Compound foundation cooperative supporting method suitable for binary structure stratum

Abstract

The application discloses a composite foundation cooperative supporting method suitable for a binary structure stratum. The method first forms a shallow reinforcement body by deep mixing construction of a plurality of first pile bodies, and after curing to the initial strength, a plurality of second pile bodies are constructed by using a non-soil-extrusion hole-forming process, so that the second pile bodies penetrate through the reinforcement body and are embedded into a deep stable bearing stratum; the second pile bodies are arranged in a grid shape, and the first pile bodies are filled in the soil between the piles to form a spatially interpenetrating cooperative stress system. The application establishes a clear division mechanism of "shallow flexible global reinforcement control settlement, deep rigid point support bearing anti-floating", and through the time-space cooperative process of "first soft and then rigid", effectively avoids construction mutual disturbance, and guarantees the pile forming quality. The process significantly improves the overall bearing capacity and anti-deformation capacity of the composite foundation, the settlement control effect is outstanding, and the process is economic and efficient, quality controllable, and especially suitable for large steel structure industrial plants and other projects with strict requirements on the foundation.

Description

Technical Field

[0001] This invention relates to the field of civil engineering foundation construction technology, and in particular to a method for collaborative support of composite foundations suitable for binary structure strata. Background Technology

[0002] In civil engineering construction, the selection and construction of the foundation directly affect the safety and durability of the superstructure. This is especially true for large-span, heavy-load steel structure industrial buildings, which place extremely high demands on the foundation's bearing capacity, anti-buoyancy performance, and settlement control.

[0003] In the middle and lower reaches of the Yangtze River in my country, especially in cities along the river such as Wuhan, the geological strata generally exhibit a typical binary or multi-element structure of "soft on top and hard on the bottom." This means that the shallow layer is widely composed of weak soil layers with poor engineering properties, such as miscellaneous fill, silty clay, and silty clay, while the deeper layer contains stable bearing layers such as moderately weathered rock with high strength and good stability. Constructing factory foundations under such complex geological conditions presents the following core challenges:

[0004] I. Single-pile foundation technology has functional limitations. If only flexible piles such as cement-soil mixing piles are used for foundation treatment, although they can effectively reinforce shallow soft soil, improve its uniformity, and reduce some settlement, the limited strength of the piles themselves prevents them from embedding into deep, stable bearing layers. This makes it difficult to provide the deep anti-buoyancy anchoring force and core vertical bearing capacity required for large-span factory buildings. Conversely, if only rigid piles such as drilled (percussion) cast-in-place piles are used, although they can effectively transfer the load to deep, stable rock layers and meet the bearing capacity and anti-buoyancy requirements, the generally large spacing between rigid piles makes it impossible to effectively treat the shallow soft soil between the piles. Under large-area loads such as those on the factory floor, excessive or uneven settlement of the soil between the piles is likely to occur, affecting the normal use of the factory building.

[0005] II. Lack of a Coordination Mechanism in Simple Pile Combinations. Although the industry has recognized the potential value of combining flexible and rigid piles, current attempts mostly remain at the conceptual level, lacking a clearly defined collaborative working mechanism for specific geological formations. Specifically, this manifests in: the failure to clearly define the core load-bearing tasks of each type of pile, leading to design redundancy or coexisting weaknesses; and the failure to optimize the relative positions and arrangement of the two types of piles in terms of spatial layout, resulting in each operating independently and failing to form a spatially coordinated load-bearing system.

[0006] Third, improper construction sequence arrangement leads to mutual interference. In the construction of composite pile foundations, the construction sequence of different pile types has a significant impact on the quality of pile formation. If the sequence of constructing rigid long piles first and then flexible short piles is adopted, the drilling and concrete pouring process of the long piles may create obstacles and stress disturbances in the subsequent short pile construction area. If the two are constructed in an alternating or simultaneous manner, the mechanical vibration of the rigid piles and the soil compression can easily damage the newly formed and low-strength flexible piles, which may lead to quality accidents such as pile breakage and displacement in severe cases, greatly weakening the synergistic reinforcement effect of the foundation.

