A bridge foundation structure and design method suitable for a string-bead karst cave

Abstract

The present application relates to the technical field of pile foundation, in particular to a hierarchical stress bridge foundation structure suitable for string-bead karst cave and a design method thereof, comprising a large-diameter concrete cast-in-place pile and a small-diameter steel pipe concrete micro pile, the large-diameter concrete cast-in-place pile is cast in the first layer of stable rock stratum of the string-bead karst cave and is used for bearing the bending moment transmitted by the structure, the small-diameter steel pipe concrete micro pile is formed in the string-bead karst cave and the top end of the small-diameter steel pipe concrete micro pile is formed in the large-diameter concrete cast-in-place pile, the pile foundation is divided into two levels of bearing structure, the upper large-diameter pile fully plays the side friction and end resistance in the complete rock stratum section and bears the main bending moment and horizontal force, and the lower small-diameter micro pile only bears the remaining axial force, the stress is clear and the reinforcement is reasonable. Compared with the traditional constant cross-section pile scheme, the amount of concrete and the amount of steel reinforcement can be reduced.

Description

Technical Field

[0001] This invention relates to the field of pile foundation technology, specifically to a bridge foundation structure and design method applicable to graded stress in beaded karst caves. Background Technology

[0002] When constructing bridges in karst regions, pile foundations often encounter multiple layers of beaded karst caves. Beaded karst caves refer to multiple layers of caves arranged vertically in a beaded pattern, characterized by alternating top and bottom slabs, with each layer separated by rock slabs of varying thickness. These geological conditions present a series of engineering challenges for pile foundations when traversing karst zones, including insufficient bearing capacity, difficulties in drilling, and severe over-pouring of concrete. Traditional methods include full-depth steel casing, grouting and filling of the caves, and backfilling with rubble and clay, but all suffer from drawbacks such as high construction costs, long construction periods, and difficulty in quality control. Therefore, this paper proposes a bridge foundation structure and design method suitable for graded stress in beaded karst caves. Summary of the Invention

[0003] In order to solve the technical problems existing in the prior art, the present invention provides a bridge foundation structure and design method applicable to graded stress in beaded karst caves.

[0004] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a bridge foundation structure suitable for graded stress in beaded karst caves, comprising large-diameter cast-in-place concrete piles and small-diameter steel-concrete micropiles;

[0005] The large-diameter concrete cast-in-place piles are cast in the first stable rock layer of the beaded karst cave to bear the bending moment transmitted by the structure.

[0006] The small-diameter steel-concrete micropile is formed in the beaded karst cave passage, and the top of the small-diameter steel-concrete micropile is formed inside the large-diameter concrete cast-in-place pile.

[0007] Preferably, the large-diameter concrete cast-in-place pile is formed by reinforcing cage and concrete pouring, the diameter of the large-diameter concrete cast-in-place pile is 1.5~2.5m, and the thickness of the pile bottom embedded in the first stable rock layer is greater than 0.5m.

[0008] Preferably, the depth by which the top of the small-diameter steel pipe concrete micropile extends into the large-diameter concrete cast-in-place pile is greater than 0.5m.

[0009] Preferably, the small-diameter steel pipe concrete micropiles are formed by casting steel pipes and concrete, the number of small-diameter steel pipe concrete micropiles is not less than 5, the diameter of the small-diameter steel pipe concrete micropiles is greater than 0.3m, and the thickness of the bottom of the small-diameter steel pipe concrete micropiles embedded in the bottom layer of the beaded karst cave is greater than 0.5m.

[0010] A design method for bridge foundation structures subjected to graded stresses in beaded karst caves includes the following steps:

[0011] Step S1: Determine the bearing capacity of the large-diameter cast-in-place concrete pile. ;

[0012] ;

[0013] In the formula, The lateral friction of the soil layer overlying the beaded caves The end resistance of the soil layer overlying the beaded karst caves;

[0014] Determine the bearing capacity of a single small-diameter steel-concrete micropile. ;

[0015] ;

[0016] In the formula, It is the sum of the lateral frictional resistance of the rock slabs in each layer of the beaded cave. The end resistance of the stable rock layer at the bottom of the beaded cave;

[0017] Step S2, based on the bearing capacity of the large-diameter cast-in-place concrete pile Determine whether to install small-diameter steel pipe concrete micropiles;

[0018] ;

[0019] In the formula, For vertical forces that require small-diameter steel-concrete micropiles to bear, Total design load;

[0020] when When the value is ≥0, small-diameter steel pipe concrete micropiles need to be installed;

[0021] when When the value is less than 0, there is no need to install small-diameter steel pipe concrete micropiles;

[0022] Step S3: When setting small-diameter steel-concrete micropiles, determine the number of small-diameter steel-concrete micropiles. :

[0023] ;

[0024] Step S4, based on the number of small-diameter steel-concrete micropiles Calculate the specifications of the steel pipes used in small-diameter steel pipe concrete micropiles.

