A kind of granular-hybrid composite pile foundation for deep soft soil reinforcement and design method

Abstract

A kind of granular body-semi-rigid combined pile composite foundation for deep soft soil reinforcement and design method, the composite foundation includes semi-rigid pile, granular body pile, deformation adjusting layer, reinforced mattress layer, deep soft foundation, hard rock soil layer;Semi-rigid pile is located in the middle of granular body pile, with granular body pile coaxial, pile passes through deep soft foundation, pile bottom enters hard rock soil layer, the upper part of pile top is provided with deformation adjusting layer to the bottom of reinforced mattress layer;Granular body pile is wrapped in the outside of semi-rigid pile upper section, and the pile bottom is placed in deep soft foundation;Deformation adjusting layer is located on the top of semi-rigid pile;Reinforced mattress layer is located on the top of soft soil layer.The invention utilizes the compaction, replacement and drainage function of granular body pile, solves the problems of difficult pile quality control, large negative frictional resistance of pile side and high pile-soil stress ratio in single semi-rigid pile reinforcement of deep soft soil, has the advantages of improving the bearing capacity of composite foundation, accelerating the consolidation settlement of soft foundation, coordinating the pile-soil stress ratio and reducing the reinforcement cost and saving the construction period.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of deep soft foundation treatment, in particular to a granular-hemirigid combined pile composite foundation for deep soft soil reinforcement and a design method thereof. BACKGROUND

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

[0003] For deep soft soil foundation, commonly used composite foundations include granular pile, sand pile, and gravel pile composite foundations, cement-soil pile and lime-soil pile composite foundations, CFG pile and lime-soil concrete pile composite foundations, reinforced concrete pile and prestressed pipe pile composite foundations, etc. The above composite foundations are single-pile-type composite foundations, and each has certain deficiencies when treating deep soft soil foundation. For example, granular pile has certain compaction, replacement, and drainage consolidation effects, but it is limited in improving the bearing capacity of the foundation, and the drainage consolidation requires a long construction period and upper load. Flexible pile has limited treatment depth, difficult construction quality control, and non-setting cement paste. Rigid pile and hemirigid pile have negative skin friction on the pile body, which has a great impact on the bearing capacity of the pile body and the settlement of the pile top, and is prone to cause problems such as pile body damage, foundation separation from the natural foundation, etc.

[0004] Composite foundations composed of multiple pile types can solve the shortcomings of composite foundations composed of single pile types. Currently, similar composite pile foundations mainly include the following: ① The flexible-dispersed-rigid ternary composite pile foundation disclosed in patent CN115538411A involves placing three permeable dispersed piles at the axial center of three mixing piles, and a permeable rigid pipe pile at the center of the overlapping area within the three mixing piles. This invention mainly expands the cross-sectional area of ​​the piles through composite piles, improving the bearing capacity of the foundation under low-cost conditions. However, the cement-soil piles enclosing the dispersed piles restrict the drainage capacity of the dispersed piles. ② The high-speed railway ballastless track foundation treatment method disclosed in patent CN115964782A uses a method of alternating arrangement of crushed stone piles and CFG piles. The crushed stone piles replace and compact the soil to solve liquefaction, while the CFG piles improve the bearing capacity and reduce liquefaction. However, this invention has problems such as long construction period and difficulty in eliminating the negative skin friction of the CFG piles when applied to deep soft soil foundations. ③ Patent CN115928694A discloses a rigid-flexible pile composite foundation for reducing settlement of soft soil under rigid foundation. Specifically, it consists of longer rigid piles and shorter flexible piles. The flexible piles are arranged around the foundation, and multiple longer rigid piles are arranged in the middle of the area enclosed by the flexible piles. This invention overcomes the problems of high cost or excessive settlement of flexible piles when using rigid piles alone. However, when applied to deep soft soil foundations, it is difficult to avoid the negative skin friction of the rigid piles. Summary of the Invention

[0005] This invention addresses the shortcomings of existing composite foundation technologies for treating deep soft soil by providing a composite foundation of granular-semi-rigid piles for reinforcing deep soft soil. It avoids the problems of excessive cost when using (semi)rigid piles alone or the inability of granular piles alone to meet design requirements in terms of bearing capacity and settlement. Furthermore, it effectively eliminates the negative skin friction of the pile side when using semi-rigid piles to treat deep soft soil, thereby improving the bearing capacity of the composite foundation, reducing settlement and uneven settlement of deep soft soil, and avoiding pile damage or tilting caused by negative skin friction.

