A composite foundation composed of controllable rigid piles and compacted sand and gravel piles and a construction method thereof

Abstract

The present invention discloses a composite foundation of controllable rigid piles and compacted gravel piles and a construction method thereof, the foundation comprising a soft soil foundation, a sand cushion layer laid on the soft soil foundation and a drainage ditch arranged outside the sand cushion layer; a plurality of rigid piles are arranged in the soft soil foundation; a sleeve is arranged on the rigid pile, the top of the sleeve extends upward to the top surface of the sand cushion layer, and the bottom end of the sleeve is contracted to tightly sleeve the rigid pile; the sleeve is respectively connected to a grouting pipeline and a grouting pipeline; cement slurry is injected into the sleeve, and a cover plate is arranged on the sleeve; a plurality of gravel piles are evenly arranged in the sand cushion layer around the cover plate, the top of the gravel pile is flush with the top surface of the sand cushion layer, and the bottom end of the gravel pile penetrates the sand cushion layer and extends into the soft soil layer. The composite foundation of controllable rigid piles and compacted gravel piles of the present invention can reduce the maximum-minimum principal stress difference and shear stress of the foundation soil, thereby improving the foundation stability.

Description

Technical Field

[0001] The invention relates to the technical field of foundation treatment, and in particular to a composite foundation composed of controllable rigid piles and compacted sandstone piles and a construction method thereof. Background Art

[0002] Soft soil has the characteristics of high water content, large porosity, poor permeability, and low shear strength. Composite foundation technology is often used in engineering to reinforce soft soil foundations to improve the bearing capacity and stability of soft soil foundations and reduce their settlement and differential settlement. At the same time, the construction period and reinforcement cost of composite foundations are also critical.

[0003] At present, among all kinds of reinforcement bodies of composite foundation, rigid piles have high stiffness and strength, and can transfer the load on the top of the pile to the deep soil. They have outstanding advantages in bearing capacity and settlement. However, rigid piles do not improve the properties of the soft soil around the piles much. Rigid piles need to bear most of the upper load, resulting in a large amount of rigid piles and a high cost. At the same time, when the soft soil around the piles undergoes lateral deformation, the rigid piles are prone to breakage. Sand and gravel piles are loose material piles, which are good drainage channels in themselves. The construction of the pile body can produce a certain lateral extrusion force on the surrounding soil, which can promote the drainage and consolidation of the soft soil around the piles, and improve the shear strength and bearing capacity. However, the bearing capacity and deformation of sand and gravel piles are affected by the soil around the piles. Their bearing capacity is relatively small, and the settlement after construction is difficult to control. Flexible piles are similar to rigid piles in that they are both made of bonding materials. Flexible piles are not as good as rigid piles in terms of bearing capacity and settlement, and are not as good as loose material piles in improving the properties of the soil around the piles. The horizontal reinforcement layer can diffuse the base stress and enhance the horizontal tensile strength of the foundation. However, the horizontal reinforcement layer is only suitable for reinforcing shallow soft soil, and has limited effect on deep soft soil.

[0004] Chinese Patent 201510798115.4 discloses a composite foundation composed of controllable rigid piles + drainage body + reinforced cushion layer, which is very good in terms of bearing capacity and settlement. However, the stability of the composite foundation is slightly poor. Under the action of load pressure, the soil between the piles is prone to shear sliding deformation, which has a certain squeezing effect on the rigid piles. In particular, when the grouting time of the controllable rigid piles is late, the soil between the piles needs to bear a large load pressure, and its shear deformation and lateral deformation are large, which may cause the rigid piles to break and the foundation soil to become unstable and damaged. Summary of the invention

[0005] The purpose of the present invention is to provide a composite foundation composed of controllable rigid piles and compacted gravel piles and a construction method thereof, so as to solve the problems of poor stability of the existing foundation and easy shear sliding deformation.

