Method for determining compressive ultimate bearing capacity of distributed post-grouting pile foundation

Abstract

The invention relates to the technical field of pile foundation ultimate bearing capacity calculation, in particular to a distributed post-grouting pile foundation compression ultimate bearing capacity determination method which comprises the following steps: S1, obtaining the structural size and material parameters of a pile foundation, the physical and mechanical parameters of soil, the grouting amount and the grouting material parameters; s2, establishing a three-dimensional finite element model according to the structural size of the pile foundation in S1; s3, selecting a soil constitutive model; s4, inputting the physical and mechanical parameters of the test soil in the S1 into the soil constitutive model in the S3; s5, pile-soil interface parameters, pile end bearing capacity and pile end spring stiffness are determined according to the physical and mechanical parameters, provided in the S1, of the pile foundation and the soil; s6, setting a finite element model according to a pile foundation loading model test process; s7, calculating and analyzing the finite element model to obtain a Q-S curve of pile top settlement and load; s8, according to the Q-S curve obtained through calculation in the S7, the ultimate bearing capacity of the distributed post-grouting pile foundation is determined; the ultimate bearing capacity of the pile foundation calculated by the method is relatively accurate.

Description

Technical Field

[0001] The invention relates to the technical field of ultimate bearing capacity calculation of pile foundations, and in particular to a method for determining the ultimate compressive bearing capacity of a distributed post-grouting pile foundation. Background Art

[0002] Piles are the most common foundation form used in buildings at present. They are widely used due to their high bearing capacity, strong adaptability, simple construction and many other characteristics. In order to improve the bearing capacity of pile foundations, pile foundation post-grouting technology came into being.

[0003] However, the current engineering practice of distributed post-grouting is ahead of theoretical research. There is still a lack of systematic research on the reinforcement mechanism of distributed post-grouting technology, the bearing mechanism of distributed post-grouting pile foundation, and the law of load transfer. Therefore, the ultimate bearing capacity of distributed post-grouting pile foundation is mainly determined by the field measurement method, but the field measurement method has the disadvantages of low efficiency, long cycle, high material consumption, and poor regularity.

[0004] The invention discloses a method for predicting the ultimate vertical bearing capacity of a pile foundation with the publication number CN113821863A, which includes the steps of step S1, establishing an initial population; step S2, using the fitness function of the improved radial movement algorithm to calculate the fitness value of each individual in the initial population, determining the contemporary optimal position by comparing the fitness values ​​of the individuals one by one, and defining it as the initial center position; step S3, using the update condition to generate a new pre-position point within the range of ±0.5(xjmax-xjmin)wk of the k-th generation center position, calculating the fitness value, and updating the position information; step S4, determining the global optimal position, and calculating the fitness value of the global optimal position; step S5, assigning the weight and threshold of the global optimal position to a BP neural network model, training and simulating the BP neural network model to obtain the optimal prediction value of the ultimate vertical bearing capacity of the pile foundation; the disclosed method is not comprehensive enough for the calculation of the pile foundation, and the established data model is not accurate enough. Summary of the invention

[0005] The technical problem to be solved by the present invention is that the calculation of the ultimate bearing capacity of the pile foundation is not comprehensive enough and the established data model is not accurate enough.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0007] Firstly, a method for determining the ultimate compressive bearing capacity of a distributed post-grouting pile foundation is provided, which is characterized by comprising the following steps:

[0008] S1. According to the pile foundation construction data, the structural dimensions and material parameters of the pile foundation, the physical and mechanical parameters of the soil, the grouting amount and the grouting material parameters are obtained;

[0009] S2, according to the pile foundation structure size in S1, establish a three-dimensional finite element model and perform meshing;

[0010] S3, select soil constitutive model;

[0011] S4, input the physical and mechanical parameters of the soil in S1 into the soil constitutive model in S3;

[0012] S5. Determine pile-soil interface parameters, pile end bearing capacity and pile end spring stiffness according to the physical and mechanical parameters of the pile foundation and soil provided in S1;

[0013] S6. According to the pile foundation loading process, set the finite element model calculation steps;

[0014] S7. Calculate and analyze the finite element model to obtain the QS curve of pile top settlement and load, and calculate the axial force distribution of the pile body;

[0015] S8. Determine the ultimate bearing capacity of the distributed post-grouting pile foundation based on the QS curve calculated in S7 and the conditions for determining the ultimate bearing capacity of the pile foundation.