[0007] Therefore, there is an urgent need to develop a new type of composite foundation technology that can establish a clear collaborative working mechanism, optimize spatial layout and construction procedures, and organically integrate shallow flexible reinforcement with deep rigid bearing capacity to meet the stringent requirements of steel structure industrial plants for foundation bearing capacity and settlement control. Summary of the Invention

[0008] In view of this, the purpose of the present invention is to provide a composite foundation collaborative support method applicable to binary structure strata and a composite foundation structure formed by the process, so as to solve the technical problems of insufficient function of single pile foundation process, lack of collaborative mechanism of simple pile type combination, and poor pile quality caused by mutual interference of construction procedures in the prior art.

[0009] The present invention provides a composite foundation collaborative support method applicable to binary structure strata, specifically strata consisting of a weak upper soil layer and a stable bearing layer below, comprising the following steps:

[0010] Step 1: In the shallow soft soil area of ​​the site to be treated, the foundation soil and solidifying agent are forcibly mixed by deep mechanical mixing, and multiple first piles are constructed to form a shallow reinforced body.

[0011] Step 2: After the shallow solidified body has cured to its initial strength, multiple second piles are constructed at the preset pile positions using a non-soil-displacement drilling process, so that the second piles penetrate the shallow solidified body and embed into the deep stable bearing layer; wherein, the multiple second piles are arranged in a grid pattern, and the first piles are filled in the soil between adjacent second piles, forming a cooperative force-bearing system in which the first piles and second piles intersect each other in space;

[0012] Step 3: Conduct a bearing capacity test on the composite foundation formed by the second pile and the first pile after the maintenance is completed.

[0013] Furthermore, the deep stable bearing layer is a moderately weathered rock layer, and the pile tip of the second pile body enters the moderately weathered rock layer to a depth greater than or equal to 1.5m, so as to mainly bear the deep anti-buoyancy and core vertical bearing capacity.

[0014] Furthermore, the first pile body is a cement-soil mixing pile, and the second pile body is a cast-in-place pile.

[0015] Furthermore, the diameter of the cast-in-place pile is larger than that of the cement-soil mixing pile, and the spacing between adjacent cast-in-place piles is larger than that between adjacent cement-soil mixing piles, forming a composite bearing structure with rigid cast-in-place piles as the main framework and flexible mixing piles to reinforce the soil between the piles.

[0016] Furthermore, the diameter of the cast-in-place pile is 700-900mm, and the pile spacing is 4-5 times the pile diameter; the diameter of the cement-soil mixing pile is 400-600mm, and the pile spacing is 2-3 times the pile diameter; the length of the cast-in-place pile is greater than the length of the cement-soil mixing pile.

[0017] Furthermore, a reinforcing cage is provided inside the cast-in-place pile. The main reinforcement bars of the reinforcing cage are made of steel bars with a diameter of 20-25mm, and the stirrups are made of steel bars with a diameter of 6-10mm. The protective layer thickness of the reinforcing cage inside the hole is greater than or equal to 50mm.

[0018] Furthermore, in step one, the shallow solidified layer formed by the cement-soil mixing pile is cured for no less than 7 days to give it the initial strength.

[0019] Furthermore, in step two, the drilling speed of the second pile body is less than or equal to 1.5 m / h, and the thickness of the sediment at the bottom of the hole after drilling is less than or equal to 50 mm; the second pile body is poured with concrete using the tremie method, the pouring speed is greater than or equal to 2 m / h, and the tremie is embedded in the concrete to a depth of 2-6 m.

[0020] Furthermore, in step one, the cement content used in the cement-soil mixing pile is 12%-20% of the mass of the reinforced soil, and the water-cement ratio is 0.45-0.65; the 28-day unconfined compressive strength of the cement-soil mixing pile is greater than or equal to 1.5 MPa.

