Low-prestress crack-free tension-compression cast-in-place pile and design and construction method thereof

Abstract

The invention provides a low-prestress crack-free tension-compression cast-in-place pile for a power transmission tower and a design method and a construction method of the low-prestress crack-free tension-compression cast-in-place pile and belongs to the technical field of civil engineering foundations. Except the ground part, the cast-in-place pile is divided into an upper pile section and a lower pile section, low prestress is applied to the upper pile section in the mode that a fastening nut at the top of the pile is tightened, only common steel bars are arranged to serve as longitudinal main steel bars of the pile, special prestressed steel bars or steel strands are not used, and complex anchorage devices and tensioning modes are not used either. Therefore, the pile foundation solution meeting the pulling-resistant and compression-resistant requirements at the same time can be achieved. The technical effects of reducing the pile diameter, reducing the construction cost and facilitating construction can be achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of civil engineering foundation, and particularly relates to a low prestressed crack-free tension-compression cast-in-place pile and a design and construction method thereof. Background Art

[0002] The cast-in-place concrete piles commonly used in transmission tower foundations are subject to both compression and significant pullout forces. In many cases, the pullout and the compressive forces are similar in value.

[0003] When the foundation soil is corrosive, according to Section 8.5.12 of the "Code for Design of Building Foundations" (GB 50007-2011), the crack control level should be Level 2, which means that the tensile stress in the concrete must not exceed the standard value of the concrete's axial tensile strength. To meet this requirement, common methods for cast-in-place piles include: prestressing the entire length of the pile using precision-rolled threaded steel bars (prestressed steel bars) or high-strength steel strands, increasing the pile diameter, raising the concrete grade, and increasing the reinforcement.

[0004] For example, prior art 1 (Chinese patent application CN 220352797 U, entitled "A Post-Tensioned Unbonded Prestressed Bored Cast-in-Place Pile") discloses a mixed reinforcement post-tensioned unbonded prestressing scheme. This scheme arranges ordinary longitudinal reinforcement around the circumference, sets a prestressed steel strand bundle inside, and applies prestress (prepressure) to the entire length of the pile body by tensioning the steel strands. Both the longitudinal reinforcement and the prestressed steel strands are full-length reinforcement. In this scheme, the entire pile body is equipped with ordinary steel bars and prestressed steel strands at the same time; the entire length of the pile body is prestressed, and the prestressed sections are relatively long; the upper and lower ends of the prestressed steel strands need to be anchored, and the upper end needs to be tensioned with a jack and locked with an anchor, which is a relatively cumbersome procedure. Symmetrical tensioning is difficult to control and can easily cause bending moments in the pile body.

[0005] The second prior art (Chinese patent application CN 118653460 A, entitled "A Prestressed Pile for a Liquefied Natural Gas Storage Tank and Its Production Method") discloses a mixed reinforcement scheme using both conventional steel and slow-bond prestressed steel. This scheme achieves this by distributing the slow-bond prestressed steel bars from long to short along the length of the pile, depending on the tensile strength. This mixed reinforcement scheme complicates the fabrication of the steel cage; the slow-bond sections are the full length of the prestressed steel bars; the prestressed steel bars vary in length, making cutting complex; and the use of clip anchors at the top of the prestressed bars complicates construction.

[0006] The third prior art (CN 114960627 B, entitled "A method for constructing super-long slow-bonding prestressed bored piles") discloses a mixed reinforcement scheme in which ordinary steel bars and slow-bonding prestressed steel strands are configured at the same time. The slow-bonding section of the steel strand is set over the entire length of the pile, and the steel strands are retracted inward at the upper and lower ends of the pile. This scheme is also a mixed reinforcement scheme, and the steel cage is relatively complex to manufacture. The full-length prestressed steel strands are retracted inward at the upper and lower ends of the pile, passing through the longitudinal main reinforcement of the pile twice, forming two bends, and the reinforcement arrangement is relatively complex. The slow-bonding section of the steel strand is set over the entire length of the pile, and the length is relatively long. The prestressed steel strand uses a clip anchor at the upper end of the pile, which makes construction more complicated; due to the two bends of the steel strand, its path is not a straight or smooth line, and the prestressing effect is poor.

[0007] In prior art four (CN 217896478 U, entitled "Prestressed Bored Cast-in-Place Pile") and prior art five (CN115288125 A, entitled "A Prestressed Bored Cast-in-Place Pile and Its Construction Method"), the pile body is divided into two sections: the upper section is an unbonded prestressed post-tensioned section, and the lower section is a bonded prestressed section serving as the anchor end. The steel bars are also divided into two sections, both of which are fine-rolled threaded steel bars, connected by a steel bar connector. The upper post-tensioned steel bars are tensioned and locked using a tensioner. This solution uses prestressed fine-rolled threaded steel bars throughout the entire length, resulting in a high cost. The steel bars are arranged in two sections, which require connection via steel bar connectors, making the process more complex. All steel bar connection points are located on the same horizontal plane, resulting in a 100% joint rate, which is unfavorable for stress bearing. The upper steel bars must be locked with an anchor after tensioning, which is more complicated to operate.

[0008] Prior Art 6 (CN 205712090 U, entitled "A New Prestressed Pull-Out and Floating Pile") discloses a hybrid reinforcement scheme for prestressed pull-out and floating piles using both ordinary steel bars and steel strands. The prestressed steel strands are installed along the entire length of the pile. The lower end of the steel strand bundle is fixed to a tension plate and a cast iron tension head system. The upper end of the steel strands passes upward through the anti-floating base plate and is prestressed and anchored there. A secondary grouting pipe is also installed on the pile side. This scheme uses a hybrid reinforcement scheme that combines ordinary steel bars and steel strands. It features an external grouting pipe, a tension plate and a cast iron tension head at the lower end, and a complex steel cage system. The prestressed section runs the entire length of the pile, which is relatively long. The prestressed steel strands must be anchored at both the upper and lower ends, with the upper end tensioned with a jack and locked with an anchor, a cumbersome and uneconomical procedure. Symmetrical tensioning is difficult to control, which can easily cause bending moments in the pile.

[0009] In the seventh prior art (CN 205224025 U, entitled "Prestressed Pullout Piles"), only prestressed steel strands are deployed longitudinally along the entire length of the pile. Tensioning maintains the pile in compression to prevent cracks. The prestressed section extends the entire length of the pile, and the strands must be anchored at both ends. The upper end is tensioned with a jack and locked with a dedicated anchor, a complex process. Because the longitudinal reinforcement of the pile is entirely steel strands, the longitudinal stiffness of the reinforcement cage is poor, making it difficult to process the cage using the inherent stiffness of the slow-bonding strands. This necessitates the addition of a cage skeleton, which increases costs and makes hoisting and drilling the cage difficult. Tensioning is also difficult to control, which can easily cause bending moments in the pile.

[0010] Prior Art 8 (CN 203729291 U, entitled "Pulling-Resistant Cast-In-Place Piles") discloses a post-tensioned, prestressed, pulling-out pile scheme using a hybrid reinforcement system with both conventional steel bars and strands. The prestressed strands run the full length of the pile, with the lower ends of the strand bundles secured to the pile base. The hybrid reinforcement cage is complex to manufacture; the prestressed section runs the full length of the pile, which is rather long; and the upper and lower ends of the prestressed strands require jacking and locking with specialized anchors, a cumbersome procedure.

