Miniature expanded-base pile group for power transmission tower construction and construction method

Abstract

The invention relates to the technical field of power transmission tower erection pile foundation construction, in particular to a micro expanded-base pile group for power transmission tower construction and a construction method. The pile comprises pile bodies, a bearing platform and a reinforcing steel bar structure, the pile bodies are expanded-base piles, the pile bodies close to each other are connected into a whole and fixed at the bottom expanded-base position, and the reinforcing steel bar structure comprises a micro pile reinforcing steel bar set, an expanded-base reinforcing steel bar set and a bearing platform reinforcing steel bar set. The bearing area of a pile foundation is increased by adopting the expanded-base design at the bottoms of the pile bodies, so that the bearing capacity of a single pile is improved, the load is effectively dispersed, the risk of foundation settlement is reduced, and a plurality of pile bodies are connected into a whole at the expanded-base positions at the bottoms to form an integral foundation system; the connection mode enhances the overall stability and lateral force resistance of the foundation, and the expanded-base steel bar group provides additional strength and stability at the expanded-base part at the lower part of the pile body, especially in the aspects of shear resistance and tension resistance.

Description

Technical Field

[0001] The invention relates to the technical field of pile foundation construction for power transmission towers, and in particular to a micro-enlarged bottom pile group for power transmission tower construction and a construction method. Background Art

[0002] Transmission tower pile foundation construction is an important part of power engineering, mainly used to provide support and protection for transmission towers. Transmission tower erection construction technology is relatively mature and widely used in urban power grid construction, including underground transmission tower tunnels and overhead transmission tower lines. Pile foundation construction is required for the erection of overhead transmission tower lines.

[0003] Micro pile is a foundation composed of cast-in-place reinforced concrete piles and a cap connected to the top of the pile with a diameter of 200-400mm and a slenderness ratio usually greater than 30. Micro piles have the characteristics of saving materials, reducing costs, strong adaptability, wide application range, simple construction, short construction period, high bearing capacity and safety and reliability. The micro pile group system has a good bearing capacity in terms of compression, tension and axial load resistance. Micro piles first appeared in soft soil foundation reinforcement projects and slope anti-slip protection projects. With the increasing demand for electricity in society, various voltage levels of transmission lines such as high voltage, ultra-high voltage and ultra-high voltage are being built at a sustained and rapid rate, and micro piles are widely used in transmission lines.

[0004] Existing micropiles are generally grouting by traditional means, that is, the traditional grouting method is to drill a hole in the foundation, then insert a grouting pipe into the hole, first grouting outside the grouting pipe, and then grouting inside the grouting pipe. Although this increases the fixing strength of the micropile to a certain extent, due to the small cross-sectional area of ​​the micropile itself, it only relies on the solidified slurry to increase its contact area with the ground, thereby increasing its bonding effect with the soil foundation, and relies on the pressure of the grouting to squeeze the soil wall to increase its bearing capacity. Therefore, when subjected to increased external pulling or squeezing force, the bearing capacity of the micropile still cannot meet the requirements.

[0005] Patent publication number CN107882022A discloses a variable diameter steel bar cage structure for expanded base piles and an expanded base pile construction method, wherein the structure includes fixed ring bars, movable ring bars and variable diameter bar groups, wherein the fixed ring bars are fixed to the outside of the main steel bar cage, and the movable ring bars are connected to the fixed ring bars through multiple groups of variable diameter bar groups, so that the movable ring bars can move closer to or away from the fixed ring bars, and other disclosed technical contents. The structure is prone to expand prematurely during use, affecting the lowering of the steel bar cage into the drilled hole.

[0006] Patent publication number CN110344397A discloses a steel cage structure for bored and expanded base piles, which includes a main steel cage, a secondary steel cage and an expanded steel cage, wherein the main steel cage and the secondary steel cage are movably connected and arranged in an upper and lower manner; the expanded steel cage includes expanded main bars and connecting bars, one end of the connecting bar is fixedly arranged with the expanded main bars, and the other end of the connecting bar is fixedly arranged with the secondary steel cage; the upper end of the expanded main bars is movably arranged with the secondary steel cage, and the lower end of the expanded main bars is movably arranged; when the secondary steel cage is subjected to force, the connecting bars are propped up, so that the expanded main bars are arranged in an expanded state, and other disclosed technical contents, because a double-layer steel cage is used, it is not suitable for micro piles, and the steel cage-in-steel cage structure is prone to contact and interference when they are stretched and contracted. Summary of the invention

[0007] In order to solve the problems of insufficient bearing capacity and weak tensile and compressive resistance of micro piles in transmission tower construction, the present invention provides a micro expanded bottom pile group for transmission tower construction and a construction method.

[0008] On the one hand, the present invention provides a micro-enlarged bottom pile group for transmission tower construction using the following technical solution:

[0009] A micro-enlarged bottom pile group for transmission tower construction comprises a pile body, a cap and a steel bar structure, wherein the pile body is constructed and buried in a soil layer, the cap is constructed and arranged on the ground and connected to the top of the pile body, the steel bar structure is constructed and cast in the pile body and the cap, the pile body is an enlarged bottom pile and pile bodies close to each other are connected as a whole and fixed at the bottom enlarged bottom, the steel bar structure comprises a micro-pile steel bar group, an enlarged bottom steel bar group and a cap steel bar group, the micro-pile steel bar group is cast in the upper main body part of the pile body, the enlarged bottom steel bar group is cast in the lower enlarged bottom part of the pile body, and the cap steel bar group is cast in the cap.

[0010] By adopting the expansion design at the bottom of the pile body, the bearing area of ​​the pile foundation is increased, thereby improving the bearing capacity of the single pile. This design effectively disperses the load and reduces the risk of foundation settlement. Multiple piles are connected into one at the bottom expansion to form an integrated foundation system. This connection method enhances the overall stability and lateral force resistance of the foundation and adapts to different geological conditions. The cap is connected to the top of the pile body to form an overall structural system to ensure that the load is evenly transferred from the upper structure to the pile foundation, reducing local stress concentration and improving the safety of the structure. The micropile reinforcement group enhances the tensile and bending resistance of the upper part of the pile body and improves the overall rigidity of the pile body. The expansion reinforcement group provides additional strength and stability in the expansion part of the lower part of the pile body, especially in terms of shear and tension. The cap reinforcement group ensures the structural strength and durability of the cap, provides the necessary tensile and shear resistance, and ensures the effective transfer and distribution of the load.

