Calculation method for enclosing structure with miniature steel pipe piles internally connected to cast-in-place piles

Abstract

The invention relates to the field of deep foundation pit supporting calculation, and discloses a calculation method for a building envelope with a miniature steel pipe pile internally connected to a cast-in-place pile, which comprises the following steps of: 1, enabling the building envelope to be equivalent to a combined model of an upper cast-in-place pile and a lower reinforced concrete non-web reinforcement pile, enabling steel pipes to be longitudinal steel bars of the non-web reinforcement pile, and enabling rock masses between the steel pipes to be concrete; secondly, when the excavation depth is not larger than hz, the inertia moment I and the elastic modulus E of the non-web reinforcement pile are calculated according to a plane bar system structure elastic fulcrum method based on an incremental method, and the same as those of a cast-in-place pile; thirdly, when the excavation depth is larger than hz, the reduction of the cross section size of the non-web reinforcement pile and the reduction of the lateral rigidity EI caused by rock mass cracking are considered; 4, re-checking whether reinforcement, section size and displacement of the supporting structure are met or not; and 5, if the adjustment parameters are not met, repeating the steps 2-4. The invention provides an elastic fulcrum method based on lateral rigidity reduction, which is based on engineering practice, is clear in mechanics concept and can be widely applied to the field of deep foundation pit supporting.

Description

Technical Field

[0001] The invention relates to the field of deep foundation pit support structure calculation, and in particular to a calculation method for a micro steel pipe pile internally connected to a cast-in-place pile enclosure structure. Background Art

[0002] The hilly area in my country accounts for one-tenth of the country's total area, mainly distributed in Liaodong, Jiaodong, Jiangnan, Fujian and Zhejiang, and Guangdong and Guangxi. When carrying out production and construction and excavating deep foundation pits in hilly areas, you will encounter an upper soil layer and a lower rock layer, a binary stratum of "weak on top and strong on the bottom". The soil layer uses reinforced concrete cast-in-place piles to embed part of the rock (not to the base), and the "hanging pile" retaining structure below the rock uses external micro steel pipe piles or composite soil nail walls. It is widely used in binary stratum deep foundation pit projects in hilly areas such as Qingdao, Fuzhou, Chongqing, Jinan, and Guangzhou in China. The "hanging pile" retaining structure is "not completely continuous" and has weak points at the soil-rock interface. The stiffness, internal force, and deformation will suddenly change at this point, especially when the cast-in-place piles are embedded in the rock at a small depth, the rock is broken, or the blasting excavation is disturbed. In actual application, in order to ensure the safety of foundation pit excavation conditions and effectively control the displacement of foundation pit soil, the embedded depth of bored piles in the rock is still relatively large, especially when the displacement control of existing structures and pipelines around the foundation pit is strict. This makes the construction of the "hanging pile" retaining structure still difficult and its economic efficiency is also reduced.

[0003] Changing the external connection of micro steel pipe piles to the internal connection of cast-in-place piles not only solves the problem of discontinuous hanging pile structure and sudden increase in displacement at the soil-rock interface, but also solves the problems of large occupied space, large number of cast-in-place piles embedded in rocks and high construction difficulty. In recent years, the application of micro steel pipe piles internally connected to cast-in-place piles in hilly areas of my country has gradually increased. However, this design method of retaining structure is still based on experience, without a suitable calculation model and a good theoretical calculation method, which greatly limits its promotion and application.

[0004] Therefore, considering the "knot effect" between micro steel pipe piles and the rock mass between piles and the similarity between the rock mass between piles and cracked concrete after excavation, the structure is equivalent to a combined model of "upper cast-in-place pile + lower unreinforced pile", the steel pipe is the longitudinal reinforcement of the lower unreinforced pile, and the rock mass between the steel pipes is concrete. A calculation method for the retaining structure of micro steel pipe piles connected to cast-in-place piles is proposed. Summary of the invention

[0005] The present invention mainly solves the problems existing in the above-mentioned existing design calculations and provides a calculation method for a micro steel pipe pile internally connected to a cast-in-place pile enclosure structure.