[0011] Prior art nine (CN 118481178 A, entitled "A Crack-free Pull-out Pile and Its Construction Method") discloses a mixed reinforcement post-tensioned prestressed pull-out pile scheme that is configured with both ordinary steel bars and prestressed steel bars. It is equipped with two sets (two circles) of longitudinal steel bar systems, the outermost circle is ordinary steel bars, and the inner circle is a post-tensioned prestressed steel bar system; the inner circle prestressed steel bars are anchored by a U-shaped connector at the bottom of the pile, and then prestress is applied to the pile by rotating the nut at the top of the pile. The steel cage is relatively complicated to make. The prestressed section is the entire length of the pile, which is relatively long, and prestress is only applied to the inner circle steel cage. It has a greater effect on resisting axial force, but has limited contribution to the bending resistance of the pile body. If there is bending moment on the pile body, cracks may still occur.

[0012] In prior art 10 (CN 117888531 A, entitled "A Combined Tension-Compression Retarded Bond Post-Tensioned Pile and Its Construction Process"), the steel bars are also divided into upper and lower sections, both of which are the same type of PSB prestressed threaded steel bars. The upper section is a composite steel retarded section, while the lower section is an anchoring section. The two sections are connected by a steel bar connector. The upper end steel bar is a single piece, tensioned with a dedicated tensioner and then locked with a locking nut and anchor plate. The entire length of the PSB prestressed threaded steel bar is relatively expensive. The two sections of steel bar need to be connected with a steel bar connector, which is a complex process. All steel bar connection points are located on the same horizontal plane, with a 100% joint rate, which is unfavorable for stress bearing. The upper end steel bar needs to be tensioned with a dedicated tensioner, which is complex to operate and difficult to control, and can easily cause bending moments in the pile body.

[0013] Prior art 11 (CN 117845891 A, entitled "A Pressure-Type Retarded Bonding Post-Tensioned Pullout Pile and Its Construction Process") discloses a pressure-type pullout pile that transmits upward pullout force from the pile bottom. The pile is constructed of PSB prestressed threaded steel bars + a retarded bonding layer. Prestress is applied along the entire length of the pile, resulting in a high cost. The prestressed steel bars are connected by a connector and a sealer, resulting in a complex process. The upper end of the steel bar is tensioned by a dedicated tensioner, which is complex to operate and difficult to control, and can easily cause bending moments in the pile body.

[0014] Prior art 12 (CN 211340824 U, entitled "Concrete Pullout Piles with Compression") discloses a method whereby only high-strength steel strands or ordinary hot-rolled steel bars are arranged longitudinally along the entire length of the pile body. Anchorage sections are provided within a small area at the bottom, and anchoring end plates are added to effectively anchor the steel bars. Except for the portion of the pile body required for bottom anchoring, the upper ordinary hot-rolled steel bars are sleeved; the steel bars are not prestressed. However, this scheme does not prestress the pile body. Instead, the pile body is subjected to forces in which the pile cap transmits the upward pull force to the steel bars or strands, which are then transmitted to the anchor at the bottom of the pile, and ultimately to the entire pile body. This force-bearing mechanism is similar to that of unprestressed compression anchors. Because there is no prestress, the entire pile body cannot resist pullout unless the pile cap generates sufficient upward pull, resulting in significant displacement of the pile cap at the top. Such large upward pull can lead to cracks, water ingress, and corrosion.

[0015] Wind turbine tower pile foundations lack post-cast caps. The tower's uplift forces are transmitted to the pile's top concrete via anchor bolts, which in turn transmit them to the pile's longitudinal reinforcement. This solution, with the pile's top concrete separated from the longitudinal reinforcement by a sheath, prevents the uplift forces from the anchor bolts from being effectively transferred to the pile. To effectively transmit these forces, either the longitudinal reinforcement at the pile's top must be cast directly into the concrete, or prestressing the pile's reinforcement or strands at the top of the pile would be necessary to transform the pile into a complete load-bearing structure. Therefore, this solution is unsuitable for wind turbine tower uplift piles.

[0016] Prior Art 13 (CN 113969576 A, entitled "A Novel Unbonded Post-Tensioned Prestressed Pullout Pile Structure") discloses a post-tensioned prestressed pullout pile scheme with mixed reinforcement. It is characterized by the arrangement of ordinary longitudinal steel bars and prestressed steel strands along the entire length of the pile. The arrangement of ordinary longitudinal steel bars is the same as that of conventional cast-in-place piles, while a separate post-tensioned unbonded steel strand system is installed along the entire length. A P-type extrusion anchor is installed at the lower end of the steel strands, and a clip-type anchor is installed at the upper end to secure the unbonded steel strands. After the pile reaches sufficient strength, the steel strands are tensioned at the top of the pile and locked with the anchor, thereby applying precompressive stress to the pile. This scheme, a hybrid reinforcement scheme that simultaneously arranges ordinary longitudinal steel bars and prestressed steel strands, is complex to manufacture and relatively costly. The prestressed section extends the entire length of the pile, which is relatively long. The control technology required for tensioning the steel strands is also high, which can easily cause bending moments in the pile body.

[0017] Prior art 14 (CN 113123332 B, title: "A post-tensioned slow-setting main reinforcement pressure-type pull-out pile and its preparation method") discloses that the longitudinal main reinforcement of the pile body is a composite slow-setting anchor cable (including pull-out anchor cable, slow-setting adhesive spacer, and casing), prestressing is applied to the entire length of the pile body, and after the prestressing is applied by tensioning, it is anchored by a clip anchor set 100mm below the pile top elevation. However, the prior art 14 sets the prestressed anchor cable along the entire length of the pile, which is complicated to manufacture and has high cost; the prestressed section is the entire length of the pile, which is relatively long; the tensioning control technology of the steel strand is high, which easily causes bending moment in the pile body; it is difficult to process the composite slow-setting anchor cable into a steel cage based on its own rigidity, and a steel cage skeleton needs to be added, which increases cost; the clip anchor anchor system is relatively complex. Moreover, this solution is more suitable for situations where tensile stress is not allowed in the concrete of the entire pile length. This is too demanding for transmission towers and the cost is also relatively high. The construction site of the transmission tower has a large span and a remote location, so it is not convenient to use tensioners for tensioning and clip anchors for anchoring.

[0018] Prior Art 15 (CN 109555119 A and CN 109555119 B, entitled "Retarded-Set Prestressed Cast-In-Place Piles and Method for Pullout Resistance") discloses a mixed reinforcement scheme using both ordinary steel bars and delayed-bond prestressed steel strands. The delayed-bond prestressed steel strands are arranged from long to short along the length of the pile, based on the strength of the pile. However, the delayed-bond section in this scheme is the full length of the prestressed steel strands; the varying lengths of the prestressed steel strands, combined with the addition of ordinary steel bars, complicate material cutting; and the use of clip anchors at the top of the prestressed steel strands complicates construction.

[0019] Prior art sixteen (CN 102839652 A, title: "A Steel Stranded Cast-in-Place Pile and Its Construction Method") discloses a cast-in-place pile in which the entire pile body is made of steel strands instead of the traditional steel cage. The steel strands (or steel bars, etc.) are fixed to the bottom of the pile and tensioned and anchored at the top of the pile to form a pile body with mainly pull-out resistance. However, the prestressed section in this scheme is the entire length of the pile, which is relatively long. The upper and lower ends of the prestressed steel strands need to be anchored, and the upper end needs to be tensioned with a jack and locked with a special anchor, and the procedure is relatively cumbersome. Since the longitudinal reinforcement of the pile is made of steel strands with poor rigidity, ordinary steel bars need to be used to make a skeleton to be processed into a steel cage. While increasing the cost, it is also difficult to hoist the steel cage and put it into the pile hole. The tensioning control is difficult, and it is easy to cause bending moment in the pile body.