[0011] Furthermore, the pile body includes a micropile and an expanded bottom end, the micropile is an elongated cylindrical shape and is vertically cast or buried underground, the bottom of the micropile is connected and fixed with the expanded bottom end, and the expanded bottom end is a downwardly expanding cone.

[0012] The conical design of the expanded bottom end increases the contact area at the bottom of the pile. This increased contact area disperses the load applied to the pile, significantly improves the bearing capacity of the pile foundation, and is suitable for bearing large vertical loads. The design of the expanded bottom end enables the pile to have a better embedding effect in the foundation, and increases the structure's anti-overturning and anti-pulling capabilities. This design performs particularly well in soft soil layers and provides additional stability. Due to the presence of the expanded bottom end, the load is more evenly distributed over a larger soil volume, thereby reducing the possibility of foundation settlement and improving the safety and reliability of the overall structure.

[0013] Furthermore, the micropile reinforcement group is cast in the micropile, and the micropile reinforcement group includes vertical reinforcements, and the vertical reinforcements are connected to the bottom expansion reinforcement group through a connecting transmission, and the upper center position of the connecting transmission is connected and fixed to the bottom end of the vertical reinforcement.

[0014] The vertical reinforcement is connected to the bottom-enlarged reinforcement group through a connecting drive to form a continuous reinforcement skeleton. This integrity enhances the structural integrity of the pile and improves its bending and shear resistance.

[0015] Furthermore, the expanded bottom steel bar group includes umbrella-shaped bars, expanded bottom steel bar cages and movable support bars. The umbrella-shaped bars are evenly arranged in a circle and are hinged to the circumference of the connecting transmission through the upper ends. The expanded bottom steel bar cage is movably installed directly below the connecting transmission. The expanded bottom steel bar cage is connected to the middle part of the umbrella-shaped bars through movable support bars hingedly arranged in a circle on the circumference. The movable support bars and the umbrella-shaped bars are hinged to each other and paired one by one.

[0016] The combined design of the umbrella-shaped bars and the expanded bottom steel cage enables the expanded bottom end to effectively disperse and transfer the load. The uniform arrangement of the umbrella-shaped bars and the structure of the expanded bottom steel cage work together to increase the bearing area at the bottom of the pile and improve the overall bearing capacity. The movable support bars are used to support the umbrella-shaped bars and cooperate with the expanded bottom steel cage to support the umbrella-shaped bars in place, thereby facilitating construction.

[0017] Furthermore, the bottom end of the umbrella-shaped reinforcement is hook-shaped, and the expanded bottom reinforcement cage includes longitudinal reinforcement and stirrups. The longitudinal reinforcement is vertically arranged and evenly arranged in a circle, and the periphery of the circumferentially arranged longitudinal reinforcement is connected and fixed into a cylindrical shape by stirrup binding.

[0018] The hook design at the bottom of the umbrella-shaped bar provides an additional anchoring effect, enabling it to be more firmly embedded in the structure of the expanded base concrete. This design increases the connection strength between the umbrella-shaped bar and the expanded base concrete and reduces the risk of slippage under load.

[0019] Furthermore, the movable support rib is connected to the middle position of the umbrella-shaped rib through a fixed connector and a movable connector. The fixed connector is cylindrical and is sleeved on the middle position of the umbrella-shaped rib. A threaded hole is opened on the side wall of the fixed connector. The threaded hole passes through the inner and outer walls of the fixed connector and is internally threaded with a fixing screw. The fixed connector and the movable connector are hinged to each other through a hinged bolt.

[0020] The fixed connection piece is cylindrical and is sleeved on the middle part of the umbrella-shaped rib. The design of threaded holes and fixed screws ensures the firmness and stability of the connection. This design can effectively prevent the movable support rib from displacement or loosening when subjected to force. The design of threaded holes and fixed screws makes the installation and adjustment process easier and reduces construction time and complexity.

[0021] On the other hand, the present invention provides a method for constructing a micro-enlarged bottom pile group for transmission tower construction, which adopts the following technical solution:

[0022] A method for constructing a micro-pile group for power transmission tower construction includes the following construction steps:

[0023] Formulate the specifications and quantity of pile group construction according to the design specifications of the transmission tower, and mark the predetermined design pile positions;

[0024] Position and arrange the drilling machine according to the marked position, start the drilling machine to drill holes according to the specifications and dimensions of the micro piles;

[0025] The bottom end of the drilled micropile construction casting hole is expanded by using bottom expansion equipment to form a bottom expansion end construction casting hole;

[0026] Insert the steel bar structure from top to bottom along the micropile construction casting hole and finally insert it into the bottom expansion end construction casting hole, and finally keep the steel bar structure in the micropile construction casting hole and the bottom expansion end construction casting hole without sticking to the wall;

[0027] Pour concrete into the micropile construction casting hole and the bottom expansion end construction casting hole and wrap the steel structure;

[0028] After the concrete solidifies, the formed micro piles and the expanded bottom ends are fully maintained. Finally, a cap is constructed on the top of the micro pile and the cap is connected to the micro pile to form an integral structure.

[0029] By expanding the bottom of the micropile, the contact area of ​​the pile base end is increased, effectively improving the bearing capacity of the single pile. This design can better disperse the load and is suitable for bearing large vertical and horizontal forces. The design of the expanded bottom end enhances the stability of the pile foundation and reduces the risk of settlement and tilting. Combined with the use of reinforced structures, the entire pile body is more stable when subjected to force and can adapt to different geological conditions.

[0030] Furthermore, when the drilling machine drills micropile construction casting holes, it drills in groups of three or four, and the drilling positions of the three or four micropile construction casting holes in each group are distributed on the four corners of a rectangle or an equilateral triangle, the bottoms of the three or four micropile construction casting holes in each group are expanded by using bottom expansion equipment, and the expanded bottom end construction casting holes formed by the expansion of the bottoms of the three or four drilled holes in each group are connected to each other, and the verticality of the drilled holes is monitored in real time during drilling.

