Calculation method of over-consolidation ratio of clay based on pore pressure static cone penetration test

Abstract

The present application relates to the calculation method of the overconsolidation ratio of clay based on the pore pressure static sounding test, and belongs to the field of geotechnical parameter evaluation in geotechnological investigation. The static sounding test equipment is used to carry out the static sounding test on the soil sample, and the static sounding data is obtained; the overconsolidation ratio OCR of the soil is preliminarily calculated by using the CPTU prediction calculation formula, and a curve graph is drawn; the error analysis is carried out by using the root mean square deviation RMSD factor, the Nash efficiency coefficient NSE factor and the determination coefficient R 2 factor, the CPTU calculation OCR formula is modified, and the calculation of the overconsolidation ratio of clay based on the pore pressure static sounding test is completed; the fitting target value is set as the consolidation test value, the empirical coefficient in the empirical formula is changed, and finally the modified prediction model closer to the soil test result is obtained. By comparing the indoor consolidation test result, the evaluation is carried out based on different evaluation factors, the original formula is modified by using the planning solution method, and the accuracy of measuring the overconsolidation ratio is obviously improved.

Description

Technical Field

[0001] This invention relates to a method for calculating the overconsolidation ratio of cohesive soil based on pore pressure static cone penetration test, belonging to the field of geotechnical parameter evaluation in geotechnical engineering investigation. Background Technology

[0002] The overconsolidation ratio (OCR) of cohesive soil is one of the important mechanical parameters in offshore wind power engineering design. The OCR reflects the sedimentary history of the soil and is an important mechanical parameter in offshore geotechnical engineering design. Traditional geotechnical testing methods use standard consolidation tests to determine the preconsolidation pressure of soil. However, standard consolidation tests using the stress path method are time-consuming and the test results are easily affected by various factors, such as soil sample disturbance and the interpretation method of preconsolidation pressure.

[0003] The cone tip resistance, side friction, and pore water pressure provided by the pore pressure static cone penetration test (CPTU) can effectively reflect the overconsolidation ratio of soil with minimal disturbance to the soil sample. However, this method relies on CPTU interpretation to obtain the corresponding clay strength. Existing standards provide estimation formulas for the overconsolidation ratio, but in practical applications, these formulas are often dependent on regional experience. Furthermore, current research lacks comparisons between different calculation formulas, making it impossible to determine which formula should be used for a specific marine area. Using default empirical parameters can lead to significant errors in the calculation results.

[0004] Existing standards provide estimation formulas for overconsolidation ratios, but in practical applications, these formulas are often related to local experience. Furthermore, existing research lacks comparisons between different calculation formulas, making it impossible to determine which formula should be used for calculation in a particular sea area. Using default empirical parameters for calculations can lead to large errors in the results and poor accuracy, making it unsuitable for use in geotechnical engineering construction. Summary of the Invention

[0005] This invention interprets the parameters in the estimation formula based on actual site conditions, and optimizes the estimation formula to provide a basis and technical support for marine engineering projects in local sea areas. Therefore, it provides a method for calculating the overconsolidation ratio of cohesive soil based on pore pressure static cone penetration tests.

[0006] The present invention adopts the following technical solution:

[0007] This invention relates to a method for calculating the overconsolidation ratio of cohesive soil based on pore pressure static cone penetration tests, the steps of which are as follows:

[0008] (1) Static cone penetration tests were conducted on the soil samples using static penetration testing equipment to obtain static penetration data: pore pressure static cone tip resistance q c Side friction resistance f sPore ​​water pressure u2; The overconsolidation ratio (OCR) of the soil was obtained by conducting geotechnical consolidation tests on the soil samples, and the soil type was determined.

