Construction method of cement mixing pile in soft soil foundation

Abstract

The application is suitable for the technical field of foundation treatment, and provides a soft soil foundation cement mixing pile construction method, which comprises the following steps: S1, on-site surveying of the soft soil foundation is performed to obtain soil physical and mechanical property data; a cement and composite cementitious material mixing ratio scheme is determined according to the soil data; S2, a satellite positioning system is used to accurately position the designed pile position, a mixing pile machine is moved to the specified pile position, is centered and leveled; S3, the mixing drill bit and the mortar pump are started, the cement slurry determined in S1 is sprayed into the soft soil foundation, and the drill rod is rotated and sunk at the same time, so that the cement slurry and the soft soil are forced to mix in situ, and a cement-soil pile body is formed. The application obtains the water content, the void ratio and the plasticity index of the soil through on-site surveying, and determines the cement and composite cementitious material mixing ratio scheme according to the same, so that the synergistic effect is fully exerted, and the slurry performance is highly matched with the on-site soil layer condition.

Description

Technical Field

[0001] This invention belongs to the field of foundation treatment technology, and particularly relates to a method for constructing cement mixing piles on soft soil foundations. Background Technology

[0002] Cement mixing pile technology, as a core process for soft soil foundation reinforcement, involves forcibly mixing cement grout with soft soil in situ to form cement-soil piles with high structural strength, thereby effectively improving the bearing capacity of the foundation and controlling uneven settlement deformation. This technology, with its advantages of short construction period, minimal environmental impact, and outstanding economic efficiency, has been widely used in the treatment of soft soil areas such as highway subgrades, municipal engineering, water conservancy facilities, and building foundations. However, current construction practices still have systemic defects in several key aspects, seriously restricting the stability and reliability of pile quality.

[0003] In terms of cement grout mix design, existing methods fail to fully incorporate the dynamic changes in the physical and mechanical properties of the soil on site. During construction, mix design schemes are often determined by fixed empirical parameters or indoor simulation tests. However, in actual working conditions, soil parameters such as moisture content, void ratio, and plasticity index fluctuate significantly due to geological conditions, resulting in a serious disconnect between indoor mix design and the on-site environment.

[0004] Construction positioning and verticality control also face severe challenges. Traditional pile location layout relies on manual measurement methods, making it difficult to guarantee positioning accuracy and often resulting in pile position deviation or cumulative errors. The verticality monitoring of the drill rod guide frame lacks a real-time dynamic feedback mechanism. Once the guide frame tilts beyond the allowable range, it is impossible to implement effective correction in time, causing the pile to bend or tilt. This deviation not only weakens the structural integrity of the pile but also directly affects its stress distribution and bearing capacity under load, creating potential safety hazards in the project.

[0005] In summary, existing methods for constructing cement-mixing piles on soft soil foundations have significant shortcomings in terms of the on-site adaptability of mix design and the precise control of the construction process. There is an urgent need to develop a construction system that can dynamically respond to soil properties, achieve precise control throughout the entire process, and ensure reliable quality. To address these issues, existing technologies urgently need improvement. Summary of the Invention

[0006] The purpose of this invention is to provide a method for constructing cement mixing piles on soft soil foundations, aiming to solve the above-mentioned problems.

[0007] This invention is implemented as follows: a method for constructing cement mixing piles on soft soil foundations, comprising the following steps: S1. Conduct on-site investigation of soft soil foundation to obtain soil physical and mechanical property data, including water content, void ratio and plasticity index; determine the mix ratio of cement and composite cementitious material based on the soil data to make the mixing pile grout suitable for the on-site soil conditions. S2. Use a satellite positioning system to accurately locate the designed pile position, move the mixing pile machine to the designated pile position, center and level it, and control the verticality deviation of the drill rod guide frame within the design allowable range. S3. Start the mixing drill bit and grout pump, and inject cement grout into the soft soil foundation according to the mix ratio determined in S1. At the same time, the drill rod rotates and sinks to force the cement grout to mix with the soft soil in situ to form a cement-soil pile. S4. After the drill rod is lowered to the designed pile bottom elevation, the drill rod is raised while maintaining the grouting state for mixing. After being raised to the designed pile top elevation, the drill rod is lowered again and a second mixing is performed to ensure that the cement grout and soil are fully and evenly mixed. S5. After pile formation, the mixing piles are sampled and tested. The test indicators include pile integrity, cement-soil uniformity and unconfined compressive strength. The pile formation quality is determined based on the test results to determine whether it meets the design requirements.

[0008] In a further technical solution, S1, the mix proportion includes a water-cement ratio and a composite cementitious material dosage; the composite cementitious material, by weight, consists of 70%-85% ordinary Portland cement, 10%-20% slag powder, and 5%-10% desulfurized gypsum; the water-cement ratio is dynamically adjusted according to the natural moisture content of the soil. When the natural moisture content is higher than the liquid limit, the water-cement ratio is 0.45-0.55; when the natural moisture content is lower than the liquid limit, the water-cement ratio is 0.55-0.65.

