Pile-raft foundation structure for controlling uneven settlement and construction method thereof

Abstract

The present application discloses a pile-raft foundation structure for controlling uneven settlement and a construction method thereof. The method comprises the following steps: reinforcing the foundation by constructing the cement-soil mixing pile from the periphery to the center; simultaneously sinking the short ribbed pile with a long rebate when the mixing pile is not solidified; and densely arranging the short ribbed pile in the stress concentration area. The upper part of the pile top is cast-in-situ cushion layer, and the raft foundation is arranged on the upper part of the cushion layer. The internal part of the raft foundation is arranged with the longitudinal and transverse intersecting stiff composite beams and double-layer and double-direction steel mesh, and the horizontal and vertical non-bonding prestressed tendons are separately arranged. The stiff composite beams are connected by the grouting sleeve / anchoring plate, and the exposed steel bars with anchoring plates are arranged on the pile top. The present application forms a composite foundation and a double-direction prestressed raft foundation collaborative stress system, and the differential arrangement of the piles offsets the difference of the foundation bearing capacity. The double-direction prestressed constraint controls the deformation of the raft foundation, effectively controls the uneven settlement of the building, and improves the anti-cracking, anti-floating and waterproof durability of the foundation.

Description

Technical Field

[0001] This invention relates to the field of pile-raft foundation structures for buildings, specifically to pile-raft foundation structures for controlling uneven settlement and their construction methods. Background Technology

[0002] Existing building settlement monitoring results indicate that the main causes of irregular settlement in buildings are improper foundation selection and differential bearing capacity of the soil. Pile-raft foundations, widely used in high-rise buildings in soft soil areas, combine the advantages of piles transferring loads to deep, stable soil layers with the raft slab distributing loads from the superstructure. The current mainstream practice is to rigidly connect the pile tops to the raft slab, forming a unified load-bearing structure. However, in recent years, for economic reasons, a "separate pile arrangement-integral pile-raft foundation" has emerged, concentrating piles under the main vertical components. This approach, however, has sparked new controversies in engineering practice.

[0003] In special geological conditions with significant soil variations, such as granite residual soil and karst landforms, conventional pile-raft foundations experience excessive differential settlement, making it difficult to meet the normal usage requirements of high-rise buildings. When using pile-raft structures to reinforce deep silt foundations in road-bridge transition sections, they may compress adjacent bridge abutment foundations or even cause abutment pile displacement. Furthermore, for end-bearing pile foundations, the significant difference in deformation between the piles and the foundation soil leads to prominent pile-soil deformation coordination issues, resulting in the foundation soil's bearing capacity not being fully utilized, thus causing waste.

[0004] Therefore, in order to solve the above-mentioned technical problems, the present invention provides a pile-raft foundation structure for controlling uneven settlement and its construction method. Summary of the Invention

[0005] The purpose of this invention is to provide a pile-raft foundation structure and its construction method for controlling uneven settlement, which solves the problem of uneven settlement on soft soil and differential foundations by combining differentiated reinforcement with prestress control.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a pile-raft foundation structure for controlling uneven settlement, comprising a cement-soil mixing pile foundation, ribbed short piles and a raft foundation, wherein the cement-soil mixing pile foundation contains a number of ribbed short piles, a cushion layer is provided on top of the ribbed short piles, and a raft foundation is provided on top of the cushion layer, wherein the raft foundation comprises crisscrossing stiffened composite beams, double-layer bidirectional steel mesh and longitudinal and transverse unbonded prestressed tendons; The rigid composite beam serves as the longitudinal and transverse intersecting foundation beam inside the raft foundation. All connection nodes are located at the top of the closely ribbed short piles, and the top of the closely ribbed short piles here has vertically arranged exposed steel bars. The exposed steel bars pass through the cushion layer and extend directly into the internal area of ​​the raft foundation, and their ends are screwed with anchor plates inside the raft foundation. The double-layer bidirectional steel mesh serves as the steel reinforcement skeleton inside the raft foundation and is located on the upper part of the stiffened composite beam. The transverse unbonded prestressing tendons traverse the stiffened composite beam, and the longitudinal unbonded prestressing tendons are located in the middle of the double-layer bidirectional steel mesh. After tensioning, the longitudinal and transverse unbonded prestressing tendons are fixed to the sides of the concrete-cast raft foundation using anchorages, forming bidirectional cross prestressing inside the raft foundation.

[0007] Preferably, the cross-sectional dimensions of the ribbed short pile are not less than 250 mm × 250 mm, and the length is 1000 mm to 1500 mm. Each side of the ribbed short pile is provided with a continuous tongue and groove along the length of the pile to strengthen the interlocking force between the pile and the soil. The tongue and groove width is not less than 80 mm and the tongue and groove depth is not less than 50 mm.

[0008] Preferably, the bottom of the ribbed short pile is provided with a pile tip, and in areas where the stress is large or concentrated or the foundation soil is uneven and relatively soft, the ribbed short piles are densely arranged.

[0009] Preferably, the rigid composite beam has no less than 6 Φ12@150 steel bars inside, and the internal stirrups are U-shaped open stirrups. I-beams are pre-embedded in the top of the rigid composite beam. The width of the I-beam flange is 1 / 4 to 1 / 3 of the width of the rigid composite beam section, the web height is not less than 300 mm, the I-beam is pre-embedded in the rigid composite beam for no less than 120 mm, the thickness of the I-beam flange and web is not less than 8 mm and the surface is roughened. It is centrally positioned, and the rigid composite beam and the I-beam above it are prefabricated as a whole in a prefabricated PC factory.

[0010] Preferably, the exposed length of the reinforcing bars at the top of the pile is not less than 300 mm and should meet the structural length requirements for anchoring into the raft foundation, while the diameter is not less than 16 mm.

[0011] Preferably, the extended reinforcing bars at the ends of two adjacent stiffened composite beams in the same direction are connected by a grouting sleeve. For the extended reinforcing bars at the ends of stiffened composite beams without a connection target, an anchor plate is added to improve the joint strength.

