A pile foundation construction method for reusing old piles

Abstract

This invention discloses a pile foundation construction method for reusing old piles. Without removing the old piles, the method involves performing integrity testing and load-bearing capacity assessment on the old piles, then excavating in layers around the old piles to a predetermined mortise and tenon connection depth. A mortise is machined along the pile axis at the center of the top section of the old pile. Fiber-reinforced concrete is then poured in situ within the mortise and the upper tenon forming area, forming an integral structure with the upper new pile bearing section concrete during the same pouring process. This creates an integrated composite pile foundation consisting of the old pile, the mortise and tenon connection area, and the new pile body. The mortise and tenon connection uses fiber-reinforced concrete to form a mechanical interlocking mechanism, eliminating the need for additional connectors. Combined with radial restraint measures such as metal hoops or fiber composite material restraint layers, the vertical ultimate bearing capacity of the composite pile can exceed the sum of the bearing capacities of the old and new piles, achieving synergistic load-bearing capacity enhancement. This method requires minimal working space, minimizes construction disturbance, and offers high connection reliability, making it suitable for in-situ extension and load-bearing capacity enhancement of existing old piles.

Description

Technical Field

[0001] This invention relates to the field of building engineering, and in particular to a pile foundation construction method for the reuse of old piles. Background Technology

[0002] With the increase in urban renewal and existing building renovation projects, a large number of old pile foundations are being abandoned or demolished, resulting in resource waste and environmental pollution. Traditional methods of dealing with old piles mostly involve direct removal or abandonment, which not only causes serious waste of resources such as steel and concrete, but also disturbs the surrounding soil, reduces the bearing capacity of subsequent new piles, and generates a large amount of carbon emissions, which is inconsistent with the concept of green construction.

[0003] Existing technologies for reusing old piles, such as CN121066137A and CN120592288A, which use composite slab connections and pile caps for co-bearing, have achieved partial utilization of old piles, but they have significant shortcomings: First, the connection between new and old piles is mostly indirect, resulting in discontinuous mechanical transmission paths, poor overall integrity, and a tendency for stress concentration; second, the shear and tensile strength of the connection parts is limited, and separation is likely to occur during long-term service; third, the problem of uneven settlement caused by the difference in stiffness between new and old piles has not been effectively solved, affecting the stability of the superstructure.

[0004] Chinese patent CN211143062U discloses a new-old combined pile for complex construction environments with high groundwater levels. This application's new-old combined pile fully utilizes the original pile, only addressing problematic sections. By retaining the original pile's outer wall as a protective shell for the new pile's construction, the new pile can be constructed in a waterless environment, ensuring its quality. Simultaneously, this new-old combined pile avoids the slow construction speed and high project costs associated with existing methods that use impact hammers to break the original pile. However, the core of this solution is using the original pile's outer wall as a protective shell to recast the new pile. The force transmission between the old and new piles mainly relies on the lateral friction of the retaining wall and potential bottom supports, failing to form a truly integral, integrated load-bearing structure along the pile axis. Under uplift loads, horizontal loads, and seismic forces, relative displacement or separation easily occurs between the old pile's outer shell and the new pile's inner core, resulting in insufficient connection reliability and failing to address the issue of extending the old pile after its upper section is cut off.

[0005] Chinese patent CN101922152A discloses a reinforced pile foundation structure, which includes an existing pile cap, with soil composite piles surrounding the existing pile cap. The pile body of the existing pile cap has rough edges, and newly installed reinforcing bars are arranged on the pile body of the existing pile cap. The newly installed reinforcing bars and the soil composite piles are interwoven. New pile cap concrete is poured around the pile cap and the soil composite piles. This application increases the load-bearing area of ​​the pile cap by setting soil composite piles around the existing pile cap. By roughening the pile body of the existing pile cap, connecting the newly installed reinforcing bars to the pile body, and then pouring new pile cap concrete around the pile cap and the soil composite piles, the existing pile cap and the soil composite piles are more tightly connected, forming a whole, allowing the new piles and the existing pile cap to share the load. However, this plan requires expanding the existing pile cap and installing reinforcement bars on the piles before pouring the new pile cap concrete. The construction requires large-area pile cap excavation and formwork support, which cannot be adapted to the extremely narrow space on both sides. At the same time, the new and old piles still indirectly transmit force through the pile cap, and the pile body itself does not form a rigid node connection. The connection between the pile and the pile cap is still a weak point under repeated loads, and it cannot fundamentally solve the problem of the new and old piles directly sharing the force and the overall settlement coordination.

[0006] In summary, how to solve the problems of limited working space and insufficient connection reliability in existing technologies, so as to further achieve synergistic efficiency and reuse, is a key requirement in the field of old pile reuse. Summary of the Invention

[0007] The purpose of this invention is to provide a pile foundation construction method for reusing old piles, which addresses the common problems in existing technologies such as limited working space, insufficient reliability of the connection method between old and new piles, and discontinuous force transmission in the reuse of old piles.