[0020] Therefore, there is an urgent need for a cast-in-place pile solution that can reduce the pile diameter, lower the project cost, and facilitate construction. Summary of the Invention

[0021] The present invention provides a low prestressed crack-free tension-compression cast-in-place pile and a design and construction method thereof, so as to overcome at least one technical problem existing in the prior art.

[0022] To achieve the above-mentioned object, the present invention provides a low prestressed crack-free tension-compression cast-in-place pile, comprising longitudinal main reinforcement arranged along the circumference of the pile, stirrups cooperating with the longitudinal main reinforcement to form a reinforcement cage, and a concrete pile body cast on the reinforcement cage; wherein,

[0023] The cast-in-place pile is divided into a lower pile section, an upper pile section and an above-ground section from bottom to top, and the dividing point between the upper pile section and the lower pile section is the dividing point between the prestressed section and the non-prestressed section of the cast-in-place pile;

[0024] A thread and a fastening nut are provided at the top of the longitudinal main reinforcement of the pile, and a pressure plate and an annular positioning plate are provided in sequence below the fastening nut; wherein the annular positioning plate is used to constrain the longitudinal main reinforcement of the pile and to disperse the pressure of the pressure plate during tensioning;

[0025] A sheath is provided outside the longitudinal main reinforcement of the pile in the upper pile section;

[0026] By controlling the length of the upper pile section and applying prestress to the upper pile section by tightening a nut at the pile top, a partially prestressed pile body is formed, so that the concrete tensile stress of the upper pile section and the lower pile section is controlled to not exceed the control limit of the concrete tensile stress.

[0027] Furthermore, an optional solution is to further include an enlarged head arranged on the top of the cast-in-place pile, and the enlarged head is used to protect the concrete on the pile top when prestress is applied to the pile top.

[0028] Furthermore, an optional solution is to further include a retarding adhesive, which is arranged between the longitudinal main reinforcement of the pile at the upper pile end and the sheath; wherein the retarder can be replaced by post-tensioning grease.

[0029] Furthermore, an optional solution is that the sheath is a steel pipe sheath or a HDPE sheath with external ribs or a plastic sheath of similar material.

[0030] Furthermore, an optional solution is to further include an anchoring piece, wherein the anchoring piece is arranged at the bottom end of the lower pile section and is fixed to the longitudinal main reinforcement of the pile.

[0031] Furthermore, an optional solution is that the anchoring member can be any one of a steel bar anchor plate, a welded anchor bar, a plug-welded anchor plate, a bent longitudinal bar, or a welded nut at the bottom of the longitudinal bar. Alternatively, if the length of the steel bar in the lower pile section exceeds the steel bar anchorage length, no special anchoring measures may be taken at the lower end of the steel bar.

[0032] Furthermore, an optional solution is to further include a tower anchor assembly, which is arranged on the above-ground section and is used to fix the tower column foot on the cast-in-place pile.

[0033] Furthermore, an optional solution is that the compressive bearing capacity of the pile body concrete of the upper pile section meets the following requirements:

[0034] N+γF≤ψ c f c A ps

[0035] The compressive bearing capacity of the pile body of the lower pile section meets the following requirements:

[0036] N+γ(G1+G3-R a上 )≤ψ c f c A ps +0.9f y 'A s '

[0037] A ps =A-λA s

[0038] Where N is the design value of the axial pressure on the pile top under the basic combination of load effects, γ is the comprehensive partial factor, standard value × γ = design value, F is the total effective prestress applied by all the longitudinal main reinforcements at the pile top, ψ c is the foundation pile construction coefficient, f c A is the design value of concrete axial compressive strength; ps is the net cross-sectional area of the pile concrete, A is the gross cross-sectional area of the pile, λ is the area expansion coefficient of the longitudinal main reinforcement of the pile, A s is the cross-sectional area of the longitudinal main reinforcement of the pile; R a上 is the characteristic value of the pull-out bearing capacity of a single pile in the upper pile section, f y ′ is the design value of the compressive strength of the longitudinal main reinforcement of the pile, A s ′ is the cross-sectional area of the longitudinal main reinforcement of the pile; G1 is the deadweight of the upper pile section, and G3 is the deadweight of the above-ground section.

[0039] On the other hand, the present invention also provides a design method for low prestressed crack-free tension-compression cast-in-place piles, which is used to design the low prestressed crack-free tension-compression cast-in-place piles as described above; wherein the concrete tensile stress at the top of the upper pile section is determined by the following formula:

[0040] σ ck -σ pc =(T-G3-F) / A ps ≤f tk

[0041] Among them, σ ck is the normal stress of the pile cross section under the standard combination of load effects, σ pc f is the prestress of the pile concrete after deducting all stress losses, tkis the standard value of the axial tensile strength of concrete, and T is the standard value of the axial tension at the top of the pile under the standard combination of load effects.

[0042] In order to solve the above problems, the present invention also provides a construction method of low prestressed crack-free tension-compression cast-in-place piles, which is used to construct the low prestressed crack-free tension-compression cast-in-place piles as described above; the method comprises:

[0043] Making the longitudinal main reinforcement of the pile; including: processing threads on the top of the longitudinal main reinforcement of the pile;

[0044] Fabricate the reinforcement cage, which includes: wrapping spiral stirrups around the longitudinal main reinforcement of the pile using a roll welder and placing spiral stirrups and reinforcing stirrups;

[0045] Place casing on the pile site;

[0046] Hole forming and hole cleaning;

[0047] Hanging and placing steel cage;

[0048] Pressure pouring concrete through the conduit;

[0049] Place the annular positioning plate and tower anchor assembly at the designed elevation;

[0050] After the pile reaches the designed strength, the bearing plate and matching nuts are placed on the upper end of the longitudinal main reinforcement of the pile;

[0051] Apply prestressing force.

[0052] Furthermore, an optional solution is that the production of the longitudinal main reinforcement of the pile also includes: applying a slow-setting adhesive to the upper pile section and covering it with a sheath, and firmly tying the sheath up and down and sealing the opening.

[0053] Furthermore, an optional solution is to use a torque wrench to tighten the fastening nuts in a symmetrical, graded, and multi-round loading manner during the process of applying prestress, so that the longitudinal main reinforcement of the pile reaches a preset tension force.

[0054] Furthermore, an optional solution is that, during the process of applying the prestress, the prestress applied each time is 10% to 20% of the design value.

[0055] The low-prestressed, crack-free, tension-and-compression cast-in-place piles of the present invention, as well as their design and construction methods, can be applied to transmission tower foundations. Based on conventional cast-in-place piles, this invention provides a pile foundation solution that simultaneously meets both pullout and compression requirements by employing only ordinary steel bars as the longitudinal main reinforcement of the pile, without using specialized prestressed steel bars or stranded wire, or complex anchors or tensioning methods. Instead, the pile is tightened with a nut at the top of the pile to apply a low prestress to the upper pile section, thereby achieving a solution. This reduces pile diameter, lowers project costs, and facilitates construction. Furthermore, if a slow-setting adhesive is used, the elongation of the prestressed steel bars will not decrease even if the anchor ends become loose, effectively maintaining the prestressed compressive stress of the pile body and increasing the steel bars' corrosion resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0057] Figure 1 A schematic structural diagram of a low prestressed, crack-free, tension-compression cast-in-place pile provided by one embodiment of the present invention;

[0058] Figure 2 for Figure 1 1-1 cross-sectional diagram;

[0059] Figure 3 for Figure 1 2-2 cross-sectional diagram in FIG;

[0060] Figure 4 for Figure 1 Schematic diagram of section 3-3 in FIG.