[0016] Furthermore, the three-dimensional finite element model includes a soil model, a pile foundation model, a pile-soil contact effect, and a grouting body model; the soil model is simulated using a hardened soil (small strain stiffness) constitutive model, the normal directions around the soil model are constrained, and the X, Y, and Z directions of the bottom of the soil model are constrained; the pile foundation model is simulated using a beam unit, and the beam unit constrains the Z-direction rotation; the pile-soil contact effect is simulated using an interface unit; the grouting body is simplified to a grouting sphere, and a solid unit is used. The calculation formula for the grouting sphere radius is:

[0017]

[0018] Where, T is the grouting volume; N is the number of grouting balls; r is the radius of the grouting balls; ρ is the grouting density.

[0019] Furthermore, the soil constitutive model is a hardened soil (small strain stiffness) constitutive model, and the mechanical parameters of the hardened soil constitutive model include soil strength parameters, deformation parameters and small strain parameters.

[0020] Furthermore, the soil strength parameters include effective cohesion c, effective internal friction angle Shear dilatancy angle ψ and failure ratio R f , the dilatancy angle ψ can be calculated by the following formula:

[0021]

[0022] Where, ψ is the soil dilatancy angle; is the effective internal friction angle of soil;

[0023] The destruction ratio R f It is related to the soil porosity ratio e. Based on indoor test data, the relationship between the damage ratio and the porosity ratio is:

[0024]

[0025] Among them, R f is the destruction ratio of the soil, and e is the porosity ratio of the soil.

[0026] Furthermore, the deformation parameters include the reference stress p ref , the tangent compression modulus corresponding to the reference stress Secant modulus corresponding to 0.5 times the deviatoric stress under reference stress Unloading and reloading modulus at reference stress Normal consolidation static lateral pressure coefficient Stiffness stress level related power exponent m;

[0027] The normal consolidation static lateral pressure coefficient The calculation formula is:

[0028]

[0029] in, is the normal consolidation static lateral pressure coefficient, is the effective internal friction angle of soil;

[0030] The value of the stiffness stress level related power exponent m is related to the soil properties, reflecting the sensitivity of the soil modulus to stress changes. For sandy soil (including sandy silt), the stiffness stress level related power exponent m is 0.7, and for clay soil (including clay silt), the stiffness stress level related power exponent m is 0.65.

[0031] Furthermore, the small strain parameter includes a small strain reference initial shear modulus The shear strain γ corresponding to the decay of the secant shear modulus to 0.7 times the initial shear modulus 0.7 ; The small strain parameters include the small strain reference initial shear modulus It can be determined by the following empirical formula:

[0032]

[0033] in, is the small strain reference initial shear modulus, e is the soil void ratio, and OCR is the overconsolidation ratio;

[0034] The shear strain γ corresponding to the decay of the secant shear modulus to 0.7 times the initial shear modulus 0.7In clay soil (including clayey silt), the range is (2.4~4.2)×10 -4 , with an average value of 3.2×10 -4 , in sandy soil (including sandy silt) the range is (2.9~5.2)×10 -4 , with an average value of 3.9×10 -4 .

[0035] Furthermore, the pile-soil contact interface unit parameters include the standard value of the pile body ultimate lateral friction resistance, the shear stiffness modulus k t , normal stiffness modulus k n The standard value of the ultimate lateral friction resistance of the pile body can be obtained according to the type of soil according to Appendix 5.3.5-1 in the Technical Specifications for Building Pile Foundations (JGJ94-2008). The shear stiffness modulus k t Take 10 times the elastic modulus of the soil, the normal stiffness modulus k n The standard value of the ultimate bearing capacity of the pile end is 100 times the elastic modulus of the soil. The standard value of the ultimate bearing capacity of the pile end can be taken according to the type of soil according to Appendix 5.3.5-2 in the Technical Specifications for Building Pile Foundations (JGJ94-2008). The spring stiffness of the pile end is the elastic resistance coefficient of the soil.