[0008] The objective of this invention can be achieved through the following technical solutions: In a first aspect, the present invention provides a pile foundation construction method for the reuse of old piles, comprising the following steps: (1) Conduct integrity testing and bearing capacity assessment on the old piles left after the demolition and renovation of the original building; (2) Excavate the soil layer by layer with the old pile as the center to the preset mortise and tenon connection depth. Cut off and discard the parts that do not meet the reuse requirements. For the parts that meet the reuse requirements, calculate the bearing capacity of the new and old composite piles. (3) Clean the surface of the top of the old pile and machine a mortise along the pile axis at the center of the top section of the old pile; (4) Clean the surface of the top of the old pile and the mortise, and build a customized template in the area around the top of the old pile and the mortise; (5) Fiber-reinforced concrete is poured in situ into the mortise and the upper tenon forming area, and then concrete is poured into the upper new pile bearing section and formed as a whole, so that the tenon and the new pile body are integrated. (6) After maintenance, a composite pile foundation structure integrating old and new piles is formed, consisting of old piles, mortise and tenon connection area and new pile body.

[0009] Furthermore, the old piles are solid concrete piles, excluding prestressed concrete pipe piles; because prestressed concrete pipe piles contain prestressed tendons in the pipe wall and have a cavity in the center of the pile body, it is difficult to process mortises at the center of the pile cross-section, so this method is not applicable.

[0010] Furthermore, the integrity test results in step (1) are graded and evaluated according to the degree of defects in the pile body; Among them, when using the low-strain method for testing, if the measured waveform is regular, the pile bottom reflection is clear, the amplitude of the reflected wave from the same phase defect does not exceed 10% of the incident wave amplitude, and the average longitudinal wave velocity of the pile body calculated from the round-trip time of the pile bottom reflection signal and the pile length meets the following conditions, the rating is Class I, indicating that the pile body is generally intact, has no obvious structural defects, and can be put into use: (1) The speed of non-prestressed concrete piles such as ordinary cast-in-place concrete piles is 3200m / s to 4200m / s; (2) The speed of high-strength concrete solid piles is 3500m / s to 4500m / s; A Class II rating is granted when all of the following conditions are met, indicating that the pile body has minor defects that do not affect its overall bearing capacity and reuse safety, and it is ready for use: (1) The measured waveform has a slight in-phase defect reflected wave with an amplitude between 10% and 30% of the incident wave amplitude; (2) The estimated value of the corresponding defect cross-sectional area shall not exceed 15% of the pile cross-sectional area; (3) Any possible slight local narrowing or mild segregation are discontinuous and non-continuous defects, and the wave velocity of the pile body has not been significantly reduced. When the reflected wave amplitude of a defect exceeds 30% of the incident wave amplitude, or the estimated cross-sectional area of ​​the defect is greater than 15% of the pile cross-sectional area, or the wave velocity is significantly reduced, or there are continuous and penetrating serious defects, it is rated as Class III or below, does not meet the requirements for reuse, and is cut off and discarded or disposed of separately.

[0011] Furthermore, the load-bearing capacity assessment in step (1) is calculated using the following formula: ; in: This represents the ultimate vertical bearing capacity of the composite pile. The ultimate vertical bearing capacity provided for the old piles shall be determined based on the test results of the old piles, the original design data, or the results of on-site verification. The ultimate vertical bearing capacity provided for the new pile shall be determined in accordance with the current pile foundation design code or through test methods; The synergistic effect coefficient is used to reflect the amplification effect of the enhanced radial constraint of the structure and the consolidation of the soil around the old pile on the vertical ultimate bearing capacity of the composite pile. Calculate using the following formula: ; in, The constraint enhancement coefficient ranges from 1.0 to 1.10, and its value is determined based on the radial constraint of the concrete in the connection zone by the metal hoop, fiber composite material constraint layer, or locally densified concrete layer. The consolidation enhancement coefficient ranges from 1.0 to 1.20, and is determined based on the effect of the consolidation of the soil around the pile on the bearing capacity of the old pile during its service life. The vertical control bearing capacity of the mortise and tenon joint area includes the comprehensive control value of the mortise and tenon joint area under compression, shear and pull-out conditions.

[0012] Furthermore, in step (2), when excavating in layers, the width of the transverse working face on one side of the excavation, that is, the distance from the outer edge of the old pile to the excavation edge, shall not exceed 0.5 times the diameter of the old pile; When there is shallow damage, material deterioration or insufficient bearing capacity in the upper part of the old pile, the old pile is cut off at the preset mortise and tenon connection depth, and the part below the cut-off is retained as a reused old pile segment. The old pile is extended and the new pile and the old pile are supported together by the mortise and tenon connection structure.