[0009] In a further technical solution, in S3, the rotational sinking speed of the mixing drill bit is 0.6-1.0 m / min, and the drill bit blade rotation speed is 40-60 r / min; the grouting pressure is controlled at 0.5-0.8 MPa, and the matching relationship between the grouting flow rate and the sinking speed is: the grouting volume corresponding to each meter of sinking depth is not less than 95% of the theoretical grouting volume required per meter of the designed pile body.

[0010] In a further technical solution, in S2, the verticality deviation of the drill rod guide frame is controlled within 0.5% of the pile length; when the deviation exceeds 0.5%, the horizontal adjustment cylinder of the pile driver chassis is used for real-time correction until the horizontal deviation of the guide frame is no greater than 0.3°.

[0011] In a further technical solution, S4, the specific operation of the secondary mixing is as follows: after the drill rod is raised to the design elevation of the pile top, it is immediately lowered to the bottom of the pile again at a speed not greater than 70% of the initial sinking speed, and then raised and mixed again at a speed of 0.8-1.2m / min. The total overlap area of ​​the two mixing operations is not less than 90% of the total length of the pile.

[0012] In a further technical solution, S5, the sampling rate for the sampling inspection is 2%-5% of the total number of piles, and not less than 5 piles; the inspection age is 28 days after pile formation, and the standard value of the unconfined compressive strength of the pile body is not less than 1.2MPa; at the same time, the core drilling method is used to test the integrity of the pile body, and the proportion of continuous, intact core samples and cement-soil in a hard plastic state is not less than 85%.

[0013] In a further technical solution, S1, the actual cement content per unit pile length of the cement grout is determined based on the soil physical properties using the following dimensionless correction formula: The actual cement consumption per unit pile length is equal to the benchmark cement consumption per unit pile length multiplied by the correction factor; The correction factor is equal to 1 plus the first empirical factor multiplied by the moisture content correction term, plus the second empirical factor multiplied by the natural porosity, plus the third empirical factor multiplied by the plasticity index correction term. The moisture content correction term is calculated by dividing the natural moisture content of the soil at the construction site by the liquid limit moisture content of the soil at the construction site. Both the natural moisture content and the liquid limit moisture content are expressed as percentages and divided directly, resulting in a dimensionless ratio. The plasticity index correction term is calculated by dividing the plasticity index of the soil by one hundred. The plasticity index is expressed as a percentage and divided by one hundred to obtain a dimensionless decimal. The natural void ratio is a dimensionless number calculated from the measured dry density and the specific gravity of the soil particles. The actual unit of cement usage per pile length is kilograms per meter. The benchmark unit of cement usage per pile length is the cement usage per pile length determined by indoor mix proportioning tests under the condition that the soil is in the plastic limit state, and its unit is also kilograms per meter. The first empirical coefficient, the second empirical coefficient, and the third empirical coefficient are all dimensionless empirical coefficients determined by indoor multi-factor orthogonal mix proportioning tests. The value range of the first empirical coefficient is 0.15 to 0.35, the value range of the second empirical coefficient is 0.05 to 0.20, and the value range of the third empirical coefficient is 0.10 to 0.30.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention obtains soil moisture content, void ratio, and plasticity index through on-site investigation, and determines the mix ratio of cement and composite cementitious material accordingly. In particular, the water-cement ratio is dynamically adjusted based on the relationship between natural moisture content and liquid limit. The composite cementitious material is composed of ordinary silicate cement, slag powder, and desulfurized gypsum in a specific ratio, giving full play to the synergistic effect and making the grout performance highly matched with the on-site soil conditions. This effectively avoids the problem of grout being too thin or too thick due to a fixed mix ratio, and improves the uniformity and later strength of cement-soil piles.

[0015] This invention uses a satellite positioning system to accurately locate the designed pile positions, replacing traditional manual layout and eliminating cumulative offset errors. At the same time, the verticality deviation of the drill rod guide frame is controlled within 0.5% of the pile length, and is dynamically corrected in real time by the horizontal adjustment cylinder of the pile driver chassis until the horizontal deviation is no more than 0.3°, ensuring the straightness of the pile body throughout the entire depth range, avoiding pile bending or tilting, and effectively ensuring the structural integrity and load transfer performance of the pile body.

[0016] This invention limits the rotational sinking speed of the mixing drill bit to 0.6-1.0 m / min, the blade rotation speed to 40-60 r / min, and the grouting pressure to 0.5-0.8 MPa. It also establishes a matching relationship between the grouting flow rate and the sinking speed, enabling efficient injection and preliminary mixing of the grout during the initial sinking stage. Simultaneously, the secondary mixing speed is no more than 70% of the initial sinking speed, the lifting speed is 0.8-1.2 m / min, and the total overlap area between the two mixing operations is no less than 90% of the total pile length. This greatly eliminates mixing dead zones and uneven mixing phenomena, and significantly improves the uniformity and density of the cement-soil in the pile body.

