A method for predicting soil weakening and pile slippage during offshore super-large pile driving

Abstract

The present invention relates to the field of data ocean engineering, and discloses a method for predicting soil weakening and pile slipping during piling of super-large piles at sea, comprising the following steps: S10, preparation stage: preparing to select marine silty clay as the test soil, putting into a fully automatic cyclic direct shear apparatus as a test device, and making steel plates with different surface roughnesses to simulate different pile-soil interface conditions; S20, test setting: first setting different initial normal stresses, and planning multiple groups of tests, each group of tests setting different interface test data, and performing test operations on the fully automatic cyclic direct shear apparatus and recording the shear characteristics of the pile and soil interface under cyclic action. By accurately simulating and predicting the behavior of the soil under dynamic loading, the method can effectively predict and control the weakening of soil strength and the phenomenon of pile slipping, which not only optimizes the use of resources and reduces construction risks, but also improves the economic benefits and engineering quality of the entire construction process, and significantly improves the safety and efficiency of the project.

Description

Technical Field

[0001] The invention relates to the technical field of data ocean engineering, and in particular to a method for predicting soil weakening and pile slippage during offshore super-large pile driving. Background Art

[0002] As a clean energy, offshore wind power has significant social and economic benefits, and it is of great significance to actively and rationally develop offshore wind power. Pile foundation is the most widely used foundation form in the currently operating offshore wind farms. At present, offshore wind power pile foundation has the characteristics of large diameter, extra-long and deep penetration. The diameter is usually not less than 5m, and the penetration depth under the seabed is often close to or even exceeds 100m. This type of offshore pile foundation can be classified as large-diameter extra-long pile.

[0003] In marine pile foundation projects, pile foundations are usually installed by hammering or vibration sinking. However, long-term dynamic pile sinking will cause damage to the soil structure around the pile and generate excess pore water pressure, which will weaken the soil strength and significantly reduce the friction resistance on the pile side. As the quality of the pile body and supporting construction equipment continue to develop towards large-scale, the difficulty of controlling the pile driving process has increased, and pile slippage has occurred from time to time. Therefore, studying the mechanism of dynamic pile sinking, revealing the weakening of soil strength during pile sinking, and accurately analyzing the driveability of piles are of great significance to ensure the safe and efficient completion of the project.

[0004] At present, there are some engineering empirical formulas for the commonly used methods of reducing the strength of soil around small-diameter piles (less than 2m in diameter). When applied to super-large diameter pile foundation projects, there are obvious errors and they cannot meet the requirements of safety and efficiency of actual projects. This is mainly because the existing empirical formulas only consider the influence of a single factor of the soil itself. However, pile driving itself is a dynamic process with pile-hammer-soil as a whole. The influence of piles and hammers can be ignored in small-diameter pile foundation projects. For super-large diameter pile foundation projects, the influence of piles and hammers cannot be ignored. However, there are still few studies on super-large diameter piles that consider multiple factors affecting the weakening of soil strength around piles and pile slippage, and a complete prediction method has not yet been formed. Summary of the invention

[0005] In view of the deficiencies of the prior art, the present invention provides a method for predicting soil weakening and pile slippage during offshore super-large pile driving, which solves the problem that the prior art does not consider the influence of multiple factors enough.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solutions: A method for predicting soil weakening and pile slippage during offshore super-large pile driving, comprising the following steps:

[0007] S10, preparation stage: prepare to select marine silty clay as the test soil, use a fully automatic cyclic direct shear apparatus as the test device, and make steel plates with different surface roughness to simulate different pile-soil interface conditions;

[0008] S20, test setup: first set different initial normal stresses, and plan multiple groups of tests, set different interface test data for each group of tests, and perform test operations on a fully automatic cyclic direct shear apparatus and record the shear characteristics of the pile-soil interface under cyclic action;

[0009] S30, data collection and analysis: by collecting shear displacement-shear stress curves and observing the behavior of soil under different cycle numbers as data basis, the influencing factors are analyzed;

[0010] S40, Result prediction: Compare the test results with the on-site piling records to verify the practicality and accuracy of the test, and formulate a method for calculating pile-hammer-soil βs with reference to the test data and back-analysis results;

[0011] S50, Project implementation: Use the prediction method formula to evaluate the piling process of the actual project, and adjust the piling plan according to the prediction results to optimize the project design;

[0012] S60, Continuous cycle: Evaluate the results after implementation to ensure the adaptability and accuracy of the method, and continuously improve and optimize the method based on feedback from engineering practice.

[0013] Preferably, the initial normal stress in step S20 includes 100 kPa, 150 kPa, 200 kPa, 250 kPa, and 300 kPa.

[0014] Preferably, the test in step S20 includes three groups, including interface roughness including smooth, relatively rough, and rough options.

