Grouting-jet pile wall construction method for thick sand layer pile foundations

Abstract

This invention discloses a method for constructing retaining walls for jet grouting piles in thick sandy soil layers, relating to the field of bridge pile foundation construction technology. The method includes the following steps: geological investigation, parameter matching, and precise pile location layout for jet grouting piles in thick sandy soil layers; drilling rig centering, grout mixing, and jet grouting for pile formation; graded jet grouting pile construction and pile overlap accuracy control; core sampling and unconfined compressive strength testing after pile formation; and curing of jet grouting piles and control of their connection with drilled cast-in-place pile drilling and mud parameters. This method solves the technical problems in existing technologies for bridge pile foundation construction in thick sandy soil layers, such as easy collapse of the borehole wall, high cost of retaining wall construction, poor integrity of the support system, and uncontrollable quality of sand layer reinforcement.

Description

Technical Field

[0001] This invention relates to the field of bridge pile foundation construction technology, and more specifically, to a method for constructing a grout-jet pile retaining wall for thick sand layer pile foundations. Background Technology

[0002] With the rapid development of transportation infrastructure construction, bridge engineering is gradually extending to areas with complex geological conditions. Thick sandy layers are widely distributed in alluvial plains and other regions, becoming one of the common complex geological types in bridge pile foundation construction. Currently, thick sandy layers are characterized by large sand layer thickness, loose soil, and high permeability. During bridge pile foundation drilling, factors such as groundwater seepage, soil self-weight, and construction disturbance can easily cause problems such as borehole wall collapse and sand inrush. This can not only prevent the smooth progress of bored pile construction but also lead to construction accidents such as drill bit burial and stuck drill bits, seriously affecting the safety and efficiency of pile foundation construction.

[0003] In existing technologies, the main methods for borehole wall protection in thick sand layer pile foundation construction include steel casing, high-pressure jet grouting, and mud slurry. Among these: The steel casing method requires specialized machinery for casing installation and removal, which incurs high rental costs and high material costs. For thick sand layers (thickness ≥ 15m), the construction cost increases significantly.

[0004] While high-pressure jet grouting piles can reinforce sand layers, the construction process is complex, requiring high standards for equipment and personnel. Furthermore, the quality of pile formation is greatly affected by construction parameters, making it prone to incomplete reinforcement.

[0005] Traditional mud slurry wall protection is only suitable for scenarios with thin sand layers and low groundwater levels. For thick sand layers, the mud is prone to loss, resulting in poor wall protection and difficulty in effectively preventing borehole collapse. At the same time, some existing grouting pile wall protection construction methods also have problems such as loose pile overlap and poor overall support system. The construction process does not carry out graded wall protection design according to the sand layer thickness, resulting in poor reinforcement effect in thick sand layer areas. Subsequent drilling is still prone to borehole collapse. Furthermore, the inspection and acceptance standards after grouting pile completion are not clear, making it impossible to effectively control the quality of sand layer reinforcement and creating safety hazards for pile foundation construction. Summary of the Invention

[0006] Therefore, the present invention provides a method for constructing a grouting pile retaining wall for thick sand layer pile foundations, in order to solve the technical problems in the prior art for bridge pile foundation construction in thick sand layer geology, such as easy collapse of the borehole wall, high cost of retaining wall construction, poor integrity of the support system, and uncontrollable quality of sand layer reinforcement.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A method for constructing a grout-jet pile retaining wall for thick sand layer pile foundations includes the following steps: S1: Geological investigation, parameter matching and precise pile location layout construction of grouting pile foundation in thick sand layer; For the construction area of ​​thick sand pile foundation, on-site geological survey was carried out to determine the thickness of the sand layer, the groundwater level and the top elevation of the rock layer. Sand soil samples were collected for indoor mix proportioning tests to determine the optimal water-cement ratio and cement content of the cement grout. Grouting pile construction equipment was brought in and debugged to complete the accurate layout of the pile positions. S2: Thick sand layer grout jetting drilling rig centering, grout mixing and jetting to form piles; Align the drilling rig with the layout pile position, center and level it, mix the cement slurry, start the drilling rig to make the mixing head rotate and cut the soil down to the top elevation of the rock layer, spray slurry from the bottom of the pile upwards, mix and lift it up, repeat the spraying and mixing to the bottom of the pile and continue spraying slurry for the preset time, finally mix and lift it to the ground, and after completing the construction of a single slurry-jet pile, move the drilling rig to the next pile position. S3: Construction of graded grouting and jet grouting piles in thick sand layer pile foundations and control of pile splicing accuracy; A first-level protective wall is formed by installing a ring of grouting piles close to the outside of the pile diameter. If the hole collapses during construction, a second-level protective wall is formed by installing a ring of grouting piles around the outside of the first-level protective wall to ensure effective overlap between the grouting piles. S4: Core sampling and unconfined compressive strength testing of thick sand layer grout jet piles at various ages after pile formation; After the grouting pile is completed, core sampling is carried out at the preset age to test the unconfined compressive strength of the pile body. If it passes the test, subsequent bored pile construction can be carried out. S5: Curing of grouting piles and control of drilling and mud parameters in connection with bored piles; After the grouting piles have been cured to the preset strength, the bored pile construction is carried out. During the construction process, the drilling speed and mud parameters are controlled to avoid damaging the grouting pile wall.