[0036] Furthermore, the finite element model calculation step includes:

[0037] S1, activate the soil, apply gravity and soil constraints, and then clear the displacement;

[0038] S2, activate the pile foundation, interface unit, pile end unit, grouting ball, and apply rotation constraint to the pile foundation in the Z direction;

[0039] S3, perform displacement clearing operation;

[0040] S4, loading at the pile top, the loading is divided into n increments, and then submitted for calculation and analysis. The more increments, the higher the calculation accuracy.

[0041] Furthermore, the conditions for determining the ultimate bearing capacity of the pile foundation are as follows:

[0042] It is determined according to the characteristics of pile top settlement changing with load, that is, the characteristics of QS curve. For the steep drop QS curve, the ultimate bearing capacity of the pile foundation is the load value corresponding to the starting point of the obvious steep drop.

[0043] When the load-settlement value of a certain level of the QS curve is greater than twice the load-settlement value of the previous level, the ultimate bearing capacity of the pile foundation shall be the load value of the previous level;

[0044] For the slowly varying QS curve, it can be determined according to the settlement. When the pile foundation diameter D is less than 800mm, the ultimate bearing capacity of the pile foundation is the load value corresponding to the settlement of 40mm; when the pile foundation diameter D is greater than or equal to 800mm, the ultimate bearing capacity of the pile foundation is the load value corresponding to the settlement of 0.05D.

[0045] Furthermore, the ultimate bearing capacity of the pile foundation must be within the compressive strength of the pile foundation itself.

[0046] The present invention has the following beneficial effects:

[0047] First, the present invention establishes a distributed post-grouting pile foundation loading calculation model, which can not only simulate the pile foundation loading process and the interaction between the pile foundation, the grouting body and the soil, but also calculate the QS curve and the pile body axial force;

[0048] 2. The present invention has the advantages of adjustable calculation accuracy, fast calculation convergence speed, high calculation efficiency, simple QS curve acquisition process, and accurate calculation results. It can also comprehensively and accurately reflect the mechanical response of distributed post-grouting pile foundations, and has a certain guiding role in the optimization of distributed post-grouting schemes and the economic rationality design of pile foundations. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0050] Figure 1 Flow chart for calculation of ultimate bearing capacity of post-grouting pile foundation;

[0051] Figure 2 Soil model diagram;

[0052] Figure 3 Post-grouting pile foundation model diagram;

[0053] Figure 4 Comparison chart of pile foundation QS curve calculation results and test results;

[0054] Figure 5 Pile axial force calculation curve. DETAILED DESCRIPTION

[0055] In order to better explain the present invention and facilitate understanding, the present invention is described in detail below through specific implementation modes in conjunction with the accompanying drawings.

[0056] It should be noted that all directional indications in the embodiments of the present invention are only used to explain the relative position relationship, movement status, etc. between various components in a certain specific posture. If the specific posture changes, the directional indication will also change accordingly.

[0057] In addition, in the present invention, descriptions such as "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" or "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0058] In the present invention, unless otherwise clearly specified and limited, the terms "connection", "fixation", etc. should be understood in a broad sense. For example, "fixation" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0059] The invention provides a method for determining the ultimate compressive bearing capacity of a distributed post-grouting pile foundation.