[0017] This invention proposes that the actual cement usage per unit pile length = benchmark usage × correction coefficient. The correction coefficient comprehensively considers three dimensionless correction terms: natural moisture content / liquid limit moisture content, natural void ratio, and plasticity index / 100, and multiplies each by an empirical coefficient determined through orthogonal regression experiments. This formula quantifies the impact of soil softness, compaction, and plasticity on cement demand, allowing for dynamic adjustment of cement usage according to soil conditions. This avoids material waste and ensures the consistency and reliability of pile strength under different geological conditions, significantly improving the adaptability and economy of the construction method. Attached Figure Description

[0018] Figure 1 A flowchart illustrating the main process of cement mixing pile construction on soft soil foundations; Figure 2 A logic diagram was developed for the mix proportion scheme; Figure 3 This is a logic diagram for drilling and mixing operations. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0020] The specific implementation of the present invention will be described in detail below with reference to specific embodiments.

[0021] like Figures 1-3As shown, an embodiment of the present invention provides a method for constructing cement mixing piles on soft soil foundations, comprising the following steps: In step S1, an on-site investigation is conducted on the soft soil foundation to obtain data on the physical and mechanical properties of the soil, including moisture content, void ratio, and plasticity index. This investigation can be carried out through on-site drilling and sampling, indoor geotechnical tests, etc. For example, the natural moisture content of the soil can be determined using the ring cutter method or the drying method; the specific gravity of soil particles can be determined using the hydrometer method and combined with dry density to calculate the void ratio; and the liquid limit and plastic limit of the soil can be determined using a combined liquid and plastic limit tester, thereby calculating the plasticity index. After obtaining these data, the mix proportion scheme of cement and composite cementitious materials is determined based on the soil data to ensure that the mixing pile grout is suitable for the on-site soil conditions. The mix proportion scheme can be determined based on empirical formulas, charts, or preliminary indoor test results. For example, a water-cement ratio range can be initially set based on the moisture content of the soil; different types of composite cementitious materials can be selected or their dosage adjusted based on the plasticity index of the soil.

[0022] In step S2, a satellite positioning system is used to accurately locate the designed pile position. The mixing pile machine is moved to the designated pile position, centered, and leveled to ensure that the verticality deviation of the drill rod guide frame is controlled within the design allowable range. Pile position positioning can be performed using a handheld GPS device or a vehicle-mounted GNSS receiver. The design coordinates are input into the device to guide the pile machine to the target position. After the pile machine reaches the designated pile position, preliminary centering and leveling are performed visually or using a simple level. The verticality of the drill rod guide frame can be checked by suspending a plumb line or using an angle gauge to ensure that it is within the allowable deviation range. For example, construction personnel can manually adjust the height of the support legs of the pile machine chassis to make the drill rod guide frame approximately vertical.

[0023] In step S3, the mixing drill bit and grout pump are started, and cement grout is injected into the soft soil foundation according to the mix ratio determined in S1. Simultaneously, the drill rod rotates and sinks, forcibly mixing the cement grout with the soft soil in situ to form a cement-soil pile. The mixing drill bit is typically started by the hydraulic system of the pile driver, rotating it at a preset speed. The grout pump delivers the pre-mixed cement grout through pipelines to the injection hole at the bottom of the drill bit. As the drill rod rotates, it drills downwards at a certain speed, and the drill blades break up the soil and initially mix it with the injected grout. For example, during construction, the operator can manually control the sinking speed of the drill rod and the injection volume of the grout pump based on experience to achieve a better mixing effect.

[0024] In step S4, after the drill rod is lowered to the designed pile bottom elevation, it is raised while maintaining the grouting state for mixing. After being raised to the designed pile top elevation, the drill rod is lowered again for secondary mixing to ensure that the cement grout and soil are fully and evenly mixed. When the drill rod reaches the designed pile bottom, lowering stops, but grouting continues while rotating, and then it is raised at a certain speed. During the raising process, the mixing drill bit re-mixes the soil that has been injected with grout, further promoting the mixing of grout and soil. After the first raising to the pile top is completed, in order to improve the uniformity of mixing, the drill rod will be lowered to the pile bottom again and raised and mixed again. For example, the operator can manually control the lowering and raising speed of the secondary mixing according to the site conditions and roughly estimate the overlap area of ​​the two mixing processes.

[0025] In step S5, after pile formation, sampling inspections are conducted on the mixed-mix piles. The inspection indicators include pile integrity, cement-soil homogeneity, and unconfined compressive strength. The results determine whether the pile quality meets design requirements. Sampling inspections can be conducted after the piles have reached a certain age. Pile integrity can be initially assessed visually or using a simple sonic logging method. Cement-soil homogeneity can be judged by observing the consistency of color and texture in the soil samples taken. Unconfined compressive strength can be obtained by taking samples on-site, preparing test blocks, and conducting compressive strength tests in a laboratory. For example, several piles can be randomly selected for simple visual inspection and strength testing to evaluate the overall construction quality.

[0026] The construction method in this embodiment, through refined mix design, high-precision construction process control, optimized mixing technology, and a comprehensive quality testing system, forms a systematic and quantifiable technical solution. This solution can effectively solve many problems existing in the prior art and significantly improve the pile quality and engineering reliability of cement mixing piles on soft soil foundations.