[0031] By arranging the four micropile construction casting holes of each group at the four corners of a rectangle or three micropile construction casting holes on an equilateral triangle, and expanding the bottom to form connected expanded bottom end construction casting holes, the load can be effectively dispersed and transferred to improve the overall bearing capacity. This layout design forms a stable structural unit between each group of piles, enhances the stability of the overall structure, and monitors the verticality of the borehole in real time during the drilling process to ensure the accuracy and verticality of each micropile construction casting hole. This monitoring improves the construction accuracy, ensures the quality and reliability of the pile foundation, and reduces the need for later adjustments and corrections.

[0032] Furthermore, the specific steps of casting the micropile casting hole and the bottom expansion end casting hole include:

[0033] Cleaning the micropile construction casting hole and the bottom expansion end construction casting hole formed by drilling, wherein the cleaning includes cleaning of gravel soil and cleaning of accumulated water;

[0034] After cleaning, a steel bar structure is placed in the micropile construction casting hole and the bottom expansion end construction casting hole;

[0035] Use the pouring pipe to insert the top and bottom of the micropile construction pouring hole and the expanded bottom end construction pouring hole and pour the expanded bottom end construction pouring hole, and use the vibrator to vibrate and compact it, and then check the pouring density and connectivity of the expanded bottom end;

[0036] Then the pouring pipe is poured at a uniform speed from bottom to top along the micropile construction hole, and a vibrator is used to follow the pouring process to vibrate and compact it.

[0037] Before pouring, the micropile construction casting holes and the expanded bottom end construction casting holes are cleaned, including removing gravel and stagnant water to ensure a clean environment in the holes, which helps to improve the adhesion between concrete and the foundation and prevent impurities from affecting the strength and stability of concrete. By using casting pipes and vibrators, the density of concrete in the expanded bottom end and micropile construction casting holes is ensured. The use of vibrators helps to remove bubbles in the concrete and prevent the formation of voids, thereby improving the density and bearing capacity of the concrete. The casting is poured at a uniform speed from bottom to top through the casting pipe to ensure that the concrete is evenly distributed and avoid stratification or unevenness.

[0038] Furthermore, the specific steps of placing the steel bar structure include:

[0039] The steel structure is lifted vertically by a hoisting machine, and then the umbrella-shaped reinforcement with the bottom of the steel structure retracted is inserted into the micropile construction casting hole from the top of the micropile construction casting hole. During the insertion process, the steel structure is kept vertically downward along the center of the micropile construction casting hole;

[0040] After the umbrella-shaped reinforcement reaches the construction pouring hole at the end of the expanded bottom, continue to lower the reinforcement structure;

[0041] The expanded bottom steel cage is fixed after contacting the bottom surface, and then the steel structure is lowered;

[0042] Under the support of the movable supporting ribs, the bottom end of the umbrella-shaped rib expands outwards and eventually forms an umbrella-shaped structure.

[0043] The folding and expanding design of the umbrella-shaped reinforcement eliminates the need for additional position adjustment during the insertion and expansion process, effectively reducing errors caused by human intervention and improving placement accuracy and construction quality. The umbrella-shaped reinforcement is expanded at the expanded bottom end to form a continuous mechanical connection with the steel structure, ensuring that the pile body is evenly stressed during the load-bearing process, further improving the overall structural strength and stability of the pile foundation.

[0044] In summary, the present invention has the following beneficial technical effects:

[0045] 1. By adopting the expansion design at the bottom of the pile body, the bearing area of ​​the pile foundation is increased, thereby improving the bearing capacity of the single pile. This design effectively disperses the load and reduces the risk of foundation settlement. Multiple piles are connected into one at the bottom expansion to form an integrated foundation system. This connection method enhances the overall stability and lateral force resistance of the foundation and adapts to different geological conditions. The cap is connected to the top of the pile body to form an integrated structural system to ensure that the load is evenly transferred from the upper structure to the pile foundation, reducing local stress concentration and improving the safety of the structure. The micropile reinforcement group enhances the tensile and bending resistance of the upper part of the pile body and improves the overall rigidity of the pile body. The expansion reinforcement group provides additional strength and stability in the expansion part of the lower part of the pile body, especially in terms of shear and tension. The cap reinforcement group ensures the structural strength and durability of the cap, provides the necessary tensile and shear resistance, and ensures the effective transfer and distribution of the load.

[0046] 2. The conical design of the expanded bottom end increases the contact area at the bottom of the pile. This increased contact area disperses the load applied to the pile, significantly improves the bearing capacity of the pile foundation, and is suitable for bearing large vertical loads. The design of the expanded bottom end enables the pile to have a better embedding effect in the foundation, and increases the structure's anti-overturning and anti-pulling capabilities. This design performs particularly well in soft soil layers and provides additional stability. Due to the presence of the expanded bottom end, the load is more evenly distributed over a larger soil volume, thereby reducing the possibility of foundation settlement and improving the safety and reliability of the overall structure.

[0047] 3. By expanding the bottom of the micropile, the contact area of ​​the pile base end is increased, effectively improving the bearing capacity of the single pile. This design can better disperse the load and is suitable for bearing large vertical and horizontal forces. The design of the expanded bottom end enhances the stability of the pile foundation and reduces the risk of settlement and tilting. Combined with the use of reinforced structures, the entire pile body is more stable when subjected to force and adapts to different geological conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 It is a schematic diagram of the construction process of the present invention;

[0049] Figure 2 It is a schematic diagram of the pile group structure of the present invention;

[0050] Figure 3 It is a schematic diagram of the steel bar structure of the present invention;

[0051] Figure 4 for Figure 3 A local enlarged schematic diagram of point A;

[0052] Figure 5 This is a schematic diagram of the grid division of a micro-enlarged base monopile;

[0053] Figure 6 This is a schematic diagram of the grid division of single pile foundation soil;

[0054] Figure 7 This is a schematic diagram of the grid division of micro-enlarged base pile groups;

[0055] Figure 8 This is a schematic diagram of the grid division of pile group foundation soil;

[0056] Fig. 9 This is the stress diagram of the micro-enlarged base single pile;

[0057] Fig.10 This is the displacement cloud diagram of a micro-enlarged single pile;

[0058] Fig.11 It is the displacement-load curve of the single installation of the expanded bottom;

[0059] Fig.12This is the stress diagram of micro-pile group with expanded bottom;

[0060] Fig.13 It is the displacement cloud diagram of micro-pile group with expanded bottom;

[0061] Fig.14 It is the displacement-load curve of the expanded base pile group;

[0062] Fig.15 is the vertical ultimate bearing displacement diagram of micro pile groups;

[0063] Fig.16 It is the vertical ultimate bearing stress cloud diagram of micro pile group;

[0064] Fig.17 This is the load-displacement curve of the expanded base pile group.

