Discrete-semi-rigid composite pile composite foundation for deep soft soil reinforcement and design method

Abstract

The invention discloses a discrete-semi-rigid combined pile composite foundation for deep soft soil reinforcement and a design method. The composite foundation comprises semi-rigid piles, discrete piles, a deformation adjusting layer, a reinforced mattress layer, a deep soft foundation and a hard rock-soil layer. The semi-rigid pile is located in the middle of the discrete pile and is coaxial with the discrete pile, a pile body penetrates through the deep soft foundation, the pile bottom enters a hard rock-soil layer, and a deformation adjusting layer is arranged on the upper portion of the pile top to the bottom of the reinforced mattress layer; the discrete pile wraps the upper section of the semi-rigid pile, and the pile bottom is arranged in the deep soft foundation; the deformation adjusting layer is located at the top of the semi-rigid pile; the reinforced mattress layer is located on the top of the soft soil layer. By means of the compaction, replacement and drainage functions of the discrete piles, the problems that when single semi-rigid piles are adopted for reinforcing deep soft soil, the pile body quality is difficult to control, the pile side negative friction resistance is large, and the pile-soil stress ratio is high are solved; the method has the advantages of improving the bearing capacity of the composite foundation, accelerating consolidation and settlement of the soft foundation, coordinating the pile-soil stress ratio, reducing the reinforcement cost, saving the construction period and the like.

Description

Technical Field

[0001] The invention relates to the technical field of deep and soft foundation treatment, in particular to a loose-semi-rigid combined pile composite foundation for reinforcing deep and soft soil and a design method thereof. Background Art

[0002] When constructing structures on deep soft soil foundations, problems such as excessive settlement and insufficient foundation bearing capacity are often encountered. Composite foundation, as a common soft foundation treatment method, can effectively eliminate or reduce soft foundation settlement and post-construction settlement, improve foundation bearing capacity, and ensure project safety.

[0003] For deep soft soil foundations, commonly used composite foundations include: composite foundations of loose piles such as sand piles, gravel piles and crushed stone piles, composite foundations of flexible piles such as cement soil piles and lime soil piles, composite foundations of semi-rigid piles such as CFG piles and fly ash concrete piles, composite foundations of rigid piles such as reinforced concrete piles and prestressed pipe piles, etc. The above composite foundations are all single pile type composite foundations. When dealing with deep soft soil foundations, each has certain shortcomings. For example, loose piles have certain compaction, replacement and drainage consolidation effects, but they are limited in improving the bearing capacity of the foundation, and drainage consolidation requires a long construction period and upper load; flexible piles have problems such as limited treatment depth, difficult construction quality control and non-condensation of cement slurry; rigid piles and semi-rigid piles have pile body friction resistance, which has a great impact on the bearing capacity of the pile body and the settlement of the pile top, and is easy to cause pile damage and separation of the foundation from the natural foundation.

[0004] Composite foundations composed of multiple pile types can solve the shortcomings of composite foundations of single pile type. At present, similar composite pile foundations mainly include the following types: ① The flexible-loose-rigid ternary combined pile composite foundation disclosed in patent CN115538411A is to set three air-permeable loose piles at the internal axis positions of the three mixing piles, and a permeable rigid pipe pile is set at the center position of the overlapping area inside the three mixing piles. This invention mainly expands the cross-sectional area of ​​the piles through composite piles to improve the foundation bearing capacity under low-cost conditions, but the cement soil piles wrapping the loose piles restrict the drainage capacity of the loose piles. ② The high-speed railway ballastless track foundation treatment method disclosed in patent CN115964782A is a method of using gravel piles and CFG piles to arrange at intervals, using the replacement and compaction of gravel piles to solve the liquefaction of sandy soil, and using CFG piles to improve the foundation bearing capacity and reduce liquefaction. When this invention is applied to deep soft soil foundations, there are problems such as long construction period and difficulty in eliminating the negative friction resistance of CFG piles. ③ Patent CN115928694A discloses a rigid-flexible pile composite foundation for reducing soft foundation settlement under a rigid foundation, which is specifically composed of longer rigid piles and shorter flexible piles, wherein the flexible piles are arranged around the foundation, and multiple longer rigid piles are arranged in the middle of the area surrounded by the flexible piles. This invention overcomes the problem of high cost or excessive settlement of flexible piles when using only rigid piles, but it is difficult to avoid the negative side friction of the rigid piles when used in deep soft soil foundations. Summary of the invention