[0027] In a preferred embodiment of the present invention, in S1, the mix proportion includes a water-cement ratio and a composite cementitious material content; the composite cementitious material, by weight, consists of 70%-85% ordinary Portland cement, 10%-20% slag powder, and 5%-10% desulfurized gypsum; the water-cement ratio is dynamically adjusted according to the natural moisture content of the soil. When the natural moisture content is higher than the liquid limit, the water-cement ratio is 0.45-0.55, and when the natural moisture content is lower than the liquid limit, the water-cement ratio is 0.55-0.65.

[0028] In this embodiment, the proposed solution refines and dynamically adjusts the mix ratio of cement and composite cementitious material in the cement mixing pile construction method for soft soil foundations, thereby ensuring that the grout performance is highly matched with the on-site soil conditions. Specifically, the mix ratio specifies the water-cement ratio and the amount of composite cementitious material, which is composed of ordinary Portland cement, slag powder, and desulfurized gypsum in a specific proportion. This ensures that the grout not only has good early strength but also improves its workability, later strength, and durability through the synergistic effect of slag powder and desulfurized gypsum. More importantly, the water-cement ratio is dynamically adjusted according to the relationship between the natural moisture content and the liquid limit of the soil. When the natural moisture content is higher than the liquid limit, the water-cement ratio is appropriately reduced to avoid the grout becoming too thin; when the natural moisture content is lower than the liquid limit, the water-cement ratio is appropriately increased to ensure the fluidity of the grout and its thorough mixing with the soil. This dynamic adjustment mechanism enables the soil physical and mechanical property data obtained in step S1 to be used more effectively, ensuring that the consistency and activity of the grout are always at their best when the cement grout is forcibly mixed with the soft soil in situ in steps S3 and S4, thereby forming a uniform, dense cement-soil pile with the required strength, and finally obtaining excellent pile quality in the test in step S5.

[0029] In a preferred embodiment of the present invention, in S3, the rotational sinking speed of the stirring drill bit is 0.6-1.0 m / min, and the drill bit blade rotation speed is 40-60 r / min; the grouting pressure is controlled at 0.5-0.8 MPa, and the matching relationship between the grouting flow rate and the sinking speed is: the grouting volume corresponding to each meter of sinking depth is not less than 95% of the theoretical grouting volume required per meter of the designed pile body.

[0030] In this embodiment, the rotational sinking speed of the mixing drill bit refers to the rate at which the drill bit penetrates the soil while rotating. This speed directly affects the degree of disturbance to the soil by the drill bit and the contact time between the grout and the soil. For example, the feed speed of the drill rod can be precisely controlled by a hydraulic system, or the sinking speed of the drill rod can be adjusted by a gear transmission mechanism driven by a servo motor. The blade rotation speed refers to the rate at which the blades on the mixing drill bit rotate around their axis. This rotation speed determines the cutting, crushing, and mixing intensity of the blades on the soil. For example, the mixing head can be driven by a variable frequency motor, and the blade rotation speed can be precisely controlled by adjusting the motor frequency, or different rotation speeds can be switched by a mechanical gearbox. The grouting pressure refers to the pressure of the cement grout when it is ejected from the grouting nozzle. This pressure affects the grouting distance, penetration ability, and mixing range with the soil. For example, the grouting pressure can be controlled by adjusting the output power or rotation speed of the grout pump, while combining it with a pressure sensor for real-time monitoring and feedback adjustment to ensure that the pressure is stable within the target range. The matching relationship between the grouting flow rate and the sinking speed refers to the proportional relationship between the amount of grout injected per unit sinking depth and the design requirements during the sinking of the drill rod. This relationship ensures that the cement slurry is supplied to the mixing area in sufficient and uniform quantity throughout the sinking process, avoiding localized shortages or excesses. For example, the grout flow rate can be monitored in real time by a flow meter and linked with data from the drill rod sinking speed sensor. The control system then dynamically adjusts the output of the grout pump according to a preset matching algorithm to meet the slurry quantity requirements.

[0031] This application's solution optimizes the in-situ forced mixing process of cement slurry and soft soil by precisely controlling the rotational sinking speed of the mixing drill bit, the drill bit blade rotation speed, the grouting pressure, and the matching relationship between the grouting flow rate and the sinking speed. Specifically, during the drill rod sinking process, the rotational sinking speed of the mixing drill bit is limited to a suitable range of 0.6-1.0 m / min, while the drill bit blade rotation speed is controlled at 40-60 r / min. This allows the drill bit to cut, break up, and initially mix the soil with appropriate intensity as it penetrates downwards, creating favorable conditions for thorough mixing of the slurry. Furthermore, by controlling the grouting pressure at 0.5-0.8 MPa, it is ensured that the cement slurry can be uniformly injected into the disturbed soil with sufficient kinetic energy and penetrating power. More importantly, by establishing a matching relationship between the grouting flow rate and the sinking speed—that is, the grouting volume per meter of sinking depth is not less than 95% of the theoretical grout volume required per meter of the designed pile—sufficient cement grout is precisely injected into the target area at each step of the drill rod sinking, avoiding uneven mixing caused by insufficient or excessive grout supply. This multi-parameter collaborative control strategy ensures that the cement grout can be efficiently and uniformly forcibly mixed with the soft soil during the drill rod sinking stage, laying a solid foundation for subsequent lifting and secondary mixing, thus effectively solving problems such as uneven mixing and unstable pile quality that may occur in traditional construction.