[0065] Description of reference numerals:

[0066] 1. Pile body, 11. Micro pile, 12. Expanded bottom end, 2. Cap, 3. Steel bar structure, 31. Micro pile steel bar group, 311. Connecting transmission device, 312. Vertical reinforcement, 32. Expanded bottom steel bar group, 321. Umbrella-shaped reinforcement, 322. Expanded bottom steel bar cage, 3221. Longitudinal reinforcement, 3222. Hoop reinforcement, 323. Movable support reinforcement, 3231. Fixed connector, 3232. Movable connector, 3233. Fixed screw, 3234. Articulated bolt, 33. Cap steel bar group. DETAILED DESCRIPTION

[0067] The following will be combined with the attached Figure 1-Figure 17 , the technical solutions in the embodiments of the present invention are clearly and completely described. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. 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.

[0068] In the description of the present invention, it should be understood that the terms "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside" and "outside" etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present invention.

[0069] Embodiment 1:

[0070] The embodiment of the present invention discloses a micro-enlarged bottom pile group for transmission tower construction, referring to Figure 1 and Figure 2, comprising a pile body 1, a cap 2 and a steel structure 3, wherein the pile body 1 is constructed and buried in the soil layer, the cap 2 is constructed and arranged on the ground and connected to the top of the pile body 1, and the steel structure 3 is constructed and cast in the pile body 1 and the cap 2;

[0071] The pile bodies 1 are expanded bottom piles and the pile bodies 1 close to each other are connected into one piece and fixed at the expanded bottom;

[0072] The steel structure 3 includes a micropile steel bar group 31, an expanded bottom steel bar group 32 and a cap steel bar group 33. The micropile steel bar group 31 is cast in the upper main part of the pile body 1, the expanded bottom steel bar group 32 is cast in the lower expanded bottom part of the pile body 1, and the cap steel bar group 33 is cast in the cap 2.

[0073] Embodiment 2:

[0074] Reference Figure 1 and Figure 2 The pile body 1 includes a micropile 11 and an expanded bottom end 12. The micropile 11 is an elongated cylindrical shape and is vertically cast or buried underground. The bottom of the micropile 11 is connected and fixed with the expanded bottom end 12, and the expanded bottom end 12 is a cone that expands downward.

[0075] According to Article 5.3.5 of the current Technical Specifications for Building Pile Foundations (JGJ94-2008), the ultimate pull-out bearing capacity of a single pile is calculated as follows (1):

[0076] Q uk =Q sk +Q pk =u∑q sik l i +q pk A p (1)

[0077] Where: Q sk , Q pk —standard value of total ultimate side resistance and standard value of total ultimate end resistance, kN respectively;

[0078] u i —Pile circumference, m;

[0079] q isk —Standard value of the ultimate lateral resistance of the i-th layer of soil on the pile side, kN / m2. If there is no local experience, refer to Table 5.3.5-1 in the Code;

[0080] l i —Thickness of the i-th layer of soil around the pile, m;

[0081] q pk —Standard value of ultimate end resistance, kN / m2. If there is no local experience, refer to Table 5.3.5-2 in the specification;

[0082] According to formula (2) combined with the pile-soil parameters, it can be concluded that the ultimate bearing capacity of the micro-enlarged bottom single installation of the present invention ranges from 266.41 kN to 341.80 kN.

[0083] The present invention is a micro-pile group with expanded bottom. Under the action of vertical load, the pile group foundation interacts with the surrounding soil. The bearing mechanism is more complicated than that of a single pile, especially the failure characteristics and bearing performance of the pile group. The bearing capacity of the pile group is not a simple addition of the bearing capacity of the single pile. This is the pile group effect. The pile group effect coefficient (η) can well reflect the strength of the pile group effect. In the present invention, the bottoms of the pile groups are connected together, so the pile group effect coefficient (η) needs to be reconsidered. The calculation formula (2) is as follows:

[0084]

[0085] Where: W is the ultimate bearing capacity of pile groups;

[0086] N is the number of single installations in the pile group;

[0087] P is the ultimate bearing capacity of a single installation;

[0088] The present invention uses ABAQUS numerical simulation software to establish a micro-enlarged base single installation model and a micro-enlarged base pile group model respectively to determine their ultimate pull-out bearing capacity and then determine their pile group effect coefficient (η).

[0089] Basic assumptions:

[0090] (1) There is no bond failure or slip between the steel bars and concrete, and the stiffness of the steel bars does not degrade;

[0091] (2) The foundation soil is regarded as an isotropic, homogeneous, single elastic-plastic continuum;

[0092] (3) The physical parameters of the foundation soil, such as compression modulus, density, and Poisson’s ratio, will not change due to the influence of construction conditions and the passage of time.

[0093] Reference Figure 2 and Figure 3 The micropile reinforcement group 31 is cast in the micropile 11, and the micropile reinforcement group 31 includes vertical reinforcement 312. The vertical reinforcement 312 is connected to the bottom reinforcement group 32 through a connecting transmission 311, and the upper center position of the connecting transmission 311 is connected and fixed to the bottom end of the vertical reinforcement 312.

[0094] Reference Figure 3 and Figure 4The bottom-enlarged steel bar group 32 includes an umbrella-shaped rib 321, a bottom-enlarged steel bar cage 322 and a movable support rib 323. The umbrella-shaped rib 321 is evenly arranged in a circle and is hinged to the circumference of the connecting transmission device 311 through the upper end. The bottom-enlarged steel bar cage 322 is movably installed directly below the connecting transmission device 311. The bottom-enlarged steel bar cage 322 is connected to the middle position of the umbrella-shaped rib 321 through the movable support rib 323 hingedly arranged in a circle on the circumference. The movable support rib 323 and the umbrella-shaped rib 321 are hinged to each other and matched one by one.