[0061] Figure 5 for Figure 1 in Enlarged image;

[0062] Figure 6 for Figure 2 middle Enlarged image;

[0063] Figure 7 for Figure 3 middle Enlarged image;

[0064] Figure 8 A schematic diagram of the positioning plate structure according to an embodiment of the present invention;

[0065] Figure 9 for Figure 8 Schematic diagram of section 4-4 in FIG.

[0066] Figure 10 Schematic diagram of the pressure plate structure according to an embodiment of the present invention;

[0067] Figure 11 for Figure 10 Schematic diagram of section 5-5;

[0068] Figure 12 、 Figure 13 Schematic diagram of structural marking for calculating the length of an upper pile segment according to an embodiment of the present invention;

[0069] Figure 14 This is an example diagram of a low prestressed crack-free tension-compression cast-in-place pile project according to an embodiment of the present invention.

[0070] Reference numerals include:

[0071] Above-ground section 1, upper pile section 2, lower pile section 3, effective pile length 4, enlarged head diameter 5, transmission tower 6, concrete protective cap 7, tower anchor assembly 8, prestressed section / non-prestressed section dividing point 12, cast-in-place pile body 13, stirrups 14, pile longitudinal main reinforcement 15, pile diameter 16, enlarged head 17, pile top expansion distance 19, steel bar anchor plate 24, annular positioning plate 41, bearing plate 42, fastening nut 43, strain gauge 51, thread 61, sheath 71, retarding adhesive 72, positioning plate anchor bar 101, positioning plate steel bar hole 102.

[0072] The purpose, features and advantages of the present invention will be further described with reference to the accompanying drawings and in conjunction with the embodiments. DETAILED DESCRIPTION

[0073] It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0074] The low prestressed crack-free tension-compression cast-in-place pile provided by the present invention is particularly suitable for the anti-pullout pile of the foundation of the power transmission tower of the collector line in a corrosive environment. Figure 1 This is a schematic diagram of the overall structure of a low prestressed crack-free tension-compression cast-in-place pile provided by one embodiment of the present invention. Figures 2 to 4 They are Figure 1 Schematic diagrams of sections 1-1, 2-2 and 3-3, Figures 5 to 7 They are Figures 1 to 3 A magnified detail in the image, Figure 8 is a schematic structural diagram of a positioning plate according to an embodiment of the present invention, Figure 9 for Figure 8 4-4 cross-sectional diagram in FIG. Figure 10 is a schematic structural diagram of a pressure plate according to an embodiment of the present invention, Figure 11 for Figure 10 Schematic diagram of section 5-5.

[0075] like Figures 1 to 11 As shown in the figure, the low prestressed crack-free tension-compression cast-in-place pile disclosed in the present invention is a crack-free cast-in-place pile suitable for use in power transmission towers (meeting the crack control level of level 2), which can withstand both downward pressure and upward pull. This type of cast-in-place pile is artificially divided into two sections during design, namely the upper pile section 2 and the lower pile section 3. The longitudinal main reinforcement 15 of the pile is ordinary hot-rolled steel bar (HPB300, HRB400, HRB500, etc.), and the longitudinal main reinforcement 15 of the pile is a full-length one. The difference is that a sheath 71 is provided outside the longitudinal main reinforcement 15 of the upper pile section 2. A lower prestress is applied to the upper pile section 2 by tightening a nut at the top of the pile to form a partially prestressed pile body.

[0076] The basic principle of the present invention is that under the action of an upward pull, the upper pile section experiences a greater tensile force. Therefore, partial prestress (precompression force) is applied to control the concrete tensile stress of the upper pile section 2 to not exceed the concrete tensile stress control limit. When subjected to the pullout force, the lower pile section experiences a smaller tensile force due to lateral resistance. Therefore, the lower pile section 3 is treated as a normal pile section and no prestress is applied to it. The upward pullout force is then shared by the concrete pile body and the longitudinal main reinforcement, controlling the concrete tensile stress of the lower pile section 3 to not exceed the concrete tensile stress control limit. This achieves the goal of achieving a full-length crack control level of Level 2 (i.e., concrete tensile stress does not exceed the standard value of concrete axial tensile strength).

[0077] Specifically, such as Figure 1 As shown, in this embodiment, the low prestressed crack-free tension-compression cast-in-place pile mainly includes pile longitudinal main reinforcement 15 arranged along the pile circumference, stirrups 14 cooperating with the pile longitudinal main reinforcement 15 to form a steel cage, and a concrete cast-in-place pile body 13 cast on the steel cage; wherein, the cast-in-place pile is divided into a lower pile section 3, an upper pile section 2 and an above-ground section 1 from bottom to top, the dividing point between the upper pile section 2 and the lower pile section 3 is the prestressed section / non-prestressed section dividing point 12 of the cast-in-place pile, the length a of the upper pile section 2 and the length b of the lower pile 3 together constitute the effective pile length l of the low prestressed crack-free tension-compression cast-in-place pile of this embodiment, that is, l=a+b; the height "s" of the above-ground section 1 is generally 200mm~300mm; the pile diameter 16 is d.

[0078] A thread 61 and a fastening nut 43 screwed to the top of the pile longitudinal main reinforcement 15 are provided at the top end of the pile longitudinal main reinforcement 15, and a pressure plate 42 and an annular positioning plate 41 are provided in sequence below the fastening nut 43; wherein the annular positioning plate 41 is used to constrain all the pile longitudinal main reinforcements 15 and to disperse the pressure of the pressure plate 42 during tensioning.

[0079] In addition, a sheath 71 is provided over the longitudinal main reinforcement 15 of the upper pile segment 2. This sheath 71 can be a ribbed HDPE sheath, or alternatively, a steel pipe or other plastic sheath. In a preferred embodiment of the present invention, a retarding adhesive 72 is applied between the longitudinal main reinforcement 15 of the upper pile segment 2 and the sheath 71. This prevents further loss of prestress in the reinforcement of the upper pile segment 2 even if the anchor head loosens.

[0080] Prestress is applied to the upper pile segment 2 by tightening a nut (tightening nut 43) at the pile top to form a partially prestressed pile body, so as to control the concrete tensile stress of the upper pile segment 2 not to exceed the control limit of the concrete tensile stress.

[0081] In order to ensure that the concrete at the top of the pile is not crushed and provide a sufficient working surface when prestressing is applied to the top of the pile, in a specific embodiment of the present invention, an enlarged head 17 is further provided at the top of the cast-in-place pile. Figure 2 As shown, the enlarged head is represented by its enlarged head diameter 5, which is denoted by D. Of course, as an alternative, the enlarged head can also be cancelled, and in this case, the pile top expansion distance 19 is 0 (i.e. e=0).

[0082] In addition, in order to better fix the lower pile segment 3, an anchor 24 can be provided at the bottom end of the lower pile segment 3, and the anchor 24 is welded or screwed to the pile longitudinal main reinforcement 15. In a specific embodiment of the present invention, the anchor 24 is a steel bar anchor plate, which is sleeved on the pile longitudinal main reinforcement 15 and welded to it. When the calculated length "b" of the lower pile segment 3 is greater than the anchorage length of the pile longitudinal main reinforcement 15, as an alternative, the anchor may not be used. When a steel bar anchor plate is required, as an alternative, end anchoring may be performed by methods such as stick welding anchor bars, plug welding anchor plates, bending longitudinal reinforcements, and welding nuts to the bottom of longitudinal reinforcements.

[0083] It should be noted that Figure 1 The placement direction of the steel anchor plate shown is for reference only. In actual operation, it can be placed in either the upright or reverse direction.