[0006] In order to achieve the above object, the present invention adopts the following technical scheme, a calculation method for a micro steel pipe pile connected to a cast-in-place pile enclosure structure, comprising the following steps:

[0007] Step 1: The retaining structure is equivalent to a combined model of "upper cast-in-place piles + lower reinforced concrete piles without web reinforcement", the steel pipes are the longitudinal reinforcement of the piles without web reinforcement, and the rock mass between the steel pipes is concrete;

[0008] Step 2: Excavation depth is not greater than h z When the elastic support method of the plane bar structure based on the incremental method is used, the moment of inertia I and elastic modulus E of the pile without web reinforcement are the same as those of the cast-in-place pile;

[0009] Step 3: Excavation depth greater than h z When considering the reduction in cross-sectional size of the pile without web reinforcement and the reduction in lateral stiffness EI caused by rock mass cracking;

[0010] Step 4: Check whether the reinforcement, cross-sectional dimensions and displacement of the supporting structure are satisfactory;

[0011] Step 5: If the adjustment parameters are not met, repeat steps 2 to 4 until the requirements are met.

[0012] Preferably, the first step retaining structure is equivalent to a combined model of "upper cast-in-place piles + lower reinforced concrete piles without web reinforcement", which is a vertically placed elastic foundation composite beam consisting of an upper and a lower part, and is a stepped variable-section pile; the upper part is a reinforced concrete cast-in-place pile with a diameter of D, which mainly supports the soil layer and is embedded in the rock under the soil layer to a certain depth; the lower part is an unreinforced pile with only longitudinal steel bars but no shear web reinforcement, the steel pipe is the longitudinal steel bar of the lower unreinforced pile, the upper part of the steel pipe is anchored to a certain length of the upper cast-in-place pile, and the lower part is embedded in the rock under the base to a certain depth; the rock mass between the steel pipes is concrete.

[0013] Preferably, the excavation depth of the second step is not greater than h z When the elastic support method of the plane bar structure based on the incremental method is used to calculate the enclosure structure, h z The depth from the top surface of the support to the soil-rock interface. n No greater than h z When , the elastic modulus of the supporting cast-in-place pile is E, and the moment of inertia is I; the elastic modulus of the moment of inertia of the pile without web reinforcement is the same as that of the cast-in-place pile; at this time, the calculation formula is (1)~(3);

[0014]

[0015]

[0016]

[0017] Where y—horizontal displacement;

[0018] z—the distance from the top of the enclosure structure to the calculation point;

[0019] e aik—Standard value of horizontal load outside the foundation pit;

[0020] b a — Calculation width of lateral earth pressure;

[0021] b0—soil resistance calculation width;

[0022] m—proportional coefficient of horizontal resistance coefficient of foundation soil;

[0023] h n —Excavation depth of foundation pit under the nth working condition.

[0024] Preferably, the excavation depth in the third step is greater than h z When considering the reduction in the cross-sectional size of the pile without web reinforcement, the moment of inertia I is reduced; z The depth from the top surface of the support to the soil-rock interface. n Greater than h z When the steel pipe is connected to the cast-in-place pile, the equivalent cross-section of the pile without web reinforcement formed by the steel pipe pile and the rock and soil between the pile becomes smaller. The equivalent rectangular cross-section, cross-section width and height are divided into b w and h0, and its moment of inertia I1 is calculated according to formula (4);

[0025]

[0026] Preferably, the excavation depth in the third step is greater than h z When calculating the pile without web reinforcement, the reduction of elastic modulus E caused by rock cracking due to rock blasting and excavation is considered; the elastic modulus is related to the compressive strength. First, its elastic modulus is obtained according to the compressive strength of the rock. The rock compressive strength f ck Compressive strength of standard cube f cu,k Convert according to formula (5) and find out its elastic modulus E1 according to the current concrete structure design code;

[0027] f ck =0.88*α c1 *α c2 *f cu，k (5)

[0028] where α c1 is the ratio of prism strength to cubic strength, and for C50 and below concrete, it is 0.76; α c2 is the brittleness reduction factor of concrete above C40, and is 1 for C40 and below; 0.88 is a reduction factor that takes into account the difference between the specimen size and the actual component size and other factors;

[0029] Due to the vibration caused by blasting and excavation, the cracks in the rock mass will crack and expand, and its elastic modulus will decrease. The comprehensive reduction coefficient β is used to characterize the reduction of the elastic modulus. After the reduction, the elastic modulus E2 = βE1; β is taken as 0.5~0.7 according to different rock types and different blasting and excavation methods.