[0005] The present invention aims at the shortcomings of the existing composite foundation technology in treating deep soft foundations, and provides a composite foundation of loose-semi-rigid combined piles for reinforcing deep soft soil, which can avoid the problems of excessively high cost of using (semi) rigid piles alone or the difficulty of using loose piles alone in meeting the design requirements in terms of bearing capacity and settlement, and can effectively eliminate the negative friction on the pile side when the semi-rigid piles are used to treat deep soft foundations, thereby improving the bearing capacity of the composite foundation, reducing the settlement and uneven settlement of the deep soft foundation, and avoiding damage or tilting of the pile body caused by negative friction.

[0006] A composite foundation of loose-semi-rigid combined piles for reinforcing thick soft soil, comprising:

[0007] Semi-rigid piles, bulk piles, deformation adjustment layers, reinforced cushion layers, deep soft foundations, and hard rock and soil layers;

[0008] The semi-rigid pile is located in the middle of the bulk pile, coaxial with the bulk pile, the pile body passes through the deep soft foundation, the pile bottom enters the hard rock and soil layer, and a deformation adjustment layer is arranged on the top of the pile to the bottom of the reinforced cushion layer;

[0009] The bulk pile is wrapped around the outside of the upper section of the semi-rigid pile to reinforce the shallow soft soil, eliminate the negative friction resistance of the semi-rigid pile, and place the pile bottom in the deep soft foundation;

[0010] The deformation adjustment layer is located at the top of the semi-rigid pile and is used to adjust the deformation of the top of the semi-rigid pile to avoid piercing and damage to the top of the pile;

[0011] The reinforced cushion layer is located on the top of the soft soil layer and is used to reduce uneven settlement and coordinate the pile-soil stress ratio.

[0012] Furthermore, the semi-rigid piles are medium-low strength concrete piles, and the thickness of the piles inserted into the hard rock and soil layer is not less than 1.0 m.

[0013] Furthermore, the deformation adjustment layer is a medium-coarse sand layer with a layer thickness of 200 mm to 400 mm, a compaction degree of not less than 95%, a bottom width of the semi-rigid pile diameter, and widens upward at 45° to the bottom of the reinforced cushion layer.

[0014] Furthermore, the reinforced cushion layer includes a cushion layer and a bidirectional geogrid, the cushion layer has a thickness of 150 to 300 mm, and the bidirectional geogrid is arranged in the middle of the cushion layer.

[0015] Furthermore, the semi-rigid pile has a diameter of 300-600 mm and a pile body strength of 10-30 MPa.

[0016] Furthermore, the pile body of the semi-rigid pile adopts a low-strength reinforced concrete precast pile, a mud wall bored pile or a cement fly ash gravel pile.

[0017] Furthermore, the diameter of the granular pile is 800 mm to 1000 mm, and the filler is made of crushed stone, pebble or slag hard material, with a mud content of no more than 5% and a maximum particle size of no more than 80 mm.

[0018] Furthermore, the diameter of the granular pile is 800 mm to 1000 mm, and the filler is made of crushed stone, pebble or slag hard material, with a mud content of no more than 5% and a maximum particle size of no more than 80 mm.