[0084] The pile longitudinal main reinforcement 15 in the present invention generally uses hot-rolled ordinary steel bars HRB400. As an alternative, hot-rolled ordinary steel bars of other grades such as HPB300 and HRB500 can also be used; in addition, the number of pile longitudinal main reinforcements should be determined by the design drawings. Figure 1 The examples are for reference only and the layout principle is to arrange them evenly around the pile.

[0085] The thread 61 in this embodiment is a rolled thread, which can improve the tensile strength of the steel bar while forming the thread. As an alternative, after calculation and verification, a cut thread can also be used.

[0086] Figure 8 and Figure 9FIG. 4 shows the structure of the annular positioning plate 41 according to an embodiment of the present invention. Figure 8 and Figure 9 As shown, a positioning plate through-steel hole 102 corresponding to the pile longitudinal main reinforcement 15 is provided on the annular positioning plate 41. The diameter of the positioning plate through-steel hole 102 is d3, and its value should be able to ensure that the pile longitudinal main reinforcement 15 passes through. Preferably, a structural positioning plate anchor 101 can also be added to the annular positioning plate. The structural positioning plate anchor 101 is evenly distributed on the annular positioning plate 41 to ensure the stability of the annular positioning plate 41. The annular positioning plate 41 is a member that serves to restrain the longitudinal reinforcement and disperse the pressure of the pressure plate 42 during tensioning. The annular positioning plate 41 can be made of a steel plate that is not less than 6 mm, and its width should not be less than the maximum plane dimension of the pressure plate 42.

[0087] In addition, in order to provide a fixing point for the tower, tower anchor bolt assemblies 8 are evenly arranged in the inner area of the annular positioning plate 41 on the ground section. The tower anchor bolt assemblies 8 are used to fix the tower column base on the cast-in-place pile.

[0088] In order to determine the torque required for the pile longitudinal main reinforcement 15 to reach the designed pre-tension under the corresponding conditions (with or without oil), so as to ensure that the pre-tension of the steel bar reaches the designed value during the tensioning process. In a specific embodiment of the present invention, the low prestressed crack-free tension-compression type cast-in-place pile also includes a strain gauge 51, such as Figure 5 As shown, the stress calculated from the strain gauge can be converted into tension and compared with the corresponding tightening torque to determine the torque required for the pile longitudinal main reinforcement 15 to reach the designed pre-tension under the corresponding conditions.

[0089] The following is an illustrative description of the design method for low-prestress, crack-free, tension-compression cast-in-place piles provided by the present invention. This method primarily addresses the following issues: It provides a method for calculating the prestress to be applied to the upper pile segment 2, verifying this prestress by attaching strain gauges to the main reinforcement, and provides a method for calculating the lengths of the upper and lower pile segments 2 and 3.

[0090] In addition, as an extension, in a non-corrosive environment (i.e., a micro-corrosive environment), the cracks in the foundation piles can also be controlled within a specified range by applying a lower prestress to the steel bars of the upper pile section 2 as described in the present invention.

[0091] It should be noted that the calculation formulas cited in the following examples are intended solely to illustrate the logic of the design and calculation methods described herein and are not intended to be exclusive. For example, when calculating the ultimate vertical bearing capacity of a single pile, this example uses one of the methods in the Technical Specification for Building Pile Foundations (JGJ 94-2008) (Formula 5.3.6 of that specification).

[0092] In the calculation method provided in this embodiment, the pile tensile stress control sections are respectively the ground elevation and the top surface of the lower pile section.

[0093] The calculation formula for the standard value of the ultimate bearing capacity of a single compressed pile is as follows:

[0094]

[0095] Parameters: Q uk is the standard value of the ultimate bearing capacity of a single pile, Q sk is the standard value of the total limit lateral resistance, Q pk is the standard value of the total ultimate end resistance, u is the circumference of the pile body, ψ si , ψ p are the size effect coefficients of the side resistance and end resistance of large diameter piles, q sik is the ultimate lateral resistance of the i-th layer of soil on the pile side, l i is the thickness of the i-th layer of soil around the pile, q pk is the standard value of the ultimate end resistance of a pile with a diameter of 800 mm, A p is the pile tip area.

[0096] The calculation method for determining the bearing capacity of a single pile is as follows:

[0097] a. Pile pullout force calculated according to the standard combination of load effects N kb , should be verified according to formula 2.

[0098] N kb ≤T uk / K+G p (Formula 2)

[0099] Parameters: N kb is the pile pullout force calculated according to the standard combination of load effects, T uk is the standard value of the ultimate bearing capacity of the pile, G p is the deadweight of the foundation pile, the floating weight is taken below the groundwater level, K is the safety factor of the foundation pile design, for the collector line project. uk , K and can be calculated according to Article 9.4.3 and Article 3.1.5 of the "Code for Design of Overhead Transmission Line Foundations" DL / T 5219-2023, which will not be repeated here.

[0100] b. The pile pressure calculated according to the load effect standard combination should not be greater than the characteristic value of the vertical downward bearing capacity of a single pile determined by formula 3, R a .

[0101] N ky ≤η p R a (Formula 3)

[0102] Parameters: N kyis the vertical pressure of the pile calculated according to the standard combination of load effects, η p The adjustment coefficient for the bearing capacity of pile foundation is 1.1 for vertical suspension towers, 1.0 for tension straight line (0° angle) and suspended angle towers, and 0.9 for tension angle, terminal and long span towers. a is the characteristic value of the vertical downward bearing capacity of a single pile, R a =Q uk / 2, Q uk is the standard value of the ultimate bearing capacity of a single pile, and 2 is the safety factor. Relevant regulations can be found in Articles 9.3.1 and 9.3.2 of the "Code for Design of Overhead Transmission Line Foundations" (DL / T 5219-2023), and will not be repeated here.

[0103] The calculation method of the compressive bearing capacity of the pile cross section is as follows:

[0104] Among them, the compressive bearing capacity of the pile body concrete of the upper pile section 2 should meet the following requirements:

[0105] N+γF≤ψ c f c A ps (Formula 4)

[0106] The compressive bearing capacity of the pile body of the lower pile section 3 shall meet the following requirements:

[0107] N+γ(G1+G3-R a上 )≤ψ c f c A ps +0.9f y 'A s ′ (Formula 5)

[0108] A ps =A-λA s (Formula 5a)

[0109] Among them, the parameters are: N is the design value of the axial pressure at the top of the pile under the basic combination of load effects, γ is the comprehensive partial coefficient, standard value × γ = design value, γ can be set between 1.25 and 1.35 according to the situation, and 1.25 is temporarily taken in this embodiment. F is the total effective prestress applied by all longitudinal reinforcements at the top of the pile, ψ c is the pile construction coefficient (the value can be determined according to Article 5.8.3 of the Technical Specification for Building Pile Foundations JGJ94-2008. In this embodiment, it is temporarily taken as 0.75), f c A is the design value of concrete axial compressive strength; ps is the net cross-sectional area of the pile concrete, A is the gross cross-sectional area of the pile, λ is the area expansion coefficient of the longitudinal main reinforcement of the pile, A s is the cross-sectional area of the longitudinal main reinforcement of the pile; R a上 is the characteristic value of the pull-out bearing capacity of the single pile in the upper pile section 2 (only the lateral resistance part), fy ′ is the design value of the compressive strength of the longitudinal main reinforcement of the pile, A s ' is the cross-sectional area of the longitudinal main reinforcement of the pile; G1 is the deadweight of the upper pile section 2, and the water buoyancy should be deducted below the groundwater level; G3 is the deadweight of the pile section above the ground (including the enlarged head 17 and the concrete protective cap 7).