[0021] A composite foundation structure is constructed using the composite foundation collaborative support method applicable to binary structure strata described in any one of the above-mentioned methods.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] I. A composite force-bearing system with clear division of labor and spatial coordination was established. This invention clearly defines the first pile body (flexible mixing pile) as responsible for shallow, comprehensive reinforcement to control settlement, and the second pile body (rigid cast-in-place pile) as responsible for deep rock embedding to bear the core bearing capacity and resist buoyancy, forming a vertical division of labor of "shallow continuous reinforcement and deep point support"; spatially, an interlaced layout of "large-diameter rigid piles arranged in a grid-like main framework and small-diameter flexible piles filling the soil between the piles in a plum blossom pattern" is adopted, which eliminates abrupt changes in pile-soil stiffness, realizes efficient and smooth load transfer, and significantly improves the overall bearing capacity of the composite foundation compared with the single pile foundation technology.

[0024] Second, a "flexible-then-rigid" spatiotemporal collaborative construction sequence was established. This invention innovatively adopts a sequence of first constructing flexible mixing piles to form a shallow solidified layer, curing it to initial strength, and then constructing rigid cast-in-place piles. This effectively avoids the mutual disturbance problems caused by improper construction sequence. The solidified layer formed in the early stage provides a stable borehole wall environment for subsequent drilling, significantly reducing the risk of borehole collapse; the "curing time window of no less than 7 days" ensures that the flexible pile has sufficient initial strength, greatly improving its ability to resist subsequent construction disturbances and guaranteeing the quality and integrity of the piles.

[0025] III. Effective Settlement Control and Structural Safety. Flexible piles provide comprehensive reinforcement of shallow, soft soil, fundamentally improving the integrity and uniformity of the foundation and significantly increasing the shallow compression modulus; rigid piles are embedded in the stable bearing layer, effectively constraining the total settlement. The synergistic effect of these two methods eliminates excessive local settlement of the soil between piles while keeping the overall settlement and uneven settlement differential of the plant building at extremely low levels, ensuring the long-term safety and normal use of the superstructure steel plant building.

[0026] IV. Economical and efficient, with controllable construction quality. This invention clarifies the key quantitative control indicators for each process, such as drilling speed, sediment thickness, pouring speed, and cement dosage, forming a standardized and replicable construction process with a complete quality assurance system. Compared to a fully rigid pile scheme, by rationally configuring flexible piles, the number of rigid piles and material consumption are significantly reduced while meeting the same bearing capacity requirements, thus lowering project costs and shortening the construction period. It possesses good engineering practicality and economic promotion value. Attached Figure Description

[0027] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0028] Figure 1 This is a schematic diagram of the construction process of the present invention. Detailed Implementation

[0029] It should be noted that in the description of this specification, the terms "upper", "lower", "top", "bottom", "inner", "outer", "axial", "radial", "circumferential", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the present invention 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. Therefore, they should not be construed as limiting the present invention.

[0030] As shown in the figure: This embodiment describes a composite foundation collaborative support method applicable to binary strata, specifically those consisting of a weak upper soil layer and a stable bearing layer below. A binary stratum refers to a geological profile exhibiting two distinct layers with vastly different mechanical properties: the upper layer typically consists of weak cohesive soil, silty soil, or loose fill deposited since the Holocene or Late Pleistocene of the Quaternary period. This layer possesses engineering characteristics such as high natural water content, large void ratio, high compressibility, and low bearing capacity, making it unsuitable as a natural bearing layer for large industrial plants; the lower layer comprises relatively intact bedrock with high strength or stiff plastic old clay, providing reliable pile end support and anchorage conditions. This type of stratum structure is widely distributed in the alluvial plains of the middle and lower reaches of the Yangtze River, the Pearl River Delta, and the Bohai Bay area in my country. The typical profile in the Wuhan area is particularly representative, featuring shallow fill and silty clay overlying moderately weathered rock layers. This invention is specifically designed for such geological conditions, aiming to solve the core problem that a single foundation treatment process cannot simultaneously address shallow reinforcement and deep bearing capacity in binary structural strata.