[0019] A design method for a composite foundation of a granular-semi-rigid composite pile for reinforcing thick soft soil as described above comprises the following steps:

[0020] Step 1: Investigate and analyze the engineering geological conditions, hydrogeological conditions and design technical requirements of the site, and clarify the constraints such as the thickness of the deep soft foundation, the physical and mechanical parameters of the soft soil, the upper additional load, the settlement required by the foundation treatment, and the foundation bearing capacity requirements;

[0021] Step 2: Determine the length of the loose pile according to the position of the neutral point of negative friction resistance when the semi-rigid pile is used to treat thick soft soil, the construction period, etc.;

[0022] Step 3: Calculate the bearing capacity f of the composite foundation composed of loose piles and semi-rigid piles k The composite foundation is composed of soil between piles, loose piles and semi-rigid piles. The calculation method is as follows:

[0023] 5) The bearing capacity of the soil foundation between piles after composite foundation treatment can be determined by on-site load test or static penetration test. If the on-site in-situ test conditions are not available, it can be estimated according to the following formula based on experience:

[0024] f ak =K a f sk (1)

[0025] Where: K a is the soil correction factor around the pile, which can be 1.0 to 1.5; f sk is the characteristic value of the bearing capacity of natural foundation;

[0026] 6) Bearing capacity characteristic value f of bulk pile bk It can be obtained through on-site single pile load test, heavy dynamic penetration test and other in-situ tests. If the conditions for on-site in-situ testing are not available, it can be determined by the empirical value of the pile-soil stress ratio based on the properties of the pile material, the density of the pile and the limit measurement capacity of the soil around the pile. The value of clay soil is generally 2.0-4.0. It can also be estimated according to the following parameters;

[0027]

[0028] f bk =n×f sk (3)

[0029] Where: C u is the undrained shear strength of the soil between piles, kPa; δ is the angle between the fracture surface and the horizontal plane; φ is the internal friction angle of the pile material; n is the pile-soil stress ratio;

[0030] 7) Bearing capacity characteristic value f of semi-rigid pile ck (kPa) is composed of pile side friction and pile end resistance, which can be obtained through on-site single pile load test. If on-site test is not available, the single pile bearing capacity can also be estimated according to the following formula;

[0031]

[0032] Where: u p is the circumference of the pile, m; A p is the cross-sectional area of ​​the pile, m 2 ;q si is the characteristic value of the side resistance of the i-th layer of soil around the pile, kPa; l si is the thickness of the i-th soil layer within the pile length, m; q p is the characteristic value of pile end resistance, kPa;

[0033] 8) Characteristic value of bearing capacity of composite pile composite foundation f k The calculation is as follows:

[0034] Gravel pile area: f k =(1-m1-m2)f ak +m1f bk +m2λ c f ck (5)

[0035] Gravel piles below area: f k =(1-m2)f ak +m2λ c f ck (6)

[0036] Where: m1 is the area replacement rate of the loose pile; m2 is the area replacement rate of the semi-rigid pile; λ c is the single pile bearing capacity exertion coefficient, which is 0.9 to 1.0 according to the stratum distribution;

[0037] Step 4: Calculate the settlement s of the composite foundation composed of loose piles and semi-rigid piles:

[0038]

[0039] Where: θ s It is the empirical coefficient for calculating the settlement of composite foundation, which is generally determined according to local observation experience, or can be obtained by looking up the table according to the equivalent value of compression modulus, 0.2~1.0; p0 is the additional load value on the cushion layer, kPa; E si is the compression modulus of the i-th natural soil layer, z i 、z i-1 is the distance from the bottom of the foundation to the bottom of the i-th soil layer and the i-1-th soil layer / m; is the average additional stress coefficient within the range from the calculation point of the foundation bottom to the bottom of the i-th soil layer and the i-1-th soil layer;

[0040] Step 5. According to the design requirements for the bearing capacity and settlement of the composite foundation, the calculation method of steps 3 and 4 is adopted, and the geological and formation parameters are taken as constant parameters. By adjusting the variable parameters such as the semi-rigid pile diameter d1, the granular pile diameter d2, and the pile spacing a, the bearing capacity and settlement values ​​under different combined working conditions are obtained, so as to select the best design scheme.