[0006] In order to solve the above technical problems, the present invention provides a composite foundation composed of controllable rigid piles and compacted gravel piles, comprising a soft soil foundation, a sand cushion layer laid on the soft soil foundation, and a drainage ditch arranged outside the sand cushion layer;

[0007] A plurality of rigid piles are evenly arranged in the soft soil foundation, and the top of the rigid pile is not higher than the surface of the soft soil foundation; a sleeve is arranged on the rigid pile, and the top of the sleeve extends upward to the top surface of the sand cushion layer, and the bottom end of the sleeve is shrunken to tightly cover the rigid pile;

[0008] The sleeves are respectively connected to the grouting pipeline and the grouting discharge pipeline; cement slurry is injected into the sleeves, and a cover plate is arranged on the sleeves;

[0009] A plurality of gravel piles are evenly arranged in the sand cushion layer around the cover plate, the top ends of the gravel piles are flush with the top surface of the sand cushion layer, and the bottom ends of the gravel piles penetrate the sand cushion layer and extend into the soft soil foundation.

[0010] As a preferred solution, the thickness of the sand cushion layer is not less than 500 mm.

[0011] As a preferred solution, the rigid pile is at least one of a CFG pile, a concrete pile, a PHC pipe pile or a PTC pipe pile.

[0012] As a preferred solution, a gravel layer with a thickness of 100 mm to 200 mm is laid under the sleeve.

[0013] As a preferred solution, the inner diameter of the sleeve is 100 mm to 300 mm larger than that of the rigid pile.

[0014] As a preferred solution, the grouting pipeline includes a steel flower pipe arranged inside the sleeve, the steel flower pipe is connected to a grouting branch pipe, the grouting branch pipe passes through the outside of the sleeve and extends upward to the sand cushion layer; the grouting branch pipes on the same row are connected through a grouting outer pipe, one end of the grouting outer pipe extends to the drainage ditch; a slidable grouting inner pipe is arranged inside the grouting outer pipe, and the grouting inner pipe is connected to a grouting valve after extending out of the grouting outer pipe.

[0015] As a preferred solution, the inner diameter of the grouting outer tube is 3 mm to 10 mm larger than the outer diameter of the grouting inner tube; the grouting outer tube is a steel tube, and the grouting inner tube is a PVC fiber reinforced hose.

[0016] As a preferred solution, the grouting outer pipe and the grouting branch pipe are covered with a sand cushion layer with a thickness of not less than 400 mm.

[0017] As a preferred embodiment, the slurry discharge pipeline includes a slurry discharge pipe and a slurry outlet pipe; two adjacent sleeves in the same row are connected by a slurry discharge pipe, wherein one end of the slurry discharge pipe is connected to the top of the sleeve away from the drainage ditch of the two sleeves, and the other end is connected to the bottom end of the other sleeve, so that the slurry flows out from the top of the sleeve away from the drainage ditch and flows into the bottom end of the adjacent sleeve through the slurry discharge pipe; the slurry outlet pipe is arranged on the top of the sleeve closest to the drainage ditch in the same row of sleeves, and the slurry outlet pipe is extended to the drainage ditch and then connected to the slurry outlet valve.

[0018] The present invention also provides a construction method of a composite foundation composed of controllable rigid piles and compacted sand and gravel piles, comprising the following steps:

[0019] S1. Level the site, lay a sand cushion layer on the surface of the soft soil foundation to be treated, and set a drainage ditch on the outer edge of the sand cushion layer;

[0020] S2. Construct gravel piles on the sand cushion layer, vertically from the shallower soft foundation to the deeper soft foundation; if there is a structure, the gravel piles are constructed from the adjacent structure to the direction away from the structure;

[0021] S3. Multiple rigid piles are evenly arranged during construction. The bottom of the rigid piles should not be higher than the surface of the soft soil foundation when they enter the bearing layer. The height difference between the top of the pile and the top of the sand cushion layer should not be less than the total settlement of the soil between the piles after bearing the load pressure.