[0031] Step 1: In the shallow, weak soil area of ​​the site to be treated, the foundation soil and solidifying agent are forcibly mixed using deep mechanical mixing. Multiple first piles are then constructed to form a shallow reinforced body. Deep mechanical mixing refers to using a specialized deep mixing pile machine to drive a drill rod with mixing blades into the soil to the designed depth while rotating. Then, while lifting, cement grout or other solidifying grout is injected under pressure through a grouting nozzle at the end of the drill rod. The rotating cutting and mixing action of the blades forces the weak soil and solidifying agent to mix deep within the foundation, thereby forming a reinforced body with a certain degree of integrity and strength. The solidifying agent is usually a cement-based material, but lime, fly ash, or composite solidifying materials can also be used depending on the soil characteristics. Through a hydration reaction with the water in the soil, a gel substance is generated, filling the pores between soil particles, encapsulating and cementing the soil particles, thus solidifying the loose soft soil into a cement-soil solidified body with a certain bearing capacity and deformation resistance. The first pile can be constructed using single-axis, dual-axis, or triple-axis mixing technology according to design requirements. The planar layout of the piles is usually in the form of an equilateral triangle (plum blossom shape) or a rectangular grid. Through the combined action of the pile group, a continuous, large-area integral solidified body is formed in the shallow layer, thereby effectively eliminating the heterogeneity of the shallow soft soil and limiting its lateral deformation and vertical compression deformation under load.

[0032] Step Two: After the shallow reinforcement has cured to its initial strength, multiple second piles are constructed at the pre-set pile locations using a non-displacement drilling process. These second piles penetrate the shallow reinforcement and embed themselves into the deep, stable bearing layer. Reaching initial strength is a critical time point in this process. Initial strength refers to the level at which, after a period of curing, the cement hydration reaction in the shallow reinforcement has reached a sufficient degree, and the pile strength has increased to a level sufficient to resist subsequent construction machinery vibrations, borehole wall lateral pressure, and other external forces without causing cracking or breakage. If the second pile is constructed before the reinforcement has reached sufficient strength, the mechanical vibrations, mud scouring forces, and borehole wall stress release during the drilling process will easily damage the still-weak first pile, rendering it ineffective in reinforcing the structure. Conversely, excessive curing time will affect the overall construction period. Engineering practice and testing have verified that, under normal temperature conditions, a curing time of approximately 7 days achieves a better balance between these two aspects. Non-displacement pile forming technology differs from driven or pressed pile forming methods. It uses drilling, percussion, or manual excavation to remove soil and form pile holes. This process minimizes the compression and disturbance to the surrounding soil, maximizing the protection of the structure of the first pile and the soil between them, ensuring the integrity and continuity of the synergistic load-bearing system. The multiple second piles are arranged in a grid pattern, with the first piles filling the soil between adjacent second piles, forming a synergistic load-bearing system where the first and second piles interweave spatially. This interweaving layout forms the spatial basis for the entire composite foundation's collaborative operation: the grid-like second piles constitute the main load-bearing framework, bearing concentrated point or linear loads, while the densely packed first piles between the grids form a continuous reinforcement layer, bearing distributed surface loads and constraining soil deformation between piles. Together, they form an organic load-bearing whole in three-dimensional space, combining rigidity and flexibility, and integrating point and surface elements, efficiently and smoothly transferring the superstructure load to the deep foundation.

[0033] Step 3: Conduct bearing capacity testing on the composite foundation formed by the second and first piles after curing. Curing and testing of the composite foundation is the final crucial step in quality assurance. After the second pile is poured, it needs to be watered or covered for curing at specified intervals to promote normal concrete strength development; the first pile needs to be cured cumulatively until its design age. After curing, a systematic bearing capacity test of the composite foundation should be conducted according to relevant specifications. Testing methods mainly include static load testing, high-strain dynamic testing, and core sampling inspection. Static load testing involves applying vertical loads in stages to the pile top or the bearing plate of the composite foundation, measuring the corresponding settlement, and plotting a load-settlement curve to determine whether the bearing capacity characteristic value of the composite foundation meets the design requirements. High-strain dynamic testing can quickly assess the integrity of the second pile and the bearing capacity of a single pile. Core sampling is used to test the uniformity of cement-soil mixing in the first pile and the unconfined compressive strength of the core sample, among other indicators. Only after all test results meet the design requirements can the composite foundation be deemed qualified and the subsequent construction phase of the foundation cap and superstructure begin.