[0008] Based on the above technical solution, the present invention is further described as follows: As a further aspect of the present invention, the on-site geological investigation in step S1, which involves geological investigation, parameter matching, and precise pile location setting out for thick sand layer pile foundation grouting, specifically includes: Verify the distribution range, density, and gradation of the thick sand layer; determine the groundwater level depth, the top elevation of the rock layer, and the location of the karst development zone; and use standard penetration tests to determine the mechanical properties of the sand layer. The pile positions are accurately laid out with a deviation of no more than 2cm. The guide piles are installed for verification during the construction process. The grouting piles are laid out at the outer side of the pile diameter, close to the pile outline.

[0009] As a further aspect of the present invention, the equipment debugging in step S1, which involves geological investigation, parameter matching, and precise pile location layout for thick sand layer pile foundation grouting, specifically includes: Check the sealing and operating status of the main unit connection, hydraulic system, electrical system and shotcrete system of the mixing pile machine, adjust the mixing blades to unfold to the preset diameter, and verify the grout tank, grouting pipeline and testing instruments to ensure stable equipment operation; The mix design trial specifically includes preparing cement-soil test blocks with different cement admixture amounts and different water-cement ratios, measuring the unconfined compressive strength of test blocks at different ages, and determining the optimal mix design parameters that meet the design requirements.

[0010] As a further aspect of the present invention, step S2, the positioning of the drilling rig during the thick sand layer grout jetting pile construction, including rig centering, grout mixing, and grout jetting and mixing, specifically includes: Align the mixing pile machine with the center of the pile position, level the pile machine body to ensure the verticality of the pile body, and the verticality deviation should not exceed 1%. After the grout tank is filled with cement, seal the inlet to ensure that there is no leakage during the grouting process. The preparation of the cement slurry specifically includes mixing the cement slurry according to the determined water-cement ratio and mix proportion, with a mixing time of not less than the preset time, to ensure that the slurry is uniform and free of lumps. After mixing, the slurry is poured into the collection hopper for later use, and the collection hopper is equipped with an anti-settling mixing device.

[0011] As a further aspect of the present invention, step S2, the drilling and sinking during the thick sand layer grout jetting pile construction, specifically includes: Start the construction machinery, fully extend the mixing blades to the preset diameter, and let the mixing head rotate to cut the soil while sinking at a uniform speed. The sinking speed is controlled within the preset range until the mixing head reaches the reinforcement depth of the top of the rock layer. The aforementioned enhancement and mixing of the sprayed slurry specifically includes: After reaching the bottom elevation of the pile, grout is sprayed upwards at a uniform speed while being stirred and lifted simultaneously. The grouting pressure and lifting speed are kept constant. Grouting is stopped when the pile is lifted to 20cm above the ground. The grouting process is continuous and uninterrupted.

[0012] As a further aspect of the present invention, the repeated grouting and mixing during the centering of the thick sand layer grouting pile drilling rig, grout mixing, and grouting and mixing in step S2 of the pile construction specifically includes: The drill bit rotates forward, and while spraying and mixing grout, it sinks to the designed depth of the pile bottom. After reaching the pile bottom, drilling stops and grout is continuously sprayed for 1 minute to ensure that the grout at the pile bottom is thoroughly mixed with the sand layer. Finally, it is stirred and lifted to the ground to complete the construction of a single grout-jet pile. The device relocation specifically includes: Turn off the power to the equipment, move the mixing pile machine to the next preset pile position, and repeat the construction procedure of step S2 until all the grouting piles in the area are completed.

[0013] As a further aspect of the present invention, the primary retaining wall in step S3, the construction of graded grouting and jet grouting piles in thick sand layer pile foundations and the control of pile overlap accuracy, specifically includes: When the pile foundation diameter is 1.2m, 11 grouting piles with a diameter of 0.5m are arranged around the outside of the pile foundation. The support length is from the original ground to the top elevation of the rock stratum, and the grouting pile support length ranges from 18 to 20m. The secondary retaining wall specifically includes: Grouting piles were added around the outer perimeter of the primary retaining wall. When the pile diameter was 1.2m, the number of grouting piles was increased to 34 with a diameter of 0.5m. The overlap spacing between the grouting piles was controlled at 25cm to ensure a seamless support system, enhance the overall integrity of the thick sand layer reinforcement, and strictly control the overlap accuracy of the piles.

