of the pile foundation and the constant water level and the fluctuating water level is used to calculate the bearing capacity of the pile foundation. On the one hand, the design method of the present application fully considers the long-term influence factors of the erosion and fracture zone above the influence range of the fluctuating water level, the influence range of the fluctuating water level or the normal water level on the end bearing capacity of the pile foundation structure and the pile foundation structure pile side resistance, effectively improving the accuracy of the bearing capacity of the pile foundation structure with a long service life and strict deformation requirements, and ensuring the use safety of the pile foundation structure in the special geomorphic environment of the erosion and fracture zone in the karst area. On the other hand, the present application simulates the environmental test treatment of the core samples taken from the erosion and fracture zone to obtain the porosity change at the end of the service life, and deduce the degree of influence on the end bearing capacity of the pile foundation and the pile foundation side resistance. The test method is simple to operate, solves the problems of difficulty and high cost in testing the pile end bearing capacity and pile side resistance of the test samples, and is easy to promote and apply.

[0030] Furthermore, in step 2, the porosity difference between adjacent layers in the dissolution and fracture zone within the first area, the dissolution and fracture zone within the second area, and the dissolution and fracture zone within the third area is ≥2%, and the soil content is ≥5%.

[0031] Furthermore, in step 3, the rainfall environment within the service life of the pile foundation includes at least one of carbon dioxide concentration, annual rainfall, hourly rainfall, rainfall frequency and groundwater fluctuation frequency.

[0032] Among them, the carbon dioxide concentration is collected based on the data of the environmental department in the project area, the annual rainfall and hourly rainfall are obtained based on the rainfall statistics of the local meteorological department, and the number of rainfalls and groundwater fluctuations are obtained based on the heavy rain statistics of the meteorological department and the values ​​are consistent.

[0033] Fluctuating water levels have nothing to do with rainfall or hourly rainfall, but only with the number and duration of action and CO 2 The environment is related; below the water level it is only related to CO 2 Concentration related.

[0034] Further, in step 3, in formula two, when n equals 1, k 1 is 0.95 to 0.98. For example, k 1 is 0.95, 0.96, 0.97, or 0.98.

[0035] Further, in step 3, in formula two, when n equals 2, k 2 is 0.92 to 0.95. For example, k 2 is 0.92, 0.93, 0.94, or 0.95.

[0036] Further, in step 3, in formula two, when n equals 3, k 3 is 0.92 to 0.94.

[0037] k 1 、k 2 、k 2 are obtained through numerical simulation.

[0038] Another object of the present invention is to provide an application of the above design method for the bearing capacity of the pile foundation structure.

[0039] The application of the above design method for the bearing capacity of the pile foundation in designing the bearing capacity of the pile foundation in the karst dissolution and fracture zone.

[0040] In summary, due to the adoption of the above technical solutions, the beneficial effects of the present invention are:

[0041] The design method for the bearing capacity of pile foundations in karst areas' dissolution and fragmentation zones provided by this application first measures the normal water level line and the water fluctuation influence line in the dissolution and fragmentation zone of the engineering area to be tested, designs the positions for installing pile foundations, and drills and cores the foundation at the designed installation positions of the pile foundations. Based on the core samples, the initial allowable bearing capacity at the bottom of the designed installation position of the pile foundation and the initial ultimate lateral resistance on the side of the designed installation position of the pile foundation are calculated. At the same time, to improve the test accuracy, the dissolution and fragmentation zones around the pile foundation are calculated and processed in layers according to the soil layer conditions. Then, the rainfall environment within the service life of the pile foundation is simulated, the cores around the pile foundation are treated for environmental impacts, and the porosity of the core samples is measured to obtain the influence coefficient of the ultimate lateral resistance of the pile foundation in the foundation of each layer of the dissolution and fragmentation zone and the influence coefficient of the allowable bearing capacity at the position where the bottom of the pile foundation is located. Then, based on the relationship between the length of the pile foundation and the normal water level and the fluctuating water level, the bearing capacity of the pile foundation is calculated. On the one hand, the design method of this application fully considers the long-term influencing factors of the dissolution and fragmentation zone above the influence range of the fluctuating water level, within the influence range of the fluctuating water level, or within the normal water level range on the end bearing capacity of the pile foundation structure and the lateral resistance of the pile circumference of the pile foundation structure, effectively improving the accuracy of the bearing capacity of pile foundation structures with long service life and strict deformation requirements, and ensuring the use safety of pile foundation structures in the special geomorphic environment of karst areas' dissolution and fragmentation zones. On the other hand, through the simulation of environmental tests on the core samples of the dissolution and fragmentation zone, this application obtains the change in porosity at the end of the service life, and infers the influence degree on the end bearing capacity of the pile foundation and the lateral resistance of the pile circumference. This test method is simple to operate, solves the problems of difficult testing and high cost of the pile end bearing capacity and the lateral resistance of the pile circumference of test samples, and is convenient for popularization and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a schematic diagram of the structural relationship between the foundation in the karst area's dissolution and fragmentation zone and the pile foundation in Embodiment 1.

[0043] Figure 2 It is a schematic diagram of the structural relationship between the foundation in the karst area's dissolution and fragmentation zone and the pile foundation in Embodiment 2.

[0044] Figure 3 It is a schematic diagram of the structural relationship between the foundation in the karst area's dissolution and fragmentation zone and the pile foundation in Embodiment 3.

[0045] Reference Signs: 1 - Dissolution and fragmentation zone; 11 - First area; 12 - Second area; 13 - Third area; 2 - Normal water level line; 3 - Water level fluctuation influence line; 4 - Pile foundation. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0046] The present invention will be described in detail below with reference to the accompanying drawings.

[0047] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0048] Embodiment 1:

[0049] (1) Determine the position of the normal water level line 2 and the position of the water level fluctuation influence line 3 in the karst dissolution and fracture zone 1 during a certain karst investigation process. The area of the karst dissolution and fracture zone 1 above the water level fluctuation influence line 3 is the first area 11, and the height of the first area is denoted as l1 = 15 m. The area between the normal water level line 2 and the water level fluctuation influence line 3 is the second area 12, and the height of the second area is denoted as l2 = 3 m; the area of the karst dissolution and fracture zone 1 below the normal water level line 2 is the third area 13.

[0050] The length of a certain pile foundation extending into the karst dissolution and fracture zone after installation is l = 25 m, and the entire pile body involves the first area 11, the second area 12 and the third area 13.

[0051] The length of the pile body in the first area 11 is 15 m, the length in the second area 12 is 3 m, and the length of the third area is 7 m.

[0052] (2) Drill cores in the 25 m karst dissolution and fracture zone area where the pile foundation extends. The pile foundation is in the first area, the second area and the third area. According to the porosity and soil content of the core samples, the first area 11 is divided into two layers, l 11 layer depth 7 m, l 12 layer depth 8 m. The contact depths of the pile bodies in the second area 12 and the third area 13 with the karst dissolution and fracture zone are considered as one layer of foundation. The designed service life of the high-speed railway is 100 years, and the pile diameter is 1.0 m. Therefore, l 21 layer depth is 3 m, l 31 layer depth is 7 m, and the average annual rainfall is 800 mm. Specifically: the initial porosities of the shallow layer 7 m and the lower layer 8 m in the first area 11 are 22% and 21% respectively, and the soil contents are 17% and 11% respectively. The initial porosities of the 3 m in the second area and the 7 m in the third area are 20% and 20% respectively, and the soil contents are 9% and 10% respectively. Therefore, the first area is divided into two layers according to the soil content exceeding 5%.

[0053] (3) Obtain the initial allowable bearing capacity and ultimate side resistance of each stratified karst dissolution and fracture zone foundation through indoor geotechnical tests.