[0060] Embodiment 1:

[0061] Taking the distributed post-grouting pile foundation loading model test as an example, the location of the pile foundation post-grouting is as follows: Figure 3 As shown, the grouting positions are 0.1m, 0.5m, 0.9m, and 1.3m of the pile foundation, the grouting amount is 8kg, the material is C30 concrete, the diameter of the test pile foundation is 0.05m, the length is 1.6m, the material is C35 concrete, the model test soil is fine sand, and the physical parameters of the fine sand are shown in the following table:

[0062]

[0063] like Figure 1 As shown, the method for calculating the ultimate bearing capacity of the distributed post-grouting pile foundation of this embodiment includes the following steps:

[0064] S1. According to the post-grouting pile foundation loading model test data, the structural dimensions and material parameters of the pile foundation, the physical and mechanical parameters of the soil, the grouting amount and the grouting material parameters are obtained;

[0065] S2, establish a pile-soil finite element model and perform mesh division. The pile-soil finite element model is as follows: Figure 1 and Figure 2 As shown;

[0066] The pile-soil finite element model includes a soil model, a pile foundation model, a pile-soil contact effect, and a grouting body model.

[0067] The pile foundation model is simulated by beam elements, and the beam elements are constrained in Z-direction rotation;

[0068] The soil model has a size of 3.6m×3.6m×3.4m (X×Y×Z). Normal constraints are applied on all sides of the soil, and X, Y, and Z constraints are applied on the bottom.

[0069] The contact friction between the pile and the soil is simulated by using interface unit;

[0070] The grouting body is simulated by solid unit, and the grouting shape is simplified to a grouting sphere. The calculation formula of the grouting sphere radius is:

[0071]

[0072] Among them, the grouting volume T is 8 kg; the number of grouting balls N is 4; the radius of the grouting ball r is 0.058 m; the grouting density ρ is 2.5×10 3 kg / m 3 .

[0073] S3. Select a soil constitutive model, which is a hardened soil (small strain stiffness) constitutive model.

[0074] The mechanical parameters of the hardened soil (small strain stiffness) constitutive model include soil strength parameters, deformation parameters and small strain parameters.

[0075] The soil strength parameters include cohesion c, internal friction angle Shear dilatancy angle ψ and failure ratio R f , the dilatancy angle ψ can be calculated by the following formula:

[0076]

[0077] Among them, the shear dilatation angle ψ of fine sand is 1.5°; the internal friction angle of fine sand is It is 31.5°.

[0078] The destruction ratio R f It is related to the soil porosity ratio e. Based on indoor test data, the relationship between the destruction ratio and the porosity ratio is:

[0079]

[0080] Among them, the porosity ratio e of fine sand is 0.75, and the damage ratio R of fine sand is f is 0.95.

[0081] The deformation parameters include the reference stress p ref , the tangent compression modulus corresponding to the reference stress Secant modulus corresponding to 0.5 times the deviatoric stress under reference stress Unloading and reloading modulus at reference stress Normal consolidation static lateral pressure coefficient Stiffness stress level dependent power exponent m.

[0082] Conventional geotechnical engineering investigation reports usually only provide the compression modulus E corresponding to the consolidation test of 100-200 kPa. s1-2 , With E s1-2 The relationship is shown in the following table:

[0083] Typical soil Experience value of

[0084]

[0085] The normal consolidation static lateral pressure coefficient The calculation formula is:

[0086]

[0087] The effective internal friction angle of fine sand is is 31.5°, normal consolidation static lateral pressure coefficient It is 0.48.

[0088] The value of the stiffness stress level-related power exponent m is related to the soil properties and reflects the sensitivity of the soil modulus to stress changes. For sandy soil (including sandy silt), the stiffness stress level-related power exponent m is 0.7, and for clay soil (including clay silt), the stiffness stress level-related power exponent m is 0.65. Therefore, the stiffness stress level-related power exponent m of the fine sand in this case is 0.7.

[0089] The small strain parameters include the small strain reference initial shear modulus The shear strain γ corresponding to the decay of the secant shear modulus to 0.7 times the initial shear modulus 0.7 The small strain parameters include the small strain reference initial shear modulus It can be determined by the following empirical formula:

[0090]

[0091] Among them, the small strain refers to the initial shear modulus It is 93.2MPa, the soil porosity ratio e is 0.75, and the overconsolidation ratio OCR is 1.0.