[0013] Furthermore, the characteristic value or allowable bearing capacity of the vertical bearing capacity of the new and old composite piles under normal use conditions mentioned in step (2) can be reduced by a safety factor based on their ultimate vertical bearing capacity. The calculation formula is as follows: ; in, This refers to the characteristic value or allowable bearing capacity of the vertical bearing capacity of the composite pile. The bearing capacity safety factor has a value range of 1 ≤ ≤2.0; where the recommended value range for actual engineering is 1.6 < ≤2.0: When the integrity level of the old pile is Class I, 1.6 < ≤1.8; when the integrity level of the old pile is Class II, 1.8 < ≤2.0; After special demonstration and verification by field load test, Can be 1≤ ≤1.6.

[0014] Further, in step (3), the mortise is determined based on the maximum aggregate particle size and fiber length of the concrete used. The specific determination method is as follows: the minimum transverse net width of the mortise should not be less than 4 times the maximum aggregate particle size and 1.2 times the fiber length. At the same time, according to the size of the old pile cross section and the thickness of the protective layer, a complete pile body concrete ring with an edge of not less than 100mm is left to avoid damage to the main reinforcement on the pile side during processing. Preferably, under typical dimensions, for an old pile with a pile diameter of 800mm, when using fiber concrete with a maximum aggregate particle size of 20mm and a fiber length of 30mm, the diameter or side length of the mortise is 250~300mm, and the depth is not less than 1.0 times the width of the mortise.

[0015] Furthermore, in step (4), a local reinforcement structure is fitted on the outside of the mortise and tenon connection area. The local reinforcement structure includes a metal hoop, a fiber composite material constraint layer, a locally densified concrete layer, or a combination thereof. The metal hoop is made of steel plate or steel strip, and the material is selected from Q235 grade steel, Q355 grade steel or Q420 grade steel. The hoop width is 80mm to 200mm and the thickness is 6mm to 12mm. The hoop is arranged axially along the mortise and tenon connection area, and the spacing is 100mm to 300mm. The gap between the hoop and the pile surface is filled with grout or structural adhesive. The fiber composite confinement layer is made of one or more of carbon fiber cloth or glass fiber cloth, and its material properties comply with the provisions of the current national standard "Code for Design of Strengthening Concrete Structures" GB50367-2013. Specifically, the polyacrylonitrile molecular weight of the carbon fiber cloth is not greater than 15K, the standard value of tensile strength is not less than 3000MPa, and the unit area mass is 200-300g / m². 2 The fiberglass cloth is selected from high-strength fiberglass, alkali-resistant fiberglass, or alkali-free fiberglass. The number of constraint layers is 1 to 3, and the interlayer and circumferential overlap length is not less than 100mm. It is impregnated and pasted on the outside of the mortise and tenon connection area with matching epoxy resin, and extends outward along the pile axis to cover not less than 200mm. The locally reinforced concrete layer is poured using fine aggregate concrete or fiber-reinforced concrete with a strength grade of not less than C40, the maximum particle size of coarse aggregate is not greater than 10mm, and the thickness is 50mm to 150mm.

[0016] Furthermore, in step (5), the fibers in the fiber-reinforced concrete are selected from steel fibers, polypropylene fibers, basalt fibers, or chopped carbon fibers. The concrete strength grade is not lower than C40. The mix proportion and fiber content are determined according to the current industry standard "Technical Specification for Application of Fiber Reinforced Concrete" JGJ / T221-2010, specifically as follows: When steel fibers are selected, the volumetric content of the steel fibers is 1.0% to 2.5%, the length of the steel fibers is 20mm to 60mm, the equivalent diameter is 0.3mm to 0.9mm, the aspect ratio is 30 to 80, the tensile strength is not less than 600MPa, and one of the end-hooked special-shaped steel fibers or the corrugated special-shaped steel fibers is used, and their performance meets the requirements of GB / T39147-2020 "Steel Fibers for Concrete"; When polypropylene fibers are selected, the volumetric content of polypropylene fibers is 0.05% to 0.3%, preferably 0.1% to 0.2%, and the fiber length is 15mm to 30mm. It is used to suppress early shrinkage cracks and improve the crack resistance and impermeability of tenon concrete. When basalt fiber is selected, the volume content of basalt fiber is 0.1% to 0.5%, and the length of basalt fiber is 12 mm to 24 mm. When chopped carbon fibers are selected, the volumetric content of the chopped carbon fibers is 0.2% to 1.0%, and the length of the chopped carbon fibers is 6 mm to 12 mm.

[0017] Furthermore, in the design or verification of the integrated composite pile foundation formed in step (5), the vertical ultimate bearing capacity of the composite pile is taken as... and The smaller value in; where The vertical ultimate bearing capacity provided for the old piles is determined based on the integrity test results and the original design data, and reduced according to the integrity level. The vertical ultimate bearing capacity provided for the new pile shall be calculated independently according to the current specifications; The synergistic effect coefficient, with a value range of 1.0 to 1.32, is used to amplify the superposition value of the vertical ultimate bearing capacity of old and new piles; The vertical control bearing capacity of the mortise and tenon joint area is determined by taking the minimum value among the local compressive bearing capacity of the tenon concrete, the shear bearing capacity of the grouting interface, and the pull-out bearing capacity of the mortise sidewall.