[0032] In a preferred embodiment of the present invention, in S2, the verticality deviation of the drill rod guide frame is controlled within 0.5% of the pile length; when the deviation exceeds 0.5%, the horizontal adjustment cylinder of the pile driver chassis is used for real-time correction until the horizontal deviation of the guide frame is no greater than 0.3°.

[0033] In this embodiment, the drill rod guide frame is a key component ensuring the drill rod remains vertical during the sinking process. Its verticality deviation refers to the degree of deviation between the actual vertical direction and the ideal vertical direction of the guide frame. Controlling this deviation within 0.5% of the pile length aims to provide a high-precision initial verticality guarantee for the formation of the mixing pile, ensuring the pile maintains good straightness throughout its depth and preventing bending or tilting due to excessive initial deviation. This can be achieved by installing a high-precision tilt sensor or laser verticality detector on the mixing pile machine to monitor the verticality of the guide frame in real time and compare it with the preset 0.5% deviation limit. Alternatively, a method based on visual recognition or inertial navigation systems can be used to accurately measure the orientation of the guide frame and calculate its deviation from the vertical direction to ensure it meets the control requirements. When the verticality deviation of the drill rod guide frame is detected to exceed the allowable range (i.e., 0.5% of the pile length), immediate corrective measures are required. Real-time correction via the leveling cylinders of the piling rig chassis enables rapid and precise adjustment of the chassis's level, thereby restoring the guide frame to the required verticality and preventing accumulated deviations from negatively impacting subsequent construction. The leveling cylinders can be driven by a hydraulic system, manually controlled by the operator or automatically adjusted by an automatic control system based on deviation data from sensors. For example, when a sensor detects tilt in a certain direction, the control system sends a command to the corresponding cylinder to extend or retract, adjusting the chassis level. Alternatively, the cylinders can be used in conjunction with high-precision displacement sensors to form a closed-loop control system. When a verticality deviation signal is input, the controller calculates the required cylinder extension or retraction based on a preset algorithm and precisely controls the cylinder movement via a proportional valve, achieving fine-tuning of the chassis. The horizontal deviation of the guide frame is a manifestation of verticality deviation, typically reflected by measuring the horizontal tilt angle of the top or bottom of the guide frame. Controlling the horizontal deviation to no more than 0.3° further refines and quantifies the precision requirements for verticality control, ensuring that the guide frame achieves extremely high vertical accuracy after correction, providing a solid foundation for high-quality mixing pile construction. This can be achieved by measuring the tilt angle of the guide frame in real time using a high-precision electronic level or a dual-axis tilt sensor mounted on the guide frame. The correction system continues to operate when the measured value is greater than 0.3°; correction stops when the measured value drops to 0.3° or lower. Alternatively, a laser rangefinder or total station can be used to measure multiple points on the guide frame externally, calculate its spatial attitude, and determine whether its levelness deviation meets the requirement of not exceeding 0.3° based on the calculation results.

[0034] In a preferred embodiment of the present invention, in S4, the specific operation of the secondary mixing is as follows: after the drill rod is raised to the design elevation of the pile top, it is immediately lowered to the bottom of the pile again at a speed not greater than 70% of the initial sinking speed, and then raised and mixed again at a speed of 0.8-1.2m / min. The total overlap area of ​​the two mixing operations is not less than 90% of the total length of the pile.

[0035] In this embodiment, "after the drill rod is raised to the designed elevation at the top of the pile, it is immediately lowered again to the bottom of the pile at a speed not exceeding 70% of the initial lowering speed" means that after the initial mixing and raising to the top of the pile, the second lowering process of the drill rod is started quickly without a prolonged pause. The word "immediately" here aims to prevent the mixed slurry and soil from initial setting or settling, maintaining its good fluidity so that subsequent secondary mixing can be carried out more effectively. The speed of the second lowering is limited to no more than 70% of the initial lowering speed, meaning that the second lowering speed will be slower than the first. For example, if the initial lowering speed is 1.0 m / min, the second lowering speed can be controlled at 0.7 m / min or lower. This decelerated lowering aims to increase the contact time between the drill bit blades and the soil, allowing for more fine shearing and breaking of the already preliminarily mixed soil, further promoting sufficient contact and mixing between cement particles and soil particles.

[0036] "Re-lifting and mixing at a speed of 0.8-1.2 m / min" refers to a second upward mixing and lifting operation at a specific speed range after the drill rod has sunk to the bottom of the pile. This speed range is optimized to ensure that the cement slurry is evenly distributed across the entire cross-section and length of the pile during the lifting process, further eliminating any potential mixing dead zones or uneven areas. For example, the lifting speed can be precisely set to 1.0 m / min, or finely adjusted between 0.8 m / min and 1.2 m / min depending on the soil conditions.