[0110] It should be noted that, regarding A ps In general, the reinforcement ratio of cast-in-place piles ranges from 0.65% to 0.2%. According to Article 6.2.15 of the "Concrete Structure Design Standard" GB / T 50010-2010 (2024 Edition), A can be taken. ps =A; if the reinforcement ratio is large (greater than or equal to 3%), the net cross-sectional area can be used. For the sake of accuracy, the net cross-sectional area is used in this embodiment.

[0111] Furthermore, λ is defined as the area expansion coefficient for the longitudinal main reinforcement 15 of the pile. For the ordinary steel bars used in this embodiment, its value can be 1.5. If rolled threaded steel bars are used, it can be 1.3. This value takes into account the presence of PVC or HPDE sheathing for post-tensioned prestressed steel bars, which further reduces the net concrete area of the pile shaft. For non-prestressed sections, λ is set to 1.0.

[0112] The calculation formula for the tensile bearing capacity of the pile cross section is as follows:

[0113] T1≤f y A s (Formula 6)

[0114] Parameters: T1 is the design value of the pile top axial tension under the basic combination of load effects, f y is the design value of tensile strength of ordinary steel bars, A s The cross-sectional area of the longitudinal main reinforcement for the pile is 15.

[0115] The calculation formula for crack resistance control is as follows:

[0116] σ ck -σ pc ≤f tk (Equation 7)

[0117] Among them, σ ck is the normal stress of the pile cross section under the standard combination of load effects, σ pc f is the prestress of the pile concrete after deducting all stress losses, tk It should be noted that, in this embodiment, the crack control is based on secondary crack control, and the crack control is calculated only according to the load effect standard combination.

[0118] As an extended application of the present invention, cracks are permitted in non-corrosive environments (i.e., micro-corrosive environments), but must be kept within a certain range. Alternatively, the present invention's method of applying a lower prestress to the steel bars of the upper pile section 2 can be used to control pile cracks within the specified range. In this case, the crack width can be calculated according to the relevant provisions of the "Concrete Structure Design Standard," and this invention will not elaborate further.

[0119] As another extended application of the present invention, the method described in the present invention can also be used for design calculations in situations where tensile stress is permitted in the concrete of the pile cross-section, but the tensile stress needs to be limited to a range stricter than the standard (e.g., requiring the tensile stress to not exceed 50% of the standard value of the concrete tensile strength). This also falls within the scope of protection of the present invention.

[0120] The calculation method for the minimum value of prestress to be applied to the top of the longitudinal main reinforcement 15 of the pile is as follows:

[0121] The minimum total effective prestress F that should be applied to the longitudinal main reinforcement 15 of the pile at the top of the upper pile section 2 (i.e., the ground position) is determined by the following formula:

[0122] σ ck -σ pc =(T-G3-F) / A ps ≤f tk (Equation 8)

[0123] Parameters: T is the standard value of the axial tension at the top of the pile under the standard combination of load effects, T = T1 / γ, T1 is the design value of the axial tension at the top of the pile under the basic combination of load effects, γ is the comprehensive partial factor, F is the total effective prestress applied by all longitudinal reinforcements at the top of the pile, f tk It is the standard value of the axial tensile strength of concrete.

[0124] The length a of the upper pile segment 2 is calculated as follows:

[0125] like Figure 12 、 Figure 13 As shown in the figure, the calculation assumes that the length of the upper pile segment 2 is a, and there are m soil layers; the length of the lower pile segment 3 is b, and there are n soil layers. The soil layers are labeled with the variable "i".

[0126] The characteristic value of the pull-out bearing capacity of a single pile in the upper pile section 2 (lateral resistance part) is:

[0127]

[0128] Parameters: R a上 is the characteristic value of the pull-out bearing capacity of the single pile in the upper pile section 2 (lateral resistance part), λ i is the pull-out coefficient, which is taken as 0.7 in the calculation of this embodiment, q sik is the ultimate lateral resistance of the i-th layer of soil on the pile side, u iis the pile perimeter at the location of the i-th layer of soil on the pile side, l i is the thickness of the i-th soil layer around the pile, K is the design safety factor of the pile foundation, and m is the number of soil layers corresponding to the upper pile section.

[0129] The upward pull force P on the critical surface BB at the uppermost end of the lower pile segment 3 is:

[0130] P=T-G1-G3-R a上 (Equation 10)

[0131] AA section position converted to cross-sectional area A eq Determine as follows:

[0132] A eq =A c +n s A s (Equation 11)

[0133] The parameters are: P is the upward pull force acting on the uppermost critical surface BB of the lower pile segment (3), A eq A is the converted cross-sectional area at the top of the lower pile segment 3. c is the cross-sectional area of the pile concrete, A s is the cross-sectional area of the longitudinally loaded ordinary steel bar, n s is the ratio of the elastic modulus of the longitudinally loaded ordinary steel bar to the elastic modulus of the pile concrete. The remaining parameters are shown in Equations 1 to 8.

[0134] The calculation formula for determining the length a of the upper pile section (2) is:

[0135] P / A eq ≤f tk

[0136] Right now:

[0137]

[0138] In this embodiment, the prestressing force of the longitudinal main reinforcement 15 of the pile is applied by tightening the nut with a torque wrench. This method is convenient to construct and has precise control, and is particularly suitable for field construction operations of wind power projects.

[0139] When applying the tightening torque, the preload force can be controlled by using the torque wrench method, nut angle method, indicator washer method, bolt elongation method and bolt pre-elongation method.

[0140] Tightening torque can be determined according to relevant manuals (such as the method outlined in the Mechanical Design Manual, 6th Edition, Volume 2, pages 6-71) and verified using strain gauges. This is a well-established method used for many years and is not within the scope of this application. It is listed here only to provide a comprehensive description of the design process.

[0141] In terms of prestress verification, this embodiment uses a method of attaching a resistance strain gauge to the unthreaded portion of the bolt to control the tension on the bolt shank, with the error being controlled within ±1%. The basic principle is:

[0142] like Figure 5 As shown, the stress calculated by back-calculating the strain of the strain gauge 51 is converted into tension and compared with the corresponding tightening torque to determine the torque required to be applied to the pile longitudinal main reinforcement 15 to achieve the designed pre-tension under the corresponding conditions (with oil or without oil) to ensure that the pre-tension of the steel bar during the tensioning process meets the design requirements.

[0143] In addition, to reduce construction costs and facilitate construction, the method of attaching strain gauges 51 to determine the tightening torque can be tested only under laboratory conditions. It is sufficient to determine the tightening torque required to achieve the designed pre-tension. It is not necessary to measure it on site, nor is it necessary to set it on each pile longitudinal main reinforcement.

[0144] The construction method of the above-mentioned crack-free tension-compression cast-in-place pile can be specifically implemented through the following steps:

[0145] Step 1: Make the longitudinal main reinforcement of the pile 15

[0146] a. Process threads on the top of the longitudinal main reinforcement 15 of the pile. Rolled threads 61 should be used first, but cut threads can also be used.

[0147] b. Connecting rebar. Rebar joints should be located in the lower pile section. When rebar joints must be located in the upper pile section, pneumatic welding is recommended. The joints should be processed so that their outer diameter is no larger than the outer diameter of the connected rebar. When other joints are used, matching sheaths should be used to ensure that the rebar can expand and contract freely when tensioned.

[0148] c. Apply slow-setting adhesive 72 to the steel bars of the upper pile section and cover them with HDPE sheath 71 with external ribs. The sheath should be firmly tied up and sealed at the top and bottom.

[0149] Step 2: Make the steel cage

[0150] Specifically, a roll welder can be used to wrap spiral stirrups around the longitudinal main reinforcement 15 and place stirrups 14 (including spiral stirrups and reinforcing stirrups). The HDPE sheath 71 with external ribs should be tied and connected firmly to the stirrups.