[0006] A composite foundation for reinforcing deep soft soil using granular-semi-rigid piles, comprising:

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

[0008] The semi-rigid pile is located in the middle of the loose pile and is coaxial with the loose pile. The pile body passes through the deep soft foundation and the bottom of the pile enters the hard rock and soil layer. A deformation adjustment layer is set on the top of the pile up to the bottom of the reinforced mattress layer.

[0009] The loose-grained pile is wrapped around the upper section of the semi-rigid pile to reinforce shallow soft soil, eliminate the negative skin friction of the semi-rigid pile, and place the pile bottom in a 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 pile top to prevent the pile top from penetrating and causing damage.

[0011] The reinforced mattress layer is located on 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 pile is a medium-to-low strength concrete pile, with a penetration thickness of not less than 1.0m into a hard rock and soil layer.

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

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

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

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

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

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

[0019] A design method for a granular-semi-rigid composite pile foundation for reinforcing deep soft soil, as described above, includes 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 additional load on the superstructure, and the settlement and bearing capacity requirements of the foundation treatment.

[0021] Step 2: Determine the length of the granular piles based on the location of the neutral point of negative skin friction when treating deep soft soil with semi-rigid piles, 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 consists 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 field load tests or static cone penetration tests. If in-situ testing is not feasible, it can be estimated based on experience using the following formula:

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

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

[0026] 6) Characteristic value of bearing capacity f of granular piles bk It can be obtained through in-situ tests such as single pile load test and heavy dynamic penetration test. If the conditions for in-situ testing are not available, it can be determined by empirical value of pile-soil stress ratio based on the properties of pile material, pile density and the measurement limit of soil around the pile. For cohesive soil, the value is generally taken as 2.0 to 4.0. It can also be estimated by the following parameters.

[0027]

[0028] f bk =n×f sk (3)

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

[0030] 7) Characteristic value of bearing capacity f of semi-rigid piles ck The bearing capacity (kPa) consists of two parts: the pile side friction and the pile end resistance. It can be obtained through field single pile load test. If field test is not available, the single pile bearing capacity can also be estimated according to the following formula.

[0031]

[0032] In the formula: u p Let A be the perimeter of the pile, in meters (m); p Let m be the cross-sectional area of ​​the pile. 2 ;q si Let l be the characteristic value of the lateral resistance of the i-th soil layer around the pile, in kPa; si q represents the thickness of the i-th soil layer within the pile length range, in meters. p The characteristic value of the pile end resistance is given in kPa.

[0033] 8) Characteristic value of bearing capacity f of composite pile foundation 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] The area below the crushed stone pile: f k =(1-m2)f ak +m2λ c f ck (6)

[0036] Where: m1 is the area replacement ratio of the granular pile; m2 is the area replacement ratio of the semi-rigid pile; λ c The single pile bearing capacity utilization coefficient is taken as 0.9 to 1.0 based on the stratum distribution;

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

[0038]

[0039] In the formula: θ s The empirical coefficient for calculating the settlement of composite foundations is generally determined based on local observation experience, or it can be obtained by looking up the equivalent value of the compression modulus in a table, ranging from 0.2 to 1.0; p0 is the additional load value above the cushion layer, in kPa; E si Let z be the compression modulus of the i-th natural soil layer. i z i-1 It is the distance in meters from the bottom surface of the foundation to the bottom surface of the i-th and (i-1)-th soil layers. It is the average additional stress coefficient within the range from the calculation point on the foundation bottom surface to the bottom surface of the i-th and i-1-th soil layers;

[0040] Step 5: Based on the design requirements for the bearing capacity and settlement of the composite foundation, the calculation methods of Step 3 and Step 4 are adopted. Geological and stratum parameters are used 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, thereby selecting the optimal design scheme.

[0041] Compared with existing technologies, the composite foundation of granular-semi-rigid piles for deep soft soil reinforcement of this invention uses a combination of granular and semi-rigid piles. This combination leverages the strong bearing capacity of semi-rigid piles while avoiding the negative skin friction problem that exists when semi-rigid piles are used in deep soft soil. After the soft soil layer on the pile side is replaced with crushed stone, it not only improves the strength of the soil on the pile side and the positive skin friction, but also accelerates the drainage and consolidation of the soft soil, thereby improving the strength of the soft soil itself. The functions of each part of the composite pile are clearly defined and complementary, achieving a "1+1>2" effect, thus achieving the goals of improving the bearing capacity of the composite foundation, reducing settlement, and coordinating the pile-soil stress ratio. Attached Figure Description

[0042] Figure 1 This is a plan view of a composite foundation of granular-semi-rigid piles for reinforcing deep soft soil according to an embodiment of the present invention.