[0037] The requirement that "the total overlap area of ​​the two mixing operations should not be less than 90% of the total pile length" refers to the effective length of the drill rod's mixing action on the pile body throughout the entire construction process. This requirement ensures that the vast majority of the pile body undergoes at least two effective mixing treatments, thereby maximizing the uniformity and density of the pile. For example, for a mixing pile with a design length of 10 meters, the effective overlap area of ​​the two mixing operations should be at least 9 meters. This can be achieved by precisely controlling the start and stop positions and stroke of the drill rod.

[0038] In a preferred embodiment of the present invention, in S5, the sampling rate of the sampling inspection is 2%-5% of the total number of piles, and not less than 5 piles; the inspection age is 28 days after pile formation, and the standard value of the unconfined compressive strength of the pile body is not less than 1.2MPa; at the same time, the core drilling method is used to test the integrity of the pile body, and the proportion of continuous, intact core samples and cement-soil in a hard plastic state is not less than 85%.

[0039] In this embodiment, the sampling rate is 2%-5% of the total number of piles, with a minimum of 5 piles, aiming to balance testing costs with the effectiveness of quality control. This range ensures a sufficient sample size for statistical analysis, while the requirement of "minimum of 5 piles" avoids insufficient sample size due to a low proportion when the total number of piles is small, thus guaranteeing the representativeness and reliability of the test results. For example, the construction party can randomly select piles for testing within the range of 2%-5% based on the project scale and the total number of piles, ensuring that the number of piles sampled is not less than 5; or, the supervision unit can specify the sampling pile locations based on the project characteristics and risk assessment, and verify that the number of piles sampled meets the above requirements. The testing age is 28 days after pile formation. This age is a key node in the strength development of cement-based materials. It is generally believed that at this age, the cement hydration reaction is basically completed, the material strength tends to stabilize, and it can better reflect its final strength performance. Testing at this time point can obtain representative strength data for evaluating the long-term bearing capacity of the piles. For example, after the pile construction is completed, the construction unit records the completion date of each pile and strictly arranges subsequent strength testing according to the 28-day curing period; or, by setting up on-site curing test blocks, curing them synchronously with the piles, and conducting strength tests on the test blocks at the 28-day curing period, the strength development of the piles can be verified. The standard value of the unconfined compressive strength of the pile is not less than 1.2 MPa. This characteristic sets the minimum qualified standard for the strength of cement-soil piles, directly affecting the bearing capacity of the piles and the effectiveness of foundation treatment. Setting a clear strength standard value provides a quantitative basis for project quality acceptance, ensuring that the treated soft soil foundation can meet the design bearing requirements. For example, by drilling core samples for indoor unconfined compressive strength testing, the measured strength value should reach or exceed 1.2 MPa; or, by using on-site static cone penetration tests or vane shear tests, combined with empirical formulas or relevant specifications, the strength of the pile can be indirectly evaluated, ensuring that its equivalent strength is not less than 1.2 MPa. The core drilling method is used to test the integrity of piles. This method is a direct and reliable testing technique. By extracting continuous core samples from the pile, the internal structure of the pile, the uniformity of the cement-soil mixture, and the presence of defects such as mud inclusions and fractures can be directly observed, thus comprehensively assessing the integrity of the pile. For example, using a professional drilling rig and core sampling equipment, representative core samples are extracted from the pile according to specifications; or, during the core drilling process, changes in drilling parameters are considered to preliminarily determine the integrity of the pile, and the results are compared with the extracted core samples. The proportion of continuous, intact core samples with cement-soil in a stiff plastic state is no less than 85%. This characteristic further refines the qualification standard for core drilling integrity testing. "Continuous and intact" core samples indicate that there are no obvious fractures or serious defects in the internal structure of the pile; "cement-soil in a stiff plastic state" indicates that the cement slurry is uniformly mixed with the soil, and the hydration reaction is sufficient, forming cement-soil with a certain strength.Setting a minimum ratio of 85% allows for a certain degree of localized non-uniformity while ensuring that the quality of the vast majority of the pile body meets requirements, thereby guaranteeing the overall performance of the pile. For example, core samples are visually inspected and tactilely assessed, recording their continuity, integrity, and the hardness of the cement-soil mixture, and calculating the proportion of standard-compliant core sample lengths to the total core sample length; alternatively, in conjunction with laboratory tests, the moisture content, density, and other indicators of the core samples are measured to help determine the hard plastic state of the cement-soil mixture.