[0009] The total stress σ of the overlying soil is determined using the following formula. v Effective stress σ′ of the overlying layer v

[0010]

[0011]

[0012] Where γ i Let h be the natural unit weight of the i-th soil layer. i Let γ be the thickness of the i-th soil layer. w The specific weight of water; and the cone tip resistance q was determined by pore pressure static penetration testing. c The corrected cone tip resistance q is obtained t ;

[0013] q t =q c +u2(1-α)

[0014] In the formula, α is the probe area ratio coefficient;

[0015] (2) Define the parameter values ​​for the initial calculation. The parameter values ​​are the desired preconsolidation cone tip factors: k1, k2, k3, k4, b, m; calculate the overconsolidation ratio (OCR) of cohesive soil using the following formula:

[0016]

[0017]

[0018]

[0019]

[0020] Using the above CPTU prediction calculation formula, the soil overconsolidation ratio OCR is initially calculated. Combined with the soil consolidation test obtained in step (1), the soil overconsolidation ratio OCR is plotted.

[0021] (3) Select evaluation factors to analyze the prediction formula, evaluate the prediction accuracy and effectiveness of each initial formula, and determine the range of values ​​for the empirical parameters in the formula.

[0022] Using the root mean square deviation (RMSD) factor, Nash efficiency coefficient (NSE) factor, and coefficient of determination (R²) 2 Error analysis using factors is expressed by the following formula:

[0023] Root mean square deviation

[0024] Nash efficiency coefficient

[0025] Coefficient of determination

[0026] Correlation coefficient

[0027] In the formula x i The value is from the geotechnical test, y i These are the fitted values ​​of the empirical formula;

[0028] (4) By using EXCEL spreadsheet software to solve the evaluation factors in a better way, the CPTU calculation OCR formula is modified, and the modified estimated overconsolidation ratio relationship is obtained as follows. The evaluation factors before and after the modification are compared, and the calculation of the overconsolidation ratio of cohesive soil based on the pore pressure static penetration test is completed.

[0029]

[0030]

[0031]

[0032]

[0033] The above steps set the fitting target value as the consolidation test value, and by changing the empirical coefficients in the empirical formula, for example, making the NSE value tend to 1, a modified prediction model formula that is closer to the geotechnical test results is finally obtained.

[0034] Similarly, set RMSD, R 2 By using rxy cells and performing planning and solving, and changing different evaluation factors, the prediction formula can be modified to obtain different expected preconsolidation cone tip factors. By comparing the curves plotted with different expected preconsolidation cone tip factors, it can be concluded that, compared with the other three evaluation factors, the modified CPTU OCR calculation formula is closer to the actual values ​​of geotechnical tests by adjusting the evaluation factor NSE. Therefore, the evaluation factor NSE is preferred to optimize the formula, and other evaluation factors are included in the comparative study.

[0035] The evaluation principles in step (3) of the method for calculating the overconsolidation ratio of cohesive soil based on pore pressure static cone penetration test described in this invention are as follows:

[0036] The root mean square deviation (RMSD) factor ranges from (0, +∞).

[0037] The calculation result with the root mean square deviation (RMSD) factor close to 0 indicates that the empirical formula has a good fit, and vice versa.

[0038] Correlation coefficient r xy The factor's value range is (-1, 1).

[0039] If the correlation coefficient r xy A value close to 0 indicates that the empirical formula fit value has a low correlation with the geotechnical test report value, and the empirical formula has low reliability.

[0040] If r xy The calculation result is close to 1, indicating that the empirical formula fit value has a high correlation with the geotechnical test value, and the empirical formula has high reliability (R0). 2 Approaching 1);

[0041] The Nash efficiency coefficient (NSE factor) ranges from (-∞, 1).

[0042] If the Nash efficiency coefficient (NSE factor) is close to 1, the model has high credibility.

[0043] If the Nash efficiency coefficient (NSE) is close to 0, the empirical formula results are close to the average level of geotechnical tests, meaning the overall results are reliable, but the process simulation error may be too large.