[0054] Since the bottom of the pile foundation structure is placed below the normal water level of the karst dissolution and fracture zone foundation, in this step, only the initial ultimate side resistance of the karst dissolution and fracture zone foundation above the fluctuation water level influence range, the fluctuation water level range and the normal water level range, and the initial allowable bearing capacity of the normal water level foundation need to be obtained. The initial porosity of the karst dissolution and fracture zone foundation is obtained through indoor tests.

[0055] f 11 = 80 kPa, f 12 = 90 kPa, f 21 = 110 kPa, f 31 = 120 kPa;

[0056] σ 31 = 1100 kPa;

[0057] e 110 = 22%, e 120 = 21%, e 210 = 20%, e 310 = 20%.

[0058] (3) Core samples are taken from the karst fractured zone above the fluctuating water level. Simulate 5000 times of rainfall with a CO2 concentration of 500 PPM, an annual rainfall of 800 mm, and an hourly rainfall of 16 mm to obtain the sample porosity at the end of 100 years.

[0059] e 11 = 25%, e 12 = 23%;

[0060] λ 11 = 22 / 25 = 0.88;

[0061] λ 12 = 21 / 23 = 0.91.

[0062] (4) Core samples are taken from the karst fractured zone of the fluctuating water level. Simulate 5000 times of groundwater fluctuations in 100 years under the condition of a CO2 concentration of 500 PPM to obtain the sample porosity at the end of the service life.

[0063] e 21 = 23%;

[0064] λ 21 = 20 / 23 = 0.87.

[0065] (5) Core samples are taken from the karst fractured zone of the constant water level. Conduct an experiment on the influence of the CO2 content in water on the fractured zone to obtain the sample porosity at the end of 100 years.

[0066] e 31 = 22%;

[0067] λ 31 = 20 / 23 = 0.91;

[0068]

[0069] (6) Calculate the allowable bearing capacity of a single pile of the pile foundation structure.

[0070] Due to L 1 +L 2 <l, the entire pile body involves the first region 11, the second region 12, and the third region 13:

[0071]

[0072] If the long-term deterioration effect of the karst dissolution and fracture zone is not considered traditionally, then

[0073]

[0074]

[0075] Considering the long-term deterioration effect of the karst dissolution and fracture zone, the actual bearing capacity of the high-speed railway pile foundation is reduced by 11.2%.

[0076] Embodiment 2

[0077] The karst dissolution and fracture zone 1 faced by Embodiment 2 is the same as the environment of Embodiment 1, and the difference lies in the different installation depths of the pile foundation. As Figure 2 shown, determine the position of the normal water level line 2 and the position of the water level fluctuation influence line 3 in the karst dissolution and fracture zone 1 in a certain karst investigation process area. The area of the karst dissolution and fracture zone 1 above the water level fluctuation influence line 3 is the first region 11, and the height of the first region is denoted as l1 = 15m. The area between the normal water level line 2 and the water level fluctuation influence line 3 is the second region 12, and the height of the second region is denoted as l2 = 3m; the area of the karst dissolution and fracture zone 1 below the normal water level line 2 is the third region 13.

[0078] The length of a certain pile foundation extending into the karst dissolution and fracture zone after installation is l = 17m, and the entire pile body involves the first region 11 and the second region 12.

[0079] The length of the pile body in the first region 11 is 15m, and the length in the second region 12 is 2m.

[0080] (2) Core drilling is carried out on the 17m karst dissolution and fracture zone area where the pile foundation extends. The pile foundation is in the first region and the second region. According to the porosity and soil content of the core samples, the first region is divided into two layers, l 11 layer depth 7m, l 12 layer depth 8m. The contact depth of the pile body in the second region with the karst dissolution and fracture zone is considered as a single-layer foundation. The design service life of the high-speed railway is 100 years, the pile diameter is 1.0m, so l 21 layer depth is 2m, and the average annual rainfall is 800 mm.