[0012] Preferably, the raft foundation contains a pre-embedded internal steel reinforcement cage for the load-bearing columns of the subsequent buildings on the foundation. The internal steel reinforcement cage of the load-bearing columns includes vertical steel bars and stirrups. The stirrups are tied to the vertical steel bars, and the vertical steel bars are spot-welded to the internal steel reinforcement skeleton of the raft foundation.

[0013] This invention also provides the following technical solution: a construction method for a pile-raft foundation structure, wherein the pile-raft foundation structure is any one of the pile-raft foundation structures described above, and the construction method includes the following steps: Step S1: Use a total station to determine the location of the cushion layer, use a deep mixing machine to reinforce the foundation soil layer below the cushion layer location, and continuously mix from the perimeter of the foundation towards the center to form a cement-soil mixing pile foundation. Step S2: While constructing the cement-soil mixing pile foundation, use a total station to accurately measure the positional relationship of the ribbed short piles on the top surface of the already mixed foundation soil according to the plan of the ribbed short piles. Drive each ribbed short pile one by one according to the measured positional relationship. At the same time, use the cement-soil mixing pile foundation that has been completed to drive the ribbed short piles before it is completely solidified. The pile heads of all ribbed short piles at the connection nodes are chiseled in advance. After the cement-soil mixing pile foundation has been cured to more than 60% of the design strength, straighten the exposed steel bars at the top of the piles at the connection nodes one by one, and clean the pre-rolled threads at their ends. Step S3: After removing rust and dirt from the exposed steel bar ends at the pile top, tighten the anchor plate one by one using a torque wrench. The tightening torque must be strictly controlled according to the design value, and the tightening torque of each steel bar should be checked three times and the average value should be taken to ensure that the anchor plate is tightly attached to the end face of the steel bar without any looseness. After tightening, lay a layer of fine stone concrete of uniform thickness as a cushion layer in the pile top area. The strength grade of the cushion layer should not be lower than C30. When pouring, use a plate vibrator to compact and level it. At the same time, take care to protect the tightened anchor plate and the exposed steel bar at the pile top from disturbance. The elevation of the top surface of the cushion layer should be precisely controlled 0-5 mm above the design elevation. After the cushion layer concrete has solidified, re-measure the anchor plate torque. If there is any decrease, tighten it again immediately. Finally, after passing the final acceptance, proceed to the next process. Step S4: After the concrete strength of the subbase reaches the design strength, use a total station to accurately measure the planar position relationship and hoisting elevation of the stiffened composite beams on the subbase surface according to the plan layout of the stiffened composite beams. Then, hoist the stiffened composite beams one by one on the subbase. When hoisting the stiffened composite beams, first use mortar to adjust the elevation of the stiffened composite beams on the subbase surface, then hoist each stiffened composite beam into position one by one. Finally, grout sleeves are used to connect the steel bars inside the connection nodes of the stiffened composite beams, or anchor plates are added. Step S5: After the rigid composite beam is hoisted into place and the connection nodes are processed and accepted, mark the position of the double-layer bidirectional steel mesh on the top surface of the rigid composite beam and mark the column position control line of the load-bearing column on the cushion layer according to the design drawings. First, the transverse unbonded prestressing tendons are laid using the perforation method. The transverse unbonded prestressing tendons are laid through the reserved holes inside the stiffened composite beam, one by one according to the design coordinates. Their horizontal positioning is fixed by special support steel bars or U-shaped clips. Then, the bottom of the steel cage inside the load-bearing column is placed above the pad layer, and the upper end of its vertical steel bars is temporarily locked with positioning stirrups and tied to the load-bearing column stirrups. Next, the double-layer bidirectional steel mesh is fixed to the vertical steel bars of the load-bearing column by spot welding. At the same time as tying the double-layer bidirectional steel mesh, the longitudinal unbonded prestressing tendons are laid. The longitudinal unbonded prestressing tendons are located in the middle of the double-layer bidirectional steel mesh and are perpendicular to the direction of the transverse unbonded prestressing tendons. The tensioning end and the fixed end of the prestressing tendon are set on the side of the raft slab according to the design position, and the matching anchor plate and spiral reinforcement are installed. The anchor plate is perpendicular to the axis of the prestressing tendon and is welded or tied firmly. Throughout the reinforcement process, the quantity, position, sag and anchor end positioning of the prestressing tendon are repeatedly checked, and the protective layer thickness, reinforcement spacing and skeleton size are checked. After the concealed works are accepted, the next process is carried out. Step S6: After all the steel bars and prestressing tendons in Step S5 have passed the acceptance inspection, the raft foundation concrete is poured. A layered continuous pouring process is adopted, and the pouring is evenly advanced from one end to the other. The pouring speed and the order of material placement are strictly controlled to ensure that the concrete smoothly fills the entire raft foundation area and the top anchorage area of ​​the stiffened composite beam and the closely ribbed short piles. At the same time, it avoids impact displacement on the unbonded prestressing tendons and the vertical steel bars of the load-bearing columns that have been placed. After the concrete is poured, it should be covered and kept moist for curing. Once the concrete strength of the raft foundation reaches 100% of the design strength grade and is not lower than the tension strength specified in the design documents, the longitudinal and transverse unbonded prestressing tendons should be tensioned and anchored. After tensioning to the design control stress and holding the load for a specified time, the anchorage should be tightened to complete the anchorage. After unloading, the excess exposed prestressing tendons at the tensioning end should be cut off, and the ends and anchorages should be coated with special anti-corrosion grease for protection. Finally, the anchorage groove at the tensioning end should be sealed and smoothed with micro-expansion fine stone concrete or waterproof mortar. After sealing, it should be flush with the side of the raft foundation, thus completing the reliable establishment of the bidirectional cross prestressed system.