[0092] The shear strain γ corresponding to the decay of the secant shear modulus to 0.7 times the initial shear modulus 0.7 In clay soil (including clayey silt), the range is (2.4~4.2)×10 -4 MPa, with an average value of 3.2×10 -4 MPa, and in sandy soil (including sandy silt) the range is (2.9~5.2)×10 -4 MPa, with an average value of 3.9×10 -4 MPa. Therefore, the shear strain of fine sand in this case is γ 0.7 Take 3.9×10 -4 MPa.

[0093] S4, inputting the constitutive model parameters of hardened soil (small strain stiffness) obtained by converting the physical and mechanical parameters of the test soil in S1 into the soil constitutive model;

[0094] S5. Determine the pile-soil interface parameters, the ultimate end resistance of the pile foundation, and the spring stiffness of the pile end according to the physical and mechanical parameters of the pile foundation and the soil provided in S1;

[0095] The pile-soil contact interface unit parameters include the ultimate lateral friction of the pile foundation, the shear stiffness modulus k t , normal stiffness modulus k n The ultimate lateral friction resistance of the pile foundation is 55 kPa according to the Appendix 5.3.5-1 of the Technical Specifications for Building Pile Foundations (JGJ94-2008). The shear stiffness modulus k t Take 720MPa, normal stiffness modulus k n Take 7200Mpa; the ultimate end resistance of the pile foundation is 1800kPa according to Appendix 5.3.5-2 in the Technical Specifications for Building Pile Foundations (JGJ94-2008); the elastic resistance coefficient of the soil body is 10MPa / m for the spring stiffness of the pile end.

[0096] S6. According to the pile foundation loading model test process, the pile foundation loading finite element model calculation steps are set.

[0097] The pile foundation loading finite element model calculation steps include:

[0098] Step 1: Activate the soil, apply gravity and soil constraints, calculate the initial ground stress, and then reset the displacement to keep the ground stress in a balanced state;

[0099] Step 2: Activate the pile foundation, interface unit, pile end unit, and grouting ball, and apply rotation constraint to the pile foundation in the Z direction;

[0100] Step 3: Perform a displacement zeroing operation to prevent the displacement calculated in step 2 from affecting subsequent displacement calculation results;

[0101] Step 4: Apply a load of 9.8 kN to the top of the pile. The loading is divided into 10 incremental steps. Set the calculation output of each step and then submit it for calculation analysis.

[0102] The more incremental steps there are, the higher the calculation accuracy will be. The corresponding incremental steps can be set according to the requirements of calculation accuracy.

[0103] S7. According to the calculation results of the pile foundation loading finite element model, the relationship curve between pile top settlement and load is obtained, also known as the QS curve, such as Figure 4 shown.

[0104] Figure 4 This is a QS comparison curve of the calculation results and the test results of this embodiment. It can be seen that the change trends of the calculation results and the test results curves are basically consistent, and the degree of agreement between the two is high, which verifies the accuracy and reliability of the calculation method provided by the present invention.

[0105] The present invention can also obtain a variation curve of the pile shaft axial force, such as Figure 5 As shown in the figure, the axial force variation curve of the pile body can clarify the bearing mechanism of the distributed post-grouting pile foundation and reveal the load transfer law, so as to promote the theoretical research on the bearing capacity of the distributed post-grouting pile foundation.

[0106] S8. Determine the ultimate bearing capacity of the distributed post-grouting pile foundation based on the QS curve calculated in S7 and the conditions for determining the ultimate bearing capacity of the pile foundation.

[0107] The conditions for determining the ultimate bearing capacity of pile foundation are as follows:

[0108] (1) For the steep drop QS curve, the ultimate bearing capacity of the pile foundation is taken as the load value corresponding to the starting point of the obvious steep drop.

[0109] (2) When the load-settlement value of a certain level of the QS curve is greater than twice the load-settlement value of the previous level, the ultimate bearing capacity of the pile foundation shall be the load value of the previous level.