[0095] The soil body adopts the Mohr-Coulomb constitutive model, which has good applicability in numerical simulation. The pile body adopts the elastic model. The pile-soil parameters are shown in Table 1:

[0096] Table 1 Pile-soil parameters

[0097]

[0098] HPB300 steel bars are used as reinforcement. When simulating the behavior of the reinforcement, an ideal elastic-plastic model is used, which means that once the reinforcement reaches its yield strength, its stress will remain unchanged even if the strain continues to increase. This simplification helps to reduce the computational complexity. Its density is 7.8E-09t·mm -3 , elastic modulus is 2100MPa, and Poisson's ratio is 0.3.

[0099] The structural parameters of the micro-pile group pile of the present invention are based on the enterprise standard "Technical Regulations for Micro-Pile Foundations of Power Transmission Lines" of the State Grid Corporation. The micro-piles are arranged in a square, the distance between the pile centers of adjacent micro-piles is 500 mm, the pile body length h of the micro-pile is 8 m, the pile diameter d is 200 mm, and the size design of the expanded bottom end portion refers to "JGJ 94-2008 Technical Specifications for Building Pile Foundations" and "JGJT225-2010 Technical Specifications for Large Diameter Expanded Bottom Cast-in-Place Piles", wherein the expanded bottom end portion 3 has an expanded bottom diameter D=600 mm, a difference b between the expanded end radius and the pile body radius is 200 mm, and the slope (b / hc) of the side surface of the expanded bottom end is 1 / 3, so hc=600 mm, the width of the soil body is 10 m, and the depth is 16 m.

[0100] Reference Figure 4 The bottom end of the umbrella-shaped reinforcement 321 is hook-shaped, and the bottom-expanded reinforcement cage 322 includes longitudinal reinforcement 3221 and stirrups 3222. The longitudinal reinforcement 3221 is vertically arranged and evenly arranged in a circle, and the outer periphery of the circumferentially arranged longitudinal reinforcement 3221 is tied and connected by stirrups 3222 to form a cylindrical shape.

[0101] Reference Figure 4The movable support rib 323 is connected to the middle position of the umbrella-shaped rib 321 through a fixed connecting piece 3231 and a movable connecting piece 3232. The fixed connecting piece 3231 is cylindrical and is sleeved on the middle position of the umbrella-shaped rib 321. A threaded hole is opened on the side wall of the fixed connecting piece 3231. The threaded hole passes through the inner and outer walls of the fixed connecting piece 3231 and is internally threadedly connected with a fixed screw 3233. The fixed connecting piece 3231 and the movable connecting piece 3232 are hinged to each other through a hinge bolt 3234.

[0102] The concrete grade of the pedestal 2 shall not be lower than C25, and the minimum thickness shall not be less than 400 mm; the length of the top of the pile body 1 embedded in the interior of the pedestal 2 should not be less than 50 mm, and the longitudinal main reinforcement 312 at the top of the pile should be anchored in the pedestal 2, and its anchoring length should not be less than 30 times the diameter of the longitudinal reinforcement 312.

[0103] The micropile body 1 is a cast-in-place reinforced concrete pile with a diameter between 200mm and 400mm. The slenderness ratio of the pile body 1 is less than 50, the concrete strength of the pile body 1 should not be lower than C15, the thickness of the steel bar protective layer is 35mm, and the center spacing of the micropile body 1 is generally not less than 2.5 times its design diameter.

[0104] The expanded bottom end portion 12 is formed by a bottom expansion drilling rig. The three expanded bottom end portions 12 are connected together after design and construction, and an umbrella-shaped steel cage is placed inside. After grouting, the three expanded bottom end portions 12 form a whole. The diameter of the expanded bottom end portion 12 should be determined compared with the diameter of the pile body 1 based on the bearing capacity and the soil characteristics of the side surface of the expanded bottom end portion and the bearing layer of the pile end and the bottom expansion construction method. The slope of the expanded bottom end portion 12 should be determined based on the actual hole formation and the soil conditions themselves.

[0105] Embodiment 3:

[0106] The embodiment of the present invention discloses a method for constructing a micro-pile group for power transmission tower construction, referring to Figure 1 , including the following construction steps:

[0107] Formulate the specifications and quantity of pile group construction according to the design specifications of the transmission tower, and mark the predetermined design pile positions;

[0108] Position and arrange the drilling machine in place according to the marked position, and start the drilling machine to drill holes according to the specifications of the micro pile 11;

[0109] Using bottom expansion equipment to expand the bottom end of the drilled micro pile 11 construction casting hole to form an expanded bottom end 12 construction casting hole;

[0110] Insert the steel bar structure 3 from top to bottom along the construction casting hole of the micro pile 11 and finally insert it into the construction casting hole of the expanded bottom end 12, and finally keep the steel bar structure 3 in the construction casting hole of the micro pile 11 and the construction casting hole of the expanded bottom end 12 without sticking to the wall;

[0111] Pour concrete into the construction casting hole of the micro pile 11 and the construction casting hole of the expanded bottom end 12 and wrap the steel structure 3;

[0112] After the concrete solidifies, the formed micro pile 11 and the expanded bottom end 12 are fully maintained, and finally the cap 2 is constructed on the top of the micro pile 11 and connected with the micro pile 11 to form an integral structure.

[0113] Under the action of the pull-out load, the load is transmitted between the interfaces through friction resistance, accompanied by the generation of relative displacement. In order to accurately simulate this complex interaction, it is very necessary to use the friction contact algorithm to define the behavior between the contact surfaces. Since the stiffness of the pile is much greater than that of the foundation soil, in the simulation, the outer surface of the pile is set as the master surface, and the inner surface of the soil is set as the slave surface. In terms of normal action, a "hard" contact model is used. This model is a special discontinuous constraint. The contact condition is considered only when the soil is in actual contact with the foundation surface. When the two surfaces are separated, the normal action is not considered. This setting can truly reflect the normal interaction between the soil and the foundation. In terms of tangential contact properties, the Coulomb friction model is used to simulate the friction behavior between the soil and the foundation. The Coulomb friction model reflects the magnitude of the friction between the two by referring to the specification to set the friction coefficient μ=0.3. When there is normal contact pressure between the contact surfaces, the model allows the transmission of tangential stress (i.e. friction force), the magnitude of which is limited by the friction coefficient. By combining the master and slave surface settings, normal "hard" contact and tangential Coulomb friction model, the interaction between the concrete foundation and the foundation soil can be fully and accurately simulated.