[0030] Preferably, the excavation depth in the third step is greater than h z When , the enclosure structure is calculated according to formulas (6) to (8);

[0031]

[0032]

[0033]

[0034] Preferably, the fourth step is to check the reinforcement and cross-sectional dimensions of the support structure, calculate the bending bearing capacity of the pile without web reinforcement according to the current concrete structure design specification, calculate the required reinforcement area, check and adjust the cross-sectional dimensions of the steel pipe according to the equal strength principle, and check and calculate according to formula (9);

[0035]

[0036] Among them A s2 —Calculate the cross-sectional area of ​​the micro steel pipe pile on the load-bearing side;

[0037] A s1 —Calculate the required steel bar cross-sectional area on the load-bearing side;

[0038] f y1 —Calculate the design value of steel bar strength;

[0039] f y2 —Design value of micro steel pipe pile strength.

[0040] Preferably, the shear bearing capacity of the pile without web reinforcement in the fourth step is calculated according to the working reinforced concrete beam without web reinforcement with cracks, and the calculation formula is shown in (10);

[0041]

[0042] Where V is the shear bearing capacity; b w is the effective width of the section; h0 is the effective height of the section; f c ' is the axial compressive strength of concrete, and the rock compressive strength f ck value; λ is the shear span ratio (λ<5); n is the ratio of longitudinal reinforcement to concrete elastic modulus; ρ is the longitudinal reinforcement ratio, which is calculated based on the cross-sectional area of ​​the steel pipe and the equivalent cross-sectional area, and the value range is 0.06% to 5.0%.

[0043] Beneficial Effects

[0044] The present invention provides a calculation method for a micro steel pipe pile connected to a cast-in-place pile enclosure structure. It has the following beneficial effects:

[0045] (1) For the retaining structure of micro steel pipe piles connected to the soil-rock binary stratum cast-in-place piles, the calculation model of the combined pile of "upper cast-in-place piles + lower piles with no web reinforcement" was proposed, considering the "knot effect" between the micro steel pipe piles and the rock and soil between the piles and the similarity between the rock between the piles and the cracked concrete after excavation. The steel pipe is the longitudinal reinforcement of the lower pile without web reinforcement, and the rock between the steel pipes is concrete.

[0046] (2) The lateral stiffness reduction is used to quantitatively characterize the effects of the reduction in the retaining structure cross section and the rock mass cracks on the stiffness of the retaining structure. The elastic support method based on the lateral stiffness reduction is proposed to calculate the internal forces of the retaining structure, and the equivalent pile shear bearing capacity is calculated by considering the effect of the longitudinal reinforcement pins.

[0047] (3) Considering the changes in the lateral stiffness of the retaining structure under different working conditions, the internal force of the retaining structure is calculated according to the incremental method.

[0048] (4) The calculation model and method are based on practical engineering considerations, with clear mechanical concepts and simple and practical calculations. They fill the gap in the calculation of cast-in-place pile micro-steel pipe pile retaining structures and lay the foundation for the promotion and application of cast-in-place pile micro-steel pipe pile retaining structures. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 This is the elevation drawing of the micro steel pipe pile connected to the cast-in-place pile enclosure structure;

[0050] Figure 2 This is a cross-sectional view of the micro steel pipe pile connected to the cast-in-place pile enclosure structure;

[0051] Figure 3 This is the calculation model diagram of micro steel pipe pile connected to cast-in-place pile;

[0052] Figure 4 Schematic diagram of the effective cross-section of the equivalent pile.

[0053] Legend:

[0054] 101. Reinforced concrete bored piles; 102. Micro steel pipe piles; 103. Rocks between steel pipe piles; 104. Inside of foundation pit; 105. Outside of foundation pit; 106. Soil layer; 107. Rock layer; 108. Soil-rock interface; 109. Simplified spring of horizontal retaining member; 110. Passive zone spring support; 111. Supporting rock and soil pressure; 201. Equivalent cross section of pile without web reinforcement; ①. Excavation condition based on incremental method, the number in the circle is the number of condition steps, and the rest are the same. DETAILED DESCRIPTION

[0055] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0056] A calculation method for the micro steel pipe pile connected to the cast-in-place pile enclosure structure, such as Figure 1 to Figure 4 As shown, the following steps are included:

[0057] Step 1: The retaining structure is equivalent to a combined model of "upper cast-in-place pile 101 + lower reinforced concrete pile without web reinforcement 201", the micro steel pipe 102 is the longitudinal steel bar in the lower pile without web reinforcement 201, and the rock mass 103 between the micro steel pipes is concrete;

[0058] Step 2: Excavation depth is not greater than h z When the elastic support method of the plane bar structure based on the incremental method is used, the moment of inertia I and elastic modulus E of the pile without web reinforcement 201 are the same as those of the cast-in-place pile 101;

[0059] Step 3: Excavation depth greater than h z When considering the reduction in cross-sectional size of the pile without web reinforcement 201 and the reduction in lateral stiffness EI caused by the cracking of the rock mass 103;

[0060] Step 4: Check whether the reinforcement, section size and displacement of the support structures 101 and 201 are satisfactory;

[0061] Step 5: If the adjustment parameters are not met, repeat steps 2 to 4 until the requirements are met.