[0025] Preferably, in step S1, the vertical ultimate bearing capacity of a single small-diameter steel-concrete micropiles is... The formula for calculation is:

[0026] ;

[0027] In the formula, The outer diameter of the steel pipe. Let be the standard value of the ultimate side friction resistance of the i-th soil / rock layer. The length of the small-diameter steel-concrete micropiles in the i-th soil / rock layer is taken as the length of the karst cave section. , This is the standard value of the ultimate end resistance of the pile. The cross-sectional area of ​​the steel-concrete composite pile is... denoted as the standard value of the side friction resistance of the i-th soil / rock layer.

[0028] Preferably, in step S4, the specifications of the steel pipe used for the small-diameter steel-concrete micropiles include: outer diameter of the steel pipe. and steel pipe wall thickness ;

[0029] The outer diameter of the steel pipe used and steel pipe wall thickness Both stiffness and strength conditions must be met simultaneously.

[0030] Preferably, in step S4, the outer diameter of the steel pipe that meets the stiffness requirement... and steel pipe wall thickness The method for determining the relationship is as follows:

[0031] Establishing the equivalent flexural stiffness of small-diameter steel-concrete micropiles The calculation formula:

[0032] ;

[0033] In the formula, The elastic modulus of steel. This refers to the elastic modulus of concrete. Let the moment of inertia of the steel pipe section be . The moment of inertia of the concrete core;

[0034] The moment of inertia of the steel pipe section and concrete core moment of inertia Substitute the equivalent bending stiffness From the calculation formula, we obtain the equivalent bending stiffness. The computational expansion:

[0035] ;

[0036] In the formula, For the steel pipe wall thickness;

[0037] Establish equivalent axial stiffness The calculation formula:

[0038] ;

[0039] In the formula, For the area of ​​the steel pipe, The area of ​​the concrete core;

[0040] Based on equivalent bending stiffness and equivalent axial stiffness Establish equivalent radius of gyration The calculation formula:

[0041] ;

[0042] Based on equivalent radius of gyration Calculating the height of caves Establish slenderness ratio The calculation formula:

[0043] ;

[0044] In the formula, The ratio of elastic modulus;

[0045] Stiffness requirements require slenderness ratio Not exceeding the allowable slenderness ratio :

[0046] ;

[0047] In the formula, the allowable aspect ratio is... The value range is 80~100;

[0048] After unfolding, the constraint relationship between the outer diameter d and the wall thickness t of the steel pipe that satisfies the stiffness condition is obtained as follows:

[0049] .

[0050] Preferably, in step S4, the outer diameter of the steel pipe that meets the strength condition is... and steel pipe wall thickness The method for determining the relationship is as follows:

[0051] Strength requirements axial force Not exceeding the design bearing capacity ,Right now:

[0052] ;

[0053] Design bearing capacity The formula for calculation is:

[0054] ;

[0055] In the formula, For the clamping coefficient, For axial compression bearing capacity, The stability coefficient;

[0056] but:

[0057] ;

[0058] After unfolding, the constraint relationship between the outer diameter d and the wall thickness t of the steel pipe that satisfies the strength condition is obtained as follows:

[0059] .

[0060] Preferably, the outer diameter d and wall thickness t of the steel pipe are obtained by trial and error based on the constraint relationship between the outer diameter d and wall thickness t of the steel pipe that meets the strength condition.

[0061] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0062] 1. This invention divides the pile foundation into upper and lower load-bearing structures. The upper large-diameter piles fully utilize lateral friction and end resistance in intact rock strata to bear the main bending moment and horizontal force; the lower small-diameter micropiles only bear the remaining axial force, resulting in clear stress distribution and reasonable reinforcement. Compared with traditional uniform cross-section pile schemes, this reduces the amount of concrete and steel reinforcement used in the pile body. Attached Figure Description

[0063] Figure 1 This is a schematic diagram of the structure of the present invention;

[0064] Figure 2 This is a schematic diagram of the construction state of the present invention;

[0065] Figure 3 This is a top sectional view of the present invention.