[0044] The method for calculating the overconsolidation ratio of cohesive soil based on pore pressure static cone penetration test (CPTU) as described in this invention involves the following steps for correcting the CPTU calculation OCR formula using EXCEL spreadsheet software:

[0045] Import the data obtained in steps (1) and (2) into Excel;

[0046] 1) In the Excel add-ins, call the Solver add-in menu;

[0047] 2) In the top data pane of Excel, click Solver to construct the cell containing the objective function (evaluation factor functions k1, k2, k3, k4, b, m), and select Set the objective to the maximum value of 0;

[0048] 3) Select the variable cell and choose the cell containing the coefficient of the undetermined expected pre-consolidation cone tip factor;

[0049] 4) Based on the expected pre-consolidated cone tip factor coefficients k1, k2, k3, k4, b, m in step 2), set the initial constraints of the variable cells using default values;

[0050] 5) Set the solution method, adopt the nonlinear GRG solution method, and set it to use multi-initial-point optimization;

[0051] 6) Once the desired preconsolidated cone tip factor value is obtained, stop solving and record it as the first solution value;

[0052] 7) Repeat sub-steps 1 to 3, reduce the upper limit of the constraint in sub-step 4 by 0.05, for example, change the constraint k1 to 0 < k1 < 0.95, solve again, and record the second solution value;

[0053] 8) Repeat the above sub-steps, change the constraints according to sub-step 7), and solve the problem multiple times. When sub-step 7 is repeated for the nth time, the solution value of the preconsolidated cone tip factor is expected to be equal to the solution value when sub-step 7 is repeated for the (n-1)th time. At this time, the solution value of the preconsolidated cone tip factor is expected to be the corrected exact solution.

[0054] The method for calculating the overconsolidation ratio of cohesive soil based on pore pressure static cone penetration test described in this invention is evaluated using the Nash efficiency coefficient (NSE factor) and the root mean square deviation (RMSD factor) and coefficient of determination (R²). 2 Factors, relation numbers r xy To assist in the evaluation.

[0055] When the Nash efficiency coefficient (NSE) is close to 1, this method is the optimal method, and its calculation results are closest to the consolidation test values. Based on this, we can make a preliminary judgment on the prediction accuracy of each initial formula and a preliminary judgment on the reasonable range of values ​​for the empirical coefficients.

[0056] The overconsolidation ratio (OCR) of cohesive soil based on pore pressure static cone penetration test described in this invention is calculated as follows: In step (1), the overconsolidation ratio (OCR) of the soil is the sum of the preconsolidation pressure Pc and the overlying effective stress σ′. v The ratio, in which the preconsolidation pressure of the soil is measured by consolidation tests.

[0057] The method for calculating the overconsolidation ratio of cohesive soil based on pore pressure static cone penetration test described in this invention has the following default value settings: the initial constraints of the variable cells are: 0 < k1, k2, k3, m < 1; -1 < b < 1; 0 < k4 < 15; and the constraints are changed in each solution by reducing them by 0.05 based on the previous constraints.

[0058] Beneficial effects:

[0059] 1. The method for calculating the overconsolidation ratio of cohesive soil based on pore pressure static cone penetration test described in this invention is based on conventional pore pressure static cone penetration test. It can conveniently measure the overconsolidation ratio of cohesive soil using test data, which is convenient for practical engineering use. The research content and results have high application and reference value, and promote cost reduction and efficiency improvement of offshore wind power in the field of offshore geotechnical exploration.

[0060] 2. The overconsolidation ratio calculation method for cohesive soil based on pore pressure static cone penetration test described in this invention can accurately obtain the overconsolidation ratio of the soil by interpreting the parameters in the estimation formula according to the actual site conditions and using the modified CPTU calculation formula. Compared with geotechnical tests or the original CPTU calculation formula, the calculation accuracy is higher and the parameters are more accurate, which can provide a basis and technical support for marine engineering projects in local sea areas.

[0061] 3. The method for calculating the overconsolidation ratio of cohesive soil based on pore pressure static cone penetration test described in this invention compares the results of indoor consolidation tests, evaluates the results based on different evaluation factors, and corrects the original formula using a programming solution method, thereby significantly improving the accuracy of measuring the overconsolidation ratio. Attached Figure Description

[0062] Figure 1 This is a flowchart of the method of the present invention;

[0063] Figure 2 This is a schematic diagram illustrating the relationship between the overconsolidation ratio (OCR) and the penetration depth (h) of a coastal site according to an embodiment of the present invention.