[0022] S4. Clear the sand cushion layer and soil around the rigid pile, lay a gravel layer as the sleeve foundation, and then place the sleeve. The upper end surface of the sleeve is flush with the surface of the sand cushion layer, and the lower end surface of the sleeve is narrowed to cover the rigid pile. The length of the rigid pile entering the sleeve should not be less than the pile diameter;

[0023] S5. After the sleeve is placed, the steel flower pipe is placed inside the sleeve. The grouting branch pipe penetrates the sleeve from the outside and is connected to the steel flower pipe. The grouting branch pipes on the same row are connected through the grouting outer pipe, and the grouting outer pipe is extended to the drainage ditch. A slidable grouting inner pipe is arranged inside the grouting outer pipe. One end of the grouting inner pipe extends to the grouting branch pipe farthest from the drainage ditch, and the other end is connected to the grouting valve.

[0024] S6. Connect the sleeves on the same row through a slurry discharge pipe, wherein one end of the slurry discharge pipe is connected to the top of the sleeve far away from the drainage ditch of the two sleeves, and the other end is connected to the bottom of the other sleeve, so that the slurry flows out from the top of the sleeve far away from the drainage ditch and flows into the bottom of the adjacent sleeve through the slurry discharge pipe; a slurry discharge pipe is arranged at the edge of the sand cushion layer, i.e., the top of the sleeve closest to the drainage ditch, and the slurry discharge pipe extends to the outside of the sand cushion layer, i.e., onto the drainage ditch;

[0025] S7. After the pipeline is installed, fill the pipeline and the surrounding of the sleeve with sand in layers and tamp it to be flush with the sand cushion layer. Then install the reinforced concrete cover plate on the sleeve. The center of the cover plate is aligned with the center of the sleeve. The filler under the cover plate is dense to ensure that the cover plate does not produce differential settlement.

[0026] S8. Use a grouting machine to grout the inner grouting tube until fresh slurry is discharged from the slurry outlet, and the sleeve grouting is completed; pull the grouting hose out a distance between piles, and grout the adjacent sleeves for the second time, and repeat this process until each sleeve is grouted for the second time to ensure that the slurry in the sleeve is full; loading and construction are prohibited during the initial setting period of the slurry.

[0027] The working steps of the composite foundation composed of controllable rigid piles and compacted sand and gravel piles are as follows:

[0028] (1) Rigid piles can reduce the bearing effect on the top of the piles before grouting, ensuring smooth drainage and consolidation of the soil between the piles and improving the bearing capacity. After grouting, the rigid piles and the soil between the piles share the load and control settlement, which saves more pile body than ordinary rigid pile composite foundations.

[0029] (2) Compacted gravel piles are used instead of the combination of drainage body and reinforced cushion layer. This is not only because compacted gravel piles are good drainage channels that can promote drainage and consolidation of foundation soil, but also because compacted gravel piles can generate a large lateral extrusion force on the foundation soil during construction. When the vertical pressure of the foundation soil gradually increases under the action of the upper load and becomes the maximum principal stress, the lateral extrusion force can reduce the maximum-minimum principal stress difference and shear stress of the foundation soil, and the shear deformation or lateral deformation of the foundation soil is also reduced accordingly. Therefore, the lateral soil pressure on the rigid piles is reduced, and the stability of the foundation is improved.

[0030] (3) The cover plate has a large area, and uneven force is likely to cause differential settlement. If the sleeve diameter is slightly larger than the diameter of the rigid pile, the cover plate and sleeve have a small degree of freedom of inclination, which may cause the pile head to bend and break. If the sleeve diameter is too large, the soft soil between the piles is easily squeezed into the sleeve from the gap between the sleeve and the rigid pile after being compressed. The sleeve is designed to have a tapered shape at the lower end to reduce the gap between the sleeve and the rigid pile. At the same time, a gravel layer is laid under the sleeve to effectively prevent the soft soil from being squeezed into the sleeve.

[0031] The present invention has the following beneficial effects:

[0032] (1) Controllable rigid piles can not only ensure smooth drainage and consolidation of the soil between the piles and improve the bearing capacity, but also share the load with the soil between the piles and control the settlement, thereby reducing the settlement of the composite foundation and the amount of piles used.