[0034] In this embodiment, the deep stable bearing layer is a moderately weathered rock layer. The pile tip of the second pile penetrates the moderately weathered rock layer to a depth greater than or equal to 1.5m, primarily to bear the deep anti-buoyancy and core vertical bearing capacity. Moderately weathered rock refers to rock layers where, under weathering, the rock has been partially weathered along joint surfaces or fissure surfaces, and the mineral composition has undergone slight alteration, but the rock mass still retains its original rock structure and has high overall strength. It is an ideal bearing layer in pile foundation engineering. Limiting the rock penetration depth to no less than 1.5m is a safe embedding depth that has been fully verified through bearing capacity calculations and engineering experience: on the one hand, this depth ensures that the pile tip completely penetrates the weathered and fractured zone on the rock surface and embeds itself into relatively intact bedrock, allowing the pile tip resistance to be fully utilized; on the other hand, sufficient embedding depth provides reliable embedding, preventing the pile from detaching from the rock layer under the action of groundwater buoyancy and uplift loads, thus undertaking the deep anti-buoyancy anchoring function. The core vertical bearing capacity refers to the main self-weight load and live load transmitted from the superstructure to the stable rock strata through the pile body, which is the foundation for ensuring the safety of the entire plant structure. By specifying that the second pile body uses moderately weathered rock as the bearing stratum and stipulating the minimum rock penetration depth, the deep bearing capacity and anti-buoyancy capacity of the composite foundation are fundamentally guaranteed.

[0035] In this embodiment, the first pile body is a cement-soil mixing pile, and the second pile body is a cast-in-place pile. Cement-soil mixing piles utilize cement as a curing agent, forcibly mixing soft soil with cement slurry deep within the foundation using deep mixing machinery. This hardens the soft soil into a pile body with integrity, water stability, and a certain strength. It is convenient to construct, economical in cost, and provides reliable reinforcement, making it a mature technology for treating weak surface soil. Cast-in-place piles are formed by mechanically drilling a hole, placing a reinforcing cage inside, and then pouring concrete. Different pile diameters and lengths can be selected as needed to adapt to various complex geological conditions. They have outstanding advantages such as high bearing capacity, high stiffness, and small deformation. Using cement-soil mixing piles as the first pile body and cast-in-place piles as the second pile body is an optimal matching scheme that comprehensively considers the technical and economic advantages of both pile types and the functional division of the composite foundation's collaborative load-bearing system.

[0036] In this embodiment, the diameter of the cast-in-place piles is larger than that of the cement-soil mixing piles, and the spacing between adjacent cast-in-place piles is larger than that between adjacent cement-soil mixing piles, forming a composite load-bearing structure with rigid cast-in-place piles as the main framework and flexible mixing piles reinforcing the soil between the piles. The cast-in-place piles have larger diameters and wider spacing, forming a sparse yet strong main load-bearing framework, similar to the skeletal system of the human body, bearing and transmitting the main structural load to the deep rock mass; the cement-soil mixing piles have smaller diameters and denser spacing, forming a continuous and uniform soil reinforcement layer between the piles, similar to muscle tissue, filling the gaps in the framework and dispersing local stress, limiting lateral extrusion and compression deformation of the soil. This spatial configuration of "large sparse and small dense, rigid and flexible" allows the two types of piles to be highly complementary in function, work synergistically in terms of force, and achieve optimized allocation of materials and construction period in terms of engineering economy, constituting the essential characteristics of a composite load-bearing structure.