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

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

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

[0046] Figure 5 This is a pile-soil stress ratio curve of a granular-semi-rigid composite pile foundation for reinforcing deep soft soil according to an embodiment of the present invention.

[0047] In the diagram: 1—semi-rigid pile; 2—fragmented pile; 3—deformation adjustment layer; 4—reinforced cushion layer; 5—deep soft foundation; 6—hard soil and rock layer. Detailed Implementation

[0048] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. 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.

[0049] Please refer to Figure 1 and Figure 2 This invention provides a composite foundation for deep soft soil reinforcement using a combination of semi-rigid and granular piles, comprising semi-rigid piles 1 and granular piles 2, a deformation adjustment layer 3, and a reinforced cushion layer 4. In this embodiment, the semi-rigid piles 1 are located in the middle of the granular piles 2, coaxial with them, and provide bearing capacity. The granular piles 2 enclose the upper section of the semi-rigid piles 1, reinforcing shallow soft soil and eliminating negative skin friction on the pile sides of the semi-rigid piles 1. The deformation adjustment layer 3 is located at the top of the semi-rigid piles 1, adjusting the deformation at the pile top to prevent pile top penetration damage. The reinforced cushion layer 4 is located at the top of the soft soil layer, reducing uneven settlement and harmonizing 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 set at the top of the pile. The pile body penetrates the deep soft foundation 5, and the thickness h3 of the pile bottom into the hard rock and soil layer 6 is ≥1.0m. The pile body can be a low-strength reinforced concrete precast pile, a mud-walled bored pile, or a cement fly ash gravel pile.

[0051] The loose-matrix pile 2 is a gravel or crushed stone pile, encasing the upper half of the semi-rigid pile 1. The pile bottom is placed in a deep soft foundation 5, and the pile top extends outwards at a 45° angle from the top elevation of the semi-rigid pile 1 to the bottom of the reinforced cushion layer 4. The diameter of the loose-matrix pile 2 is 800–1000 mm. The fill material can be hard materials such as crushed stone, pebbles, or slag, with a mud content not exceeding 5% and a maximum particle size not exceeding 80 mm. The length of the loose-matrix pile 2 should be greater than the neutral point of the negative skin friction of the semi-rigid pile 1.

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

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

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

[0055] The tensile yield strength of the bidirectional geogrid 4-2 should not be less than 30kN / m and not less than the maximum tensile force per unit width. The linear density of the geogrid reinforcement should be 0.15 to 0.35.

[0056] Please refer to Figure 4 This invention provides a stress analysis diagram for a granular-semi-rigid composite pile foundation used for reinforcing deep soft soil. The additional load on the upper part is mainly composed of the soil between the piles. ak , loose pile bk and semi-rigid piles f ck It consists of three parts, of which the semi-rigid pile 1 is the main load-bearing pile; the loose pile 2 can eliminate the negative skin friction 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 5This invention provides a pile-soil stress ratio for a composite foundation of granular-semi-rigid piles used for reinforcing deep soft soil. Through indoor model tests, the pile-soil stress ratios of three types of composite foundations—granular piles, semi-rigid piles, and composite piles—were obtained. As shown in the figure, under top load, the pile-soil stress ratio of the composite piles ranges from 9.7 to 19.6, and it increases with increasing top load. Compared to granular piles, the pile-soil stress ratio of the composite piles is significantly higher. Compared to semi-rigid piles, when the top load is small, the pile-soil stress ratio of the composite piles is greater than that of the semi-rigid piles. However, when the top load exceeds 132 kPa, the increase in the pile-soil stress ratio of the composite piles is less than that of the semi-rigid piles. This indicates that the composite pile foundation can better coordinate the pile-soil stress ratio and utilize the bearing capacity of the surrounding soil.

[0058] This invention discloses a composite foundation of granular-semi-rigid piles for reinforcing deep soft soil, comprising semi-rigid piles 1, granular piles 2, a deformation adjustment layer 3, and a reinforced cushion layer 4. The semi-rigid piles 1 are the main load-bearing piles. The granular piles 2 are used to replace and compact the soft soil around the piles, and the granular piles 2 are used for drainage. This invention solves the problems of difficult pile quality control, high negative skin friction on the pile side, and high pile-soil stress ratio that exist when using semi-rigid piles alone to reinforce deep soft soil. This invention has the advantages of improving the bearing capacity of the composite foundation, accelerating the consolidation and settlement of soft soil, coordinating the pile-soil stress ratio, reducing reinforcement costs, and saving construction time.

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

[0060] Step 1: Determine the constraints such as the thickness of the deep soft soil foundation, the physical and mechanical parameters of the soft soil, the additional load on the superstructure, and the settlement and bearing capacity requirements of the foundation treatment.