[0064] Figure 3 This is a schematic diagram of the parameter settings for the Mayne (1990) method in an embodiment of the present invention.

[0065] Figure 4 The normalized cone tip resistance Q in this embodiment of the invention t A schematic diagram showing the relationship between penetration depth h and the penetration depth h;

[0066] Figure 5 The net cone tip resistance q in this embodiment of the invention net A schematic diagram showing the relationship between penetration depth h and the penetration depth h;

[0067] Figure 6 This is a schematic diagram showing the relationship between the overconsolidation ratio (OCR) and the penetration depth (h) in the 21# wind turbine section of this invention. Detailed Implementation

[0068] To make the objectives and technical solutions 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, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0069] like Figure 1 As shown, the method for calculating the overconsolidation ratio of cohesive soil based on pore pressure static cone penetration test includes the following steps:

[0070] Step (1): Import static penetration data: pore pressure static cone tip resistance q c Side friction resistance f s The soil overconsolidation ratio (OCR) obtained from consolidation tests is used to determine the soil type, and the corrected cone tip resistance (q) is determined based on this. t Total stress σ of the overlying soil v Effective stress σ of the overlying layer v ';

[0071] According to the above method, the maximum pressure for the consolidation test is 3200 kPa, with the first stage at 5 kPa. Subsequent loading stages are as follows: for soil samples at depths of 0–40 m, loading is applied at 50 kPa, 100 kPa, 150 kPa, 300 kPa, 600 kPa, 1200 kPa, 2400 kPa, and 3200 kPa; for soil samples at depths of 40–75 m, loading is applied at 50 kPa, 150 kPa, 300 kPa, 600 kPa, 1200 kPa, 2400 kPa, and 3200 kPa. This study involved drilling boreholes at five different wind turbine locations. The boreholes for the static cone penetration test were numbered JT5, JT21, JT27, JT56, and JT62, with the borehole spacing generally within 6.0 m. The exploration used a Vandenberg static cone penetration test system imported from the Netherlands. There are three probe models: I-CFXYP20-15, I-CFXYP100-15, and I-C2xFXYP100-10, with a probe area ratio coefficient α of 0.75.

[0072] Step (2): Calculate the overconsolidation ratio of the soil using the data from the previous step. Use the initially calculated overconsolidation ratio of the cohesive soil to create a scatter plot with a smooth line. Then, plot the scatter plot based on the measured values ​​from the geotechnical test. This includes the following sub-steps:

[0073] The calculation formula is determined as follows. The initial empirical coefficient is determined, and the overconsolidation ratio of cohesive soil is calculated according to the initial prediction formula. Based on the calculation and the overconsolidation ratio obtained from geotechnical tests, a schematic diagram of the relationship between the overconsolidation ratio OCR and the penetration depth h of the coastal site is drawn. Figure 2 )

[0074]

[0075]

[0076]

[0077]

[0078] Step (3): Preprocess the data, and then select evaluation factors to perform error analysis on the initial prediction formula and triaxial CU test data. The following evaluation factors are selected:

[0079] Root mean square deviation

[0080] Correlation coefficient

[0081] Nash efficiency coefficient

[0082] Coefficient of determination

[0083] Calculate the evaluation factors for the initial prediction models of the modified cone tip resistance method and the stress history method. Use the evaluation factors to preliminarily determine the effectiveness of the formula prediction and the reasonable range of the empirical coefficients.