[0037] In this embodiment, the diameter of the cast-in-place piles is 700-900mm, and the pile spacing is 4-5 times the pile diameter; the diameter of the cement-soil mixing piles is 400-600mm, and the pile spacing is 2-3 times the pile diameter; the length of the cast-in-place piles is greater than that of the cement-soil mixing piles. The 700-900mm pile diameter is considered a medium-diameter pile, providing sufficient side friction area and end bearing area without excessive concrete and steel reinforcement due to an overly large diameter, resulting in good economic efficiency. The 4-5 times pile diameter pile spacing is an optimal spacing determined based on the pile-soil stress distribution pattern and group pile effect analysis: too small a spacing leads to severe stress overlap between piles, reducing the group pile bearing efficiency; too large a spacing increases the need for soil reinforcement between piles, potentially exceeding the effective reinforcement range of the mixing piles. The diameter of cement-soil mixing piles is 400-600mm, which is the commonly used construction diameter for current twin-axis mixing pile machines, making it easy to ensure pile quality. The pile spacing is 2-3 times the pile diameter, which ensures that the effective reinforcement range of adjacent mixing piles overlaps or is adjacent to each other, forming a continuous reinforced body without weak points. The length of cast-in-place piles is longer than that of mixing piles, reflecting the functional stratification of the two in vertical depth: mixing piles are concentrated in the shallow soft soil area for reinforcement, while cast-in-place piles extend through the reinforcement area to the deep bearing rock layer, forming a vertical collaborative pattern of "shallow continuous reinforcement and deep point support".

[0038] In this embodiment, a reinforcing cage is installed inside the cast-in-place pile. The main reinforcement bars of the cage are made of steel bars with a diameter of 20-25mm, and the stirrups are made of steel bars with a diameter of 6-10mm. The protective layer thickness of the reinforcing cage inside the hole is greater than or equal to 50mm. The reinforcing cage is the core load-bearing component of the cast-in-place pile resisting bending moment and tensile force. The main reinforcement bars are arranged longitudinally in the cage, bearing the axial tensile force and bending tensile stress of the pile. A diameter of 20-25mm provides sufficient reinforcement area and structural ductility, ensuring the safety and deformation capacity of the pile under complex stress conditions. The stirrups are spirally or circularly wound around the outside of the main reinforcement bars, bearing the shear force of the pile and providing lateral restraint to the main reinforcement bars, preventing buckling under compression. A diameter of 6-10mm meets the structural and load-bearing requirements. The protective layer thickness refers to the thickness of the concrete from the outer edge of the reinforcing cage to the borehole wall or the outer surface of the pile body concrete. It is set to be no less than 50mm. This is to ensure that the reinforcing bars are fully wrapped and bound by a sufficiently thick layer of concrete inside the pile body, to isolate them from the erosion of corrosive media in groundwater and soil, to ensure the durability of the reinforcing bars and the reliable bond and anchorage between the reinforcing bars and the concrete. It is an important structural measure to ensure the long-term bearing capacity and service life of cast-in-place piles.

[0039] In this embodiment, in step one, the shallow solidified layer formed by the cement-soil mixing pile is cured for no less than 7 days to achieve the required initial strength. The determination of the 7-day curing period is based on engineering considerations of the strength development law of cement-soil. The hydration and hardening process of cement-soil exhibits a significant age effect: under standard curing conditions, the unconfined compressive strength of cement-soil at 7 days can typically reach 40%-60% of the strength at 28 days of standard age. At this point, the mixing pile already possesses basic self-supporting ability and strength to resist minor external disturbances. Furthermore, the 7-day timeframe is reasonable within the general construction schedule and will not significantly extend the construction period. If unfavorable weather conditions such as low temperature or high humidity are encountered, the curing time can be appropriately extended according to the actual situation, or auxiliary curing measures such as covering and insulation can be adopted to ensure that the solidified layer reaches the required initial strength level.