[0061] Step 2: Determine the length of the granular pile 2 based on the location of the neutral point of negative skin friction, settlement requirements, and construction period when treating deep soft soil with semi-rigid piles;

[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: the soil between the piles, 2 loose piles, and 1 semi-rigid pile, with a bearing capacity f. k The calculation method is as follows:

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

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

[0065] 2) Characteristic value of bearing capacity f of granular pile 2 bk Generally, the results are obtained through in-situ tests such as single pile load tests and heavy dynamic penetration tests. If in-situ testing conditions are not available, the pile-soil stress ratio can be determined by empirical values ​​based on the pile material properties, pile density, and the testing capacity of the soil around the pile. For cohesive soil, the value is generally taken as 2.0 to 4.0. It can also be estimated based on the following parameters.

[0066]

[0067] f bk =n×f sk (3)

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

[0069] 4) Characteristic value of bearing capacity f of semi-rigid pile 1 ck The bearing capacity of a single pile is generally composed of two parts: pile side skin friction and pile end resistance. The following formula can be used to estimate the bearing capacity of a single pile:

[0070]

[0071] In the formula: u p Let A be the perimeter of the pile, in meters (m); p Let m be the cross-sectional area of ​​the pile. 2 ;q si Let l be the characteristic value of the lateral resistance of the i-th soil layer around the pile, in kPa; si q represents the thickness of the i-th soil layer within the pile length range, in meters. p The characteristic value of the pile end resistance is given in kPa.

[0072] 3) Characteristic value of bearing capacity f of composite pile foundation 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] The area below the crushed stone pile: f k =(1-m2)f ak +m2λ c f ck(6)

[0075] Where: m1 is the area replacement ratio of the granular pile; m2 is the area replacement ratio of the semi-rigid pile; λ c The single pile bearing capacity utilization coefficient can be taken as 0.9 to 1.0 depending on the soil stratum distribution;

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

[0077]

[0078] In the formula: θ s The empirical coefficient for calculating the settlement of composite foundations is generally determined based on local observation experience, or it can be obtained by looking up the equivalent value of the compression modulus in a table, ranging from 0.2 to 1.0; p0 is the additional load value above the cushion layer, in kPa; E si Let z be the compression modulus of the i-th natural soil layer. i z i-1 It is the distance in meters from the bottom surface of the foundation to the bottom surface of the i-th and (i-1)-th soil layers. It is the average additional stress coefficient within the range from the calculation point on the foundation bottom surface to the bottom surface of the i-th and i-1-th soil layers;

[0079] Step 5: Based on the design requirements for the bearing capacity and settlement of the composite foundation, the calculation methods of Steps 3 and 4 are used. Geological and stratum parameters are treated as constant parameters. Variable parameters such as the semi-rigid pile diameter d1, the granular pile diameter d2, and the pile spacing s are adjusted to obtain the optimal design scheme. Compared with traditional granular material piles, the granular-semi-rigid composite pile foundation has a significantly improved pile-soil stress ratio, allowing the piles to bear more load. This composite foundation exhibits higher bearing capacity and smaller settlement, effectively solving the problem of insufficient bearing capacity or excessive settlement in deep soft soil. Compared with traditional semi-rigid piles, the granular-semi-rigid composite pile foundation, by replacing the soft soil layer along the pile with crushed stone, not only eliminates the negative skin friction of the upper soft soil pile side, increasing the strength of the soil along the pile side and the positive skin friction, but also accelerates the drainage and consolidation of the soft soil, improving the strength of the soft soil itself.

[0080] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A design method for a granular-semi-rigid composite pile foundation for reinforcing deep soft soil, characterized in that, The composite foundation of granular-semi-rigid piles used for deep soft soil reinforcement includes: semi-rigid piles (1), granular piles (2), deformation adjustment layer (3), reinforced cushion layer (4), deep soft foundation (5), and hard rock and soil layer (6). The semi-rigid pile (1) is located in the middle of the loose pile (2), coaxial with the loose pile (2), the pile body passes through the deep soft foundation (5), the bottom of the pile enters the hard rock and soil layer (6), and a deformation adjustment layer (3) is set on the top of the pile to the bottom of the reinforced mattress layer (4); The loose pile (2) is wrapped around the upper part of the semi-rigid pile (1) to reinforce the shallow soft soil, eliminate the negative skin friction of the semi-rigid pile (1), and the bottom of the pile is placed in the 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 pile top of the semi-rigid pile (1) to prevent the pile top from penetrating and causing damage. 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; the design method includes the following steps: 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 additional load on the superstructure, and the settlement and bearing capacity requirements that the foundation treatment needs to achieve. Step 2: Determine the length of the granular piles based on the location of the neutral point of negative skin friction and the construction period when using semi-rigid piles 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 consists 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 is determined by field load tests or static cone penetration tests. If in-situ testing is not feasible, it can be estimated based on experience using the following formula: （1）； In the formula: The characteristic value of the bearing capacity of the soil foundation between piles. The correction factor for the soil around the pile is 1.0 to 1.