[0041] Compared with the prior art, the composite foundation of the loose-semi-rigid combined pile for deep soft soil reinforcement of the present invention adopts a combined pile composed of two types of loose and semi-rigid piles, which not only gives full play to the strong bearing capacity characteristics of the semi-rigid piles, but also avoids the problem of negative friction on the pile side when the semi-rigid piles are used in deep soft soil; after the soft soil layer on the pile side is replaced with crushed stone, not only the strength of the soil on the pile side and the positive friction on the pile side are improved, but also the drainage and consolidation of the soft soil are accelerated, and the strength of the soft soil itself is improved; the functions of the various parts of the combined pile are clearly played, and the advantages are complementary, achieving the effect of "1+1>2", thereby achieving the purpose of improving the bearing capacity of the composite foundation, reducing settlement, and coordinating the pile-soil stress ratio. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a plan layout diagram of a composite foundation of a granular-semi-rigid composite pile for reinforcing thick soft soil according to an embodiment of the present invention;

[0043] Figure 2 It is a cross-sectional view of a composite foundation of a bulk-semi-rigid composite pile for reinforcing deep soft soil according to an embodiment of the present invention;

[0044] Figure 3 It is a detailed drawing of a reinforced cushion layer of a granular-semi-rigid composite pile composite foundation for reinforcing deep soft soil according to an embodiment of the present invention;

[0045] Figure 4 This is a force distribution diagram of a composite foundation of a granular-semi-rigid composite pile for reinforcing thick soft soil according to an embodiment of the present invention.

[0046] Figure 5 It is a pile-soil stress ratio curve diagram of a bulk-semi-rigid combined pile composite foundation for deep soft soil reinforcement according to an embodiment of the present invention;

[0047] In the figure: 1—semi-rigid piles; 2—bulk piles; 3—deformation adjustment layer; 4—reinforced cushion layer; 5—thick soft foundation; 6—hard rock and soil layer. DETAILED DESCRIPTION

[0048] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are 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.

[0049] Please refer to Figure 1 and Figure 2 The embodiment of the present invention provides a composite foundation of a bulk-semi-rigid combined pile for reinforcing deep soft soil, including a semi-rigid pile 1 and a bulk pile 2, a deformation adjustment layer 3 and a reinforced cushion layer 4. In the embodiment of the present invention, the semi-rigid pile 1 is located in the middle of the bulk pile 2, coaxial with the bulk pile 2, and is used to provide bearing capacity; the bulk pile 2 is wrapped around the outside of the upper section of the semi-rigid pile 1, and is used to reinforce the shallow soft soil and eliminate the negative friction resistance of the semi-rigid pile 1 on the pile side; the deformation adjustment layer 3 is located at the top of the semi-rigid pile 1, and is used to adjust the deformation of the top of the semi-rigid pile 1 to avoid damage caused by the pile top; the reinforced cushion layer 4 is located at the top of the soft soil layer, and is used to reduce uneven settlement and coordinate the pile-soil stress ratio.

[0050] Among them, the semi-rigid pile 1 is a medium-low strength concrete pile with a pile diameter of 300-600mm and a pile body strength of 10-30MPa. A deformation adjustment layer 3 is arranged on the top of the pile. The pile body passes through a deep soft foundation 5, and the thickness of the pile bottom entering the hard rock and soil layer 6 is h3≥1.0m. The pile body can be a low-strength reinforced concrete precast pile, a mud wall bored pile or a cement fly ash gravel pile.

[0051] The bulk pile 2 is a gravel pile or a crushed stone pile, which is wrapped in the upper part of the semi-rigid pile 1. The pile bottom is placed in the deep soft foundation 5, and the pile top part expands outward at 45 degrees from the top elevation of the semi-rigid pile 1 to the bottom of the reinforced cushion layer 4. The pile diameter of the bulk pile 2 is 800-1000mm, and the filler can be hard materials such as crushed stone, pebbles, slag, etc., with a mud content of no more than 5% and a maximum particle size of no more than 80mm. The length of the bulk pile 2 should be greater than the neutral point of the negative friction resistance of the semi-rigid pile 1.