[0018] Furthermore, the ambient temperature for curing in step (6) should not be lower than 5°C; the relative humidity of the concrete surface should not be lower than 95% by covering with plastic film, spraying curing agent or sprinkling water, and the curing time should not be less than 14 days, and the grouting reinforcement area should be moisturized and cured simultaneously.

[0019] This invention draws upon the "mechanical interlocking" characteristic of traditional mortise and tenon structures, achieving a stable connection without the need for additional connectors. Furthermore, the construction process requires minimal working space, perfectly suited to the limited space requirements of reusing old piles. Therefore, based on the principles of mortise and tenon structures, this invention designs a method for combining new and old piles that requires a small working area and provides a stable connection, thereby solving the problems of limited working space and insufficient connection reliability in existing technologies.

[0020] Compared with the prior art, the present invention has the following beneficial effects: 1) The tenon of this invention is cast integrally in the mortise, without the need for insertion into the space. The lateral expansion width of the working surface is only 0.5 times the outer diameter of the old pile, reducing the disturbance to the soil around the old pile. At the same time, the new pile is directly grafted onto the old pile, without the need to set a new pile cap on the top of the old pile, further reducing the working surface and the disturbance to the soil around the old pile.

[0021] 2) The present invention uses fiber-reinforced concrete to cast tenons, whose tensile strength, combined with the natural interlocking structure, can meet the bearing requirements of pile foundation without reinforcement.

[0022] 3) This invention eliminates the need for tenon joints and reinforcement processing, shortening the construction cycle compared to traditional solutions and eliminating the need for large equipment on site.

[0023] 4) This invention reduces the amount of concrete and steel reinforcement used, thereby lowering engineering costs. The ultra-narrow excavation reduces the amount of soil excavation, effectively reducing carbon emissions.

[0024] 5) This invention achieves synergistic effect of strengthening the radial constraint of the structure and restoring the old pile over time, so that the vertical ultimate bearing capacity of the composite pile can be greater than the direct superposition value of the bearing capacity of the new and old piles, thereby realizing synergistic efficiency in bearing capacity, improving the utilization efficiency of a single pile under the same pile conditions, and further promoting the resource utilization of old piles. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the process flow of the method of the present invention; Figure 2 This is a schematic diagram of the integrated composite pile foundation of new and old piles in an embodiment of the method of the present invention; Figure 3 This is a top view of the mortise and tenon connection area of ​​the integrated new and old pile composite foundation in an embodiment of the method of the present invention; In the diagram: 1-New pile, 2-Old pile, 3-Metal hoop, 4-Tortoise and tenon joint area, 5-Tenon. Detailed Implementation

[0026] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0027] For simplicity, this application only explicitly discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form a range not explicitly stated; and any lower limit can be combined with other lower limits to form a range not explicitly stated, just as any upper limit can be combined with any other upper limit to form a range not explicitly stated. Furthermore, although not explicitly stated, every point or individual value between the endpoints of the range is included within that range. Therefore, each point or individual value can be used as its own lower or upper limit and combined with any other point or individual value or with other lower or upper limits to form a range not explicitly stated. In the description of this application, it should be noted that, unless otherwise stated, "above" includes the stated number, and "multiple" in "one or more" means two or more. The terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. The terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, unless otherwise expressly specified and limited, the terms "installed," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two elements. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0028] The foregoing description of this application is not intended to describe every disclosed implementation or method. Instead, the following description provides more specific examples of exemplary embodiments. Throughout the application, guidance is provided through a series of embodiments that can be used in various combinations. The examples listed are representative only and should not be construed as exhaustive.

[0029] Numerous details are explored in the following description to provide a more thorough explanation of embodiments of this application; however, it will be apparent to those skilled in the art that embodiments of this application may be practiced without these specific details.

[0030] To further understand the present invention, the following embodiments are provided. It is worth noting that, unless otherwise specified, all raw materials used in the present invention are commercially available; and all methods and equipment employed are common in the art.

[0031] To address the problems of limited working space, insufficient reliability of the connection between old and new piles, and discontinuous force transmission in existing technologies, this application provides a pile foundation construction method for reusing old piles and its construction method.

[0032] like Figure 2-3 As shown, the present invention provides a pile foundation for the reuse of old piles, which includes an old pile 2 and a new pile 1. The new pile 1 is fixed to the top of the old pile 2. The tenon 5 on the new pile 1 is connected to the mortise of the old pile 2. The tenon 5 is located in the mortise and tenon connection area 4. The metal hoop 3 is used to apply radial restraint outside the mortise and tenon connection area 4.