[0040] In a preferred embodiment of the present invention, in S1, the actual cement content per unit pile length of the cement grout is determined according to the soil physical properties using the following dimensionless correction formula: The actual cement consumption per unit pile length is equal to the benchmark cement consumption per unit pile length multiplied by the correction factor; The correction factor is equal to 1 plus the first empirical factor multiplied by the moisture content correction term, plus the second empirical factor multiplied by the natural porosity, plus the third empirical factor multiplied by the plasticity index correction term. The moisture content correction term is calculated by dividing the natural moisture content of the soil at the construction site by the liquid limit moisture content of the soil at the construction site. Both the natural moisture content and the liquid limit moisture content are expressed as percentages and divided directly, resulting in a dimensionless ratio. The plasticity index correction term is calculated by dividing the plasticity index of the soil by one hundred. The plasticity index is expressed as a percentage and divided by one hundred to obtain a dimensionless decimal. The natural void ratio is a dimensionless number calculated from the measured dry density and the specific gravity of the soil particles. The actual unit of cement usage per pile length is kilograms per meter. The benchmark unit of cement usage per pile length is the cement usage per pile length determined by indoor mix proportioning tests under the condition that the soil is in the plastic limit state, and its unit is also kilograms per meter. The first empirical coefficient, the second empirical coefficient, and the third empirical coefficient are all dimensionless empirical coefficients determined by indoor multi-factor orthogonal mix proportioning tests. The value range of the first empirical coefficient is 0.15 to 0.35, the value range of the second empirical coefficient is 0.05 to 0.20, and the value range of the third empirical coefficient is 0.10 to 0.30.

[0041] In this embodiment, the technical solution aims to address the problem of uneven soil properties in soft soil foundations. By introducing a dynamic adjustment mechanism, the actual amount of cement grout used can be accurately calculated and adjusted based on the specific physical properties of the soil on site. This ensures the performance consistency of cement-soil piles under different soil layer conditions, avoiding fluctuations in engineering quality or waste of resources due to soil differences. This determination process can be based on pre-calculation using layered exploration data of the soil layers at the pile location before construction, or it can be dynamically corrected by combining real-time soil parameters acquired during drilling. The correction formula provides a quantitative method for calculating the actual cement usage per unit pile length. It multiplies the baseline cement usage by a correction coefficient, which comprehensively considers the influence of key soil physical properties such as moisture content correction, natural void ratio correction, and plasticity index correction. This multi-factor-based correction mechanism makes the determination of cement usage more scientific and accurate, and can more comprehensively reflect the soil's demand for cement. This formula can be integrated into the control system of the mixing pile machine through a preset program for automated calculation and adjustment, or it can be manually calculated by on-site technicians based on measured data.

[0042] The moisture content correction term is obtained by dividing the natural moisture content of the soil at the construction site by the liquid limit moisture content. Natural moisture content is a key factor affecting cement hydration and the formation of cement-soil strength, while the liquid limit moisture content characterizes the boundary between the plastic and liquid states of the soil. The ratio of these two values ​​provides a dimensionless reflection of the soil's softness, allowing for a more accurate assessment of its cement requirements. For example, a higher natural moisture content indicates softer soil, potentially requiring more cement to achieve the desired solidification effect; conversely, a lower natural moisture content allows for a reduction in cement usage. The plasticity index correction term is obtained by dividing the soil's plasticity index by one hundred. The plasticity index reflects the extent of the soil's plasticity and is typically related to the clay content and mineral composition. Highly plastic soils usually have a larger specific surface area and stronger adsorption capacity, potentially requiring more cement to neutralize their surface charge and form a stable cemented structure. Introducing this correction term allows for differentiated adjustments to cement usage for soils with varying plasticity characteristics, optimizing the mechanical properties of cement-soil.

[0043] The natural void ratio is the ratio of pore volume to solid particle volume in soil, directly reflecting the soil's density. A higher void ratio indicates looser soil, requiring more pore space to be filled. Therefore, more cement grout may be needed to fill the pores and form a continuous, consolidated body. Introducing this indicator allows the amount of cement used to be directly linked to the initial density of the soil, ensuring that the cement grout can fully penetrate and consolidate the soil, improving the overall uniformity and strength of the pile.

[0044] The baseline cement content per unit pile length is determined through laboratory mix design tests when the soil is in a plastic limit state. It provides a standardized starting point for the modified formula, representing the amount of cement required under ideal or standard soil conditions. This baseline content can be optimized by conducting a series of cement content tests on representative soil samples in the laboratory, combined with indicators such as strength and permeability.

[0045] The first, second, and third empirical coefficients are dimensionless and were determined through regression analysis using indoor multi-factor orthogonal mix design experiments. They quantify the relative contributions of the moisture content correction term, the natural void ratio correction term, and the plasticity index correction term to the actual cement usage per unit pile length, respectively. The range of these coefficients (0.15 to 0.35 for the first empirical coefficient, 0.05 to 0.20 for the second empirical coefficient, and 0.10 to 0.30 for the third empirical coefficient) provides flexibility for calibration and optimization in different engineering practices, ensuring that the correction formula can adapt to the specific needs of different regions and soil conditions.