[0151] Step 3: Place casing on the pile

[0152] Step 4: Hole formation and hole cleaning. Hole formation can be done by using slurry wall forward and reverse circulation, rotary drilling, long spiral drilling and other technologies.

[0153] Step 5: Hang the steel cage.

[0154] Step 6: Pour concrete into the conduit.

[0155] Step 7: Place the annular positioning plate 41 and the tower anchor assembly 8 at the designed elevation.

[0156] Step 8: After the pile reaches the designed strength, a pressure plate and matching nuts are placed on the upper end of the longitudinal main reinforcement 15 of the pile.

[0157] Step 9: Apply prestress.

[0158] The fastening nuts 43 are tightened with a torque wrench in a symmetrical, graded, and multi-round loading manner, so that the longitudinal main reinforcement 15 of the pile reaches the designed tension force.

[0159] The torque wrench should not be tightened to the required final torque value in one go. The prestress applied each time should be 10% to 20% of the design value to prevent the pile body from generating excessive unbalanced torque.

[0160] Step 10: Install the transmission tower 6 and pour the pile top concrete protection cap 7 to complete the work.

[0161] Figure 14 The following is an example of a low prestressed crack-free tension-compression cast-in-place pile project according to an embodiment of the present invention. Figure 14 As shown in the figure, the basic design conditions of this engineering example are as follows:

[0162] A collector line tension corner tower is planned to utilize a cast-in-place pile foundation, with a designed concrete strength grade of C35. The foundation root depth is 5670 mm, requiring the piles to extend 300 mm above the ground. The standard uplift force to be borne by the pile top is 863.5 kN, and the standard downforce is 840 kN. The groundwater is located 3.4 m below the surface, within a soil layer consisting of four layers of fine sand and silt, with the fifth silt layer not drilled through. The geotechnical parameters for each layer of pile foundation are shown in the table below.

[0163] Recommended design parameter table for mud-wall drilled (driven) piles

[0164]

[0165] The calculation principles are as follows:

[0166] a. The calculation method in this embodiment should be performed by trial calculation.

[0167] b. The application principle of the calculation method in this embodiment is: under the premise of meeting the requirements of the specifications or relevant demand parties, minimize the length a of the upper pile segment and minimize the prestress value F to be applied to the longitudinal main reinforcement of the pile, because a higher prestress is not conducive to resisting external environmental erosion.

[0168] c. For a given soil layer, the length a of the upper pile section and the length b of the lower pile section are directly related to the magnitude of the prestress applied to the top of the longitudinal main reinforcement 15 of the pile. The result of the trial calculation is to determine several groups of data that meet the requirements, which will be optimized by the designer.

[0169] First, determine the effective pile length, pile diameter, and reinforcement according to Formula 1 and Formula 2.

[0170] When the pile diameter d = 600mm, according to Article 3.1.5 and Article 9.3.2 of the "Code for Design of Overhead Transmission Line Foundations" DL / T 5219-2023, for tension corner towers, the pile pullout safety factor and compressive safety factor are 2.3 and 2.0 respectively. After calculation, when the pile length is 27m, the characteristic value of the pullout bearing capacity of a single pile is 873kN, and the characteristic value of the compressive bearing capacity of a single pile (by η p After correction, the load is 1260 kN. The pile length is controlled by the pull-out resistance. Therefore, the pile length is determined to be 27 m.

[0171] The required reinforcement is HRB400 3006mm 2 . 12 φ18HRB400 steel bars, As=3054mm 2 The total tensile strength design value of the actual steel bars is 1099.3kN.

[0172] Secondly, the crack resistance control calculation of the upper pile section is carried out according to conventional methods.

[0173] When the technical solution of the present invention is not adopted, the tensile stress of the pile concrete calculated by formula 7 is 2.89N / mm 2 , exceeding the standard value of C35 concrete axial tensile strength of 2.2N / mm 2 , which does not meet the requirements of crack control level 2.

[0174] Adjust the pile diameter to d = 700 mm, and calculate the pile concrete tensile stress according to formula 7 to be 2.15 N / mm 2 , meeting the crack control level 2 requirement. At this point, compared with the d = 600mm pile, the pile concrete consumption increases by 36%.

[0175] If the commonly used diameters of pile foundations are 600mm and 800mm, and a pile with d=800mm is used, the tensile stress of the pile concrete calculated by formula 7 is 1.66N / mm 2 At this time, compared with the d=600mm pile, the pile concrete consumption increases by 78%.

[0176] Since the method of increasing the pile diameter is less economical, the crack resistance control calculation of the upper pile section is carried out according to the scheme of the present invention (using piles with d = 600 mm), which specifically includes:

[0177] a. Using the calculation formula (Equation 8) provided by the present invention, the prestress value required to be applied to the pile top and the normal tensile stress at the AA section at the top of the upper pile segment 2 are shown in the following table:

[0178]

[0179]

[0180] The results in the table above show that the ratio of the total prestress F at the pile top to the total tensile strength of the steel bar is 0.3, that is, the total prestress F is 30% of the total tensile strength of the steel bar, which is 1099.3kN×

[0181] When 0.3=327.79kN, the normal stress of the pile body concrete in the upper pile section can be controlled to be no greater than the standard value of the concrete axial tensile strength, that is, the requirement of no cracks in the pile body is met, which is easy to achieve with ordinary steel bars. This is also the reason why the present invention is called a low prestressed crack-free cast-in-place pile.

[0182] b. Using the calculation formula (Formula 12) provided in the preceding embodiment, the minimum value of the upper pile length a is determined as shown in the following table:

[0183]

[0184]

[0185] The results in the table above show that when the length of the upper pile section reaches 10m, the tensile stress of the concrete in the BB section is 2.16N / mm 2 , which is less than the standard value of axial tensile strength of C35 concrete 2.2N / mm 2 .

[0186] In summary, in this example, the final pile foundation scheme is as follows: there is no need to increase the pile diameter, the pile diameter is still 600mm, the effective pile length is 27m, 12 φ18HRB400 steel bars are installed, the length of the upper pile section (prestressed section) is 10m, and the effective prestress value applied to the pile top is 30% of the design value of the tensile strength of all longitudinal main bars, that is, 327.79kN. This can ensure that the crack design level is level 2 under corrosive conditions along the entire length of the pile.

[0187] In the calculations of the above examples, some conventional calculations also meet the requirements, such as the calculation of the compressive bearing capacity of the pile body of the lower pile section 3 according to Formula 5, which will not be listed here one by one.

[0188] It should be understood that the embodiments are for illustrative purposes only and are not intended to limit the scope of the application. It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the present invention may be implemented in other specific forms without departing from the spirit or essential features of the present invention.

[0189] Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive. The scope of the present invention is defined by the appended claims rather than the foregoing description, and all changes that come within the meaning and range of equivalents of the claims are intended to be embraced therein. Any reference to any figure or reference numeral in a claim should not be construed as limiting the claim.

[0190] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described in detail with reference to preferred embodiments, it should be understood by those skilled in the art that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A low prestressed crack-free tension-compression cast-in-place pile, characterized in that: It includes longitudinal main reinforcements arranged along the circumference of the pile, stirrups that cooperate with the longitudinal main reinforcements to form a reinforcement cage, and a concrete pile body poured on the reinforcement cage; wherein, The cast-in-place pile is divided into a lower pile section, an upper pile section and an above-ground section from bottom to top, and the dividing point between the upper pile section and the lower pile section is the dividing point between the prestressed section and the non-prestressed section of the cast-in-place pile; A thread and a fastening nut are provided at the top of the longitudinal main reinforcement of the pile, and a pressure plate and an annular positioning plate are provided in sequence below the fastening nut; wherein the annular positioning plate is used to constrain the longitudinal main reinforcement of the pile and to disperse the pressure of the pressure plate during tensioning; A sheath is provided outside the longitudinal main reinforcement of the pile in the upper pile section; By controlling the length of the upper pile section and applying prestress to the upper pile section by tightening a nut at the pile top, a partially prestressed pile body is formed, so that the concrete tensile stress of the upper pile section and the lower pile section is controlled to not exceed the control limit of the concrete tensile stress.