[0033] Before forming an integrated composite pile foundation structure consisting of old pile 2, mortise and tenon connection area 4, and new pile 1, this invention proposes a method for evaluating the vertical bearing capacity of the composite pile to facilitate engineering design and safety verification. Based on the stress characteristics of the old and new piles working together, this method decomposes the vertical bearing capacity of the composite pile into the bearing contribution of old pile 2, the bearing contribution of new pile 1, and the collaborative bearing contribution of mortise and tenon connection area 4. A coordination coefficient is used to reflect the influence of the stiffness difference and deformation coordination between the old and new piles on the bearing performance.

[0034] The vertical ultimate bearing capacity of composite piles can be evaluated using the following formula: in: This represents the ultimate vertical bearing capacity of the composite pile. The vertical ultimate bearing capacity provided for old pile 2 shall be determined based on the test results of old pile 2, the original design data, or the results of on-site verification. The ultimate vertical bearing capacity provided for new pile 1 shall be determined in accordance with the current pile foundation design code or through test methods; The synergistic effect coefficient is calculated using the following formula: in, The constraint enhancement coefficient ranges from 1.0 to 1.10, and its value is determined based on the radial constraint of the concrete in the connection area by the metal hoop 3, the fiber composite material constraint layer, or the locally densified concrete layer. The consolidation enhancement coefficient has a value range of 1.0 to 1.20, and is determined based on the effect of the consolidation of the soil around the old pile on the bearing capacity of the old pile 2 during its service period. The vertical control bearing capacity of the mortise and tenon joint area 4 includes the comprehensive control value of the mortise and tenon joint area 4 under compression, shear and pull-out conditions. Its magnitude is related to the depth-to-width ratio and size of the mortise, the surface roughness, and the grouting and sealing quality.

[0035] The above calculation formula is a simplified evaluation framework that is feasible for engineering projects, and is applicable to the preliminary design, comparison, and verification of the vertical bearing capacity of composite piles. In specific engineering applications, relevant parameters can be reasonably selected and verified by combining engineering experience, test data, or monitoring results.

[0036] The characteristic value or allowable bearing capacity of the vertical bearing capacity of a composite pile under normal service conditions can be reduced by a safety factor based on its ultimate vertical bearing capacity. The calculation formula is as follows: in, This refers to the characteristic value or allowable bearing capacity of the vertical bearing capacity of the composite pile. The bearing capacity safety factor has a value range of 1 ≤ ≤2.0; where the recommended value range for actual engineering is 1.6 < ≤2.0: When the integrity level of old pile 2 is Class I, 1.6 < ≤1.8; when the integrity level of old pile 2 is Class II, 1.8 < ≤2.0; After special demonstration and verification by field load test, Can be 1≤ ≤1.6.

[0037] To facilitate engineers in determining key parameters during the preliminary design phase, this invention provides the following criteria for selecting design parameters: (1) Bearing capacity Q of old pile 2 old The value is preferentially adopted from the static load test results of old pile 2; when there is no static load test data, the bearing capacity of bored piles is estimated according to the empirical parameter method in the current "Technical Specification for Building Pile Foundations" JGJ94-2008, based on the original pile design drawings and the pile diameter, pile length and stratum conditions verified on site, and multiplied by the reduction factor according to the integrity test level, where the reduction factor is 0.95 for Class I piles and 0.85 for Class II piles.

[0038] (2) Bearing capacity Q of new pile 1 new The bearing capacity of a single pile is calculated independently based on the cross-sectional dimensions and length of the newly designed pile segment and the formula for single pile bearing capacity in the current national standard "Code for Design of Building Pile Foundations" JGJ94-2008. The constraint enhancement effect of the old pile 2 is not taken into account for the time being, and it can be regarded as the ultimate bearing capacity of an independent single pile.

[0039] (3) Constraint efficiency coefficient The value range is 1.0 to 1.10, and the specific value can be selected according to Table 1 based on the constraint type: Table 1 Constraint Enhancement Coefficients Reference value Consolidation efficiency coefficient The value range is 1.0 to 1.20, and the specific value can be determined according to Table 2 based on the integrity level of old pile 2: Table 2 Consolidation Enhancement Coefficient Reference value (4) Vertical control bearing capacity Q of the mortise and tenon joint area j The value should be determined by the minimum of the following three calculations: 1) Local bearing strength of concrete at tenon 5: It can be estimated by multiplying the design value of concrete compressive strength at tenon 5 by 0.7 times the cross-sectional area of ​​tenon 5; 2) Grouting interface shear strength: The shear strength of the interface bond can be taken as 1.5MPa, and the product of the shear strength and the inner area of ​​the mortise can be used for verification. 3) Pull-out bearing capacity of tenon 5: can be verified by multiplying the mortise sidewall area by the shear strength of the bond at the grouting interface.

[0040] The above parameters are recommended values ​​based on the experimental and engineering experience of this invention. In actual design, they can be appropriately adjusted according to the site geological conditions, material properties and superstructure requirements, and should be verified through on-site monitoring data.