[0040] In this embodiment, in step two, the drilling speed of the second pile body is less than or equal to 1.5 m / h, and the thickness of the sediment at the bottom of the hole after drilling is less than or equal to 50 mm. The second pile body is cast using the tremie method with a casting speed greater than or equal to 2 m / h, and the tremie is embedded in the concrete to a depth of 2-6 m. Limiting the drilling speed to within 1.5 m / h is for strict control of the hole wall stability and drilling quality: excessively fast drilling speeds can cause a sudden release of stress in the hole wall soil, easily leading to hole wall collapse or necking. Especially in this process, the hole wall portion is a pre-constructed cement-soil mixing pile reinforcement; rapid drilling disturbance may weaken the bonding performance between the reinforcement and the cast-in-place pile. The sediment at the bottom of the hole is a weak interlayer formed by the sedimentation and accumulation of broken rock and soil debris during the drilling process. If the thickness is too large, the pile tip will first compress the sediment layer after the cast-in-place pile is stressed, causing abnormal pile settlement and a decrease in bearing capacity. Using 50 mm as a control standard is a quality red line to ensure the full utilization of pile tip resistance. Underwater concrete pouring using the tremie pipe method is a key process to ensure the continuity and compactness of the cast-in-place pile. A pouring speed of not less than 2 m / h can keep the concrete flowing quickly in the tremie pipe and avoid blockage of the tremie pipe due to loss of concrete fluidity caused by too slow pouring. The tremie pipe should be buried in the concrete at a depth of 2-6 m. This can prevent the tremie pipe from being pulled out of the concrete surface and the mud from flowing back and causing the pile to break due to the burial depth being too shallow, and it can also prevent the tremie pipe from being pulled out of the concrete surface due to the burial depth being too shallow, which would cause the tremie pipe to be pulled out of the concrete surface and the mud to flow back and cause the pile to break. It can also prevent the tremie pipe from being pulled out of the concrete surface due to the burial depth being too deep, which would cause the pouring resistance to be too high and the tremie pipe to be difficult to pull out. These are the core process control parameters to ensure the integrity of the pile body.

[0041] In this embodiment, in step one, the cement content used in the cement-soil mixing pile is 12%-20% of the mass of the soil being reinforced, and the water-cement ratio is 0.45-0.65; the 28-day unconfined compressive strength of the cement-soil mixing pile is greater than or equal to 1.5 MPa. Cement content is a core material parameter determining the reinforcement effect of the cement-soil mixing pile, and is measured as a percentage of the wet mass of the reinforced soft soil. 12% is the lower economic limit for achieving a significant reinforcement effect; below this value, the cement hydration products are insufficient to effectively fill soil pores and cement soil particles, resulting in minimal strength increase. 20% is the upper limit for overall cost-effectiveness; exceeding this value leads to a slower increase in strength while material costs increase linearly. The water-cement ratio is set between 0.45 and 0.65, balancing the pumpability of the cement slurry with the density of the hardened body. A water-cement ratio that is too low results in an overly thick slurry, making pumping and uniform mixing difficult; a water-cement ratio that is too high results in an overly thin slurry, leaving excessive capillary pores in the hardened body after excess water evaporates, reducing density and strength. A 28-day unconfined compressive strength of not less than 1.5 MPa is the core indicator for assessing the pile strength corresponding to the designed composite foundation bearing capacity achieved by the mixing pile reinforcement. 28 days is the standard curing age, at which point the cement hydration reaction is essentially complete and the strength tends to stabilize. Using this as a quality acceptance benchmark allows for a scientific and accurate evaluation of whether the mixing pile reinforcement quality meets the standards.

[0042] A composite foundation structure is constructed using the composite foundation collaborative support method for binary soil strata described above. This composite foundation structure consists of multiple first piles and multiple second piles arranged in a specific spatial relationship. The first piles form a continuous reinforced body within the shallow, weak soil area, while the second piles are arranged in a grid pattern and penetrate the reinforced body to embed into the deep, stable bearing layer. Together, the two types of piles constitute an integral bearing system with a clear functional division and collaborative force-bearing mechanism. This structure can be widely applied to large steel structure industrial plants, warehousing and logistics centers, large-span exhibition halls, heavy-duty factories, and other construction projects with strict requirements for foundation bearing capacity, buoyancy resistance, and settlement control. It is particularly suitable for the Wuhan area and other alluvial plains such as the middle and lower reaches of the Yangtze River and the Pearl River Delta, which have similar binary soil structures with a soft upper layer and a hard lower layer, demonstrating significant regional adaptability and broad engineering application prospects.