[0110] (3) For the slowly varying QS curve, the ultimate bearing capacity of the pile foundation can be determined based on the settlement. When the pile foundation diameter D is less than 800 mm, the ultimate bearing capacity of the pile foundation is the load value corresponding to a settlement of 40 mm. When the pile foundation diameter D is greater than or equal to 800 mm, the ultimate bearing capacity of the pile foundation is the load value corresponding to a settlement of 0.05 D.

[0111] The ultimate bearing capacity of the pile foundation under the above conditions must be within the compressive strength of the pile foundation itself.

[0112] from Figure 4The QS curve characteristic is a steep drop curve, so the ultimate bearing capacity of the test pile foundation in this case is 8.82kN.

[0113] It should be understood that the above description of the specific embodiments of the present invention is only for illustrating the technical route and features of the present invention, and its purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly, but the present invention is not limited to the above specific implementation methods. Any changes or modifications made within the scope of the claims of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for determining the ultimate compressive bearing capacity of a distributed post-grouting pile foundation, characterized in that: The steps include: S1. According to the pile foundation construction data, the structural dimensions and material parameters of the pile foundation, the physical and mechanical parameters of the soil, the grouting amount and the grouting material parameters are obtained; S2, according to the pile foundation structure size in S1, establish a three-dimensional finite element model and perform meshing; S3, select soil constitutive model; S4, input the physical and mechanical parameters of the soil in S1 into the soil constitutive model in S3; S5. Determine pile-soil interface parameters, pile end bearing capacity and pile end spring stiffness according to the physical and mechanical parameters of the pile foundation and soil provided in S1; S6. According to the pile foundation loading process, set the finite element model calculation steps; S7. Calculate and analyze the finite element model to obtain the QS curve of pile top settlement and load, and calculate the axial force distribution of the pile body; S8. Determine the ultimate bearing capacity of the distributed post-grouting pile foundation based on the QS curve calculated in S7 and the conditions for determining the ultimate bearing capacity of the pile foundation.

2. A method for determining the ultimate compressive bearing capacity of a distributed post-grouting pile foundation according to claim 1, characterized in that: The three-dimensional finite element model includes a soil model, a pile foundation model, a pile-soil contact effect, and a grouting body model; the soil model is simulated using a hardened soil constitutive model, the normal directions around the soil model are constrained, and the X, Y, and Z directions of the bottom of the soil model are constrained. The pile foundation model is simulated using a beam unit, and the beam unit constrains the Z-direction rotation. The pile-soil contact effect is simulated using an interface unit. The grouting body is simplified to a grouting sphere, and a solid unit is used. The calculation formula for the grouting sphere radius is: Where, T is the grouting volume; N is the number of grouting balls; r is the radius of the grouting balls; ρ is the grouting density.

3. The method for determining the ultimate compressive bearing capacity of a distributed post-grouting pile foundation according to claim 1, characterized in that: The soil constitutive model is a hardened soil constitutive model, and the mechanical parameters of the hardened soil constitutive model include soil strength parameters, deformation parameters and small strain parameters.

4. A method for determining the ultimate compressive bearing capacity of a distributed post-grouting pile foundation according to claim 3, characterized in that: The soil strength parameters include effective cohesion c, effective internal friction angle Shear dilatancy angle ψ and failure ratio R f , the dilatancy angle ψ can be calculated by the following formula: Where, ψ is the soil dilatancy angle; is the effective internal friction angle of soil; The destruction ratio R f It is related to the soil porosity ratio e. Based on indoor test data, the relationship between the damage ratio and the porosity ratio is: Among them, R f is the destruction ratio of the soil, and e is the porosity ratio of the soil.