[0041] The method of the present invention will be described in detail below with reference to a specific engineering embodiment.

[0042] A renovation project for an old residential area in the core area of ​​a city has 36 existing cast-in-place round piles. Each pile has a cross-sectional radius of 400mm, a length of 24m, and a service life of approximately 30 years. Due to constraints imposed by surrounding buildings, the site space is extremely narrow, with only about 0.9m between buildings. The geological conditions are silty clay. A new 6-story residential building is planned, requiring a vertical bearing capacity of no less than 1400kN for the pile foundations, while also requiring that the width of the lateral working face during construction not exceed 0.14m.

[0043] S1. First, the integrity and bearing capacity of the old piles 2 were tested. The test results showed that 34 of the old piles 2 were of Class I integrity and 2 were of Class II integrity, all of which met the requirements for reuse.

[0044] S2. Excavate in layers around old pile 2, with a depth of 7.2m, approximately 0.3 times the length of old pile 2. During the layered excavation, control the width of the transverse working face to 140mm, not exceeding 0.5 times the outer diameter of old pile 2. Temporary support is provided using sheet piles during the excavation process to prevent soil collapse and reduce disturbance to the soil surrounding old pile 2.

[0045] S3. At a predetermined height at the top of the old pile 2, a mortise is machined at the center of the cross-section of the old pile 2. The mortise is positioned along the axis of the old pile 2 to form a mortise and tenon connection structure between the old and new piles. In this embodiment, the mortise diameter is 280mm and the depth is 300mm, meeting the structural requirements that the net width of the mortise is not less than 4 times the maximum aggregate particle size and 1.2 times the fiber length, to ensure the compactness and molding quality of the concrete poured within the mortise. The mortise is machined using a small diamond cutting device to avoid adverse effects on the main structure of the old pile 2.

[0046] S4. Roughen the top of the old pile 2 and the inside of the mortise, and build a customized template around the top of the old pile 2 and the area around the mortise. The template is set around the top of the old pile 2 and ensures that the axis of the new pile 1 is aligned with that of the old pile 2.

[0047] S5. In this embodiment, steel fiber reinforced concrete with end hooks is used to pour the inside of the mortise and the upper tenon 5. The steel fiber is 35mm long, with a volumetric admixture of 1.5%, and the concrete strength grade is C50. Micro-vibration is used to ensure that the fiber is evenly dispersed and densely filled. Then, concrete is poured onto the bearing section of the new pile 1 above, and the tenon 5 structure is integrally formed in the same pouring process. Considering the actual engineering conditions and coordinating with the subsequent grouting process, the demolding strength was reached after 12 days of curing. After the subsequent grouting is completed, moist curing will continue for more than 14 days.

[0048] S6. A grouting channel is pre-set between the tenon 5 and the mortise, and micro-expansion high-strength grouting material is injected into the tenon-mortise connection area 4 to fill the tiny gaps at the interface and improve the overall reliability of the connection area. Simultaneously, a metal hoop 3 is fitted onto the outside of the tenon-mortise connection area 4. In this embodiment, Q355 steel plate hoops with a width of 120mm and a thickness of 8mm are used, arranged at 150mm intervals along the axial direction. The space between the hoop and the pile body is filled densely with micro-expansion grouting material, and circumferential prestress is applied to the hoop. In this embodiment, the circumferential tension stress is... ;according to calculate, , ,have to ,satisfy The concrete strength requirement is not less than 0.3 MPa. After grouting, the concrete should be covered and kept moist for curing, with the ambient temperature maintained above 5℃ and the relative humidity not less than 95%. After curing for 12 days, the concrete will reach the demolding strength. After removing the formwork, curing should continue for a total curing time of not less than 14 days, with the grouting area kept moist for curing.

[0049] Monitoring results after project completion show that the average vertical bearing capacity of the integrated new and old pile foundation reaches 1650kN, meeting the design requirements; the width of the transverse working surface during construction is stably controlled within 0.14m, without adversely affecting the surrounding existing buildings; the construction period is shortened by 62% compared with the traditional scheme, and the project cost is reduced; long-term monitoring shows that the cumulative settlement of the composite pile is 3.5mm, and the uneven settlement is 0.3mm, which can meet the safety requirements of a 6-story residential building.

[0050] The foregoing has shown and described the basic process, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A pile foundation construction method for reusing old piles, characterized in that, The old pile (2) is a solid concrete pile, excluding prestressed concrete pipe piles, and includes the following steps: (1) Conduct integrity testing and load-bearing capacity assessment on the old piles left after the demolition and renovation of the original building; (2) Excavate the soil layer by layer with the old pile (2) as the center to the preset mortise and tenon connection depth. Cut off and discard the parts that do not meet the reuse requirements. For the parts that meet the reuse requirements, calculate the bearing capacity of the new and old composite piles in combination with the design parameters of the proposed new pile (1). (3) Clean the surface of the top of the old pile (2) and make a mortise along the pile axis at the center of the top section of the old pile (2); (4) Clean the surface of the top of the old pile (2) and the mortise, and build a customized template in the area around the top of the old pile (2) and the mortise; (5) Fiber-reinforced concrete is poured in situ into the mortise and the upper tenon (5) forming area, and then concrete is poured into the upper new pile (1) bearing section and formed as a whole, so that the tenon (5) and the new pile (1) body are integrated in the same pouring process; (6) After maintenance, a composite pile foundation structure integrating old and new piles is formed, consisting of old piles (2), mortise and tenon connection area (4) and new piles (1).