[0014] As a further aspect of the present invention, the preset age test in step S4, which involves core sampling and unconfined compressive strength testing of thick sand layer grout-jet piles after pile formation, specifically includes: The first core sampling test is conducted 7 days after pile completion, and the second core sampling test is conducted 28 days after pile completion. The core samples are taken from the middle and end of the grout-jet pile body, and the integrity of the core samples is not less than 80%. The unconfined compressive strength index of the pile body is as follows: the unconfined compressive strength of the pile body after 7 days is not less than 720 kPa, and the unconfined compressive strength of the pile body after 28 days is not less than 1.5 MPa. If the test fails, the grouting and spraying piles should be added at the corresponding location and the test should be repeated to ensure that the test results meet the design requirements.

[0015] As a further aspect of the present invention, the quality inspection in step S4, which involves core sampling and unconfined compressive strength testing of thick sand layer grout-jet piles after pile formation, also includes real-time monitoring during construction, specifically including: Monitor the construction parameters such as the rotation speed of the mixing blades, the grouting pressure, the drilling depth, and the lifting speed. At the same time, check the pile position, pile diameter, verticality, and overlap of the grouting piles, and correct construction deviations in a timely manner. The preset strength mentioned in step S5, which involves the curing of grout-jet piles and the connection and control of drilling and mud parameters with bored piles, specifically includes: After the grouting pile is completed, it must be cured for no less than 14 days and reach more than 70% of the design strength before the construction of bored pile can be carried out. The drilling speed control specifically includes: When using rotary drilling rigs, the drilling speed should not exceed the preset value to avoid impact damage to the grout-jet pile retaining wall; The control of the mud parameters specifically includes: High-quality mud slurry is used for wall protection, with the mud slurry specific gravity controlled at 1.2~1.3 to prevent sand layer seepage and deformation. At the same time, the mud slurry level is monitored in real time, and mud slurry is replenished in time to ensure the stability of the borehole wall, so as to achieve a smooth connection between the grouting pile wall protection and the construction of the bored pile. As a further aspect of the present invention, the preset strength in step S5, which involves the curing of the grouting pile and the control of drilling and mud parameters in connection with the bored pile, specifically includes: Real-time monitoring of mud loss in the borehole; combined with the characteristics of the karst area, if rapid mud loss is detected, drilling should be stopped immediately to determine whether it is caused by groundwater seepage in the karst area. Grouting and sealing measures should be taken before construction can continue to avoid borehole collapse accidents. When drilling above the karst area, reduce the drilling speed to within 0.5 m / min to avoid impact that could cause the sand layer to connect with the karst cave and lead to borehole collapse. In mud parameter control, the mud specific gravity is adjusted to 1.25~1.3 to enhance mud wall protection.

[0016] The present invention has the following beneficial effects: This construction method involves a complete construction process for grouting and jet grouting pile foundations in thick sand layers. This process includes geological investigation, parameter matching and precise pile location layout, drilling rig alignment, grout mixing and jet grouting pile formation, graded grouting and jet grouting pile construction in thick sand layers and control of pile overlap accuracy, core sampling and unconfined compressive strength testing after pile formation, grouting and jet grouting pile curing and its connection with drilled and cast-in-place pile drilling, and mud parameter control. This method standardizes the technical requirements and operating procedures for each process, effectively reinforcing thick sand layers. It solves the technical problems of easy borehole wall collapse, high construction costs, poor overall support integrity, uncontrollable quality, and poor construction connection in existing technologies. It improves the safety and quality of pile foundation construction, and the construction technology is mature, easy to operate, and low in cost. It is suitable for thick sand layers of different thicknesses, especially for bridge pile foundation construction in areas with composite geology of thick sand layers and karst development, and has broad engineering application prospects. Attached Figure Description

[0017] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. The structures, proportions, sizes, etc., drawn in this specification are only used to complement the content disclosed in the specification, so that those skilled in the art can understand and read them. Any modifications to the structure, changes in the proportional relationships, or adjustments to the size, without affecting the effects and objectives that the present invention can produce, should still fall within the scope of the technical content disclosed in the present invention.

[0018] Figure 1 This is a schematic diagram of the overall process of the grouting and spraying pile wall construction method for thick sand layer pile foundations provided in an embodiment of the present invention. Detailed Implementation

[0019] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] The terms "upper," "lower," "left," "right," and "middle" used in this specification are merely for clarity of description and are not intended to limit the scope of the invention. Any changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.