[0015] Preferably, in step S30, the number of cycles, initial normal stress, interface roughness, soil physical parameters, relationship between shear displacement and shear stress, and hysteresis loop characteristics obtained through the test in step S20 are analyzed for influencing factors.

[0016] Preferably, the pile-hammer-soil βs calculation method in step S40 is:

[0017] β s =α(0.1-0.26lnμ+3×10 -4 σ ′ +4σ ′2 )δ

[0018]

[0019] Among them, α is the soil correction factor, which takes a value of 1 to 2; μ is the friction coefficient of dirty soil; σ′ is the effective overburden pressure at the calculation point; K0 is the lateral earth pressure coefficient; Z i is the buried depth of the i-th soil layer; N is the total number of hammer blows when the pile penetrates to the designed depth; H is the final penetration depth of the pile body; N i is the number of hammer blows required for the pile to penetrate the i-th layer of soil; δ is the difficulty coefficient of pile penetration, which is related to the penetration depth, the number of hammer blows, and the weight of the pile and hammer, and ranges from 0 to 1.

[0020] Preferably, the back analysis in step S40 is performed based on the influence of factors such as initial normal stress, number of reciprocating cycles, interface roughness, etc. on the weakening of soil strength during the piling process.

[0021] The present invention provides a method for predicting soil weakening and pile slippage during offshore super-large pile driving. It has the following beneficial effects:

[0022] The present invention can effectively predict and control soil strength weakening and pile slippage by accurately simulating and predicting the behavior of soil under dynamic loading, thereby providing reliable data support for engineering design. It not only optimizes the use of resources and reduces construction risks, but also improves the economic benefits and engineering quality of the entire construction process, and significantly improves the safety and efficiency of the project. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of the process of the present invention;

[0024] Figure 2 is a cyclic shear displacement-shear stress curve diagram of the present invention;

[0025] Figure 3 It is a schematic diagram of the influence of normal stress of the present invention;

[0026] Figure 4 It is a schematic diagram of the relationship curve between βs and normal stress of the present invention;

[0027] Figure 5 It is a schematic diagram of the effect of interface roughness of the present invention;

[0028] Figure 6 It is a schematic diagram showing the influence of soil properties and pile penetration depth of the present invention;

[0029] Figure 7 It is a schematic diagram of the pile body quality influence of the present invention;

[0030] Figure 8 It is a schematic diagram of the impact of hammer energy of the present invention;

[0031] Fig. 9 It is a schematic diagram for comparing the measured piling records of the present invention. DETAILED DESCRIPTION

[0032] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0033] Example:

[0034] A method for predicting soil weakening and pile slippage during offshore super-large pile driving, comprising the following steps:

[0035] S10, preparation stage: prepare to select marine silty clay as the test soil, use a fully automatic cyclic direct shear apparatus as the test device, and make steel plates with different surface roughness to simulate different pile-soil interface conditions;

[0036] S20, test setup: first set different initial normal stresses, and plan multiple groups of tests, each group of tests set different interface test data, and perform test operations on the fully automatic cyclic direct shear instrument and record the shear characteristics of the pile and soil interface under cyclic action; the initial normal stresses in step S20 include 100kPa, 150kPa, 200kPa, 250kPa, and 300kPa. The test in step S20 includes three groups, including interface roughness including smooth, relatively rough, and rough options.

[0037] S30, data collection and analysis: by collecting shear displacement-shear stress curves and observing the behavior of the soil at different numbers of cycles as the data basis, the influencing factors are analyzed; in step S30, the number of cycles, initial normal stress, interface roughness, soil physical parameters, the relationship between shear displacement and shear stress, and hysteresis loop characteristics obtained through the test in step S20 are used to analyze the influencing factors.

[0038] S40, result prediction: Compare the test results with the on-site piling records to verify the practicality and accuracy of the test. Refer to the test data and back-analysis results to develop a method for calculating the pile-hammer-soil βs. The calculation method for the pile-hammer-soil βs in step S40 is:

[0039] β s =α(0.1-0.26lnμ+3×10 -4 σ ′ +4σ ′2 )δ

[0040]

[0041] Among them, α is the soil correction factor, which takes a value of 1 to 2; μ is the friction coefficient of dirty soil; σ′ is the effective overburden pressure at the calculation point; K0 is the lateral earth pressure coefficient; Z i is the buried depth of the i-th soil layer; N is the total number of hammer blows when the pile penetrates to the designed depth; H is the final penetration depth of the pile body; N i is the number of hammer blows required for the pile to penetrate the i-th soil layer; δ is the difficulty coefficient of pile penetration, which is related to the penetration depth, the number of hammer blows, and the weight of the pile and hammer, and has a value range of 0 to 1. In step S40, the back analysis is conducted through the initial normal stress, the number of reciprocating cycles, the interface roughness and other factors that affect the weakening of soil strength during the piling process.