5. Characteristic value of natural foundation bearing capacity; 2) Characteristic value of bearing capacity f of granular piles bk The stress ratio is obtained through on-site single pile load tests and heavy dynamic penetration tests. If on-site in-situ testing conditions are not available, it can be obtained through either of the following two methods: First, it is determined by empirical values ​​of the pile-soil stress ratio based on the pile material properties, pile density, and the testing capacity of the soil around the pile. When the pile soil is cohesive, the empirical value of the pile-soil stress ratio is in the range of 2.0 to 4.

0. Second, it is estimated based on the following parameters. （2）； （3）； In the formula: C u The shear strength of the undrained soil between piles is given in kPa. The angle between the fracture surface and the horizontal plane; θ is the internal friction angle of the pile material; η is the pile-soil stress ratio. 3) Characteristic value of bearing capacity of semi-rigid piles It consists of two parts: pile side friction and pile end resistance. It is obtained through field single pile load test. When field test is not available, the single pile bearing capacity is estimated according to the following formula. （4）； In the formula: The characteristic value of the bearing capacity of a semi-rigid pile is given in kPa; u p Let A be the perimeter of the pile, in meters (m); p The cross-sectional area of ​​the pile is in meters. 2 ; Let be the characteristic value of the lateral resistance of the i-th layer of soil around the pile, in kPa; The thickness of the i-th soil layer within the pile length range, in meters; The characteristic value of the pile end resistance is given in kPa. 4) Characteristic value of bearing capacity of composite pile foundation The calculation is as follows: Crushed stone pile area: (5); The area below the gravel piles: (6); In the formula: The area replacement ratio of the loose-body pile; The area replacement ratio of the semi-rigid pile; The single pile bearing capacity utilization factor is taken as 0.9~1.0 according to the stratum distribution; Step 4: Calculate the settlement s of the composite foundation composed of loose piles and semi-rigid piles: （7）； In the formula: The empirical coefficients for calculating the settlement of composite foundations are obtained by looking up the equivalent values ​​of the compression modulus in a table. The value range is 0.2 to 1.0; The additional load value on the upper part of the subbase is given in kPa. Let be the compression modulus of the i-th natural soil layer. , It is the distance in meters from the bottom surface of the foundation to the bottom surface of the i-th and (i-1)-th soil layers. , It is the average additional stress coefficient within the range from the calculation point on the foundation bottom surface to the bottom surface of the i-th and i-1-th soil layers; Step 5: Based on the design requirements for the bearing capacity and settlement of the composite foundation, the calculation methods of Step 3 and Step 4 are adopted. The geological and stratum parameters are used 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 optimal design scheme.

2. The design method for granular-semi-rigid composite pile foundations for deep soft soil reinforcement according to claim 1, 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 1.0m.

3. The design method for granular-semi-rigid composite pile foundations for deep soft soil reinforcement according to claim 1, characterized in that: The loose pile (2) is a gravel pile or crushed stone pile, and the top part of the pile extends outward at 45° from the top elevation of the semi-rigid pile (1) to the bottom of the reinforced mattress layer (4).

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

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

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

7. The design method for granular-semi-rigid composite pile foundations for deep soft soil reinforcement according to claim 6, characterized in that: The semi-rigid pile (1) is made of low-strength reinforced concrete precast pile, mud wall-supported bored pile or cement fly ash crushed stone pile.

8. The design method of the composite foundation of granular-semi-rigid combined piles for deep soft soil reinforcement according to claim 3 is characterized in that: the granular pile (2) has a pile diameter of 800mm~1000mm, the filler material is hard material such as crushed stone, pebbles or slag, the mud content is not greater than 5%, and the maximum particle size is not greater than 80mm.

9. The design method for granular-semi-rigid composite pile foundations for deep soft soil reinforcement according to claim 5, characterized in that: The cushion layer (4-1) material is medium-coarse sand with a particle size not greater than 20mm and a compaction degree of less than 0.9; the tensile yield strength of the bidirectional geogrid (4-2) should not be less than 30kN / m and not less than the maximum tensile force per unit width; the reinforcement linear density of the geogrid is 0.15~0.35.

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