[0021] Example 1 like Figure 1 As shown, this embodiment of the invention provides a method for constructing a grout-jet pile retaining wall for thick sand layer pile foundations. The method is applied to the construction conditions of thick sand layer pile foundations, specifically including a 0-17.5m thick sand layer, a stable groundwater level at 7.10m, a saturated, loose-to-slightly dense sand layer with general gradation, a pile design diameter of 1.2m, and a pile length of 24m. The construction method specifically includes the following steps: S1: Geological investigation, parameter matching and precise pile location layout construction of grouting pile foundation in thick sand layer; The specific process is as follows: Conduct on-site geological surveys in the thick sand layer pile foundation construction area of ​​the tunnel bridge to determine the construction conditions; Using methods such as engineering geological mapping, drilling, and standard penetration tests, it was verified that the distribution range of the thick sand layer is the entire bridge pile foundation construction area. The sand layer is 17.5m thick, with a density of loose to slightly dense. The sand is relatively pure, with relatively uniform particle size and general gradation. The groundwater level was determined to be 7.10m deep, and the top elevation of the rock layer is 18~20m below the original ground surface. There is no obvious karst development area in this region. The mechanical properties of the sand layer were determined by standard penetration test (SPT), with a SPT blow count (N) of 8-15. Sand samples were collected and sent to the laboratory for indoor mix design trials to provide an accurate basis for determining the subsequent cement slurry parameters.

[0022] The incoming grouting pile construction equipment, including the mixing pile machine, grout tank, grouting pipeline, aggregate hopper, and core sampling testing instruments, underwent comprehensive commissioning. This included checking the connections of each part of the mixing pile machine, the operating status of the hydraulic system and electrical system, and adjusting the sealing of the grouting system to ensure no leaks in the grouting pipeline. The mixing blades were then deployed to a preset diameter of 50cm and verified to ensure the blade deployment accuracy met construction requirements. The grout tank and aggregate hopper were sealed, and the unconfined compressive strength tester and core sampling testing instruments were verified to ensure stable operation of all equipment and to meet the construction needs for parameter matching, grouting pile formation, and subsequent quality testing.

[0023] Cement-soil mix design trials were conducted. Ordinary Portland cement of grade 42.5 was used to prepare cement-soil test blocks with cement admixtures of 15%, 20%, and 25%, and water-cement ratios of 0.5, 0.6, and 0.7. The 7-day and 28-day unconfined compressive strengths of the test blocks were measured. Based on the geological parameters of the thick sand layer and design requirements, the optimal mix design was determined to be a cement admixture of 20% and a water-cement ratio of 0.6. Under this mix design, the 7-day unconfined compressive strength of the test blocks was ≥720 kPa, and the 28-day unconfined compressive strength was ≥1.5 MPa, ensuring the quality of shotcrete pile formation. Complete the precise layout of the pile positions; Based on the design center coordinates of the pile foundation, the position of the grouting piles is laid out using a total station. The layout deviation is controlled within 2cm. The layout position of the grouting piles is close to the outer side of the pile diameter and the outline of the pile foundation. Guide piles are installed for pile position verification during construction to avoid pile position deviation during construction and ensure that the pile position accuracy meets the specifications.

[0024] S2: Thick sand layer grout jetting drilling rig centering, grout mixing and jetting to form piles; The specific process is as follows: Move the mixing pile machine to the marked pile position, align it with the center of the pile position, adjust the pile machine body to keep the pile machine horizontal, and ensure the verticality of the grouting pile construction. The verticality deviation should be controlled within 1%. After filling the grout tank with 42.5 grade ordinary silicate cement, the inlet is sealed to prevent leakage during the grouting process, thereby stabilizing the grouting pressure and laying the equipment foundation for grouting and mixing to form piles.

[0025] Prepare the cement slurry according to the determined optimal mix ratio (20% cement content, 0.6 water-cement ratio). Pour the cement and water into the mixing equipment and mix for no less than 5 minutes to ensure that the slurry is uniform and free of lumps. After mixing, pour the cement slurry into the collection hopper for later use. The collection hopper is equipped with an anti-settling mixing component to keep the slurry in a mixing state, prevent the slurry from settling and separating, which would affect the spraying effect and ensure that the slurry quality meets the construction requirements.

[0026] Start the mixing pile machine and fully extend the mixing blades to the preset diameter of 50cm. Control the operating parameters of the mixing pile machine and make the mixing head rotate and cut the soil while sinking at a uniform speed as required. The sinking speed range is controlled within 0.3~0.5m / min. Monitor the drilling depth in real time during the sinking process to make the drilling depth precise and controllable until the mixing head reaches the top elevation of the rock layer (e.g., 18~20m below the original ground level) for reinforcement. Stop sinking to ensure that the drilling depth meets the reinforcement requirements of the thick sand layer.