[0114] In finite element modeling, there are two ways to load mechanical problems: force loading and displacement loading.

[0115] Force loading is to load volume load, surface load, line load, point load and other load forms on the model directly or through coupling. Force loading is suitable for simulating the stress performance of the structure when it is subjected to various forces, but the simulation of the loading process often encounters convergence problems and cannot be carried out. Displacement loading is to impose displacement boundary conditions on the structure and force the structure to deform to realize the loading process.

[0116] After comprehensive consideration, the present invention selects the displacement value method as the criterion for determining the ultimate pull-out bearing capacity. The displacement loading adopts point-surface coupling loading. The point-surface coupling loading method couples a reference point with the structural force surface. Applying a translational displacement to the reference point is equivalent to applying the same translational displacement to each point of the coupling surface. Taking into account the influence of the structural size and foundation characteristics on the foundation bearing performance, in order to ensure that the ultimate pull-out bearing capacity finally obtained is in the plastic region, the present invention uses the unidirectional pull-out load test displacement of 100mm as the termination loading condition in accordance with the provisions of the building foundation design code, that is, the pull-out load corresponding to the pull-out displacement of 100mm is taken as the final ultimate pull-out bearing capacity.

[0117] When finite element modeling is performed, the model is first meshed, as follows: Figure 5-Figure 8 shown.

[0118] Then the stress and displacement cloud diagram of the micro-enlarged pile is calculated, as shown in Fig. 9 and Fig.10 As shown, the maximum stress corresponding to the pile displacement of 100mm is 92.37MPa. Through further analysis of the calculation results, the load-displacement curve is obtained to determine its ultimate pull-out bearing capacity.

[0119] like Fig.11 As shown in the figure, the overall curve presents a "C"-shaped slow variation. The ultimate pull-out load corresponding to the upward displacement of 100 mm is 312.391 kN, which is within the calculation range of the above formula (2). This proves the reliability of the modeling analysis. Next, the vertical ultimate bearing capacity of the micro-expanded pile group will be determined, and then the pile group effect coefficient will be determined.

[0120] Then the stress and displacement cloud diagram of the micro-pile group with expanded bottom is calculated, as shown in Fig.12 and Fig.13 As shown, it can be seen that the maximum stress corresponding to the pile displacement of 100mm is 125.4MPa. Through further analysis of the calculation results, the load-displacement curve is obtained to determine its ultimate pull-out bearing capacity.

[0121] like Fig.14 As shown in the figure, the overall curve presents a "C"-shaped slow variation, and the ultimate pull-out load corresponding to the upward displacement of 100mm is 1037.138kN.

[0122] The ultimate pull-out bearing capacities of the single micro-pile and pile group at a displacement of 100 mm are 312.391 kN and 1037.138 kN respectively. According to formula (2):

[0123]

[0124] Where: W is the ultimate bearing capacity of pile groups; N is the number of single piles in the pile group; P is the ultimate bearing capacity of single piles. The calculation results show that the pile group effect coefficient η = 0.83.

[0125] Therefore, a calculation formula (3) suitable for the ultimate bearing capacity of the present invention (micro-enlarged bottom pile group) is proposed:

[0126] T uk =ηQ uk (3)

[0127] Q uk =Q sk +Q pk =u∑q sik l i +q pk A p (1)

[0128] Where: η—micro-pile group effect coefficient;

[0129] Q sk , Q pk —standard value of total ultimate side resistance and standard value of total ultimate end resistance, kN respectively;

[0130] u i —Pile circumference, m;

[0131] q isk —Standard value of the ultimate lateral resistance of the i-th layer of soil on the pile side, kN / m2. If there is no local experience, refer to Table 5.3.5-1 in the Code;

[0132] l i —Thickness of the i-th layer of soil around the pile, m;

[0133] q pk —Standard value of ultimate end resistance, kN / m2. If there is no local experience, refer to Table 5.3.5-2 in the specification.

[0134] On the basis of determining the vertical ultimate bearing capacity of the micro-pile group with expanded bottom, a micro-pile group model is established under the same conditions to determine its vertical ultimate bearing capacity, such as Fig.15 and 16 As shown, it can be seen that the maximum stress corresponding to the pile displacement of 100mm is 100.9MPa. Through further analysis of the calculation results, the load-displacement curve is obtained to determine its ultimate pull-out bearing capacity.

[0135] like Fig.17 As shown, the overall curve presents a "C"-shaped slow variation, and the ultimate pull-out load corresponding to the upward displacement of 100mm is 803.876kN.

[0136] Calculation and analysis show that the vertical ultimate bearing capacity of the micro-enlarged bottom pile group is 1.29 times that of the micro-enlarged bottom pile group, and the bearing capacity is increased by 233.262 kN. It can be concluded that the micro-enlarged bottom pile group of the present invention improves the overall stability and bearing capacity of the foundation, and the enlarged bottom end provides additional strength and stability at the lower part of the pile body.

[0137] When the drilling machine drills the construction casting holes of the micro piles 11, the drilling is carried out in groups of three or four, and the drilling positions of the three or four micro pile 11 construction casting holes in each group are distributed on the four corners of a rectangle or an equilateral triangle, the bottoms of the three or four micro pile 11 construction casting holes in each group are expanded by using bottom expansion equipment, and the expanded bottom ends 12 construction casting holes formed by the expansion of the bottoms of the three or four drilled holes in each group are connected to each other, and the verticality of the drilled holes is monitored in real time during drilling.

[0138] The specific steps of casting the micro pile 11 casting hole and the bottom end 12 casting hole include:

[0139] Cleaning the micropile 11 construction casting hole and the expanded bottom end 12 construction casting hole formed by drilling, wherein the cleaning includes cleaning of gravel and soil and cleaning of accumulated water;

[0140] After cleaning, a steel bar structure 3 is placed in the construction casting hole of the micro pile 11 and the construction casting hole of the expanded bottom end 12;

[0141] Use a pouring pipe to insert the micropile 11 construction pouring hole and the expanded bottom end 12 construction pouring hole top and bottom and pour the expanded bottom end 12 construction pouring hole, and use a vibrator to vibrate and compact it, and then check the pouring density and connectivity of the expanded bottom end 12;

[0142] Then the pouring pipe is poured at a uniform speed from bottom to top along the pouring hole of the micro pile 11, and a vibrator is used to follow and vibrate the pouring process to make it dense.