[0014] Preferably, the foundation soil layer reinforcement adopts the cement-soil mixing pile foundation treatment method, which uses a method of continuous mixing from the perimeter of the foundation towards the center. The specific construction process is as follows: While the deep mixer is pre-mixing and settling, the backstage mixes cement slurry according to the design mix ratio, using P·O42.5 grade cement, with the water-cement ratio controlled between 0.45 and 0.55. The slurry is then placed in the aggregate hopper for later use before grouting. Start the mixing pile machine turntable. After the mixing head rotates at normal speed, let the drill rod sink along the guide frame while mixing. The sinking speed is controlled by the gear. The working current should not exceed the rated value. During the sinking process, the grout pump is turned on to spray cement slurry into the soil. The two sets of blades rotate in opposite directions to cut and mix the soil until the design depth is reached. Continuously spray grout and mix at the bottom of the pile for no less than 10 seconds. After the pile reaches the designed depth, the grout pump is turned on and the grout is delivered to the outlet of the mixing head through the pipeline. The drill rod is lifted while spraying and mixing the grout at the lifting speed determined by the design, so that the cement grout is fully mixed with the soil. After the mixing head is lifted to a height of 500mm above the top of the pile, the grout pump is turned off. The mixing, sinking and spraying and lifting are repeated as required by the design until the design elevation of the top of the pile is reached. During construction, the pile position deviation shall not exceed 50 mm, and the amount of cement used per meter and the amount of grout delivered per pile shall meet the design requirements. If grouting is stopped for any reason, the mixing head shall be lowered to 1.0 m below the grouting stop surface, and grouting shall be resumed and then the pile shall be raised. After a single pile is completed, the mud on the blades and drill rod is removed, the pile driver is moved to the next pile location, and the above steps are repeated to construct the next pile. It is also important to ensure that the mixing piles interlock to improve the foundation reinforcement effect.

[0015] Preferably, in step S6, the tensioning strictly follows the control principles of symmetry, gradation, and synchronization, proceeding one tendon and one bundle at a time according to the tensioning sequence given in the design. Each bundle of prestressed tendons is tensioned simultaneously from both ends, with the tension force as the main control parameter and the measured elongation value as the verification basis. The deviation between the actual elongation value and the theoretical elongation value is controlled within ±6%. For unbonded prestressed tendons exceeding 70 m, segmented tensioning should be adopted. If the deviation exceeds the limit, tensioning should be immediately suspended, the cause investigated and dealt with before continuing the operation.

[0016] Compared with the prior art, the beneficial effects of this invention are as follows: 1. This invention targets undisturbed foundations of sandy and silty soils. It employs cement-soil mixing piles to solidify the soil as a whole, and simultaneously installs closely spaced ribbed short piles during the unsolidified stage of the mixing piles. At the same time, it follows a sequential construction method from the perimeter of the foundation pit towards the center, and densifies the pile arrangement in areas with weak soil layers and concentrated loads to achieve differentiated foundation reinforcement. Furthermore, the continuous tongue and groove joint of the closely spaced ribbed short piles can significantly improve the pile-soil interlocking force. Combined with the interlocking of the mixing piles to form a continuous reinforcement layer, it forms a dual reinforcement system that can match the differentiated bearing capacity of various parts of the foundation, resolve uneven compression deformation of the soil layer in advance, reduce settlement difference from the root of the foundation, and effectively avoid irregular settlement defects in buildings.

[0017] 2. This invention only sets exposed reinforcing bars with anchor plates at the top of the short ribbed piles at the connection nodes. The exposed reinforcing bars cross the cushion layer and anchor into the raft foundation. Together with the integrated precast stiffened composite beam, it forms a longitudinal and transverse stress skeleton. The beams are adapted to various intersection nodes through grouting sleeves and anchor plates. The vertical load at the top of the pile can be directly and evenly transferred to the raft foundation stress system, avoiding the separation of the pile and raft interface and local stress concentration. Moreover, the precast stiffened composite beam has embedded I-beams to improve the overall bending stiffness. Compared with the traditional raft beam with on-site tied reinforcing bars, it greatly reduces the bending deformation of the raft foundation and further helps to control uneven settlement.

[0018] 3. The present invention has longitudinal and transverse unbonded prestressed tendons arranged inside the raft foundation. After tensioning, they form a bidirectional compressive stress field, which offsets the additional tensile stress of the raft foundation caused by uneven settlement and significantly reduces the risk of settlement cracking throughout the foundation's life cycle. The continuous compressive stress formed by prestressing can seal the capillary channels inside the concrete and improve the raft foundation's impermeability and waterproofing ability. The bidirectional prestressing constrains the raft foundation as a whole, and simultaneously strengthens the overall pull-out and buoyancy resistance of the foundation. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of an embodiment of the pile-raft foundation structure of the present invention; Figure 2 This is a schematic diagram of a structural embodiment of the closely ribbed short pile in the pile-raft foundation structure of the present invention; Figure 3 This is a schematic diagram of a structural embodiment of the pile-raft foundation structure of the present invention, which adopts a densely ribbed short pile densification layout. Figure 4 This is a schematic diagram of a structural embodiment of the raft foundation in the pile-raft foundation structure of the present invention; Figure 5 This is a schematic diagram of a structural embodiment of the stiffened composite beam in the pile-raft foundation structure of the present invention; Figure 6 This is a schematic diagram of the connection node structure of two stiffened composite beams in the pile-raft foundation structure of the present invention. Figure 7 This is a schematic diagram of the connection node structure of the corner connection between two stiffened composite beams in the pile-raft foundation structure of the present invention; Figure 8 This is a schematic diagram of the connection node structure of the three stiffened composite beams connected in a T-shape in the pile-raft foundation structure of the present invention; Figure 9 This is a schematic diagram of the connection node structure of the four stiffened composite beams intersecting in the pile-raft foundation structure of the present invention; Figure 10 yes Figure 1 A schematic diagram of the internal structure of the transversely arranged stiffened composite beam in the embodiment; Figure 11 yes Figure 1 A schematic diagram of the internal structure of the longitudinally arranged stiffened composite beam in the embodiment; Figure 12 This is a flowchart of the overall construction process of the pile-raft foundation structure construction method of the present invention. Detailed Implementation

[0020] This invention discloses a pile-raft foundation structure for controlling uneven settlement. The pile-raft foundation structure is composed of a cement-soil mixing pile foundation, ribbed short piles 1, and a raft foundation 2. (See reference...) Figure 1 .