[0062] The first step of the retaining structure is equivalent to the combined model of "upper cast-in-place pile 101 + lower reinforced concrete unreinforced pile 201", which is a vertically placed elastic foundation composite beam, consisting of an upper and a lower part, and is a stepped variable cross-section pile; the upper part is a reinforced concrete cast-in-place pile 101 with a diameter of D, which mainly supports the soil layer and is embedded in the rock under the soil layer to a certain depth; the lower part is an unreinforced pile 201 with only longitudinal steel bars but no shear web reinforcement, and the steel pipe 102 is the longitudinal steel bar of the lower unreinforced pile 201. The upper part of the steel pipe 102 is anchored to a certain length of the upper cast-in-place pile 101, and the lower part is embedded in the rock under the base to a certain depth; the rock mass 102 between the steel pipes is concrete.

[0063] The second step is to excavate to a depth not greater than h z When the elastic support method of the plane bar structure based on the incremental method is used to calculate the enclosure structure, h z The depth from the top surface of the support to the soil-rock interface. n No greater than h zWhen the excavation condition is ①~, the elastic modulus of the supporting cast-in-place pile 101 is E, and the moment of inertia is I; the elastic modulus of the moment of inertia of the pile without web reinforcement 201 is the same as that of the cast-in-place pile 101; at this time, the calculation formula is (1)~(3);

[0064]

[0065]

[0066]

[0067] Where y—horizontal displacement;

[0068] z—the distance from the top of the enclosure structure to the calculation point;

[0069] e aik —Standard value of horizontal load outside the foundation pit;

[0070] b a — Calculation width of lateral earth pressure;

[0071] b0—soil resistance calculation width;

[0072] m—proportional coefficient of horizontal resistance coefficient of foundation soil;

[0073] h n —Excavation depth of foundation pit under the nth working condition.

[0074] The third step is to excavate a depth greater than h z When considering the reduction of the cross-sectional size of the pile without web reinforcement 201, the moment of inertia I is reduced; h z The depth from the top surface of the support to the soil-rock interface. n Greater than h z When the excavation condition is When the steel pipe 102 is connected to the cast-in-place pile 101, the equivalent cross-section of the pile without web reinforcement 201 formed by the steel pipe pile 102 and the rock and soil 103 between the piles becomes smaller, and its equivalent rectangular cross-section, cross-section width and height are divided into b w and h0, and its moment of inertia I1 is calculated according to formula (4);

[0075]

[0076] The third step is to excavate a depth greater than h z When calculating the pile without web reinforcement 201, the reduction of elastic modulus E caused by rock cracking due to rock blasting and excavation is considered; the elastic modulus is related to the compressive strength. First, its elastic modulus is obtained according to the rock compressive strength. The rock compressive strength f ck Compressive strength of standard cube f cu,k Convert according to formula (5) and find out its elastic modulus E1 according to the current concrete structure design code;

[0077] f ck =0.88*α c1 *α c2 *f cu，k (5)

[0078] where α c1 is the ratio of prism strength to cubic strength, and for C50 and below concrete, it is 0.76; α c2 is the brittleness reduction factor of concrete above C40, and is 1 for C40 and below; 0.88 is a reduction factor that takes into account the difference between the specimen size and the actual component size and other factors;

[0079] Due to the vibration caused by blasting and excavation, the cracks in the rock mass will crack and expand, and its elastic modulus will decrease. The comprehensive reduction coefficient β is used to characterize the reduction of the elastic modulus. After the reduction, the elastic modulus E2 = βE1; β is taken as 0.5~0.7 according to different rock types and different blasting and excavation methods.

[0080] The third step is to excavate a depth greater than h z When , the enclosure structures 101 and 201 are calculated according to formulas (6) to (8);

[0081]

[0082]

[0083]

[0084] The fourth step is to review the reinforcement and cross-sectional dimensions of the support structures 101 and 201. The flexural bearing capacity of the unreinforced pile 201 is calculated according to the current concrete structure design specifications. The required reinforcement area is calculated. The cross-sectional dimensions of the steel pipe are reviewed and adjusted according to the equal strength principle. The calculation is reviewed according to formula (9).