[0143] The specific steps of placing the steel bar structure 3 include:

[0144] The steel structure 3 is lifted vertically by a hoisting machine, and then the umbrella-shaped rib 321 in the retracted state at the bottom end of the steel structure 3 is inserted into the micropile 11 construction casting hole from the top of the micropile 11 construction casting hole. During the insertion process, the steel structure 3 is kept vertically downward along the center position of the micropile 11 construction casting hole;

[0145] After the umbrella-shaped reinforcement 321 reaches the construction casting hole of the expanded bottom end 12, the reinforcement structure 3 continues to be lowered;

[0146] The bottom-enlarged steel cage 322 is fixed after contacting the bottom surface, and then the steel structure 3 is lowered;

[0147] Under the support of the movable support ribs 323 , the bottom end of the umbrella-shaped rib 321 expands outwards and eventually forms an umbrella-shaped structure.

[0148] Verify the engineering geological data of the tower site and necessary hydrogeological data, design construction plans, and inspect various construction machinery and their supporting equipment.

[0149] According to the designed pile diameter, designed pile position, designed pile length and bottom expansion size, a bottom expansion drill is used to drill a hole. After the hole reaches the designed depth, drill 100 to 200 mm further and use a drill rod to clean the hole once. During the drilling process, check the verticality of the pile hole and check whether the bottom expansion ends 12 of the pile group are connected after construction.

[0150] When hoisting the steel structure 3, it is necessary to align it with the hole position and lower it slowly. When the bottom is blocked, it must not hit the cage, pier cage, or twist cage. When the expanded bottom steel bar group 32 descends to the expanded bottom end 12, it is pushed by its own gravity and the expanded bottom steel bar cage 322 to form a radial shape, and then the vertical bars 312 are fixed to prevent it from falling, so that the umbrella-shaped bars 321 are always in a radial state. After the radiation, the umbrella-shaped bars 321 of two adjacent expanded bottom ends 12 can overlap each other, because the umbrella-shaped bars 321 can rotate the vertical bars 312 by the lower end hooks so that the lower end hooks of adjacent umbrella-shaped bars 321 are hooked with each other.

[0151] After the steel structure 3 is successfully hoisted, concrete is first poured into the expanded bottom end 12 through the PVC grouting pipe to prevent the soil layer from sinking due to the excessive size of the expanded bottom. After that, PVC grouting pipes of different lengths can be used to pour concrete into the pile hole section by section, and vibrated and compacted in sections with a vibrating rod until the concrete is filled to the designed height.

[0152] After all piles are constructed and the curing time is reached, the cap 2 is poured. Before the cap steel bar group 33 is tied, the concrete on the pile top is chiseled to expose the steel bars, which are anchored into the poured cap 2 to ensure that the length of the pile body 1 buried in the cap 2 meets the design requirements. The concrete pouring of the cap 2 is completed in one go.

[0153] According to the State Grid Corporation of China's corporate standard "Technical Regulations for Micropile Foundations of Transmission Lines", as shown below: the micropile body 1 is arranged in a square, the distance between the pile centers of adjacent micropile bodies 1 is 500mm, the pile body length of the micropile body 1 is h=8m, the pile diameter is d=200mm, and the vertical reinforcement 312 of the micropile body 1 uses HPB400 steel bars with a diameter of 30mm.

[0154] The size design of the expanded bottom end portion 12 refers to "JGJ 94-2008 Technical Specifications for Building Pile Foundations" and "JGJT225-2010 Technical Specifications for Large Diameter Expanded Bottom Cast-in-Place Piles", where the expanded bottom diameter D of the expanded bottom end portion 12 is 600mm, the difference between the expanded end radius and the pile body 1 radius b is 200mm, and the slope of the side surface of the expanded bottom end portion 12 (b / hc) is 1 / 3, so hc is 600mm.

[0155] Among them, the umbrella-shaped reinforcement 321 and the movable support reinforcement 323 are both made of first-class steel bars with a diameter of 10 mm, the longitudinal reinforcement 3221 of the expanded bottom steel bar cage 322 is made of first-class steel bars with a diameter of 20 mm, and the stirrups 3222 of the expanded bottom steel bar cage 322 are made of first-class steel bars with a diameter of 8 mm. The diameter of the expanded bottom steel bar cage 322 is 150 mm, and the arrangement spacing of the stirrups 3222 is 100 mm. The expanded bottom steel bar cage 322 is formed by binding ten longitudinal reinforcements 3221 and two stirrups 3222, and the concrete or cement mortar uses C30 concrete or M20 cement mortar.

[0156] The above contents are merely examples and explanations of the structure of the present invention. The technicians in this technical field may make various modifications or additions to the specific embodiments described or replace them in a similar manner. As long as they do not deviate from the structure of the invention or exceed the scope defined by the claims, they should all fall within the protection scope of the present invention.

Claims

1. A micro-pile group with expanded bottom for power transmission tower construction, comprising a pile body (1), a cap (2) and a steel structure (3), wherein the pile body (1) is constructed and buried in a soil layer, the cap (2) is constructed and arranged on the ground and connected to the top of the pile body (1), and the steel structure (3) is constructed and cast in the pile body (1) and the cap (2), characterized in that: The pile bodies (1) are expanded-bottom piles, and the pile bodies (1) close to each other are connected into one piece and fixed at the expanded bottom. The steel bar structure (3) comprises a micropile steel bar group (31), a bottom-enlarged steel bar group (32) and a cap steel bar group (33); the micropile steel bar group (31) is cast in the upper main body of the pile body (1), the bottom-enlarged steel bar group (32) is cast in the bottom-enlarged part of the pile body (1), and the cap steel bar group (33) is cast in the cap (2).