[0027] After reaching the pile bottom elevation, the shotcrete system is started. According to the requirements for lifting and mixing shotcrete, shotcrete is sprayed upwards at a uniform speed from the pile bottom. At the same time, the mixing head rotates and mixes while being lifted. The shotcrete pressure is controlled at 1.5~2.0MPa, and the lifting speed and shotcrete pressure are kept constant. The lifting speed is 0.5~0.8m / min. The shotcrete process is continuous and uninterrupted to avoid grout interruption or leakage. When the mixing head is lifted to 20cm above the ground, the shotcrete system is turned off to stop spraying cement grout, so that the cement grout is fully mixed with the thick sand layer.

[0028] After the initial lifting and mixing of the grout, the drill bit is rotated forward according to the requirements for repeated grouting and mixing. The grouting system is restarted, and the grouting is mixed while being sprayed to the designed depth of the pile bottom. Drilling is stopped after reaching the pile bottom, and the grouting is maintained and sprayed continuously for 1 minute to ensure that the grout at the pile bottom is mixed and compacted with the sand layer, further improving the bearing capacity of the pile bottom. Finally, the mixing pile machine is restarted again to mix and lift the mixing head to the ground, completing the construction of a single grouting pile and ensuring the quality of the single grouting pile.

[0029] Turn off the power to the mixing pile machine, move the mixing pile machine to the next preset pile position, and repeat the drilling rig centering, grout mixing, drilling sinking, lifting and grouting mixing, and repeating the mixing construction process until all grouting pile construction in the area is completed, so that the thick sand layer area is fully covered and reinforced, and prepared for the subsequent graded wall protection construction.

[0030] S3: Construction of graded grouting and jet grouting piles in thick sand layer pile foundations and control of pile splicing accuracy; The specific process is as follows: First, the primary retaining wall construction is carried out. In accordance with the requirements for the primary retaining wall construction and the design parameters of 1.2m diameter for the bridge pile foundation, a ring of shotcrete piles is constructed along the outer side of the pile foundation outline to form the primary retaining wall. The primary retaining wall uses grouting piles with a diameter of 0.5m. The support length of the grouting piles is from the original ground surface to the top elevation of the rock layer, i.e., 18~20m. During construction, the pile position deviation of the grouting piles is strictly controlled to be no more than 2cm and the verticality deviation is no more than 1%, so that the primary retaining wall can achieve the foundation reinforcement of the thick sand layer and the initial protection of the borehole wall.

[0031] After the first-level retaining wall construction is completed and the test is qualified, the trial construction of bored piles is carried out. When the construction reaches below the original ground level, the hole collapse occurs, which proves that the grouting piles of the first-level retaining wall have not achieved effective overlap and there are gaps, resulting in poor overall integrity of the support system.

[0032] Therefore, in accordance with the requirements for secondary retaining wall construction, secondary retaining wall construction was carried out in a timely manner. A ring of grouting piles was added around the perimeter of the primary retaining wall grouting piles to form the secondary retaining wall. After the addition of the secondary retaining wall, the total number of grouting piles was 34 with a diameter of 0.5m. The overlap spacing between the grouting piles was strictly controlled at 25cm to ensure the accuracy of the pile overlap, make the support system seamless, enhance the integrity of the thick sand layer reinforcement, and ensure the stability of the borehole wall.

[0033] S4: Core sampling and unconfined compressive strength testing of thick sand layer grout jet piles at various ages after pile formation; The specific process is as follows: This embodiment strictly adheres to quality inspection requirements, establishing a dual quality control system that combines real-time monitoring of the construction process with core sampling after pile completion to ensure the construction quality of the grouting pile wall protection. This ensures that the grouting pile reinforcement effect meets design requirements and provides quality assurance for subsequent bored pile construction.