[0085]

[0086] Among them A s2 —Calculate the cross-sectional area of ​​the micro steel pipe pile on the load-bearing side;

[0087] A s1 —Calculate the required steel bar cross-sectional area on the load-bearing side;

[0088] f y1 —Calculate the design value of steel bar strength;

[0089] f y2 —Design value of micro steel pipe pile strength.

[0090] Step 4 The shear bearing capacity of the unreinforced pile 201 is calculated according to the working unreinforced reinforced concrete beam with cracks. The calculation formula is shown in (10);

[0091]

[0092] Where V is the shear bearing capacity; b w is the effective width of the section; h0 is the effective height of the section; f c ' is the axial compressive strength of concrete, and the rock compressive strength f ck value; λ is the shear span ratio (λ<5); n is the ratio of longitudinal reinforcement to concrete elastic modulus; ρ is the longitudinal reinforcement ratio, which is calculated based on the cross-sectional area of ​​the steel pipe and the equivalent cross-sectional area, and the value range is 0.06% to 5.0%.

[0093] Working principle of the present invention:

[0094] In the present invention, the "knot effect" between the micro steel pipe pile and the rock and soil between the piles and the similarity between the rock and the cracked concrete between the piles after excavation are considered, and a calculation model of the "upper cast-in-place pile + lower pile with no web reinforcement" combined pile is proposed. The steel pipe is the longitudinal reinforcement of the lower pile without web reinforcement, and the rock mass between the steel pipes is concrete.

[0095] In the present invention, the lateral stiffness reduction is used to quantitatively characterize the influence of the reduction of the retaining structure cross section and the rock mass cracks on the stiffness of the retaining structure, and an elastic support method based on the lateral stiffness reduction is proposed to calculate the internal force of the retaining structure;

[0096] In the present invention, the change of the lateral stiffness of the enclosure structure under different working conditions is considered, and the internal force of the enclosure structure is calculated according to the incremental method principle;

[0097] In the present invention, the shear bearing capacity of the equivalent pile is double-checked and calculated by considering the effect of the longitudinal reinforcement pins and similar shear performance after concrete cracking;

[0098] In the present invention, the equal strength principle is utilized and the micro steel pipes in the equivalent piles are used as longitudinal reinforcements.

[0099] The above shows and describes the basic principles and main features of the present invention and the advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A calculation method for a micro steel pipe pile connected to a cast-in-place pile enclosure structure, characterized in that: The following steps are involved: Step 1: The retaining structure is equivalent to a combined model of "upper cast-in-place piles + lower reinforced concrete piles without web reinforcement", the steel pipes are the longitudinal reinforcement of the piles without web reinforcement, and the rock mass between the steel pipes is concrete; Step 2: Excavation depth is not greater than h z When the elastic support method of the plane bar structure based on the incremental method is used, the moment of inertia I and elastic modulus E of the pile without web reinforcement are the same as those of the cast-in-place pile; Step 3: Excavation depth greater than h z When considering the reduction in cross-sectional size of the pile without web reinforcement and the reduction in lateral stiffness EI caused by rock mass cracking; Step 4: Check whether the reinforcement, cross-sectional dimensions and displacement of the supporting structure are satisfactory; Step 5: If the adjustment parameters are not met, repeat steps 2 to 4 until the requirements are met.

2. The calculation method of a micro steel pipe pile internally connected to a cast-in-place pile enclosure structure according to claim 1 is characterized in that: The first step retaining structure is equivalent to a combined model of "upper cast-in-place piles + lower reinforced concrete piles without web reinforcement", which is a vertically placed elastic foundation composite beam, consisting of an upper and a lower part, and is a stepped variable-section pile; the upper part is a reinforced concrete cast-in-place pile with a diameter of D, which mainly supports the retaining soil layer and is embedded in the rock under the soil layer to a certain depth; the lower part is an unreinforced pile with only longitudinal steel bars but no shear web reinforcement, and the steel pipe is the longitudinal steel bar of the lower unreinforced pile, the upper part of the steel pipe is anchored to a certain length of the upper cast-in-place pile, and the lower part is embedded in the rock under the base to a certain depth; the rock mass between the steel pipes is concrete.