[0042] S50, Project implementation: Use the prediction method formula to evaluate the piling process of the actual project, and adjust the piling plan according to the prediction results to optimize the project design;

[0043] S60, Continuous cycle: Evaluate the results after implementation to ensure the adaptability and accuracy of the method, and continuously improve and optimize the method based on feedback from engineering practice.

[0044] The specific experimental operation steps are: study the shear characteristics of the pile-soil interface under cyclic action, use a fully automatic cyclic direct shear apparatus, study the effects of initial normal stress, number of reciprocating cycles, interface roughness and other factors on the weakening of soil strength during piling, and select marine silty clay. The physical parameters of the soil used are as follows:

[0045]

[0046] In order to study the influence of different interface roughness on soil weakening during shearing, three kinds of steel plates with smooth, relatively rough and rough surface roughness were customized respectively. Based on this, the influence of factors such as interface roughness, initial normal stress and number of reciprocating cycles on soil strength weakening during piling was studied. Multiple groups of cyclic direct shear tests were carried out. The table of the research test plan is as follows:

[0047]

[0048] Test results:

[0049] (1) Cyclic shear displacement-shear stress curve

[0050] According to the test plan, the cyclic shear displacement-shear stress curve under different conditions is obtained. Please refer to Figure 2From the table, it can be concluded that the cyclic shear test can effectively simulate the weakening of soil strength around the pile during the piling process. With the increase of the number of cyclic shear tests, the hysteresis loop gradually decreases, indicating that the shear stress continues to decrease during the cyclic shear process. At the same time, the gap between the hysteresis loops after each cycle also gradually decreases, indicating that the shear stress reduction caused by each cycle is gradually decreasing, and finally reaches a stable residual strength.

[0051] (2) Influence of initial normal stress

[0052] Compare the residual strength and βs of the soil after cyclic shearing under different normal stresses in each test group to study the influence of different pile penetration depths on the weakening of soil strength around the pile. For the cycle number-shear stress curve, please refer to Figure 3 , the shear stress gradually decreases with the increase of the number of cycles. The curves of each test group show a downward trend as a whole, but the downward trend gradually slows down and is approximately horizontal at the end of the curve, indicating that after the cyclic shearing action, the soil strength of each test group tends to be stable residual strength;

[0053] For the relationship between βs and normal stress, please refer to Figure 4 , it can be seen from the figure that with the increase of normal stress, βs continues to increase, indicating that the greater the normal stress, the lower the degree of soil strength weakening. The main reason for the analysis is that the greater the normal stress, the greater the overburden pressure of the soil, the denser the soil is compressed, and the tighter and more reliable the soil structure is. Therefore, in the reciprocating shear process, the soil structure is less likely to be damaged, which leads to a decrease in the degree of soil strength weakening.

[0054] (3) Influence of interface roughness

[0055] Compare the βs of different test groups under different interface roughness to study the effect of interface roughness on the weakening of soil strength around the pile during pile driving. Figure 5 It can be seen that under the same normal stress, the higher the interface roughness, the smaller the βs, which means that the rougher the interface, the higher the degree of soil strength weakening. The main reason for this is that the rougher the interface, the higher the degree of damage to the pile-soil interface during the cyclic shear process, the more serious the damage to the soil structure, and the greater the degree of soil strength weakening.

[0056] Summary judgment analysis

[0057] In actual piling projects, the weakening of the soil around the pile is closely related to the initial normal stress, interface roughness, and the number of reciprocating cycles. During the piling process, the strength of the soil around the pile will continue to decrease with the increase in the number of cycles and eventually stabilize to the residual strength. The greater the normal stress, the lower the weakening of the soil around the pile, and the higher the roughness of the pile-soil interface, the higher the weakening of the soil around the pile.

[0058] Then, the analysis of a large number of measured piling records shows that the difficulty of piling is not only related to the depth of the pile, the number of hammer strikes, the pile-soil interface and other factors, but also affected by the soil properties, pile quality, and hammer strike energy. It is the result of the combined effect of multiple factors. Therefore, the following reverse analysis of the piling records of actual projects is carried out to obtain the influence of different factors on the weakening of soil strength during piling.

[0059] (1) Influence of soil properties and pile penetration depth

[0060] In conjunction with the offshore wind farm pile foundation project, a pile driving back analysis was conducted on piles with different penetration depths at different machine positions in similar sites. The obtained βs values ​​and pile driving analysis results can be found in Figure 6 The βs obtained by back analysis can better reflect the degree of soil strength weakening during the pile sinking process. At the same depth, the strength weakening degree of cohesive soil is greater than that of non-cohesive soil. The deeper the soil layer is buried, the greater the βs is. At a depth close to the design depth of the pile, the soil strength basically does not weaken.