[0046] Step S1 no longer simply involves determining a fixed mix proportion; instead, it allows for dynamic adjustment of the cement dosage based on the actual soil conditions. This ensures that when the mixing drill and grout pump are activated in step S3 to inject cement grout, the amount of grout injected is precisely tailored to the specific conditions of the current soil layer. This refined control of cement dosage results in cement-soil piles with more uniform strength and more stable performance after the cement grout is forcibly mixed in situ with the soft soil. This effectively overcomes the challenges posed by the variability of soft soil properties, thereby improving the adaptability and reliability of the entire construction method.

[0047] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for constructing cement mixing piles on soft soil foundations, characterized in that, Includes the following steps: S1. Conduct on-site investigation of soft soil foundation to obtain soil physical and mechanical property data, including water content, void ratio and plasticity index; determine the mix ratio of cement and composite cementitious material based on the soil data to make the mixing pile grout suitable for the on-site soil conditions. S2. Use a satellite positioning system to accurately locate the designed pile position, move the mixing pile machine to the designated pile position, center and level it, and control the verticality deviation of the drill rod guide frame within the design allowable range. S3. Start the mixing drill bit and grout pump, and inject cement grout into the soft soil foundation according to the mix ratio determined in S1. At the same time, the drill rod rotates and sinks to force the cement grout to mix with the soft soil in situ to form a cement-soil pile. S4. After the drill rod is lowered to the designed pile bottom elevation, the drill rod is raised while maintaining the grouting state for mixing. After being raised to the designed pile top elevation, the drill rod is lowered again and a second mixing is performed to ensure that the cement grout and soil are fully and evenly mixed. S5. After pile formation, the mixing piles are sampled and tested. The test indicators include pile integrity, cement-soil uniformity and unconfined compressive strength. The pile formation quality is determined based on the test results to determine whether it meets the design requirements.

2. The construction method for cement mixing piles on soft soil foundations according to claim 1, characterized in that, In S1, the mix proportion scheme includes the water-cement ratio and the amount of composite cementitious material; the composite cementitious material, by weight, consists of 70%-85% ordinary Portland cement, 10%-20% slag powder and 5%-10% desulfurized gypsum; the water-cement ratio is dynamically adjusted according to the natural moisture content of the soil. When the natural moisture content is higher than the liquid limit, the water-cement ratio is 0.45-0.55, and when the natural moisture content is lower than the liquid limit, the water-cement ratio is 0.55-0.

65.

3. The method for constructing cement mixing piles on soft soil foundations according to claim 1, characterized in that, In S3, the rotational sinking speed of the stirring drill bit is 0.6-1.0 m / min, and the drill bit blade rotation speed is 40-60 r / min; the grouting pressure is controlled at 0.5-0.8 MPa, and the matching relationship between the grouting flow rate and the sinking speed is: the grouting volume corresponding to each meter of sinking depth is not less than 95% of the theoretical grouting volume required per meter of the designed pile body.

4. The method for constructing cement mixing piles on soft soil foundations according to claim 1, characterized in that, In S2, the verticality deviation of the drill rod guide frame is controlled within 0.5% of the pile length; when the deviation exceeds 0.5%, the horizontal adjustment cylinder of the pile driver chassis is used for real-time correction until the horizontal deviation of the guide frame is no greater than 0.3°.

5. The method for constructing cement mixing piles on soft soil foundations according to claim 1, characterized in that, In S4, the specific operation of the secondary mixing is as follows: after the drill rod is raised to the design elevation of the pile top, it is immediately lowered to the bottom of the pile again at a speed not greater than 70% of the initial sinking speed, and then raised and mixed again at a speed of 0.8-1.2m / min. The total overlap area of ​​the two mixing is not less than 90% of the total length of the pile.

6. The method for constructing cement mixing piles on soft soil foundations according to claim 1, characterized in that, In S5, the sampling rate for the sampling inspection is 2%-5% of the total number of piles, and not less than 5 piles; the inspection age is 28 days after pile formation, and the standard value of the unconfined compressive strength of the pile body is not less than 1.2MPa; at the same time, the core drilling method is used to test the integrity of the pile body, and the proportion of continuous, intact core samples and cement-soil in a hard plastic state is not less than 85%.

7. The method for constructing cement mixing piles on soft soil foundations according to claim 1, characterized in that, In S1, the actual cement content per unit pile length of the cement grout is determined based on the soil physical properties using the following dimensionless correction formula: The actual cement consumption per unit pile length is equal to the benchmark cement consumption per unit pile length multiplied by the correction factor; The correction factor is equal to 1 plus the first empirical factor multiplied by the moisture content correction term, plus the second empirical factor multiplied by the natural porosity, plus the third empirical factor multiplied by the plasticity index correction term. The moisture content correction term is calculated by dividing the natural moisture content of the soil at the construction site by the liquid limit moisture content of the soil at the construction site. Both the natural moisture content and the liquid limit moisture content are expressed as percentages and divided directly, resulting in a dimensionless ratio. The plasticity index correction term is calculated by dividing the plasticity index of the soil by one hundred. The plasticity index is expressed as a percentage and divided by one hundred to obtain a dimensionless decimal. The natural void ratio is a dimensionless number calculated from the measured dry density and the specific gravity of the soil particles.

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