[0033] (2) Compacted gravel piles are good drainage channels that can promote drainage and consolidation of foundation soil. At the same time, during construction, compacted gravel piles can produce a large lateral extrusion force on the foundation soil, which reduces the maximum-minimum principal stress difference and shear stress of the foundation soil. The shear deformation or lateral deformation of the foundation soil is also reduced accordingly. Therefore, the stability of the foundation soil is improved, the lateral soil pressure on the rigid pile is reduced, and the pile body is not easy to break.

[0034] (3) The sleeve is designed with a tapered shape at the lower end, which not only prevents the pile head from being bent and broken, but also effectively prevents soft soil from being squeezed into the sleeve under pressure, affecting the penetration of the rigid pile into the sleeve. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic diagram of the half-width cross-sectional arrangement of the present invention.

[0036] Figure 2 It is a half-width plan layout schematic diagram of the present invention.

[0037] Figure 3 It is a structural schematic diagram of the connection between the grouting pipeline and the sleeve.

[0038] Figure 4 It is a schematic diagram of the layout of the grouting pipeline.

[0039] Figure 5 It is a schematic diagram of the layout of the slurry discharge pipeline.

[0040] Figure numerals: 1. soft soil foundation; 2. sand cushion layer; 3. drainage ditch; 4. rigid pile; 5. sleeve; 6. cover plate; 7. gravel pile; 8. steel flower pipe; 9. grouting branch pipe; 10. grouting outer pipe; 11. grouting inner pipe; 12. grouting pipe; 13. grouting pipe; 14. grouting valve; 15. grouting valve; 16. gravel layer; 17. embankment. DETAILED DESCRIPTION

[0041] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0042] Embodiment 1:

[0043] See also Figure 1 and Figure 2 In a preferred embodiment of the present invention, a composite foundation composed of a controllable rigid pile 4 and a compacted gravel pile 7 comprises a soft soil foundation 1, a sand cushion layer 2 laid on the soft soil foundation 1, and a drainage ditch 3 arranged outside the sand cushion layer 2;

[0044] A plurality of rigid piles 4 are evenly arranged in the soft soil foundation 1, and the top of the rigid pile 4 is not higher than the surface of the soft soil foundation 1; a sleeve 5 is arranged on the rigid pile 4, and the top of the sleeve 5 extends upward to the top surface of the sand cushion layer 2, and the bottom end of the sleeve 5 is shrunken to tightly cover the rigid pile 4;

[0045] The sleeve 5 is connected to the grouting pipeline and the grouting pipeline respectively; cement slurry is injected into the sleeve 5, and a cover plate 6 is arranged on the sleeve 5;

[0046] A plurality of gravel piles 7 are evenly arranged in the sand cushion layer 2 around the cover plate 6 , the top ends of the gravel piles 7 are flush with the top surface of the sand cushion layer 2 , and the bottom ends of the gravel piles 7 penetrate the sand cushion layer 2 and extend into the soft soil foundation 1 .

[0047] In a preferred embodiment of the present invention, the thickness of the sand cushion layer 2 is not less than 500 mm.

[0048] In a preferred embodiment of the present invention, the rigid pile 4 is at least one of a CFG pile, a concrete pile, a PHC pipe pile or a PTC pipe pile.

[0049] In a preferred embodiment of the present invention, a gravel layer 16 with a thickness of 100 mm to 200 mm is laid under the sleeve 5 .

[0050] In a preferred embodiment of the present invention, the inner diameter of the sleeve 5 is 100 mm to 300 mm larger than that of the rigid pile 4 .

[0051] See also Figure 3 and Figure 4 In a preferred embodiment of the present invention, the grouting pipeline includes a steel flower pipe 8 arranged inside the sleeve 5, and the steel flower pipe 8 is connected to a grouting branch pipe 9, which passes through the outside of the sleeve 5 and extends upward to the sand cushion layer 2; the grouting branch pipes 9 on the same row are connected through a grouting outer pipe 10, and one end of the grouting outer pipe 10 extends to the drainage ditch 3; a slidable grouting inner pipe 11 is arranged in the grouting outer pipe 10, and the grouting inner pipe 11 extends out of the grouting outer pipe 10 and is connected to a grouting valve 14.