[0034] Real-time monitoring of the construction process: Throughout the construction process, monitoring parameters such as the rotation speed of the mixing blades, grouting pressure, drilling depth, sinking speed, and lifting speed is essential. If any abnormalities occur, such as a sudden increase or decrease in grouting pressure, abnormal drilling speed, or uneven lifting speed, the machine must be stopped immediately for investigation. Construction can only continue after the problem is identified and resolved. Simultaneously, after each grouting pile is completed, its position, diameter, verticality, and overlap are promptly checked to correct any deviations and ensure the quality of the grouting piles meets design requirements. This lays the foundation for subsequent core sampling and unconfined compressive strength testing. Core sampling and testing at different ages after pile completion: According to the age testing requirements, core sampling tests are conducted on the grout-jet piles at the preset age after pile formation. The first core sampling test is conducted 7 days after pile formation, and the second core sampling test is conducted 28 days after pile formation. A core sampling machine is used to take core samples from the middle and end of the grout-jet pile body. The number of core samples is 1 for every 10 grout-jet piles. The integrity of the core sample should not be less than 80%, with no obvious cracks or loose layers, so as to visually judge the compactness of the grout-jet pile. Unconfined compressive strength tests were conducted on the core samples after core extraction. The test indicators followed the strength requirements, namely, the unconfined compressive strength of the pile body ≥720Kpa after 7 days and the unconfined compressive strength of the pile body ≥1.5Mpa after 28 days. In this embodiment, after the construction of the first-level retaining wall, the core sample testing of some grouting piles revealed insufficient strength at the overlapping parts. In response to this problem, additional core samples were promptly driven. After the construction of the second-level retaining wall, the integrity of all grouting pile core samples was ≥85%, the 7-day unconfined compressive strength was ≥750Kpa, and the 28-day unconfined compressive strength was ≥1.6Mpa. The test results were qualified and met the design requirements.

[0035] S5: Curing of grouting piles and control of drilling and mud parameters in connection with bored piles; The specific process is as follows: After the grouting piles are completed, they are cured for no less than 14 days. The grouting piles are regularly watered to ensure the curing effect. After the grouting piles have reached more than 70% of the design strength, the construction of bored piles is carried out to ensure that the grouting pile wall has sufficient bearing capacity and to avoid damage to the wall during subsequent construction.

[0036] The bored piles are constructed using rotary drilling. During construction, the drilling speed is controlled to be no more than 0.8 m / min to avoid the rotary drilling bit impacting and damaging the grouting pile wall. At the same time, high-quality mud is used for auxiliary wall protection. The mud parameters are controlled, with the mud specific gravity maintained at 1.2~1.3 and the mud viscosity controlled at 18~22s. The mud level is monitored in real time, and mud is replenished in time to ensure the stability of the borehole wall and prevent sand layer seepage and deformation. During the drilling process, the situation inside the hole is observed in real time. If any signs of collapse are found in the hole wall, drilling is stopped immediately, and measures such as supplementary grouting reinforcement are taken to stabilize the hole wall before construction can continue, so as to achieve a smooth connection between grouting pile wall protection and bored pile construction. Example 2 Regarding step S5, the maintenance of the grouting piles and the control of drilling and mud parameters in connection with the bored pile construction, in addition to strictly controlling the drilling speed and mud parameters, the loss of mud in the hole is monitored in real time. Considering the characteristics of the karst area, if rapid mud loss is detected, drilling is immediately stopped to determine whether it is caused by groundwater seepage in the karst area. Grouting and sealing measures are then taken before construction continues to avoid hole collapse. At the same time, when drilling above the karst area, the drilling speed is reduced to within 0.5 m / min to avoid impact that could cause the sand layer to connect with the karst cave and lead to hole collapse. In terms of mud parameter control, the mud specific gravity is adjusted to 1.25~1.3 to enhance the wall protection effect of the mud, prevent sand layer seepage deformation and mud loss, and achieve a smooth connection between the grouting pile wall protection and the bored pile construction, thus ensuring construction safety and quality.

[0037] In this embodiment, the construction method of grouting pile wall protection for thick sand layer pile foundation was strictly followed according to the requirements of the present invention. Combined with the complex geological characteristics of the karst development area, the construction parameters and operation specifications of each key process were optimized, and the pile foundation borehole wall reinforcement at the junction of thick sand layer and karst development area was successfully realized. This effectively prevented the borehole wall collapse caused by groundwater seepage and construction disturbance, and the construction of bored pile was successfully completed.

[0038] In summary, this invention provides a complete construction process for grouting and jet grouting piles in thick sand layers. This process includes geological investigation, parameter matching and precise pile location layout, drilling rig alignment, grout mixing and jet grouting for pile formation, graded grouting and jet grouting construction and pile overlap accuracy control, post-construction core sampling and unconfined compressive strength testing, grouting and jet grouting pile curing and its connection with drilled and cast-in-place pile drilling, and mud parameter control. This process standardizes the technical requirements and operational procedures for each step, effectively reinforcing thick sand layers. It solves the technical problems of easy borehole wall collapse, high construction costs, poor overall support integrity, uncontrollable quality, and poor construction coordination in existing technologies. This improves the safety and quality of pile foundation construction. Furthermore, the construction technology is mature, easy to operate, and low-cost, suitable for thick sand layers of varying thicknesses, especially for bridge pile foundation construction in areas with composite geology of thick sand layers and karst development, and has broad engineering application prospects.