[0052] The deformation adjustment layer 3 is a medium-coarse sand layer, located on the top of the semi-rigid pile 1, using medium-coarse sand, with a layer thickness of 200mm to 400mm, a bottom width of the semi-rigid pile 1 diameter, and widens upward at 45° to the bottom of the reinforced cushion layer 4.

[0053] Please refer to Figure 3 The embodiment of the present invention provides a reinforced cushion layer 4 of a bulk-semi-rigid composite pile composite foundation for reinforcing deep soft soil, including a cushion layer 4-1 and a bidirectional geogrid 4-2. The thickness of the cushion layer 4-1 can be 150 to 300 mm, and the bidirectional geogrid 4-2 is arranged in the middle of the cushion layer 4-1.

[0054] Among them, the cushion layer 4-1 material is medium-coarse sand, graded sand and gravel, etc., with a particle size not greater than 20mm, and its tamping degree should be less than 0.9.

[0055] The tensile yield strength of the bidirectional geogrid 4-2 should be no less than 30kN / m and no less than the maximum tension per unit width. The grid reinforcement line density should be 0.15~0.35.

[0056] Please refer to Figure 4 The embodiment of the present invention provides a force analysis diagram of a composite foundation of a granular-semi-rigid composite pile for reinforcing deep soft soil. The upper additional load is mainly composed of the soil between the piles f ak , scattered pile f bk and semi-rigid piles ck The pile is composed of three parts, wherein the semi-rigid pile 1 is the main load-bearing pile; the loose pile 2 can eliminate the negative friction resistance on the pile side of the semi-rigid pile 1, and can also compact, replace and accelerate the consolidation of the soft soil around the pile.

[0057] Please refer to Figure 5, an embodiment of the present invention provides a pile-soil stress ratio of a bulk-semi-rigid composite pile composite foundation for deep soft soil reinforcement. By conducting indoor model tests, the pile-soil stress ratios of three composite foundations, namely bulk piles, semi-rigid piles and composite piles, are obtained. As can be seen from the figure, when subjected to an attached load, the pile-soil stress ratio of the composite pile is 9.7 to 19.6, and as the attached load increases, the pile-soil stress ratio tends to increase. Compared with bulk piles, the pile-soil stress ratio of composite piles is significantly improved; compared with semi-rigid piles, when the attached load is small, the pile-soil stress ratio of the composite pile is greater than that of the semi-rigid pile. After the attached load is greater than 132kPa, the increase in the pile-soil stress ratio of the composite pile is less than that of the semi-rigid pile, indicating that the composite pile composite foundation can better coordinate the pile-soil stress ratio and give full play to the bearing characteristics of the soil around the pile.

[0058] The present invention discloses a composite foundation of a dispersed-semi-rigid combined pile for reinforcing deep soft soil, comprising a semi-rigid pile 1, a dispersed pile 2, a deformation adjustment layer 3 and a reinforced cushion layer 4, wherein the semi-rigid pile 1 is a main load-bearing pile body, and the displacement and compaction functions of the dispersed pile 2 are utilized to reinforce the soft soil around the pile, and the drainage function of the dispersed pile 2 is utilized to solve the problems of difficult pile body quality control, large pile side negative friction and high pile-soil stress ratio existing in reinforcing deep soft soil with only semi-rigid piles. The present invention has the advantages of improving the bearing capacity of the composite foundation, accelerating the consolidation settlement of the soft foundation, coordinating the pile-soil stress ratio, reducing the reinforcement cost and saving the construction period.

[0059] Please refer to further Figures 1 to 5 The embodiment of the present invention also provides a design method for a granular-semi-rigid composite pile composite foundation for deep soft soil reinforcement, comprising the following steps:

[0060] Step 1: Determine the constraints such as the thickness of the deep soft foundation in the site, the physical and mechanical parameters of the soft soil, the upper additional load, the settlement required by the foundation treatment, and the foundation bearing capacity requirements;

[0061] Step 2: Determine the length of the loose pile 2 according to the position of the neutral point of negative friction resistance when the semi-rigid pile is used to treat deep soft soil, the settlement requirements and the construction period;