[0052] In a preferred embodiment of the present invention, the inner diameter of the grouting outer tube 10 is 3 mm to 10 mm larger than the outer diameter of the grouting inner tube 11; the grouting outer tube 10 is a steel tube, and the grouting inner tube 11 is a PVC fiber reinforced hose.

[0053] In a preferred embodiment of the present invention, the grouting outer pipe 10 and the grouting branch pipe 9 are covered with a sand cushion layer 2 with a thickness of not less than 400 mm.

[0054] See also Figure 5 In a preferred embodiment of the present invention, the slurry discharge pipeline includes a slurry discharge pipe 12 and a slurry outlet pipe 13; two adjacent sleeves 5 in the same row are connected by a slurry discharge pipe 12, wherein one end of the slurry discharge pipe 12 is connected to the top of the sleeve 5 away from the drainage ditch 3 of the two sleeves 5, and the other end is connected to the bottom end of the other sleeve 5, so that the slurry flows out from the top of the sleeve 5 away from the drainage ditch 3 and flows into the bottom end of the adjacent sleeve 5 through the slurry discharge pipe 12; the slurry outlet pipe 13 is arranged on the top of the sleeve 5 closest to the drainage ditch 3 among the sleeves 5 in the same row, and the slurry outlet pipe 13 is extended to the drainage ditch 3 and then connected to the slurry outlet valve 15.

[0055] Embodiment 2:

[0056] Taking the soft soil foundation 1 under the embankment 17 as the implementation object, the construction method of the composite foundation composed of controllable rigid piles 4 and compacted gravel piles 7 includes the following steps:

[0057] (1) First, level the site, and lay a sand cushion layer 2 with a thickness of not less than 500 mm on the surface of the soft soil foundation 1. The outer edge of the sand cushion layer 2 is connected to the drainage ditch 3;

[0058] (2) Construct gravel piles 7 on the sand cushion layer 2. Vertically, construct from the shallower part of the soft soil foundation 1 to the deeper part. For structures such as bridge heads or culverts, construct from the nearest structure to the direction away from the structure. Horizontally, construct in a zigzag manner back and forth, or first on the two sides and then in the middle. The designed length of the gravel pile 7 should not be less than the calculated settlement depth of the foundation pressure zone, and the pile top should be flush with the sand cushion layer 2.

[0059] (3) The rigid pile 4 is constructed so that the bottom of the pile enters the bearing layer; the top of the pile is not higher than the surface of the soft soil foundation 1, and the height difference between the top of the pile and the top surface of the sand cushion layer 2 is not less than the total settlement of the soil between the piles after bearing the load pressure. The pile diameter, pile spacing, and pile length meet the design load requirements of the rigid pile 4;

[0060] (4) Remove the sand cushion layer 2 and soil around the pile near the pile position, lay a 100mm to 200mm thick gravel material as the foundation of the sleeve 5, and then install the sleeve 5. The sleeve 5 is made of reinforced concrete. The inner diameter of the sleeve 5 is 100mm to 300mm larger than the rigid pile 4. The upper end surface of the sleeve 5 is flush with the surface of the sand cushion layer 2, and the lower end surface of the sleeve 5 is narrowed to just cover the rigid pile 4. The length of the rigid pile 4 entering the sleeve 5 is not less than the pile diameter value;

[0061] (5) After the sleeve 5 is placed, the grouting branch pipe 9 and the grouting outer pipe 10 are symmetrically installed with the road centerline as the boundary, and the steel flower pipe 8 is placed in the sleeve 5. The grouting branch pipe 9 passes through the sleeve 5 from the outside and is connected to the steel flower pipe 8. The grouting branch pipes 9 on the same row are connected through the grouting outer pipe 10, and the grouting outer pipe 10 extends to the drainage ditch 3; a slidable grouting inner pipe 11 is arranged in the grouting outer pipe 10, one end of the grouting inner pipe 11 extends to the grouting branch pipe 9 farthest from the drainage ditch 3, and the other end is connected to the grouting valve 14;