[0081] (3) Obtain the initial allowable bearing capacity and ultimate side resistance of the karst dissolution and fracture zone foundation for each layer through indoor geotechnical tests.

[0082] Since the bottom of the pile foundation structure is placed within the fluctuating water level range of the karst fractured zone foundation, in this step, only the initial ultimate lateral resistance of the karst fractured zone foundation above the influence range of the fluctuating water level and the foundation within the fluctuating water level range, as well as the initial allowable bearing capacity of the foundation at the fluctuating water level, need to be obtained. The initial porosity of the karst fractured zone foundation is obtained through laboratory tests.

[0083] f 11 = 80 kPa, f 12 = 90 kPa, f 21 = 110 kPa

[0084] σ 21 = 980 kPa

[0085] e 110 = 22%, e 120 = 21%, e 210 = 20%

[0086] (3) For the core samples of the karst fractured zone above the fluctuating water level, simulate the rainfall action 5000 times with a CO2 concentration of 500 PPM, an annual rainfall of 800 mm, and an hourly rainfall of 16 mm to obtain the sample porosity at the end of 100 years.

[0087] e 11 = 25%, e 12 = 23%

[0088] λ 11 = 22 / 25 = 0.88

[0089] λ 12 = 21 / 23 = 0.91

[0090] (4) For the core samples of the karst fractured zone at the fluctuating water level, simulate 5000 times of groundwater fluctuations in 100 years under the condition of a CO2 concentration of 500 PPM to obtain the sample porosity at the end of the service life.

[0091] e 21 = 23%

[0092] λ 21 = 20 / 23 = 0.87

[0093]

[0094] (5) Calculate the allowable bearing capacity of a single pile of the pile foundation structure.

[0095] Since L 1 <l ≤ L 1 + L 2 : The entire pile body only involves the first region 11 and the second region 12;

[0096]

[0097] Without considering the long-term deterioration effect of the dissolution fracture zone, then

[0098]

[0099] Considering the long-term deterioration effect of the dissolution fracture zone, the actual bearing capacity of the high-speed rail pile foundation is reduced by 13.0%.

[0100] Example 3

[0101] The dissolution fracture zone 1 faced by Example 3 is the same as the environment of Example 1, the difference is that the installation depth of the pile foundation is different. As Figure 3 shown, determine the position of the normal water level line 2 and the position of the water level fluctuation influence line 3 in the dissolution fracture zone 1 of a certain karst investigation area. The area of the dissolution fracture zone 1 above the water level fluctuation influence line 3 is the first area 11, and the height of the first area is recorded as l1 = 15m. The area between the normal water level line 2 and the water level fluctuation influence line 3 is the second area 12, and the height of the second area is recorded as l2 = 3m; the area of the dissolution fracture zone 1 below the normal water level line 2 is the third area 13.

[0102] The length of a certain pile foundation extending into the dissolution fracture zone after installation is l = 14m, and the entire pile body only involves the first area 11.

[0103] The length of the pile body located in the first area 11 is 14m.

[0104] (2) Core drilling is carried out on the 14m dissolution fracture zone area where the pile foundation extends. The pile foundation is in the first area and the second area. According to the porosity and soil content of the core samples, the first area is divided into two layers, l 11 layer depth 7m, l 12 layer depth 7m. The design service life of the high-speed railway is 100 years, the pile diameter is 1.0m, and the average annual rainfall is 800 mm.

[0105] (3) Obtain the initial allowable bearing capacity and ultimate side resistance of the foundation of each stratified dissolution fracture zone through indoor geotechnical tests.

[0106] Since the bottom of the pile foundation structure is placed above the fluctuating water level of the foundation of the dissolution fracture zone, this step only needs to obtain the initial ultimate side resistance and initial allowable bearing capacity of the foundation of the dissolution fracture zone above the influence range of the fluctuating water level, and obtain the initial porosity of the foundation of the dissolution fracture zone through indoor tests.