[0062] Step 3: Calculate the bearing capacity f of the composite foundation composed of loose pile 2 and semi-rigid pile 1 k The composite foundation consists of three parts, namely, soil between piles, loose piles 2 and semi-rigid piles 1, with a bearing capacity of f k The calculation method is as follows:

[0063] 1) The bearing capacity of the soil foundation between piles after composite foundation treatment is generally determined by on-site load test and static penetration test. If the on-site in-situ test conditions are not available, it can be estimated based on experience according to the following formula: ak =K a f sk (1)

[0064] Where: K a is the soil correction factor around the pile, which can be 1.0 to 1.5; f sk is the characteristic value of the bearing capacity of natural foundation;

[0065] 2) Bearing capacity characteristic value f of granular pile 2 bk It is usually obtained through on-site single pile load test, heavy dynamic penetration test and other in-situ tests. If the conditions for on-site in-situ testing are not available, it can be determined by the empirical value of the pile-soil stress ratio based on the properties of the pile material, the density of the pile and the limit measurement capacity of the soil around the pile. The value of clay soil is generally 2.0-4.0. It can also be estimated according to the following parameters:

[0066]

[0067] f bk =n×f sk (3)

[0068] Where: C u is the undrained shear strength of the soil between piles, kPa; δ is the angle between the fracture surface and the horizontal plane; φ is the internal friction angle of the pile material; n is the pile-soil stress ratio;

[0069] 4) Bearing capacity characteristic value f of semi-rigid pile 1 ck Generally, it is composed of two parts: pile side friction and pile end resistance. The single pile bearing capacity can be estimated according to the following formula:

[0070]

[0071] Where: u p is the circumference of the pile, m; A p is the cross-sectional area of ​​the pile, m 2 ;q si is the characteristic value of the side resistance of the i-th layer of soil around the pile, kPa; l si is the thickness of the i-th soil layer within the pile length, m; q p is the characteristic value of pile end resistance, kPa.

[0072] 3) Characteristic value of bearing capacity of composite pile composite foundation f k The calculation is as follows:

[0073] Gravel pile area: f k =(1-m1-m2)f ak +m1f bk +m2λ c f ck (5)

[0074] Gravel piles below area: f k =(1-m2)f ak +m2λ c f ck(6)

[0075] Where: m1 is the area replacement rate of the loose pile; m2 is the area replacement rate of the semi-rigid pile; λ c is the single pile bearing capacity exertion coefficient, which can be 0.9 to 1.0 according to the stratum distribution;

[0076] Step 4: Calculate the settlement s of the composite foundation composed of loose piles 2 and semi-rigid piles 1:

[0077]

[0078] Where: θ s It is the empirical coefficient for calculating the settlement of composite foundation, which is generally determined according to local observation experience, or can be obtained by looking up the table according to the equivalent value of compression modulus, 0.2~1.0; p0 is the additional load value on the cushion layer, kPa; E si is the compression modulus of the i-th natural soil layer, z i 、z i-1 is the distance from the bottom of the foundation to the bottom of the i-th soil layer and the i-1-th soil layer / m; is the average additional stress coefficient within the range from the calculation point of the foundation bottom to the bottom of the i-th soil layer and the i-1-th soil layer;

[0079] Step 5: According to the design requirements for the bearing capacity and settlement of the composite foundation, the calculation method of step 3 and step 4 is adopted, and the geological and formation parameters are taken as constant parameters. By adjusting the variable parameters such as the semi-rigid pile diameter d1, the granular pile diameter d2, and the pile spacing s, the best design scheme is obtained. Compared with the traditional granular material pile, the pile-soil stress ratio of the granular-semi-rigid composite pile composite foundation is greatly improved, and the pile body bears more load. Its composite foundation has a higher bearing capacity and a smaller settlement, which can effectively solve the problem of insufficient bearing capacity or excessive settlement of deep soft soil. Compared with the traditional semi-rigid pile, the granular-semi-rigid composite pile composite foundation replaces the soft soil layer on the pile side with gravel, which not only eliminates the negative friction resistance of the upper soft soil pile side, but also improves the strength of the soil body on the pile side and the positive friction resistance on the pile side, and accelerates the drainage and consolidation of the soft soil, and improves the strength of the soft soil itself.