[0062] (6) The sleeves 5 on the same row are connected by a slurry discharge pipe 12, wherein one end of the slurry discharge pipe 12 is connected to the top of the sleeve 5 far away from the drainage ditch 3 of the two sleeves 5, and the other end is connected to the bottom of the other sleeve 5, so that the slurry flows out from the top of the sleeve 5 far away from the drainage ditch 3 and flows into the bottom of the adjacent sleeve 5 through the slurry discharge pipe 12; a slurry discharge pipe 13 is arranged at the edge of the sand cushion layer 2, i.e., the top of the sleeve 5 closest to the drainage ditch 3, and the slurry discharge pipe 13 extends to the outside of the sand cushion layer 2, i.e., onto the drainage ditch 3; it is worth noting that two steel flower pipes 8 are arranged in the sleeve, and the slurry discharge pipe 12 and the grouting branch pipe 9 are respectively connected to different steel flower pipes 8;

[0063] (7) After the pipeline is installed, sand is backfilled in layers on the pipeline and around the sleeve 5 and compacted until it is flush with the sand cushion layer 2. Then, a reinforced concrete cover plate 6 is installed on the sleeve 5. The center of the cover plate 6 is aligned with the center of the sleeve 5. The filler under the cover plate 6 is dense to ensure that the cover plate 6 does not produce a large differential settlement;

[0064] (8) Grouting the grouting pipe with a grouting machine. When the grouting valve 15 discharges fresh grout, the grouting of the sleeve 5 is completed. The grouting inner tube 11 can be pulled out a distance between piles to perform secondary grouting on the adjacent sleeve 5. This process is repeated until each sleeve 5 is filled with grout to ensure that the grout in the sleeve 5 is full. During the initial setting period of the grout, loading and construction of the embankment 17 are prohibited. The injected slurry is cement slurry or cement mortar.

[0065] At the same time, the combined use of grouting pipelines and grouting pipelines can improve the grouting efficiency, shorten the grouting time interval of each sleeve 5, avoid uneven force between the ungrouted rigid piles 4 and the grouted rigid columns 4, and avoid shear deformation and lateral deformation caused by different soil loads between the piles, thereby causing a certain impact on the instability of the rigid piles 4 and the foundation.

[0066] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and substitutions can be made without departing from the technical principles of the present invention. These improvements and substitutions should also be regarded as the scope of protection of the present invention.

Claims

1. A composite foundation composed of controllable rigid piles and compacted gravel piles, characterized in that: It includes a soft soil foundation, a sand cushion layer laid on the soft soil foundation, and a drainage ditch arranged outside the sand cushion layer; A plurality of rigid piles are evenly arranged in the soft soil foundation, and the top of the rigid pile is not higher than the surface of the soft soil foundation; a sleeve is arranged on the rigid pile, and the top of the sleeve extends upward to the top surface of the sand cushion layer, and the bottom end of the sleeve is shrunken to tightly cover the rigid pile; The sleeves are respectively connected to the grouting pipeline and the grouting discharge pipeline; cement slurry is injected into the sleeves, and a cover plate is arranged on the sleeves; A plurality of gravel piles are evenly arranged in the sand cushion layer around the cover plate, the top ends of the gravel piles are flush with the top surface of the sand cushion layer, and the bottom ends of the gravel piles penetrate the sand cushion layer and extend into the soft soil foundation; The thickness of the sand cushion layer is not less than 500 mm; gravel piles are constructed on the sand cushion layer; The rigid pile is at least one of a CFG pile, a concrete pile, a PHC pipe pile or a PTC pipe pile; A gravel layer with a thickness of 100 mm to 200 mm is laid under the sleeve; The inner diameter of the sleeve is 100 mm to 300 mm larger than that of the rigid pile; The grouting pipeline comprises a steel flower pipe arranged inside the sleeve, the steel flower pipe is connected with a grouting branch pipe, the grouting branch pipe penetrates outside the sleeve and extends upward to the sand cushion layer; the grouting branch pipes on the same row are connected through a grouting outer pipe, one end of the grouting outer pipe extends to the drainage ditch; a slidable grouting inner pipe is arranged inside the grouting outer pipe, and the grouting inner pipe is connected to a grouting valve after extending out of the grouting outer pipe; The inner diameter of the grouting outer tube is 3 mm to 10 mm larger than the outer diameter of the grouting inner tube; the grouting outer tube is a steel tube, and the grouting inner tube is a PVC fiber reinforced hose; The grouting outer pipe and the grouting branch pipe are covered with a sand cushion layer with a thickness of not less than 400 mm; The slurry discharge pipeline includes a slurry discharge pipe and a slurry outlet pipe; two adjacent sleeves in the same row are connected by a slurry discharge pipe, wherein one end of the slurry discharge pipe is connected to the top of the sleeve far away from the drainage ditch of the two sleeves, and the other end is connected to the bottom of the other sleeve, so that the slurry flows out from the top of the sleeve far away from the drainage ditch and flows into the bottom of the adjacent sleeve through the slurry discharge pipe; the slurry outlet pipe is arranged on the top of the sleeve closest to the drainage ditch in the sleeves in the same row, and the slurry outlet pipe is connected to the slurry outlet valve after extending to the drainage ditch; Sand and gravel piles are used to reduce the principal stress difference and shear stress on rigid piles.