[0021] The cement-soil mixing pile foundation contains several closely ribbed short piles 1. The cross-sectional dimensions of each closely ribbed short pile 1 are not less than 250 mm × 250 mm, and the length is 1000 mm to 1500 mm. (See reference...) Figure 2Each ribbed short pile 1 has a continuous tongue and groove along its length to enhance the interlocking force between the pile and the soil. The tongue and groove width is not less than 80 mm, and the tongue and groove depth is not less than 50 mm. The bottom of the ribbed short pile 1 has a pile tip. The cross-sectional dimensions, reinforcement, length, and spacing between adjacent ribbed short piles 1 within the foundation area must be determined through calculation based on the specific project's foundation bearing capacity, geological conditions, and surrounding environment. Furthermore, in areas with high or concentrated stress, or where the foundation soil is uneven and relatively soft, the ribbed short piles 1 should be densely arranged. (See [reference needed]). Figure 3 .

[0022] A cushion layer 3 is provided on top of the closely spaced short piles 1. To further enhance the collaborative bearing capacity of the pile-raft foundation, a crisscrossing stiffened composite beam 21 is provided on top of the cushion layer 3. (See reference...) Figure 4 The reinforced composite beam 21 has at least 6 Φ12@150 steel bars inside, and the internal stirrups are U-shaped open stirrups. (See reference...) Figure 5 An I-beam is pre-embedded in the top of the rigid composite beam 21. The width of the I-beam flange is 1 / 4 to 1 / 3 of the cross-sectional width of the rigid composite beam 21, the web height is not less than 300 mm, the I-beam is pre-embedded in the rigid composite beam 21 for not less than 120 mm, the thickness of the I-beam flange and web is not less than 8 mm and the surface is roughened, and it is centrally positioned. The rigid composite beam 21 and the I-beam above it are prefabricated as a whole in the prefabricated PC factory.

[0023] The rigid composite beam 21 serves as the longitudinal and transverse intersecting foundation beam within the raft foundation 2. Two adjacent rigid composite beams 21 in the same direction are connected by grouting sleeves 4 to join the extended reinforcing bars at their ends. For rigid composite beams 21 without a joining target, anchor plates 5 are added to enhance the joint strength. Specifically, at a connection joint, if there is only one longitudinally and / or one transversely arranged rigid composite beam 21, its extended reinforcing bars are joined with anchor plates 5, and the remaining extended reinforcing bars are joined via grouting sleeves 4. This divides several connection joints into two joints (see [reference]). Figure 6 ), two corner connections (see) Figure 7 ), three T-type connections (see) Figure 8 ) and four cross connections (see Figure 9 There are four types.

[0024] All connection nodes of the stiffened composite beam 21 are located at the top of the closely ribbed short piles 1, and the top of the closely ribbed short piles 1 at the connection nodes has the following characteristics: Figure 2The exposed steel bars 11 arranged vertically shown in the diagram have an exposed length of not less than 300 mm at the pile top and should meet the structural length requirements for anchoring into the raft foundation 2, while having a diameter of not less than 16 mm. The exposed steel bars 11 at the pile top pass through the cushion layer 3 and extend directly into the internal area of ​​the raft foundation 2, intersecting with the extended steel bars at the ends of the internal stiffened composite beam 21 of the connection node, and the ends of the exposed steel bars 11 at the pile top are anchored with anchor plates 5 inside the raft foundation 2.

[0025] participate Figure 10 and Figure 11 The upper part of the rigid composite beam 21 is provided with a double-layer bidirectional steel mesh 22, which serves as the steel reinforcement skeleton inside the raft foundation 2. The raft foundation 2 also has a pre-embedded steel reinforcement cage for the load-bearing columns of the subsequent buildings on the foundation. The steel reinforcement cage of the load-bearing columns includes vertical steel bars and stirrups. The stirrups are tied to the vertical steel bars, and the vertical steel bars are spot-welded to the steel reinforcement skeleton inside the raft foundation 2.

[0026] The raft foundation 2 also includes transverse unbonded prestressing tendons 6 and longitudinal unbonded prestressing tendons 7. The transverse unbonded prestressing tendons 6 traverse the stiffened composite beam 21, while the longitudinal unbonded prestressing tendons 7 are located in the middle of the double-layer bidirectional steel mesh 22, and are fixed and supported at the bottom by stirrups, which are welded to the double-layer bidirectional steel mesh 22. To facilitate the drilling process of the prestressing tendons, some stiffened composite beams 21 have pre-drilled holes 23 inside. The pre-drilled holes 23 are perpendicular to the length of the stiffened composite beam 21, and their positions correspond to the locations of the transverse unbonded prestressing tendons 6.

[0027] After tensioning, the longitudinal unbonded prestressing tendons 7 and the transverse unbonded prestressing tendons 6 are fixed to the sides of the concrete-cast raft foundation 2 using anchorages. This creates a bidirectional cross-prestressing structure within the raft foundation 2, thus forming a bidirectional cross-prestressed pile-raft foundation structure that can control uneven settlement. The overall technical solution follows the principle of "stabilizing the foundation soil layer with cement-soil mixing piles, secondary strengthening of the foundation with closely ribbed short piles, denser arrangement of closely ribbed short piles in areas with greater stress, hoisting of longitudinal and transverse crisscross stiffened composite beams, enhancing the pile-raft joint bearing capacity with exposed steel bars and anchor plates at the pile tops, and applying longitudinal and transverse bidirectional unbonded prestressing tendons." This pile-raft foundation effectively solves the problem of uneven settlement of buildings and is suitable for original sandy or silty soil foundations.