2. The micro-base-enlarged pile group for transmission tower construction according to claim 1, characterized in that: The pile body (1) comprises a micropile (11) and an expanded bottom end (12); the micropile (11) is in the shape of an elongated cylinder and is vertically cast or buried underground; the expanded bottom end (12) is connected and fixed to the bottom of the micropile (11); the expanded bottom end (12) is in the shape of a cone that expands downward.

3. The micro-base-enlarged pile group for transmission tower construction according to claim 2, characterized in that: The micropile reinforcement group (31) is cast in the micropile (11), and the micropile reinforcement group (31) comprises vertical reinforcements (312). The vertical reinforcements (312) are connected to the bottom-expanded reinforcement group (32) via a connecting transmission (311), and the upper center position of the connecting transmission (311) is connected and fixed to the bottom end of the vertical reinforcement (312).

4. The micro-base-enlarged pile group for transmission tower construction according to claim 3 is characterized by: The bottom-enlarged steel bar group (32) comprises an umbrella-shaped rib (321), a bottom-enlarged steel bar cage (322) and movable support ribs (323); the umbrella-shaped ribs (321) are evenly arranged in a circle and hinged at the circumference of the connecting transmission device (311) through the upper end; the bottom-enlarged steel bar cage (322) is movably installed directly below the connecting transmission device (311); the bottom-enlarged steel bar cage (322) is connected to the middle position of the umbrella-shaped rib (321) through the movable support ribs (323) hingedly arranged in a circle on the circumference; the movable support ribs (323) and the umbrella-shaped ribs (321) are hinged to each other and are paired one by one.

5. The micro-base-enlarged pile group for transmission tower construction according to claim 4, characterized in that: The bottom end of the umbrella-shaped reinforcement (321) is hook-shaped, and the bottom-expanded reinforcement cage (322) comprises longitudinal reinforcement (3221) and stirrups (3222). The longitudinal reinforcement (3221) is vertically arranged and evenly arranged in a circumferential manner, and the periphery of the circumferentially arranged longitudinal reinforcement (3221) is tied and connected by stirrups (3222) to form a cylindrical shape.

6. A micro-base-enlarged pile group for power transmission tower construction according to claim 4 or 5, characterized in that: The movable support rib (323) is connected to the middle position of the umbrella-shaped rib (321) through a fixed connecting piece (3231) and a movable connecting piece (3232); the fixed connecting piece (3231) is cylindrical and sleeved on the middle position of the umbrella-shaped rib (321); a threaded hole is opened on the side wall of the fixed connecting piece (3231); the threaded hole passes through the inner and outer walls of the fixed connecting piece (3231) and is internally threadedly connected with a fixed screw (3233); the fixed connecting piece (3231) and the movable connecting piece (3232) are hinged to each other through a hinge bolt (3234).

7. A method for constructing a micro-pile group for power transmission tower construction, characterized in that: The construction steps include: Formulate the specifications and quantity of pile group construction according to the design specifications of the transmission tower, and mark the predetermined design pile positions; Positioning and arranging a drilling machine according to the marked position, and starting the drilling machine to drill holes according to the specifications of the micro pile (11); Using bottom expansion equipment to expand the bottom end of the drilled micropile (11) construction casting hole to form an expanded bottom end (12) construction casting hole; Insert the steel bar structure (3) from top to bottom along the construction casting hole of the micro pile (11) and finally insert it into the construction casting hole of the expanded bottom end (12), and finally keep the steel bar structure (3) in the construction casting hole of the micro pile (11) and the construction casting hole of the expanded bottom end (12) without being attached to the wall; Pouring concrete into the construction casting hole of the micro pile (11) and the construction casting hole of the expanded bottom end (12) and wrapping the steel structure (3); After the concrete solidifies, the formed micro pile (11) and the expanded bottom end (12) are fully maintained, and finally a cap (2) is constructed on the top of the micro pile (11) and the cap (2) is connected to the micro pile (11) to form an integral structure.

8. The method for constructing a micro-pile group for power transmission tower construction according to claim 7, characterized in that: When the drilling machine drills the micro pile (11) construction casting holes, the drilling is performed in groups of three or four, and the drilling positions of the three or four micro pile (11) construction casting holes in each group are distributed on the four corners of a rectangle or an equilateral triangle. The bottoms of the three or four micro pile (11) construction casting holes in each group are expanded by using bottom expansion equipment, and the expanded bottom ends (12) formed by the expansion of the bottoms of the three or four drilled holes in each group are connected to each other. The verticality of the drilled holes is monitored in real time during the drilling process.

9. The method for constructing a micro-pile group for power transmission tower construction according to claim 7, characterized in that: The specific steps of casting the micropile (11) casting hole and the bottom-enlarged end (12) casting hole include: Cleaning the micropile (11) construction casting hole and the expanded bottom end (12) construction casting hole formed by drilling, wherein the cleaning includes cleaning of gravel and soil and cleaning of accumulated water; After the cleaning is completed, a steel bar structure (3) is placed in the construction casting hole of the micro pile (11) and the construction casting hole of the expanded bottom end (12); Inserting a casting pipe into the casting hole of the micropile (11) and the top and bottom of the casting hole of the expanded bottom end (12), casting the casting hole of the expanded bottom end (12), vibrating and compacting it with a vibrator, and then checking the casting density and connectivity of the expanded bottom end (12); Then, the pouring pipe is poured along the pouring hole of the micro pile (11) from bottom to top at a uniform speed, and a vibrator is used to follow and vibrate the pouring process to make it compact.

10. The method for constructing a micro-pile group for power transmission tower construction according to claim 9, characterized in that: The specific steps of placing the steel bar structure (3) include: The steel structure (3) is lifted vertically by a hoisting machine, and then the umbrella-shaped ribs (321) at the bottom of the steel structure (3) are inserted from the top of the micropile (11) construction casting hole into the micropile (11) construction casting hole, and during the insertion process, the steel structure (3) is kept vertically downward along the center of the micropile (11) construction casting hole; After the umbrella-shaped reinforcement (321) reaches the construction casting hole of the expanded bottom end (12), the reinforcement structure (3) continues to be lowered; The bottom-enlarged steel cage (322) is fixed after contacting the bottom surface, and then the steel structure (3) is lowered; Under the support of the movable support ribs (323), the bottom end of the umbrella-shaped rib (321) expands outwards and eventually forms an umbrella-shaped structure.

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