[0080] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A composite foundation of loose-semi-rigid combined piles for reinforcing thick soft soil, characterized in that: include: Semi-rigid piles (1), bulk piles (2), deformation adjustment layer (3), reinforced cushion layer (4), deep soft foundation (5), hard rock and soil layer (6); The semi-rigid pile (1) is located in the middle of the bulk pile (2) and is coaxial with the bulk pile (2). The pile body passes through a thick soft foundation (5), and the bottom of the pile enters a hard rock and soil layer (6). A deformation adjustment layer (3) is arranged on the top of the pile to the bottom of a reinforced cushion layer (4). The bulk pile (2) is wrapped around the outside of the upper section of the semi-rigid pile (1) to reinforce the shallow soft soil and eliminate the negative friction resistance on the pile side of the semi-rigid pile (1). The pile bottom is placed in a deep soft foundation (5). The deformation adjustment layer (3) is located at the top of the semi-rigid pile (1) and is used to adjust the deformation of the top of the semi-rigid pile (1) to avoid piercing and damage to the top of the pile; The reinforced cushion layer (4) is located on the top of the soft soil layer and is used to reduce uneven settlement and coordinate the pile-soil stress ratio.

2. The granular-semi-rigid composite pile composite foundation for deep soft soil reinforcement according to claim 1 is characterized in that: The semi-rigid pile (1) is a medium-low strength concrete pile, and the thickness of the pile inserted into the hard rock and soil layer (6) is not less than 1.0 m.

3. The granular-semi-rigid composite pile composite foundation for deep soft soil reinforcement according to claim 1 is characterized in that: The bulk pile (2) is a gravel pile or a crushed stone pile, and the pile top portion expands outward at 45 degrees at the pile top elevation of the semi-rigid pile (1) to the bottom of the reinforced cushion layer (4).

4. The granular-semi-rigid composite pile composite foundation for deep soft soil reinforcement according to claim 1 is characterized in that: The deformation adjustment layer (3) is a medium-coarse sand layer with a layer thickness of 200 mm to 400 mm and a compaction degree of not less than 95%. The bottom width is the pile diameter of the semi-rigid pile (1) and widens upward at 45 degrees to the bottom of the reinforced cushion layer (4).

5. The granular-semi-rigid composite pile composite foundation for deep soft soil reinforcement according to claim 1 is characterized in that: The reinforced cushion layer (4) comprises a cushion layer (4-1) and a bidirectional geogrid (4-2); the cushion layer (4-1) has a thickness of 150 to 300 mm, and the bidirectional geogrid (4-2) is arranged in the middle of the cushion layer (4-1).

6. The granular-semi-rigid composite pile composite foundation for deep soft soil reinforcement according to claim 2 is characterized in that: The semi-rigid pile (1) has a pile diameter of 300-600 mm and a pile body strength of 10-30 MPa.

7. The granular-semi-rigid composite pile composite foundation for deep soft soil reinforcement according to claim 6 is characterized in that: The pile body of the semi-rigid pile (1) is made of a low-strength reinforced concrete prefabricated pile, a mud wall bored pile or a cement fly ash gravel pile.

8. The granular-semi-rigid composite pile composite foundation for reinforcing deep soft soil according to claim 3 is characterized in that: the granular pile (2) has a pile diameter of 800 mm to 1000 mm, and the filler is made of hard material such as crushed stone, pebble or slag, with a mud content of no more than 5% and a maximum particle size of no more than 80 mm.

9. The granular-semi-rigid composite pile composite foundation for deep soft soil reinforcement according to claim 5 is characterized in that: The cushion layer (4-1) is made of medium-coarse sand and graded gravel with a particle size of no more than 20 mm and a tamping degree of less than 0.9; the tensile yield strength of the bidirectional geogrid (4-2) should be no less than 30 kN / m and no less than the maximum tensile force per unit width; the grid reinforcement line density is 0.15 to 0.