3. The calculation method of a micro steel pipe pile connected to a cast-in-place pile enclosure structure according to claim 1, characterized in that: The excavation depth of the second step is not greater than h z When the elastic support method of the plane bar structure based on the incremental method is used to calculate the enclosure structure, h z The depth from the top surface of the support to the soil-rock interface. n No greater than h z When , the elastic modulus of the supporting cast-in-place pile is E, and the moment of inertia is I; the elastic modulus of the moment of inertia of the pile without web reinforcement is the same as that of the cast-in-place pile; at this time, the calculation formula is (1)~(3); Where y—horizontal displacement; z—the distance from the top of the enclosure structure to the calculation point; e aik —Standard value of horizontal load outside the foundation pit; b a — Calculation width of lateral earth pressure; b0—soil resistance calculation width; m—proportional coefficient of horizontal resistance coefficient of foundation soil; h n —Excavation depth of foundation pit under the nth working condition.

4. The calculation method of a micro steel pipe pile internally connected to a cast-in-place pile enclosure structure according to claim 1 is characterized in that: The excavation depth in the third step is greater than h z When considering the reduction in the cross-sectional size of the pile without web reinforcement, the moment of inertia I is reduced; z The depth from the top surface of the support to the soil-rock interface. n Greater than h z When the steel pipe is connected to the cast-in-place pile, the equivalent cross-section of the pile without web reinforcement formed by the steel pipe pile and the rock and soil between the pile becomes smaller, and its equivalent rectangular cross-section, cross-section width and height are divided into b w and h0, and its moment of inertia I1 is calculated according to formula (4); 5. The calculation method of a micro steel pipe pile connected to a cast-in-place pile enclosure structure according to claim 1 is characterized in that: The excavation depth in the third step is greater than h z When calculating the pile without web reinforcement, the reduction of elastic modulus E caused by rock cracking due to rock blasting and excavation is considered; the elastic modulus is related to the compressive strength. First, its elastic modulus is obtained according to the rock compressive strength. The rock compressive strength f ck Compressive strength of standard cube f cu,k Convert according to formula (5) and find out its elastic modulus E1 according to the current concrete structure design code; f ck =0.88*a c1 *a c2 *f cu，k (5) where α c1 is the ratio of prism strength to cubic strength, and for C50 and below concrete, it is 0.76; α c2 is the brittleness reduction factor of concrete above C40, and is 1 for C40 and below; 0.88 is a reduction factor that takes into account the difference between the specimen size and the actual component size and other factors; Due to the vibration caused by blasting and excavation, the cracks in the rock mass will crack and expand, and its elastic modulus will decrease. The comprehensive reduction coefficient β is used to characterize the reduction of the elastic modulus. After the reduction, the elastic modulus E2 = βE1; β is taken as 0.5~0.7 according to different rock types and different blasting and excavation methods.

6. The method for calculating the enclosure structure of a micro steel pipe pile connected to a cast-in-place pile according to claim 1, characterized in that: The excavation depth in the third step is greater than h z When , the enclosure structure is calculated according to formulas (6) to (8); 7. The method for calculating the enclosure structure of a micro steel pipe pile connected to a cast-in-place pile according to claim 1, characterized in that: The fourth step is to check the reinforcement and cross-sectional dimensions of the support structure. The bending bearing capacity of the pile without web reinforcement is calculated according to the current concrete structure design specifications. The required reinforcement area is calculated. The cross-sectional dimensions of the steel pipe are checked and adjusted according to the equal strength principle. The calculation is checked and calculated according to formula (9). Among them A s2 —Calculate the cross-sectional area of ​​the micro steel pipe pile on the load-bearing side; A s1 —Calculate the required steel bar cross-sectional area on the load-bearing side; f y1 —Calculate the design value of steel bar strength; f y2 —Design value of micro steel pipe pile strength.

8. The method for calculating the enclosure structure of a micro steel pipe pile connected to a cast-in-place pile according to claim 1, characterized in that: The fourth step is to calculate the shear bearing capacity of the unreinforced pile according to the calculation of the unreinforced reinforced concrete beam with cracks, and the calculation formula is shown in (10); Where V is the shear bearing capacity; b w is the effective width of the section; h0 is the effective height of the section; f c ’ is the axial compressive strength of concrete, and the rock compressive strength f ck value; λ is the shear span ratio (λ<5); n is the ratio of longitudinal reinforcement to concrete elastic modulus; ρ is the longitudinal reinforcement ratio, which is calculated based on the cross-sectional area of ​​the steel pipe and the equivalent cross-sectional area, and the value range is 0.06% to 5.0%.

Images