[0107] f 11 = 80kPa, f 12 = 90kPa

[0108] σ 12 = 960kPa

[0109] e 110 = 22%, e 120 = 21%

[0110] (3) For the cored samples of the karst fractured zone above the fluctuating water level, simulate the rainfall effect 5000 times with a CO2 concentration of 500 PPM, an annual rainfall of 800 mm, and an hourly rainfall of 16 mm, and obtain the porosity of the samples at the end of 100 years.

[0111] e 11 = 25%, e 12 = 23%

[0112] λ 11 = 22 / 25 = 0.88

[0113] λ 12 = 21 / 23 = 0.91

[0114]

[0115] (4) Calculation of the allowable bearing capacity of a single pile of the pile foundation structure.

[0116] Since l ≤ L 1 : The entire pile body only involves the first region 11.

[0117]

[0118] If the long-term deterioration effect of the karst fractured zone is not considered traditionally, then

[0119]

[0120] Considering the long-term deterioration effect of the karst fractured zone, the actual bearing capacity of the high-speed rail pile foundation is reduced by 11.4%.

[0121] The design method for the bearing capacity of the pile foundation structure in the karst fractured zone provided by this application, on the one hand, fully considers the long-term influence factors of the karst fractured zone above the fluctuating water level influence range, within the fluctuating water level influence range or at the normal water level range on the end bearing capacity of the pile foundation structure and the side resistance of the pile circumference of the pile foundation structure, effectively improves the accuracy of the bearing capacity of the pile foundation structure with a long service life and strict deformation requirements, and ensures the use safety of the pile foundation structure in the special geomorphic environment of the karst fractured zone in the karst area. On the other hand, through the simulation environmental test treatment of the cored samples of the karst fractured zone, the porosity change at the end of the service life is obtained, and the influence degree on the end bearing capacity of the pile foundation and the side resistance of the pile circumference is deduced. This test method is simple to operate, solves the problems of difficult testing and high cost of the end bearing capacity of the test samples and the side resistance of the pile circumference, and is convenient for popularization and application.