[0043] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for collaborative support of composite foundations suitable for binary structure strata, characterized in that: Includes the following steps: Step 1: In the shallow soft soil area of ​​the site to be treated, the foundation soil and solidifying agent are forcibly mixed by deep mechanical mixing, and multiple first piles are constructed to form a shallow reinforced body. Step 2: After the shallow solidified body has cured to its initial strength, multiple second piles are constructed at the preset pile positions using a non-soil-displacement drilling process, so that the second piles penetrate the shallow solidified body and embed into the deep stable bearing layer; wherein, the multiple second piles are arranged in a grid pattern, and the first piles are filled in the soil between adjacent second piles, forming a cooperative force-bearing system in which the first piles and second piles intersect each other in space; Step 3: Conduct a bearing capacity test on the composite foundation formed by the second pile and the first pile after the maintenance is completed.

2. The method for collaborative support of composite foundations applicable to binary structure strata according to claim 1, characterized in that: The deep stable bearing layer is a moderately weathered rock layer. The pile tip of the second pile body enters the moderately weathered rock layer to a depth greater than or equal to 1.5m, so as to mainly bear the deep anti-buoyancy and core vertical bearing capacity.

3. The method for collaborative support of composite foundations applicable to binary structure strata according to claim 1, characterized in that: The first pile is a cement-soil mixing pile, and the second pile is a cast-in-place pile.

4. The method for collaborative support of composite foundations applicable to binary structure strata according to claim 3, characterized in that: The diameter of the cast-in-place pile is larger than that of the cement-soil mixing pile, and the spacing between adjacent cast-in-place piles is larger than that between adjacent cement-soil mixing piles, forming a composite bearing structure with rigid cast-in-place piles as the main framework and flexible mixing piles to reinforce the soil between the piles.

5. The method for collaborative support of composite foundations applicable to binary structure strata according to claim 4, characterized in that: The diameter of the cast-in-place piles is 700-900mm, and the pile spacing is 4-5 times the pile diameter; The diameter of the cement-soil mixing pile is 400-600mm, and the pile spacing is 2-3 times the pile diameter; The length of the cast-in-place pile is greater than the length of the cement-soil mixing pile.

6. The method for collaborative support of composite foundations applicable to binary structure strata according to claim 3, characterized in that: The cast-in-place pile is equipped with a steel cage. The main reinforcement bars of the steel cage are steel bars with a diameter of 20-25mm, and the stirrups are steel bars with a diameter of 6-10mm. The protective layer thickness of the steel cage in the hole is greater than or equal to 50mm.

7. The method for collaborative support of composite foundations applicable to binary structure strata according to claim 1, characterized in that: In step one, the shallow solidified layer formed by the cement-soil mixing pile is cured for no less than 7 days to give it the initial strength.

8. The method for collaborative support of composite foundations applicable to binary structure strata according to claim 1, characterized in that: In step two, the drilling speed of the second pile body is less than or equal to 1.5 m / h, and the thickness of the sediment at the bottom of the hole after drilling is less than or equal to 50 mm; the second pile body is poured with concrete using the tremie method, the pouring speed is greater than or equal to 2 m / h, and the tremie is buried in the concrete to a depth of 2-6 m.

9. The method for collaborative support of composite foundations applicable to binary structure strata according to claim 3, characterized in that: In step one, the cement content used in the cement-soil mixing pile is 12%-20% of the mass of the reinforced soil, and the water-cement ratio is 0.45-0.65; the 28-day unconfined compressive strength of the cement-soil mixing pile is greater than or equal to 1.5 MPa.

10. A composite foundation structure, characterized in that: The composite foundation collaborative support method applicable to binary structure strata, as described in any one of claims 1 to 9, is constructed using this method.

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