2. The method according to claim 1, characterized in that, In step (1), the integrity test results are graded and evaluated according to the degree of pile defects; When using the low-strain method for testing, if the measured waveform is regular, the pile bottom reflection is clear, the amplitude of the reflected wave from the same-phase defect does not exceed -10% of the incident wave amplitude, and the average longitudinal wave velocity of the pile body calculated from the round-trip time of the pile bottom reflection signal and the pile length meets the following conditions, the rating is Class I, indicating that the pile body is generally intact, has no obvious structural defects, and can be put into use: (1) The speed of non-prestressed concrete piles such as ordinary cast-in-place concrete piles is 3200m / s to 4200m / s; (2) High-strength concrete solid piles have a speed range of 3500m / s to 4500m / s. A Class II rating is granted when all of the following conditions are met, indicating that the pile body has minor defects that do not affect its overall bearing capacity and reuse safety, and it is ready for use: (1) The measured waveform has a slight in-phase defect reflected wave with an amplitude between 10% and 30% of the incident wave amplitude; (2) The estimated value of the corresponding defect cross-sectional area shall not exceed 15% of the pile cross-sectional area; (3) Any possible slight local narrowing or mild segregation are discontinuous and non-continuous defects, and the wave velocity of the pile body has not been significantly reduced. When the reflected wave amplitude of a defect exceeds 30% of the incident wave amplitude, or the estimated cross-sectional area of ​​the defect is greater than 15% of the pile cross-sectional area, or the wave velocity is significantly reduced, or there are continuous and penetrating serious defects, it is rated as Class III or below, does not meet the requirements for reuse, and is cut off and discarded or disposed of separately.

3. The method according to claim 1, characterized in that, The load-bearing capacity assessment in step (1) is calculated using the following formula: ； in: This represents the ultimate vertical bearing capacity of the composite pile. The vertical ultimate bearing capacity provided for the old pile (2) shall be determined based on the test results of the old pile (2), the original design data or the results of on-site verification; The vertical ultimate bearing capacity provided for the new pile (1) shall be determined in accordance with the current pile foundation design code or by test method; The synergistic effect coefficient is used to reflect the amplification effect of the enhanced radial constraint of the structure and the consolidation of the soil around the old pile (2) on the vertical ultimate bearing capacity of the composite pile; Calculate using the following formula: ； in, The constraint enhancement coefficient is taken in the range of 1.0 to 1.10, based on the radial constraint of the concrete in the connection area by the metal hoop (3), the fiber composite material constraint layer or the locally densified concrete layer. The consolidation enhancement coefficient is 1.0 to 1.20, and its value is determined based on the effect of the consolidation of the soil around the old pile (2) on the bearing capacity of the old pile (2) during its service period. The vertical control bearing capacity of the mortise and tenon joint area (4) includes the comprehensive control value of the mortise and tenon joint area (4) under compression, shear and pull-out conditions. Its size is related to the depth-to-width ratio and size of the mortise, the surface roughness and the grouting and sealing quality.

4. The method according to claim 1, characterized in that, In step (2), when excavating in layers, the width of the transverse working face on one side of the excavation, that is, the distance from the outer edge of the old pile (2) to the excavation edge, shall not exceed 0.5 times the diameter of the old pile (2); When there is shallow damage, material deterioration or insufficient bearing capacity in the upper part of the old pile (2), the old pile (2) is cut off at the preset mortise and tenon connection depth, and the part below the cut-off is retained as a reused old pile (2) segment. The old pile (2) is extended and the new pile (1) and the old pile (2) are supported by the mortise and tenon connection structure.

5. The method according to claim 1, characterized in that, The characteristic value or allowable bearing capacity of the vertical bearing capacity of the new and old composite piles under normal use conditions mentioned in step (2) can be reduced by a safety factor based on their ultimate vertical bearing capacity. The calculation formula is as follows: ； in, This refers to the characteristic value or allowable bearing capacity of the vertical bearing capacity of the composite pile. The bearing capacity safety factor has a value range of 1 ≤ ≤2.0; where the recommended value range for actual engineering is 1.6 < ≤2.0: When the integrity level of the old pile (2) is Class I, 1.6 < ≤1.8; when the integrity level of the old pile (2) is Class II, 1.8 < ≤2.0; After special demonstration and verification by field load test, Take 1≤ ≤1.