[0066] Figure 4 This is a schematic diagram of the small-diameter steel tube concrete micropile structure of the present invention.

[0067] The numbers in the image represent:

[0068] 1. Large-diameter cast-in-place concrete piles; 2. Small-diameter steel-concrete micropiles. Detailed Implementation

[0069] The present invention will be further described below with reference to the accompanying drawings and embodiments, which illustrate the above and other technical features and advantages of the present invention. However, the following embodiments are merely preferred embodiments of the present invention and are not exhaustive.

[0070] Example:

[0071] like Figure 1-4 As shown, the present invention provides a graded stress bridge foundation structure suitable for beaded karst caves, including large-diameter cast-in-place concrete piles 1 and small-diameter steel-concrete micropiles 2.

[0072] Large-diameter concrete cast-in-place pile 1 is cast and formed in the first stable rock layer of the beaded karst cave to bear the bending moment transmitted by the structure;

[0073] Small-diameter steel-concrete micropiles 2 are formed in the beaded karst cave passage, and the top of the small-diameter steel-concrete micropiles 2 is formed inside the large-diameter concrete cast-in-place pile 1.

[0074] The large-diameter concrete cast-in-place pile 1 is formed by reinforcing cage and concrete pouring. The diameter of the large-diameter concrete cast-in-place pile 1 is 1.5~2.5m. The thickness of the first stable rock layer embedded in the bottom of the large-diameter concrete cast-in-place pile 1 is greater than 0.5m.

[0075] The top of the small-diameter steel pipe concrete micropile 2 extends into the large-diameter concrete cast-in-place pile 1 to a depth greater than 0.5m.

[0076] Small-diameter steel pipe concrete micropiles 2 are formed by casting steel pipes and concrete. The number of small-diameter steel pipe concrete micropiles 2 is not less than 5. The diameter of the small-diameter steel pipe concrete micropiles 2 is greater than 0.3m. The thickness of the bottom of the small-diameter steel pipe concrete micropiles 2 embedded in the bottom stable rock layer of the beaded karst cave is greater than 0.5m.

[0077] A design method for bridge foundation structures subjected to graded stresses in beaded karst caves includes the following steps:

[0078] Step S1: Determine the bearing capacity of the large-diameter concrete cast-in-place pile 1. ;

[0079] ;

[0080] In the formula, The lateral friction of the soil layer overlying the beaded caves The end resistance of the soil layer overlying the beaded karst caves;

[0081] Determine the bearing capacity of a single small-diameter steel-concrete micropile 2 ;

[0082] ;

[0083] In the formula, It is the sum of the lateral frictional resistance of the rock slabs in each layer of the beaded cave. The end resistance of the stable rock layer at the bottom of the beaded cave;

[0084] Vertical ultimate bearing capacity of a single small-diameter steel-concrete micropile 2 The formula for calculation is:

[0085] ;

[0086] In the formula, The outer diameter of the steel pipe. Let be the standard value of the ultimate side friction resistance of the i-th soil / rock layer. The length of the small-diameter steel-concrete micropiles 2 in the i-th soil / rock layer is taken as the length of the karst cave section. , This is the standard value of the ultimate end resistance of the pile. The cross-sectional area of ​​the steel-concrete composite pile is... Let be the standard value of the side friction resistance of the i-th soil / rock layer;

[0087] Step S2, based on the bearing capacity of the large-diameter concrete cast-in-place pile 1 Determine whether to install small-diameter steel pipe concrete micropiles 2;

[0088] ;

[0089] In the formula, For vertical forces that require small-diameter steel-concrete micropiles to bear, Total design load;

[0090] when When the value is ≥0, small-diameter steel pipe concrete micropiles 2 need to be installed;

[0091] when When <0, there is no need to install small-diameter steel pipe concrete micropiles 2;

[0092] Step S3: When setting up the small-diameter steel-concrete micropiles 2, determine the number of small-diameter steel-concrete micropiles 2. :

[0093] ;

[0094] Step S4, based on the number of small-diameter steel-concrete micropiles 2 Calculate the specifications of the steel pipe used in the small-diameter steel pipe concrete micropile 2;

[0095] The specifications of the steel pipes used in small-diameter steel pipe concrete micropiles 2 include: outer diameter of the steel pipe. and steel pipe wall thickness ;

[0096] The outer diameter of the steel pipe used and steel pipe wall thickness The steel pipe needs to simultaneously meet both stiffness and strength requirements; the outer diameter of the steel pipe that meets the stiffness requirement is... and steel pipe wall thickness The method for determining the relationship is as follows:

[0097] Establish the equivalent bending stiffness of small-diameter steel-concrete micropiles 2 The calculation formula:

[0098] ;

[0099] In the formula, The elastic modulus of steel. This refers to the elastic modulus of concrete. Let the moment of inertia of the steel pipe section be . The moment of inertia of the concrete core;

[0100] The moment of inertia of the steel pipe section and concrete core moment of inertia Substitute the equivalent bending stiffness From the calculation formula, we obtain the equivalent bending stiffness. The computational expansion:

[0101] ;

[0102] In the formula, For the steel pipe wall thickness;

[0103] Establish equivalent axial stiffness The calculation formula:

[0104] ;

[0105] In the formula, For the area of ​​the steel pipe, The area of ​​the concrete core;

[0106] Based on equivalent bending stiffness and equivalent axial stiffness Establish equivalent radius of gyration The calculation formula:

[0107] ;

[0108] Based on equivalent radius of gyration Calculating the height of caves Establish slenderness ratio The calculation formula:

[0109] ;

[0110] In the formula, The ratio of elastic modulus;

[0111] Stiffness requirements require slenderness ratio Not exceeding the allowable slenderness ratio :

[0112] ;

[0113] In the formula, the allowable aspect ratio is... The value range is 80~100;

[0114] After unfolding, the constraint relationship between the outer diameter d and the wall thickness t of the steel pipe that satisfies the stiffness condition is obtained as follows:

[0115] .

[0116] Outer diameter of steel pipe that meets strength requirements and steel pipe wall thickness The method for determining the relationship is as follows:

[0117] Strength requirements axial force Not exceeding the design bearing capacity ,Right now:

[0118] ;

[0119] Design bearing capacity The formula for calculation is:

[0120] ;

[0121] In the formula, For the clamping coefficient, For axial compression bearing capacity, The stability coefficient;

[0122] but:

[0123] ;

[0124] After unfolding, the constraint relationship between the outer diameter d and the wall thickness t of the steel pipe that satisfies the strength condition is obtained as follows:

[0125] .

[0126] Based on the constraint relationship between the outer diameter d and the wall thickness t of the steel pipe that meets the strength condition, the outer diameter d and the wall thickness t of the steel pipe are obtained by trial and error.

[0127] The above description is merely a preferred embodiment of the present invention and is illustrative rather than restrictive. Those skilled in the art will understand that many changes, modifications, and even equivalents can be made within the spirit and scope defined by the claims of the present invention, all of which will fall within the protection scope of the present invention.

Claims

1. A bridge foundation structure suitable for graded stress in beaded karst caves, characterized in that, Including large-diameter cast-in-place concrete piles (1) and small-diameter steel-concrete micropiles (2); The large-diameter concrete cast-in-place pile (1) is cast in the first stable rock layer of the beaded karst cave to bear the bending moment transmitted by the structure. The small-diameter steel pipe concrete micropile (2) is formed in the beaded karst cave, and the top of the small-diameter steel pipe concrete micropile (2) is formed inside the large-diameter concrete cast-in-place pile (1).

2. A bridge foundation structure suitable for graded stress in beaded karst caves as described in claim 1, characterized in that, The large-diameter concrete cast-in-place pile (1) is formed by reinforcing cage and concrete pouring. The diameter of the large-diameter concrete cast-in-place pile (1) is 1.5~2.5m. The thickness of the first stable rock layer embedded in the bottom of the large-diameter concrete cast-in-place pile (1) is greater than 0.5m.

3. A bridge foundation structure suitable for graded stress in beaded karst caves as described in claim 1, characterized in that, The top of the small-diameter steel pipe concrete micropile (2) extends into the large-diameter concrete cast-in-place pile (1) to a depth greater than 0.5m.

4. A bridge foundation structure suitable for graded stress in beaded karst caves as described in claim 1, characterized in that, The small-diameter steel pipe concrete micropile (2) is formed by pouring steel pipe and concrete. The number of small-diameter steel pipe concrete micropile (2) is not less than 5. The diameter of the small-diameter steel pipe concrete micropile (2) is greater than 0.3m. The thickness of the bottom of the small-diameter steel pipe concrete micropile (2) embedded in the bottom stable rock layer of the beaded karst cave is greater than 0.5m.