[0122] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A design method for the bearing capacity of pile foundations in karst areas with solution and fractured zones, characterized in that, it includes the following steps: Step 1: Determine the position of the constant water level line (2) and the position of the water level fluctuation influence line (3) in the erosion and fracture zone (1) of the investigation project area. The erosion and fracture zone (1) area above the water level fluctuation influence line (3) is recorded as the first area (11), the water level fluctuation influence area is recorded as the second area (12), and the erosion and fracture zone (1) area below the constant water level line (2) is recorded as the third area (13). The height of the first area is L 1 , the height of the second region (12) is L 2 ; Step 2: Record the depth of the pile foundation (4) extending into the solution and fractured zone (1) after installation as l, and drill and take cores from the area to be extended; stratify the core samples of the first area (11) according to the porosity and soil content of the solution and fractured zone. If any, stratify the core samples of the second area (12). If any, stratify the core samples of the third area (13); and based on the core samples, test the initial allowable bearing capacity of the foundation of the solution and fractured zone (1) at the position where the bottom end of the pile foundation (4) is located after installation, the initial ultimate side resistance of the foundation of each layer of the solution and fractured zone (1) where the entire pile body of the pile foundation (4) is located after installation, and the initial porosity of the foundation of each layer of the solution and fractured zone (1) where the entire pile body of the pile foundation (4) is located after installation; Among them, σ ni is the initial allowable bearing capacity (kPa) of the foundation of the i-th layer of karst fractured zone; f ni is the initial ultimate lateral resistance (kPa) of the foundation of the i-th layer of solution-corroded and fractured zone; e ni0 is the initial porosity of the foundation of the i-th layer of solution-corroded and fractured zone i is the number of layers, and i is a positive integer ≥ 1; n takes 1, 2, or 3; n = 1 represents the solution and fractured zone within the first area, n = 2 represents the solution and fractured zone within the second area, and n = 3 represents the solution and fractured zone within the third area; Step 3: Simulate the rainfall environment during the service life of the pile foundation (4), measure the porosity of the core samples of the foundation of each layer of the solution and fractured zone (1) where the entire pile body of the pile foundation (4) is located after installation, and calculate the influence coefficient of the ultimate side resistance of the foundation of each layer of the solution and fractured zone (1) where the entire pile body of the pile foundation (4) is located after installation and the influence coefficient of the allowable bearing capacity of the foundation of the solution and fractured zone (1) at the position where the bottom end of the pile foundation (4) is located after installation; the calculation process is shown in Formula 1 and Formula 2; Formula 1: λ ni = f' ni / f ni ≈ e ni0 / e ni ; Formula 2: Among them, λ ni is the influence coefficient of the ultimate lateral resistance of the foundation in the i-th layer of solution-corroded and fractured zone; ζ ni is the allowable bearing capacity influence coefficient of the foundation of the i-th layer of solution fractured zone; f' ni is the ultimate side resistance (kPa) at the end of the service life of the foundation of the i-th layer of karst fractured zone σ' ni is the allowable bearing capacity (kPa) at the end of the service life of the foundation of the i-th layer of karst fractured zone e ni is the porosity at the end of the service life of the foundation of the i-th layer of solution-corroded and fractured zone; k n Coefficient of influence of porosity on foundation bearing capacity of fracture zone i is the number of layers, and i is a positive integer ≥ 1; n takes 1, 2, or 3; n = 1 represents the solution and fractured zone within the first area, n = 2 represents the solution and fractured zone within the second area, and n = 3 represents the solution and fractured zone within the third area; Step 4: Calculate the bearing capacity of the pile foundation (4); specifically as follows: When l ≤ L 1 : When L 1 <l ≤ L 1 + L 2 : When L 1 +L 2 <l: In the formula, l is the length (m) of the area where the pile foundation extends into the karst corrosion and fracture zone after installation; [P] is the bearing capacity (kN) of a single pile foundation; U is the perimeter (m) of the pile body cross-section; l 1i , l 2i and l 3i are respectively the thicknesses (m) of different layers of the karst corrosion and fracture zone within the first area, the thicknesses of different layers of the karst corrosion and fracture zone within the second area, and the thicknesses of different layers of the karst corrosion and fracture zone within the third area; m 0 is the reduction coefficient of the pile bottom bearing force; A is the area (m 2 ²).

2. The design method for the bearing capacity of pile foundations in karst areas with solution and fractured zones according to claim 1, characterized in that, in the said Step 2, the difference in porosity between adjacent layers in the solution and fractured zone within the first area, the solution and fractured zone within the second area, and the solution and fractured zone within the third area is ≥ 2%, and the soil content is ≥ 5%.

3. The design method for the bearing capacity of pile foundations in karst areas with solution and fractured zones according to claim 2, characterized in that, in the said Step 3, the rainfall environment during the service life of the pile foundation (4) includes at least one of carbon dioxide concentration, annual rainfall, hourly rainfall, number of rainfall events, and number of groundwater fluctuations.

4. The design method for the bearing capacity of pile foundations in karst areas with solution and fractured zones according to any one of claims 1 - 3, characterized in that, In step 3, in formula two, when n equals 1, k 1 is 0.95 to 0.

98.

5. The design method for the bearing capacity of pile foundations in karst areas with solution and fractured zones according to claim 4, characterized in that, In step 3, in formula two, when n is equal to 2, k 2 is 0.92 to 0.

95.

6. The design method for the bearing capacity of pile foundations in karst areas with solution and fractured zones according to claim 5, characterized in that, In step 3, in formula two, when n is equal to 3, k 3 is 0.92 to 0.

94.

7. Application of the design method for pile foundation bearing capacity according to any one of claims 1 - 6 in designing the pile foundation bearing capacity within the karst corrosion and fracture zone.

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