6.

6. The method according to claim 1, characterized in that, The transverse net width of the mortise in step (3) shall not be less than 4 times the maximum aggregate size of the concrete used and not less than 1.2 times the fiber length, and the width of the complete pile body concrete ring between the edge of the mortise and the outer edge of the old pile (2) shall not be less than 100mm.

7. The method according to claim 1, characterized in that, In step (4), a local reinforcement structure is fitted on the outside of the mortise and tenon connection area (4). The local reinforcement structure includes a metal hoop (3), a fiber composite material constraint layer, a locally densified concrete layer, or a combination thereof. The metal hoop (3) is made of steel plate or steel strip, and the material is selected from one of Q235 grade steel, Q355 grade steel or Q420 grade steel. The hoop width is 80mm to 200mm and the thickness is 6mm to 12mm. The hoop is arranged axially along the mortise and tenon connection area (4) with a spacing of 100mm to 300mm. The hoop and the pile surface are filled with grout or structural adhesive. The fiber composite confinement layer is made of one or more of carbon fiber cloth or glass fiber cloth, and its material properties comply with the provisions of the current national standard "Code for Design of Strengthening Concrete Structures" GB50367-2013. Specifically, the polyacrylonitrile molecular weight of the carbon fiber cloth is not greater than 15K, the standard value of tensile strength is not less than 3000MPa, and the unit area mass is 200-300g / m². 2 ; The fiberglass cloth is selected from high-strength fiberglass, alkali-resistant fiberglass or alkali-free fiberglass; The number of the constraint layers is 1 to 3 layers, the interlayer and circumferential overlap length is not less than 100mm, and it is impregnated and pasted with matching epoxy resin on the outside of the mortise and tenon connection area (4), and extends outward along the pile axis to cover not less than 200mm. At least one of the metal hoop (3) or fiber composite confinement layer can be subjected to circumferential prestress to form radial confinement stress on the concrete of the mortise and tenon joint area (4). , Not less than 0.3 MPa; The locally reinforced concrete layer is poured using fine aggregate concrete or fiber-reinforced concrete with a strength grade of not less than C40, the maximum particle size of coarse aggregate is not greater than 10mm, and the thickness is 50mm to 150mm.

8. The method according to claim 1, characterized in that, In step (5), the fiber in the fiber-reinforced concrete is selected from one of steel fiber, polypropylene fiber, basalt fiber, or chopped carbon fiber. The concrete strength grade is not lower than C40, the maximum particle size of the coarse aggregate is not greater than 10mm, and the thickness is 50mm to 150mm. The mix proportion and fiber content are determined according to the current industry standard "Technical Specification for Application of Fiber Reinforced Concrete" JGJ / T221-2010, specifically as follows: When steel fibers are selected, the volumetric content of the steel fibers is 1.0% to 2.5%, the length of the steel fibers is 20mm to 60mm, the equivalent diameter is 0.3mm to 0.9mm, the aspect ratio is 30 to 80, the tensile strength is not less than 600MPa, and one of the end-hooked special-shaped steel fibers or the corrugated special-shaped steel fibers is used, and their performance meets the requirements of GB / T39147-2020 "Steel Fibers for Concrete"; When polypropylene fibers are selected, the volume content of polypropylene fibers is 0.05% to 0.3%, and the fiber length is 15mm to 30mm. They are used to suppress early shrinkage cracks and improve the crack resistance and impermeability of tenon (5) concrete. When basalt fiber is selected, the volume content of basalt fiber is 0.1% to 0.5%, and the length of basalt fiber is 12 mm to 24 mm. When chopped carbon fibers are selected, the volumetric content of the chopped carbon fibers is 0.2% to 1.0%, and the length of the chopped carbon fibers is 6 mm to 12 mm.

9. The method according to claim 1, characterized in that, When designing or verifying the integrated composite pile foundation formed in step (5), the vertical ultimate bearing capacity of the composite pile should be taken as... and The smaller value in; where The vertical ultimate bearing capacity provided for the old pile (2) is determined based on the integrity test results and the original design data and reduced according to the integrity level; The vertical ultimate bearing capacity provided for the new pile (1) shall be calculated independently according to the current specifications; The synergistic effect coefficient has a value range of 1.0 to 1.32 and is used to amplify the superposition value of the vertical ultimate bearing capacity of the old pile (2) and the new pile (1); To determine the vertical control bearing capacity of the mortise and tenon joint area, the minimum value among the local compressive bearing capacity of the tenon (5) concrete, the shear bearing capacity of the grouting interface, and the pull-out bearing capacity of the mortise sidewall is taken.

10. The method according to claim 1, characterized in that, In step (6), the ambient temperature for curing should not be lower than 5°C; the relative humidity of the concrete surface should not be lower than 95% by covering with plastic film, spraying curing agent or sprinkling water, and the curing time should not be less than 14 days, and the grouting reinforcement area should be moisturized and cured simultaneously.

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