5. A design method for a graded stress bridge foundation structure applicable to beaded karst caves as described in any one of claims 1-4, characterized in that, Includes the following steps: Step S1: Determine the bearing capacity of the large-diameter cast-in-place concrete pile (1). ; ； In the formula, The lateral friction of the soil layer overlying the beaded caves The end resistance of the soil layer overlying the beaded karst caves; Determine the bearing capacity of a single small-diameter steel-concrete micropile (2). ; ； In the formula, It is the sum of the lateral frictional resistance of the rock slabs in each layer of the beaded cave. The end resistance of the stable rock layer at the bottom of the beaded cave; Step S2, based on the bearing capacity of the large-diameter concrete cast-in-place pile (1) Determine whether to install small-diameter steel pipe concrete micropiles (2); ； In the formula, For vertical forces that require small-diameter steel-concrete micropiles to bear, Total design load; when When ≥0, small-diameter steel pipe concrete micropiles need to be installed (2); when When <0, there is no need to install small-diameter steel pipe concrete micropiles (2); Step S3: When setting up small-diameter steel-concrete micropiles (2), determine the number of small-diameter steel-concrete micropiles (2). : ； Step S4, based on the number of small-diameter steel-concrete micropiles (2) , calculate the specifications of the steel pipe used for the small-diameter steel pipe concrete micropile (2).

6. A bridge foundation structure suitable for graded stress in beaded karst caves as described in claim 5, characterized in that, In step S1, the vertical ultimate bearing capacity of a single small-diameter steel-concrete micropile (2) is... The formula for calculation is: ； In the formula, The outer diameter of the steel pipe. Let be the standard value of the ultimate side friction resistance of the i-th soil / rock layer. The length of the small-diameter steel-concrete micropiles (2) in the i-th soil / rock layer is taken as the length of the karst section. , This is the standard value of the ultimate end resistance of the pile. The cross-sectional area of ​​the steel-concrete composite pile is... denoted as the standard value of the side friction resistance of the i-th soil / rock layer.

7. A bridge foundation structure suitable for graded stress in beaded karst caves as described in claim 5, characterized in that, In step S4, the specifications of the steel pipe used for the small-diameter steel-concrete micropiles (2) include: outer diameter of the steel pipe. and steel pipe wall thickness ; The outer diameter of the steel pipe used and steel pipe wall thickness Both stiffness and strength conditions must be met simultaneously.

8. A bridge foundation structure suitable for graded stress in beaded karst caves as described in claim 7, characterized in that, In step S4, the outer diameter of the steel pipe that meets the stiffness condition and steel pipe wall thickness The method for determining the relationship is as follows: Establish the equivalent flexural stiffness of small-diameter steel-concrete micropiles (2). The calculation formula: ； In the formula, The elastic modulus of steel. This refers to the elastic modulus of concrete. Let the moment of inertia of the steel pipe section be . The moment of inertia of the concrete core; The moment of inertia of the steel pipe section and concrete core moment of inertia Substitute the equivalent bending stiffness From the calculation formula, we obtain the equivalent bending stiffness. The computational expansion: ； In the formula, For the steel pipe wall thickness; Establish equivalent axial stiffness The calculation formula: ； In the formula, For the area of ​​the steel pipe, The area of ​​the concrete core; Based on equivalent bending stiffness and equivalent axial stiffness Establish equivalent radius of gyration The calculation formula: ； Based on equivalent radius of gyration Calculating the height of caves Establish slenderness ratio The calculation formula: ； In the formula, The ratio of elastic modulus; Stiffness requirements require slenderness ratio Not exceeding the allowable slenderness ratio : ； In the formula, the allowable aspect ratio is... The value range is 80~100; After unfolding, the constraint relationship between the outer diameter d and the wall thickness t of the steel pipe that satisfies the stiffness condition is obtained as follows: 。 9. A bridge foundation structure suitable for graded stress in beaded karst caves as described in claim 7, characterized in that, In step S4, the outer diameter of the steel pipe that meets the strength condition and steel pipe wall thickness The method for determining the relationship is as follows: Strength requirements axial force Not exceeding the design bearing capacity ,Right now: ； Design bearing capacity The formula for calculation is: ； In the formula, For the clamping coefficient, For axial compression bearing capacity, The stability coefficient; but: ； After unfolding, the constraint relationship between the outer diameter d and the wall thickness t of the steel pipe that satisfies the strength condition is obtained as follows: 。 10. A bridge foundation structure suitable for graded stress in beaded karst caves as described in claim 9, characterized in that, In step S4, based on the constraint relationship between the outer diameter d and the wall thickness t of the steel pipe that satisfies the strength condition, the outer diameter d and the wall thickness t of the steel pipe are obtained by trial and error.

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