[0039] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for constructing a grout-jet pile retaining wall for thick sand layer pile foundations, characterized in that, Includes the following steps: S1: Geological investigation, parameter matching and precise pile location layout construction of grouting pile foundation in thick sand layer; For the construction area of ​​thick sand pile foundation, on-site geological survey was carried out to determine the thickness of the sand layer, the groundwater level and the top elevation of the rock layer. Sand soil samples were collected for indoor mix proportioning tests to determine the optimal water-cement ratio and cement content of the cement grout. Grouting pile construction equipment was brought in and debugged to complete the accurate layout of the pile positions. S2: Thick sand layer grout jetting drilling rig centering, grout mixing and jetting to form piles; Align the drilling rig with the layout pile position, center and level it, mix the cement slurry, start the drilling rig to make the mixing head rotate and cut the soil down to the top elevation of the rock layer, spray slurry from the bottom of the pile upwards, mix and lift it up, repeat the spraying and mixing to the bottom of the pile and continue spraying slurry for the preset time, finally mix and lift it to the ground, and after completing the construction of a single slurry-jet pile, move the drilling rig to the next pile position. S3: Construction of graded grouting and jet grouting piles in thick sand layer pile foundations and control of pile splicing accuracy; A first-level protective wall is formed by installing a ring of grouting piles close to the outside of the pile diameter. If the hole collapses during construction, a second-level protective wall is formed by installing a ring of grouting piles around the outside of the first-level protective wall to ensure effective overlap between the grouting piles. S4: Core sampling and unconfined compressive strength testing of thick sand layer grout jet piles at various ages after pile formation; After the grouting pile is completed, core sampling is carried out at the preset age to test the unconfined compressive strength of the pile body. If it passes the test, subsequent bored pile construction can be carried out. S5: Curing of grouting piles and control of drilling and mud parameters in connection with bored piles; After the grouting piles have been cured to the preset strength, the bored pile construction is carried out. During the construction process, the drilling speed and mud parameters are controlled to avoid damaging the grouting pile wall.

2. The method for constructing a grout-jet pile retaining wall for thick sand layer pile foundations according to claim 1, characterized in that, The on-site geological investigation mentioned in step S1, which involves geological investigation, parameter matching, and precise pile location layout for thick sand layer pile foundation grouting, specifically includes: Verify the distribution range, density, and gradation of the thick sand layer; determine the groundwater level depth, the top elevation of the rock layer, and the location of the karst development zone; and use standard penetration tests to determine the mechanical properties of the sand layer. The pile positions are accurately laid out with a deviation of no more than 2cm. The guide piles are installed for verification during the construction process. The grouting piles are laid out at the outer side of the pile diameter, close to the pile outline.

3. The method for constructing a grout-jet pile retaining wall for thick sand layer pile foundations according to claim 2, characterized in that, Step S1, the equipment debugging during the geological investigation, parameter matching, and precise pile location setting of the thick sand layer pile foundation grouting and jet grouting construction, specifically includes: Check the sealing and operating status of the main unit connection, hydraulic system, electrical system and shotcrete system of the mixing pile machine, adjust the mixing blades to unfold to the preset diameter, and verify the grout tank, grouting pipeline and testing instruments to ensure stable equipment operation; The mix design trial specifically includes preparing cement-soil test blocks with different cement admixture amounts and different water-cement ratios, measuring the unconfined compressive strength of test blocks at different ages, and determining the optimal mix design parameters that meet the design requirements.

4. The method for constructing a grout-jet pile retaining wall for thick sand layer pile foundations according to claim 3, characterized in that, Step S2, the positioning of the drilling rig during the thick sand layer grout jetting pile construction, specifically includes: Align the mixing pile machine with the center of the pile position, level the pile machine body to ensure the verticality of the pile body, and the verticality deviation should not exceed 1%. After the grout tank is filled with cement, seal the inlet to ensure that there is no leakage during the grouting process. The preparation of the cement slurry specifically includes mixing the cement slurry according to the determined water-cement ratio and mix proportion, with a mixing time of not less than the preset time, to ensure that the slurry is uniform and free of lumps. After mixing, the slurry is poured into the collection hopper for later use, and the collection hopper is equipped with an anti-settling mixing device.

5. The method for constructing a grout-jet pile retaining wall for thick sand layer pile foundations according to claim 4, characterized in that, Step S2, the drilling and sinking of the thick sand layer grout jet rig during drilling, grout mixing, and jet grouting pile construction, specifically includes: Start the construction machinery, fully extend the mixing blades to the preset diameter, and let the mixing head rotate to cut the soil while sinking at a uniform speed. The sinking speed is controlled within the preset range until the mixing head reaches the reinforcement depth of the top of the rock layer. The aforementioned enhancement and mixing of the sprayed slurry specifically includes: After reaching the bottom elevation of the pile, grout is sprayed upwards at a uniform speed while being stirred and lifted simultaneously. The grouting pressure and lifting speed are kept constant. Grouting is stopped when the pile is lifted to 20cm above the ground. The grouting process is continuous and uninterrupted.