[0028] The above-mentioned pile-raft foundation is achieved through the following construction methods. For the overall construction process, please refer to [link / reference needed]. Figure 12 The specific construction methods are as follows: Step S1: Measurement and layout to determine location, construction of cement-soil mixing pile foundation: First, the location of the foundation layer was determined using a total station. Then, a deep mixing machine was used to reinforce the foundation soil layer beneath the foundation layer. The foundation soil layer reinforcement adopted the cement-soil mixing pile foundation treatment method, using a method of continuous mixing from the perimeter of the foundation towards the center. The specific construction process is as follows: While the deep mixer is pre-mixing and settling, the backstage mixes cement slurry according to the design mix ratio, using P·O42.5 grade cement, with the water-cement ratio controlled between 0.45 and 0.55. The slurry is then placed in the aggregate hopper for later use before grouting.

[0029] Start the mixing pile machine turntable. After the mixing head rotates at normal speed, let the drill rod sink along the guide frame while mixing. The sinking speed is controlled by the gear. The working current should not exceed the rated value. During the sinking process, the grout pump is turned on to spray cement slurry into the soil. The two sets of blades rotate in opposite directions to cut and mix the soil until the design depth is reached. Continuously spray grout and mix at the bottom of the pile for no less than 10 seconds.

[0030] After the pile reaches the designed depth, the grout pump is turned on and the grout is delivered to the outlet of the mixing head through the pipeline. The drill rod is lifted while spraying and mixing the grout at the lifting speed determined by the design (generally 0.5 to 0.8 m / min) to ensure that the cement grout is fully mixed with the soil. After the mixing head is lifted to a height of 500 mm above the top of the pile, the grout pump is turned off. The mixing, sinking and spraying and lifting are repeated as required by the design until the design elevation of the top of the pile is reached.

[0031] During construction, the deviation of the pile position should not exceed 50 mm, and the amount of cement used per meter and the amount of grout delivered per pile should meet the design requirements. If grouting is stopped for any reason, the mixing head should be lowered to 1.0 m below the grouting stop surface, and grouting should be resumed and then the head should be raised.

[0032] After a single pile is completed, the mud on the blades and drill rod is removed, the pile driver is moved to the next pile location, and the above steps are repeated to construct the next pile. It is also important to ensure that the mixing piles interlock to improve the foundation reinforcement effect.

[0033] Step S2: Drive ribbed short piles and straighten the exposed reinforcing bars at the pile top: While the cement-soil mixing pile foundation is continuously mixed from the perimeter to the center, a total station is used again to accurately determine the positional relationship of the ribbed short piles on the top surface of the already mixed foundation soil according to the plan of the ribbed short piles. Then, each ribbed short pile is driven one by one according to the determined positional relationship. In areas with high stress, concentrated stress, or uneven and soft foundation soil, the ribbed short piles are densely installed.

[0034] During on-site construction, the driving of ribbed short piles in various locations adopts a process that is carried out simultaneously with the cement-soil mixing pile foundation reinforcement construction. That is, after the cement-soil mixing pile foundation is continuously mixed from the periphery to the center, ribbed short piles are driven simultaneously in the reinforced area. The ribbed short piles are driven before the cement-soil mixing pile foundation that has been completed is fully solidified, so as to reduce the construction intensity of the ribbed short pile driving process, improve efficiency, and reduce the construction cost of ribbed short piles.

[0035] It should be noted that, according to the actual situation of the project, the pile heads of all ribbed short piles at the connection nodes were chipped off in advance, while the pile heads of the ribbed short piles at non-connection nodes did not need to be chipped off. That is, there was no exposed steel reinforcement at the top of the pile, so as not to affect the hoisting and positioning of the composite beam. At the same time, it was ensured that the pile top elevation at the connection node after chipping off the pile head was consistent with the pile top elevation at the non-connection node.

[0036] During on-site construction, after the cement-soil mixing pile foundation has been cured to more than 60% of the design strength, the exposed steel bars at the top of the piles at the connection nodes are straightened one by one, and the threads that have been mechanically rolled at their ends are cleaned.

[0037] Step S3: Tighten the anchor plate and pour the foundation concrete: To enhance the combined bearing capacity of the pile-raft foundation, all exposed steel bars at the top of the piles at the connection nodes are anchored into the raft foundation, and 100% of their ends are secured with anchor plates.

[0038] During on-site construction, after removing rust and dirt from the exposed steel bar ends at the pile top, the anchor plates are tightened one by one using a torque wrench. The tightening torque is strictly controlled according to the design value (deviation not exceeding ±5%), and the tightening torque of each steel bar is checked three times and the average value is taken to ensure that the anchor plate and the steel bar end face are tightly attached without loosening. After tightening, a layer of fine stone concrete of uniform thickness is laid as a cushion layer in the pile top area. The cushion layer strength grade is not lower than C30. During pouring, a plate vibrator is used to compact and level it. At the same time, care is taken to protect the tightened anchor plates and the exposed steel bars at the pile top from disturbance. The elevation of the top surface of the cushion layer should be accurately controlled 0-5 mm above the design elevation. After the cushion layer concrete has solidified, the anchor plate torque is checked again. If there is a decrease, tightening is immediately added. Only after the final acceptance is qualified can the next process be carried out, thereby ensuring reliable anchor force transmission and uniform stress at the bottom of the raft slab.

[0039] Step S4: Hoist the intersecting composite beams and treat the connection nodes: After the concrete of the subbase reaches its design strength, a total station is used to accurately measure the planar position and hoisting elevation of the composite beams on the subbase surface according to the plan layout diagram. Then, the composite beams are hoisted one by one onto the subbase. During hoisting, mortar is first applied to the subbase surface to adjust the elevation of the composite beams for hoisting. Then, each composite beam is hoisted into position. Finally, the reinforcing bars inside the connection nodes of the composite beams are connected by grouting sleeves or additional anchor plates are added.