[0061] (2) Influence of pile quality

[0062] For comparison of pile driving records of different weights of piles driven by the same hammer at the same site, please refer to Figure 7 , the same as the indoor model test, the greater the pile mass, the greater the degree of soil strength weakening. Since the greater the pile mass, the greater the inertia force and impact force generated when penetrating the soil, the range and degree of disturbance to the soil around the pile also increase accordingly.

[0063] (3) Impact of hammer energy

[0064] For comparison of similar pile driving records of different pile driving hammers at the same site, please refer to Figure 8 ,During the piling process, the greater the hammering energy of the same soil layer, the greater the degree of soil strength weakening.

[0065] Summary: In sand, when the relative burial depth dre l (the ratio of soil depth to the final penetration depth of the pile body) ∈ (0.1~0.35), βs is the minimum of 0.4, when dre l∈ (0.35~0.8), βs is about 0.7, and when dre l>0.8, the soil strength is almost not weakened. In clay, when dre l∈ (0.1~0.3), βs≤0.3, even in the surface layer βs<0.1; when dre l∈ (0.35~0.8), βs≈0.5, when dre l>0.8, βs∈0.5~0.8.

[0066] Using the S40 step method, a pile driving analysis of an offshore wind power pile foundation was carried out and compared with the measured pile driving records. For the test comparison table, please refer to Fig. 9 , based on β s =α(0.1-0.26lnμ+3×10-4 σ ′ +4σ ′2 The pile driving analysis results of )δ are in good agreement with the on-site pile driving records. The result in Figure (b) simulates the occurrence of pile slippage at around 40m, demonstrating the universality of the above research results.

[0067] A pile analysis of 30 groups of single piles with a diameter greater than 5 m, including the above project, showed that β s =α(0.1-0.26lnμ+3×10-4σ′+4σ′2δ) The results are in good agreement with the actual engineering piling records.

[0068] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for predicting soil weakening and pile slippage during offshore super-large pile driving, characterized in that: The following steps are involved: S10, preparation stage: prepare to select marine silty clay as the test soil, use a fully automatic cyclic direct shear apparatus as the test device, and make steel plates with different surface roughness to simulate different pile-soil interface conditions; S20, test setup: first set different initial normal stresses, and plan multiple groups of tests, set different interface test data for each group of tests, and perform test operations on a fully automatic cyclic direct shear apparatus and record the shear characteristics of the pile-soil interface under cyclic action; S30, data collection and analysis: by collecting shear displacement-shear stress curves and observing the behavior of soil under different cycle numbers as data basis, the influencing factors are analyzed; S40, Result prediction: Compare the test results with the on-site piling records to verify the practicality and accuracy of the test, and formulate a method for calculating pile-hammer-soil βs with reference to the test data and back-analysis results; The pile-hammer-soil βs calculation method is: in, is the soil property correction factor, ranging from 1 to 2 is the pile-soil friction coefficient; is the effective overburden pressure at the calculation point; is the lateral earth pressure coefficient; For the The depth of soil layer; is the total number of hammer blows when the pile penetrates to the designed depth; is the final penetration depth of the pile; For pile penetration Number of hammer blows required for each soil layer; is the difficulty coefficient of pile penetration, which is related to the penetration depth, the number of hammer blows, and the weight of the pile and hammer, and has a value range of 0~1; S50, Project implementation: Use the prediction method formula to evaluate the piling process of the actual project, and adjust the piling plan according to the prediction results to optimize the project design; S60, Continuous cycle: Evaluate the results after implementation to ensure the adaptability and accuracy of the method, and continuously improve and optimize the method based on feedback from engineering practice.

2. A method for predicting soil weakening and pile slippage during offshore super-large pile driving according to claim 1, characterized in that: The initial normal stress in the step S20 includes 100 kPa, 150 kPa, 200 kPa, 250 kPa, and 300 kPa.

3. The method for predicting soil weakening and pile slippage during offshore super-large pile driving according to claim 1, characterized in that: The test in step S20 includes three groups, including interface roughness including smooth, relatively rough, and rough options.

4. The method for predicting soil weakening and pile slippage during offshore super-large pile driving according to claim 1, characterized in that: In the step S30, the number of cycles, initial normal stress, interface roughness, soil physical parameters, relationship between shear displacement and shear stress, and hysteresis loop characteristics obtained through the test in the step S20 are analyzed for influencing factors.

5. The method for predicting soil weakening and pile slippage during offshore super-large pile driving according to claim 1, characterized in that: The back analysis in step S40 is obtained by determining the influence of factors such as initial normal stress, number of reciprocating cycles, and interface roughness on the weakening of soil strength during the piling process.

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