6. The method for constructing a grout-jet pile retaining wall for thick sand layer pile foundations according to claim 5, characterized in that, Step S2, the repeated grouting and mixing during the thick sand layer grouting pile drilling rig centering, grout mixing, and grouting pile construction, specifically includes: The drill bit rotates forward, and while spraying and mixing grout, it sinks to the designed depth of the pile bottom. After reaching the pile bottom, drilling stops and grout is continuously sprayed for 1 minute to ensure that the grout at the pile bottom is thoroughly mixed with the sand layer. Finally, it is stirred and lifted to the ground to complete the construction of a single grout-jet pile. The device relocation specifically includes: Turn off the power to the equipment, move the mixing pile machine to the next preset pile position, and repeat the construction procedure of step S2 until all the grouting piles in the area are completed.

7. The method for constructing a grout-jet pile retaining wall for thick sand layer pile foundations according to claim 6, characterized in that, The primary retaining wall in step S3, the construction of graded grouting and jet grouting piles in thick sand layers and the control of pile overlap accuracy, specifically includes: When the pile foundation diameter is 1.2m, 11 grouting piles with a diameter of 0.5m are arranged around the outside of the pile foundation. The support length is from the original ground to the top elevation of the rock stratum, and the grouting pile support length ranges from 18 to 20m. The secondary retaining wall specifically includes: Grouting piles were added around the outer perimeter of the primary retaining wall. When the pile diameter was 1.2m, the number of grouting piles was increased to 34 with a diameter of 0.5m. The overlap spacing between the grouting piles was controlled at 25cm to ensure a seamless support system, enhance the overall integrity of the thick sand layer reinforcement, and strictly control the overlap accuracy of the piles.

8. The method for constructing a grout-jet pile retaining wall for thick sand layer pile foundations according to claim 7, characterized in that, The preset age test in step S4, core sampling and unconfined compressive strength testing of thick sand layer grout-jet piles after pile formation, specifically includes: The first core sampling test is conducted 7 days after pile completion, and the second core sampling test is conducted 28 days later. The core samples are taken from the middle and end of the grout-jet pile body, and the integrity of the core samples is not less than 80%. The unconfined compressive strength index of the pile body is as follows: the unconfined compressive strength of the pile body after 7 days is not less than 720 kPa, and the unconfined compressive strength of the pile body after 28 days is not less than 1.5 MPa. If the test fails, the grouting and spraying piles should be added at the corresponding location and the test should be repeated to ensure that the test results meet the design requirements.

9. The method for constructing a grout-jet pile retaining wall for thick sand layer pile foundations according to claim 8, characterized in that, The quality inspection in step S4, core sampling and unconfined compressive strength testing of thick sand layer grout-jet piles after pile formation, also includes real-time monitoring during construction, specifically including: Monitor the construction parameters such as the rotation speed of the mixing blades, the grouting pressure, the drilling depth, and the lifting speed. At the same time, check the pile position, pile diameter, verticality, and overlap of the grouting piles, and correct construction deviations in a timely manner. The preset strength mentioned in step S5, grouting pile curing and its connection with drilled pile drilling and mud parameter control, specifically includes: After the grouting pile is completed, it must be cured for no less than 14 days and reach more than 70% of the design strength before the bored pile can be constructed. The drilling speed control specifically includes: When using rotary drilling rigs, the drilling speed should not exceed the preset value to avoid impact damage to the grout-jet pile retaining wall; The control of the mud parameters specifically includes: High-quality mud slurry is used for wall protection, with the mud slurry specific gravity controlled at 1.2~1.3 to prevent sand layer seepage and deformation. At the same time, the mud slurry level is monitored in real time, and mud slurry is replenished in time to ensure the stability of the borehole wall, so as to achieve a smooth connection between the grouting pile wall protection and the construction of the bored pile.

10. The method for constructing a grout-jet pile retaining wall for thick sand layer pile foundations according to claim 8, characterized in that, The preset strength mentioned in step S5, grouting pile curing and its connection with drilled pile drilling and mud parameter control, specifically includes: Real-time monitoring of mud loss in the borehole; combined with the characteristics of the karst area, if rapid mud loss is detected, drilling should be stopped immediately to determine whether it is caused by groundwater seepage in the karst area. Grouting and sealing measures should be taken before construction can continue to avoid borehole collapse accidents. When drilling above the karst area, reduce the drilling speed to within 0.5 m / min to avoid impact that could cause the sand layer to connect with the karst cave and lead to borehole collapse. In mud parameter control, the mud specific gravity is adjusted to 1.25~1.3 to enhance mud wall protection.

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