[0040] Step S5: Installation of the internal steel reinforcement cage of the load-bearing column and the internal steel reinforcement skeleton of the raft foundation; layout of longitudinal and transverse unbonded prestressed tendons: After the rigid composite beam is hoisted into place and the connection nodes are completed and accepted, firstly, according to the design drawings, the positions of the double-layer bidirectional steel mesh (internal steel skeleton of the raft foundation) are marked on the top surface of the rigid composite beam, and the column position control lines of the load-bearing columns are marked on the cushion layer. Then, the longitudinal and transverse unbonded prestressing tendons are laid out, the internal steel skeleton of the raft foundation is tied, and the internal steel cage of the load-bearing columns is installed. The transverse unbonded prestressing tendons are laid out before the internal steel skeleton of the raft foundation is tied and the internal steel cage of the load-bearing columns is installed. The longitudinal unbonded prestressing tendons are laid out simultaneously with the internal steel skeleton of the raft foundation.

[0041] On-site installation followed the following sequence: first, perforating the transverse unbonded prestressing tendons; then, installing the load-bearing column reinforcement cage; and finally, tying the double-layer bidirectional steel mesh and longitudinal unbonded prestressing tendons. The transverse unbonded prestressing tendons were installed using a perforation method, passing through pre-drilled holes inside the stiffened composite beam. Each tendon was installed according to the design coordinates, and its horizontal positioning was secured using specialized support steel bars or U-shaped clamps, with spacing deviation controlled within ±10 mm. For the curved sections of the prestressing tendons (where there are inflection points), spurs were used for positioning according to the design sag to ensure smooth alignment and accurate curvature.

[0042] When installing the reinforcing cage of the load-bearing column, first place the bottom of the internal reinforcing cage of the load-bearing column above the foundation layer. Temporarily lock the upper ends of the vertical reinforcing bars using positioning stirrups. Then, tie the stirrups of the load-bearing column to ensure that the verticality and axial deviation of the vertical reinforcing bars meet the specifications. The double-layer bidirectional reinforcing mesh is spot-welded to the vertical reinforcing bars of the load-bearing column, while simultaneously ensuring that the vertical reinforcing bars of the load-bearing column pass through the double-layer bidirectional reinforcing mesh. The longitudinal unbonded prestressing tendons are arranged perpendicular to the transverse unbonded prestressing tendons. The lower part of the longitudinal unbonded prestressing tendons is fixed and supported by stirrups, which are welded to the double-layer bidirectional reinforcing mesh to fix the longitudinal unbonded prestressing tendons in the middle of the double-layer bidirectional reinforcing mesh.

[0043] The tensioning and fixing ends of the prestressing tendons are respectively set on the sides of the raft foundation according to the design positions, and matching anchor plates and spiral reinforcements are installed. The anchor plates should be perpendicular to the axis of the prestressing tendons and be firmly welded or tied. Throughout the reinforcement process, the quantity, position, sag, and anchorage end positioning of the prestressing tendons must be repeatedly checked, and the thickness of the protective layer, the spacing of the reinforcement bars, and the dimensions of the skeleton must be inspected. Only after the concealed works have passed the acceptance inspection can the next process be carried out.

[0044] Step S6: Concrete pouring for the raft foundation, tensioning and anchoring of unbonded prestressing tendons in the longitudinal and transverse directions: After all the reinforcing bars and prestressing tendons in step S5 have passed the acceptance inspection, the raft foundation concrete can be poured. A layered continuous pouring process is adopted, advancing evenly from one end to the other, and the pouring speed and placement sequence are strictly controlled to ensure that the concrete smoothly fills the entire raft foundation area and the anchorage area at the top of the stiffened composite beams and ribbed short piles, avoiding impact displacement of the already positioned unbonded prestressing tendons and the vertical reinforcing bars of the load-bearing columns. During the pouring process, an immersion vibrator is used as the main vibrator, supplemented by a plate vibrator for coordinating compaction. The vibrator rod must avoid direct contact with the prestressing tendons, and manual tamping is used in the anchorage area and areas with dense reinforcing bars to ensure that the concrete is dense and free of honeycomb. Test blocks cured under the same conditions and standard curing conditions are simultaneously prepared.

[0045] After the concrete is poured, it should be covered and kept moist for curing. Only after the concrete strength of the raft foundation reaches 100% of the design strength grade (and is not lower than the tension strength specified in the design documents) can the tensioning and anchoring of the longitudinal and transverse unbonded prestressed tendons be carried out.

[0046] Tensioning must strictly adhere to the principles of symmetry, gradation, and synchronization. It should be carried out strand by strand and bundle by bundle according to the design-given tensioning sequence (usually tensioning transverse prestressing tendons first, then longitudinal prestressing tendons, or in an alternating sequence). Each bundle of prestressing tendons should be tensioned simultaneously from both ends. Tension force is used as the primary control parameter, and measured elongation is used as the verification basis. The deviation between the actual elongation and the theoretical elongation should be controlled within ±6%. For unbonded prestressing tendons exceeding 70 m, segmented tensioning is recommended. If any deviation exceeds the tolerance, tensioning must be immediately suspended, the cause investigated, and the issue addressed before resuming operation.

[0047] After tensioning to the design control stress and holding the load for a specified period, the anchorage is tightened to complete the anchorage. After unloading, excess exposed prestressing tendons at the tensioning end are cut off (leaving a length of not less than 30 mm). The ends and anchorages are coated with special anti-corrosion grease for protection. Finally, the anchorage groove at the tensioning end is sealed and smoothed with micro-expansion fine stone concrete or waterproof mortar. After sealing, it is flush with the side of the raft slab to ensure overall waterproof and durability performance, thereby completing the reliable establishment of the bidirectional cross-prestressed system.