2. A construction method of a composite foundation composed of controllable rigid piles and compacted gravel piles as claimed in claim 1, characterized in that : Includes the following steps: S1. Level the site, lay a sand cushion layer on the surface of the soft soil foundation to be treated, and set a drainage ditch on the outer edge of the sand cushion layer; S2. Construct gravel piles on the sand cushion layer, vertically from the shallower soft foundation to the deeper soft foundation; if there is a structure, the gravel piles are constructed from the adjacent structure to the direction away from the structure; S3. Multiple rigid piles are evenly arranged, with the bottom of the rigid piles entering the bearing layer, the top of the piles not higher than the surface of the soft soil foundation, and the height difference between the top of the piles and the top surface of the sand cushion layer not less than the total settlement of the soil between the piles after bearing the load pressure; S4. Clear the sand cushion layer and soil around the rigid pile, lay a gravel layer as the sleeve foundation, and then place the sleeve. The upper end surface of the sleeve is flush with the surface of the sand cushion layer, and the lower end surface of the sleeve is narrowed to cover the rigid pile. The length of the rigid pile entering the sleeve should not be less than the pile diameter; S5. After the sleeve is placed, the steel flower pipe is placed inside the sleeve. The grouting branch pipe penetrates the sleeve from the outside and is connected to the steel flower pipe. The grouting branch pipes on the same row are connected through the grouting outer pipe, and the grouting outer pipe is extended to the drainage ditch. A slidable grouting inner pipe is arranged inside the grouting outer pipe. One end of the grouting inner pipe extends to the grouting branch pipe farthest from the drainage ditch, and the other end is connected to the grouting valve. S6. Connect the sleeves on the same row through a slurry discharge pipe, wherein one end of the slurry discharge pipe is connected to the top of the sleeve far away from the drainage ditch of the two sleeves, and the other end is connected to the bottom of the other sleeve, so that the slurry flows out from the top of the sleeve far away from the drainage ditch and flows into the bottom of the adjacent sleeve through the slurry discharge pipe; a slurry discharge pipe is set at the edge of the sand cushion layer, that is, the top of the sleeve closest to the drainage ditch, and the slurry discharge pipe extends to the outside of the sand cushion layer, that is, to the drainage ditch; S7. After the pipeline is installed, fill the pipeline and the surrounding of the sleeve with sand in layers and tamp it to be flush with the sand cushion layer. Then install the reinforced concrete cover plate on the sleeve. The center of the cover plate is aligned with the center of the sleeve. The filler under the cover plate is dense to ensure that the cover plate does not produce differential settlement. S8. Use a grouting machine to grout the inner grouting tube until fresh slurry is discharged from the slurry outlet, and the sleeve grouting is completed; pull the grouting hose out a distance between piles, and grout the adjacent sleeves for the second time, and repeat this process until each sleeve is grouted for the second time to ensure that the slurry in the sleeve is full; loading and construction are prohibited during the initial setting period of the slurry.

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