5. The method for determining the ultimate compressive bearing capacity of a distributed post-grouting pile foundation according to claim 3 is characterized in that: The deformation parameters include the reference stress p ref , the tangent compression modulus corresponding to the reference stress Secant modulus corresponding to 0.5 times the deviatoric stress under reference stress Unloading and reloading modulus at reference stress Normal consolidation static lateral pressure coefficient Stiffness stress level related power exponent m; The normal consolidation static lateral pressure coefficient The calculation formula is: in, is the normal consolidation static lateral pressure coefficient, is the effective internal friction angle of soil; The value of the stiffness stress level related power exponent m is related to the soil properties and reflects the sensitivity of the soil modulus to stress changes. For sandy soil, the stiffness stress level related power exponent m is 0.7, and for clay soil, the stiffness stress level related power exponent m is 0.

65.

6. A method for determining the ultimate compressive bearing capacity of a distributed post-grouting pile foundation according to claim 3, characterized in that: The small strain parameters include the small strain reference initial shear modulus The shear strain γ corresponding to the decay of the secant shear modulus to 0.7 times the initial shear modulus 0.7 ; The small strain parameters include the small strain reference initial shear modulus It can be determined by the following empirical formula: in, is the small strain reference initial shear modulus, e is the soil void ratio, and OCR is the overconsolidation ratio; The shear strain γ corresponding to the decay of the secant shear modulus to 0.7 times the initial shear modulus 0.7 In clay soil, the range is 2.4~4.2×10 -4 , with an average value of 3.2×10 -4 , in sandy soil, the range is 2.9~5.2×10 -4 , with an average value of 3.9×10 -4 .

7. The method for determining the ultimate compressive bearing capacity of a distributed post-grouting pile foundation according to claim 1, characterized in that: The pile-soil contact interface unit parameters include the standard value of the pile body's ultimate lateral friction resistance, the shear stiffness modulus k t , normal stiffness modulus k n The standard value of the ultimate lateral friction resistance of the pile body can be obtained according to the type of soil according to Appendix 5.3.5-1 in the Technical Specifications for Building Pile Foundations (JGJ94-2008). The shear stiffness modulus k t Take 10 times the elastic modulus of the soil, the normal stiffness modulus k n The standard value of the ultimate bearing capacity of the pile end is 100 times the elastic modulus of the soil. The standard value of the ultimate bearing capacity of the pile end can be taken according to the type of soil in accordance with Appendix 5.3.5-2 of the Technical Specifications for Building Pile Foundations (JGJ94-2008). The spring stiffness of the pile end is the elastic resistance coefficient of the soil.

8. The method for determining the ultimate compressive bearing capacity of a distributed post-grouting pile foundation according to claim 1, characterized in that: The finite element model calculation step comprises: S1, activate the soil, apply gravity and soil constraints, and then clear the displacement; S2, activate the pile foundation, interface unit, pile end unit, grouting ball, and apply rotation constraint to the pile foundation in the Z direction; S3, perform displacement clearing operation; S4, loading at the pile top, the loading is divided into n increments, and then submitted for calculation and analysis. The more increments, the higher the calculation accuracy.

9. The method for determining the ultimate compressive bearing capacity of a distributed post-grouting pile foundation according to claim 1, characterized in that: The conditions for determining the ultimate bearing capacity of pile foundation are as follows: It is determined according to the characteristics of pile top settlement changing with load, that is, the characteristics of QS curve. For the steep drop QS curve, the ultimate bearing capacity of the pile foundation is the load value corresponding to the starting point of the obvious steep drop. When the load-settlement value of a certain level of the QS curve is greater than twice the load-settlement value of the previous level, the ultimate bearing capacity of the pile foundation shall be the load value of the previous level; For the slowly varying QS curve, it can be determined according to the settlement. When the pile foundation diameter D is less than 800mm, the ultimate bearing capacity of the pile foundation is the load value corresponding to the settlement of 40mm; when the pile foundation diameter D is greater than or equal to 800mm, the ultimate bearing capacity of the pile foundation is the load value corresponding to the settlement of 0.05D.

10. A method for determining the ultimate compressive bearing capacity of a distributed post-grouting pile foundation according to claim 9, characterized in that: The ultimate bearing capacity of the pile foundation must be within the compressive strength of the pile foundation itself.

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