[0048] In summary, this invention addresses the problem of uneven settlement in sandy and silty soil foundations by constructing a composite foundation consisting of interlocking cement-soil mixing piles constructed from the periphery towards the center, plus differentiated dense-ribbed short piles with tongue and groove joints. This is combined with a bidirectional cross-bonded prestressed raft slab with precast stiff composite beams and pile top anchoring force transmission structure, forming an integrated pile-raft foundation. This allows for coordinated control of uneven settlement from the foundation to the raft slab, while simultaneously improving the foundation's crack resistance, waterproofing, buoyancy resistance, and pull-out resistance.

Claims

1. A pile-raft foundation structure for controlling uneven settlement, characterized in that: It includes a cement-soil mixing pile foundation, ribbed short piles and a raft foundation. The cement-soil mixing pile foundation has several ribbed short piles inside, and a cushion layer is provided on the top of the ribbed short piles. The raft foundation is provided on the upper part of the cushion layer. The raft foundation includes a rigid composite beam arranged in a crisscross pattern, a double-layer bidirectional steel mesh and longitudinal and transverse unbonded prestressed tendons. The rigid composite beam serves as the longitudinal and transverse intersecting foundation beam inside the raft foundation. All connection nodes are located at the top of the closely ribbed short piles, and the top of the closely ribbed short piles here has vertically arranged exposed steel bars. The exposed steel bars pass through the cushion layer and extend directly into the internal area of ​​the raft foundation, and their ends are screwed with anchor plates inside the raft foundation. The double-layer bidirectional steel mesh serves as the steel reinforcement skeleton inside the raft foundation and is located on the upper part of the stiffened composite beam. Transverse unbonded prestressing tendons traverse the stiffened composite beam, while longitudinal unbonded prestressing tendons are located in the middle of the double-layer bidirectional steel mesh. After tensioning, the longitudinal and transverse unbonded prestressing tendons are fixed to the sides of the concrete-cast raft foundation using anchors, forming bidirectional cross-prestressing within the raft foundation.

2. The pile-raft foundation structure according to claim 1, characterized in that: The cross-sectional dimensions of the ribbed short pile are not less than 250mm×250mm, and the length is 1000mm~1500mm. Each side of the ribbed short pile is provided with a continuous tongue and groove along the length of the pile to strengthen the interlocking force between the pile and the soil. The tongue and groove width is not less than 80mm and the tongue and groove depth is not less than 50mm.

3. The pile-raft foundation structure according to claim 1, characterized in that: The bottom of the ribbed short pile is equipped with a pile tip, and in areas where the stress is large or concentrated, or where the foundation soil is uneven and relatively soft, the ribbed short piles are densely arranged.

4. The pile-raft foundation structure according to claim 1, characterized in that: The rigid composite beam has no less than 6 Φ12@150 steel bars inside, and the internal stirrups are U-shaped open stirrups. I-beams are pre-embedded at the top of the rigid composite beam. The width of the I-beam flange is 1 / 4 to 1 / 3 of the width of the cross section of the rigid composite beam, the web height is not less than 300 mm, the I-beam is pre-embedded inside the rigid composite beam for no less than 120 mm, the thickness of the I-beam flange and web is not less than 8 mm and the surface is roughened. It is centrally positioned, and the rigid composite beam and the I-beam above it are prefabricated as a whole in the prefabricated PC factory.

5. The pile-raft foundation structure according to claim 1, characterized in that: The exposed length of the reinforcing bars at the top of the pile shall not be less than 300 mm and shall meet the structural length requirements for anchoring into the raft foundation, while the diameter shall not be less than 16 mm.

6. The pile-raft foundation structure according to claim 1, characterized in that: The extended reinforcing bars at the ends of two adjacent stiffened composite beams in the same direction are connected by grouting sleeves. For the extended reinforcing bars at the ends of stiffened composite beams without a connection target, anchor plates are added to improve the joint strength.

7. The pile-raft foundation structure according to claim 1, characterized in that: The raft foundation contains pre-embedded internal steel reinforcement cages for the load-bearing columns of the subsequent buildings on the foundation. The internal steel reinforcement cages of the load-bearing columns include vertical steel bars and stirrups. The stirrups are tied to the vertical steel bars, and the vertical steel bars are spot-welded to the internal steel reinforcement skeleton of the raft foundation.