35.

10. A design method for a granular-semi-rigid composite pile composite foundation for deep soft soil reinforcement according to any one of claims 1 to 9, characterized in that: The steps include: Step 1: Investigate and analyze the engineering geological conditions, hydrogeological conditions and design technical requirements of the site, and clarify the thickness of the deep soft foundation, the physical and mechanical parameters of the soft soil, the upper additional load, the settlement required by the foundation treatment, and the foundation bearing capacity requirements; Step 2: Determine the length of the loose pile according to the position of the neutral point of negative friction and the construction period when the semi-rigid pile is used to treat deep soft soil; Step 3: Calculate the bearing capacity f of the composite foundation composed of loose piles and semi-rigid piles k The composite foundation is composed of soil between piles, loose piles and semi-rigid piles. The calculation method is as follows: 1) The bearing capacity of the soil foundation between piles after composite foundation treatment can be determined by on-site load test or static penetration test. If the on-site in-situ test conditions are not available, it can be estimated according to the following formula based on experience: f ak =K a f sk (1) Where: K a is the soil correction factor around the pile, which can be 1.0 to 1.5; f sk is the characteristic value of the bearing capacity of natural foundation; 2) Characteristic value of bearing capacity of bulk pile f bk It can be obtained through on-site single pile load test and heavy dynamic penetration test. If the on-site in-situ test conditions are not available, it can be determined by the empirical value of the pile-soil stress ratio based on the pile material properties, pile density and the limit measurement capacity of the soil around the pile. The value of clay soil is generally 2.0-4.

0. It can also be estimated according to the following parameters: f bk =n×f sk (3) Where: C u is the undrained shear strength of the soil between piles, kPa; δ is the angle between the fracture surface and the horizontal plane; φ is the internal friction angle of the pile material; n is the pile-soil stress ratio; 3) Bearing capacity characteristic value f of semi-rigid pile ck (kPa) is composed of pile side friction and pile end resistance, which can be obtained through on-site single pile load test. If on-site test is not available, the single pile bearing capacity can also be estimated according to the following formula; Where: u p is the circumference of the pile, m; A p is the cross-sectional area of ​​the pile, m 2 ;q si is the characteristic value of the side resistance of the i-th layer of soil around the pile, kPa; l si is the thickness of the i-th soil layer within the pile length, m; q p is the characteristic value of pile end resistance, kPa; 4) Characteristic value of bearing capacity of composite pile composite foundation f k The calculation is as follows: Gravel pile area: f k =(1-m1-m2)f ak +m1f bk +m2λ c f ck (5) Gravel piles below area: f k =(1-m2)f ak +m2λ c f ck (6) Where: m1 is the area replacement rate of the loose pile; m2 is the area replacement rate of the semi-rigid pile; λ c is the single pile bearing capacity exertion coefficient, which is 0.9 to 1.0 according to the stratum distribution; Step 4: Calculate the settlement s of the composite foundation composed of loose piles and semi-rigid piles: Where: θ s It is the empirical coefficient for calculating the settlement of composite foundation, which is generally determined according to local observation experience, or can be obtained by looking up the table according to the equivalent value of compression modulus, 0.2~1.0; p0 is the additional load value on the cushion layer, kPa; E si is the compression modulus of the i-th natural soil layer, z i 、z i-1 is the distance from the bottom of the foundation to the bottom of the i-th soil layer and the i-1-th soil layer / m; is the average additional stress coefficient within the range from the calculation point of the foundation bottom to the bottom of the i-th soil layer and the i-1-th soil layer; Step 5. According to the design requirements for the bearing capacity and settlement of the composite foundation, the calculation method of steps 3 and 4 is adopted, and the geological and stratigraphic parameters are taken as constant parameters. By adjusting the semi-rigid pile diameter d1, the granular pile diameter d2, and the pile spacing a, the bearing capacity and settlement values ​​under different combined working conditions are obtained, thereby selecting the best design scheme.

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