2. The low prestressed crack-free tension-compression cast-in-place pile according to claim 1, characterized in that: The utility model also comprises an enlarged head arranged on the top of the cast-in-place pile, and the enlarged head is used for protecting the concrete on the pile top when prestress is applied to the pile top.

3. The low prestressed crack-free tension-compression cast-in-place pile according to claim 1, characterized in that: It also includes a retarded setting adhesive, which is arranged between the longitudinal main reinforcement of the pile in the upper pile section and the sheath.

4. The low prestressed crack-free tension-compression cast-in-place pile according to claim 3, characterized in that: The sheath is a steel pipe sheath or a HDPE sheath with external ribs.

5. The low prestressed crack-free tension-compression cast-in-place pile according to claim 1, characterized in that: It also includes an anchoring piece, which is arranged at the bottom end of the lower pile section and fixed to the longitudinal main reinforcement of the pile.

6. The low prestressed crack-free tension-compression cast-in-place pile according to claim 5, characterized in that: The anchor piece is any one of a steel bar anchor plate, a welded anchor bar, a plug welded anchor plate, a bent longitudinal bar, and a welded nut at the bottom of the longitudinal bar.

7. The low prestressed crack-free tension-compression cast-in-place pile according to claim 5, characterized in that: It also includes an iron tower anchor bolt assembly, which is arranged on the above-ground section and is used to fix the iron tower column foot on the cast-in-place pile.

8. The low prestressed crack-free tension-compression cast-in-place pile according to any one of claims 1 to 7, characterized in that: The longitudinal main reinforcement of the pile is made of ordinary steel bars.

9. The low prestressed crack-free tension-compression cast-in-place pile according to any one of claims 1 to 7, characterized in that: The compressive bearing capacity of the pile body concrete of the upper pile section meets the following requirements: N+γF≤ψ c f c A ps The compressive bearing capacity of the pile body of the lower pile section meets the following requirements: N+γ(G1+G3-R a上 )≤ψ c f c A ps +0.9f y ′A s ′ A ps =A-λA s Where N is the design value of the axial pressure on the pile top under the basic combination of load effects, γ is the comprehensive partial factor, standard value × γ = design value, F is the total effective prestress applied by all the longitudinal main reinforcements at the pile top, ψ c is the pile construction coefficient, f c A is the design value of concrete axial compressive strength; ps is the net cross-sectional area of the pile concrete, A is the gross cross-sectional area of the pile, λ is the area expansion coefficient of the longitudinal main reinforcement of the pile, A s is the cross-sectional area of the longitudinal main reinforcement of the pile; R a上 is the characteristic value of the pull-out bearing capacity of a single pile in the upper pile section, f y ′ is the design value of the compressive strength of the longitudinal main reinforcement of the pile, A s ′ is the cross-sectional area of the longitudinal main reinforcement of the pile; G1 is the deadweight of the upper pile section, and G3 is the deadweight of the above-ground section.

10. A design method for low prestressed crack-free tension-compression cast-in-place piles, characterized in that: Used for designing a low prestressed crack-free tension-compression cast-in-place pile according to any one of claims 1 to 9; wherein the concrete tensile stress at the top of the upper pile section is determined by the following formula: s ck -s pc =(T-G3-F) / A ps ≤f tk Among them, σ ck is the normal stress of the pile cross section under the standard combination of load effects, σ pc f is the prestress of the pile concrete after deducting all stress losses, tk is the standard value of the axial tensile strength of concrete, and T is the standard value of the axial tension at the top of the pile under the standard combination of load effects.

11. The design method of low prestressed crack-free tension-compression cast-in-place pile according to claim 10, characterized in that: The length a of the upper pile section is determined as follows: Assume that the length of the upper pile section is a, corresponding to a total of m soil layers, the length of the lower pile section is b, corresponding to a total of n soil layers, and the soil layers are all marked with the variable "i", then, The characteristic value of the pull-out bearing capacity of a single pile in the upper pile section is: Among them, R a上 is the characteristic value of the pull-out bearing capacity of a single pile in the upper pile section, λ i is the pull-out coefficient, q sik is the ultimate lateral resistance of the i-th layer of soil on the pile side, u i is the pile perimeter at the location of the i-th layer of soil on the pile side, l i is the thickness of the i-th soil layer around the pile, K is the design safety factor of the pile foundation, and m is the number of soil layers corresponding to the upper pile section; The upward pull force P on the critical surface at the upper end of the lower pile section is: P=T-G1-G3-R a上 Cross-sectional area A converted from the critical surface position at the top of the lower pile segment eq Determine as follows: A eq =A c +n s A s Where P is the upward pull force acting on the critical surface at the upper end of the lower pile segment, T is the standard value of the axial tension at the top of the pile under the standard combination of load effects, and A is eq A is the cross-sectional area converted from the top cross-sectional position of the lower pile segment. c is the cross-sectional area of the pile concrete, A s is the cross-sectional area of the longitudinally loaded ordinary steel bar, n s It is the ratio of the elastic modulus of the longitudinally stressed ordinary steel bar to the elastic modulus of the pile concrete; The calculation formula for the upper pile section length a is: P / A eq ≤f tk Among them, f tk It is the standard value of the axial tensile strength of concrete.

12. A construction method for low prestressed crack-free tension-compression cast-in-place piles, characterized in that: Used for constructing low prestressed crack-free tension-compression cast-in-place piles as described in any one of claims 1 to 9; the method comprises: Making the longitudinal main reinforcement of the pile; including: processing threads on the top of the longitudinal main reinforcement of the pile; Fabricate the reinforcement cage, which includes: wrapping spiral stirrups around the longitudinal main reinforcement of the pile using a roll welder and placing spiral stirrups and reinforcing stirrups; Place casing on the pile site; Hole forming and hole cleaning; Hanging and placing steel cage; Pressure pouring concrete through the conduit; Place the annular positioning plate and tower anchor assembly at the designed elevation; After the pile reaches the designed strength, the bearing plate and matching nuts are placed on the upper end of the longitudinal main reinforcement of the pile; Apply prestressing force.

13. The construction method of low prestressed crack-free tension-compression cast-in-place pile according to claim 11, characterized in that: The method of making the longitudinal main reinforcement of the pile further includes: applying a slow-setting adhesive to the upper pile section and covering it with a sheath, and firmly tying the sheath up and down and sealing the opening.

14. The construction method of low prestressed crack-free tension-compression cast-in-place pile according to claim 12, characterized in that: During the process of applying prestress, a torque wrench is used to tighten the fastening nuts in a symmetrical, graded, and multi-round loading manner so that the longitudinal main reinforcement of the pile reaches a preset tension force.

15. The construction method of low prestressed crack-free tension-compression cast-in-place pile according to claim 11, characterized in that: During the process of applying prestress, the prestress applied each time is 10% to 20% of the design value.

16. The construction method of low prestressed crack-free tension-compression cast-in-place pile according to claim 11, characterized in that: After applying prestressing, it also includes: Installing transmission towers; Pour the concrete protective cap on the top of the pile.

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