8. A construction method for a pile-raft foundation structure, characterized in that, The pile-raft foundation structure is the pile-raft foundation structure according to any one of claims 1-7, and the construction method includes the following steps: Step S1: Use a total station to determine the location of the cushion layer, use a deep mixing machine to reinforce the foundation soil layer below the cushion layer location, and continuously mix from the perimeter of the foundation towards the center to form a cement-soil mixing pile foundation. Step S2: While constructing the cement-soil mixing pile foundation, use a total station to accurately measure the positional relationship of the ribbed short piles on the top surface of the already mixed foundation soil according to the plan of the ribbed short piles. Drive each ribbed short pile one by one according to the measured positional relationship. At the same time, use the cement-soil mixing pile foundation that has been completed to drive the ribbed short piles before it is completely solidified. The pile heads of all ribbed short piles at the connection nodes are chiseled in advance. After the cement-soil mixing pile foundation has been cured to more than 60% of the design strength, straighten the exposed steel bars at the top of the piles at the connection nodes one by one, and clean the pre-rolled threads at their ends. Step S3: After removing rust and dirt from the exposed steel bar ends at the pile top, tighten the anchor plate one by one using a torque wrench. The tightening torque must be strictly controlled according to the design value, and the tightening torque of each steel bar should be checked three times and the average value should be taken to ensure that the anchor plate is tightly attached to the end face of the steel bar without any looseness. After tightening, lay a layer of fine stone concrete of uniform thickness as a cushion layer in the pile top area. The strength grade of the cushion layer should not be lower than C30. When pouring, use a plate vibrator to compact and level it. At the same time, take care to protect the tightened anchor plate and the exposed steel bar at the pile top from disturbance. The elevation of the top surface of the cushion layer should be precisely controlled 0-5 mm above the design elevation. After the cushion layer concrete has solidified, re-measure the anchor plate torque. If there is any decrease, tighten it again immediately. Finally, after passing the final acceptance, proceed to the next process. Step S4: After the concrete strength of the subbase reaches the design strength, use a total station to accurately measure the planar position relationship and hoisting elevation of the stiffened composite beams on the subbase surface according to the plan layout of the stiffened composite beams. Then, hoist the stiffened composite beams one by one on the subbase. When hoisting the stiffened composite beams, first use mortar to adjust the elevation of the stiffened composite beams on the subbase surface, then hoist each stiffened composite beam into position one by one. Finally, grout sleeves are used to connect the steel bars inside the connection nodes of the stiffened composite beams, or anchor plates are added. Step S5: After the rigid composite beam is hoisted into place and the connection nodes are processed and accepted, mark the position of the double-layer bidirectional steel mesh on the top surface of the rigid composite beam and mark the column position control line of the load-bearing column on the cushion layer according to the design drawings. First, the transverse unbonded prestressing tendons are laid using the perforation method. The transverse unbonded prestressing tendons are laid through the reserved holes inside the stiffened composite beam, one by one according to the design coordinates. Their horizontal positioning is fixed by special support steel bars or U-shaped clips. Then, the bottom of the steel cage inside the load-bearing column is placed above the pad layer, and the upper end of its vertical steel bars is temporarily locked with positioning stirrups and tied to the load-bearing column stirrups. Next, the double-layer bidirectional steel mesh is fixed to the vertical steel bars of the load-bearing column by spot welding. At the same time as tying the double-layer bidirectional steel mesh, the longitudinal unbonded prestressing tendons are laid. The longitudinal unbonded prestressing tendons are located in the middle of the double-layer bidirectional steel mesh and are perpendicular to the direction of the transverse unbonded prestressing tendons. The tensioning end and the fixed end of the prestressing tendon are set on the side of the raft slab according to the design position, and the matching anchor plate and spiral reinforcement are installed. The anchor plate is perpendicular to the axis of the prestressing tendon and is welded or tied firmly. Throughout the reinforcement process, the quantity, position, sag and anchor end positioning of the prestressing tendon are repeatedly checked, and the protective layer thickness, reinforcement spacing and skeleton size are checked. After the concealed works are accepted, the next process is carried out. Step S6: After all the steel bars and prestressing tendons in Step S5 have passed the acceptance inspection, the raft foundation concrete is poured. A layered continuous pouring process is adopted, and the pouring is evenly advanced from one end to the other. The pouring speed and the order of material placement are strictly controlled to ensure that the concrete smoothly fills the entire raft foundation area and the top anchorage area of ​​the stiffened composite beam and the closely ribbed short piles. At the same time, it avoids impact displacement on the unbonded prestressing tendons and the vertical steel bars of the load-bearing columns that have been placed. After the concrete is poured, it should be covered and kept moist for curing. Once the concrete strength of the raft foundation reaches 100% of the design strength grade and is not lower than the tension strength specified in the design documents, the longitudinal and transverse unbonded prestressing tendons should be tensioned and anchored. After tensioning to the design control stress and holding the load for a specified time, the anchorage should be tightened to complete the anchorage. After unloading, the excess exposed prestressing tendons at the tensioning end should be cut off, and the ends and anchorages should be coated with special anti-corrosion grease for protection. Finally, the anchorage groove at the tensioning end should be sealed and smoothed with micro-expansion fine stone concrete or waterproof mortar. After sealing, it should be flush with the side of the raft foundation, thus completing the reliable establishment of the bidirectional cross prestressed system.

9. The construction method according to claim 8, characterized in that, The foundation soil layer reinforcement adopts the cement-soil mixing pile foundation treatment method, which uses a method of continuous mixing from the perimeter of the foundation towards the center. The specific construction process is as follows: While the deep mixer is pre-mixing and settling, the backstage mixes cement slurry according to the design mix ratio, using P·O42.5 grade cement, with the water-cement ratio controlled between 0.45 and 0.

55. The slurry is then placed in the aggregate hopper for later use before grouting. Start the mixing pile machine turntable. After the mixing head rotates at normal speed, let the drill rod sink along the guide frame while mixing. The sinking speed is controlled by the gear. The working current should not exceed the rated value. During the sinking process, the grout pump is turned on to spray cement slurry into the soil. The two sets of blades rotate in opposite directions to cut and mix the soil until the design depth is reached. Continuously spray grout and mix at the bottom of the pile for no less than 10 seconds. After the pile reaches the designed depth, the grout pump is turned on and the grout is delivered to the outlet of the mixing head through the pipeline. The drill rod is lifted while spraying and mixing the grout at the lifting speed determined by the design, so that the cement grout is fully mixed with the soil. After the mixing head is lifted to a height of 500 mm above the top of the pile, the grout pump is turned off. The mixing, sinking and spraying and lifting are repeated as required by the design until the design elevation of the top of the pile is reached. During construction, the pile position deviation shall not exceed 50 mm, and the amount of cement used per meter and the amount of grout delivered per pile shall meet the design requirements. If grouting is stopped for any reason, the mixing head shall be lowered to 1.0 m below the grouting stop surface, and grouting shall be resumed and then the pile shall be raised. After a single pile is completed, the mud on the blades and drill rod is removed, the pile driver is moved to the next pile location, and the above steps are repeated to construct the next pile. It is also important to ensure that the mixing piles interlock to improve the foundation reinforcement effect.

10. The construction method according to claim 8, characterized in that, In step S6, tensioning strictly follows the control principles of symmetry, gradation, and synchronization. Tensioning is carried out one tendon and one bundle at a time according to the tensioning sequence given in the design. Each bundle of prestressed tendons is tensioned synchronously at both ends. The tension force is used as the main control parameter, and the measured elongation value is used as the verification basis. The deviation between the actual elongation value and the theoretical elongation value is controlled within ±6%. For unbonded prestressed tendons exceeding 70 m, segmented tensioning should be adopted. If the deviation exceeds the limit, tensioning should be stopped immediately, the cause should be identified and dealt with, and then the operation can continue.

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