[0084] Step (4): Based on the new prediction formula, plot the normalized cone tip resistance Q. t Schematic diagram of the relationship between penetration depth h and penetration depth h Figure 4 and net cone tip resistance q net Schematic diagram of the relationship between penetration depth h and penetration depth ( Figure 5 Compare the normalized cone tip resistance with the net cone tip resistance q. net Trend of change, normalized cone tip resistance Q t With net cone tip resistance q net The trends are consistent, and the normalized cone tip drag and the overconsolidation ratio show a linear relationship with depth. Furthermore, a schematic diagram of the relationship between the overconsolidation ratio (OCR) and penetration depth (h) at the 21# wind turbine site is drawn. Figure 6 ).

[0085] Step (5): For cohesive soils, a method for estimating the overconsolidation ratio of cohesive soils based on pore pressure static cone penetration tests is proposed using mathematical programming methods. Specifically:

[0086]

[0087]

[0088]

[0089]

[0090] The calculated values ​​of each evaluation factor are shown in Tables 1 and 2. To verify the rationality of the results, the Binhai No. 21 wind turbine site was selected for verification analysis. The evaluation factors before and after the correction of the overconsolidation ratio of cohesive soil were estimated based on four methods of pore pressure static cone penetration test, as shown in Tables 1-4 below:

[0091] The static cone tip resistance q net Model and normalized cone tip resistance Q t A comparison of the models shows that the coefficients of determination R0 of the Chanmee method, the NGI method, and the Santiago method are...2 The results are all quite close, meaning that all three methods can effectively predict the overconsolidation ratio (OCR) of cohesive soil. The three modified prediction models are arranged from largest to smallest according to the evaluation factor NSE value: NGI, Chanmee, and Santiago. From this perspective, the NGI method is slightly better than the Santiago method.

[0092] Table 1. Evaluation factors for overconsolidation ratio (OCR) (Mayne (1990))

[0093]

[0094]

[0095] Table 2. Evaluation factor values ​​for overconsolidation ratio OCR (Chanmee (2017))

[0096]

[0097] Table 3. Evaluation factor values ​​for overconsolidation ratio OCR (NGI(2019))

[0098]

[0099] Table 4. Evaluation factor values ​​for overconsolidation ratio OCR (Santiago (2019))

[0100]

[0101] Table 5.21 Comparison of OCR Fitting Results for Overconsolidation Ratio of Cohesive Soil in Wind Turbine Section (Engineering Example 2)

[0102]

[0103] As can be seen from the data in Table 5, the fitting results of NGI (2019) are generally greater than those of the consolidation test results, while the fitting results of the other three methods are close to the measured values ​​of the consolidation test. Among them, Mayne (1990) is also the method recommended by the Chinese "Technical Specification for Static Convection Penetration Testing" (TCCES1-2017) and the water transport industry standard "Technical Specification for Static Convection Testing of Water Transport Engineering" (JTS / T242-2020). Therefore, this invention verifies the feasibility of the standard-recommended method and provides reasonable values ​​for the empirical coefficients. In summary, the method of this invention calculates the overconsolidation ratio of cohesive soil with smaller errors than the measured values ​​of the geotechnical test, and is closer to the measured values ​​of the geotechnical test. For soil layers in the coastal area of ​​Jiangsu, the method of this invention has higher reliability.

[0104] The above description is merely a preferred 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 method for calculating the overconsolidation ratio of cohesive soil based on pore pressure static cone penetration test, characterized in that: The steps are as follows: (1) Static cone penetration tests were conducted on the soil samples using static penetration testing equipment to obtain static penetration data: pore pressure static cone tip resistance q c Side friction resistance f s Pore ​​water pressure u2; The overconsolidation ratio (OCR) of the soil was obtained by conducting geotechnical consolidation tests on the soil samples, and the soil type was determined. The total stress of the overlying soil is determined using the following formula. Effective stress of the overlying layer : , , Where γ i Let h be the natural unit weight of the i-th soil layer. i Let γ be the thickness of the i-th soil layer. w The specific weight of water; and the cone tip resistance q was determined by pore pressure static penetration testing. c The corrected cone tip resistance q is obtained t ; what t =q c +u2(1-α) In the formula, α is the probe area ratio coefficient; (2) Define the parameter values ​​for the initial calculation. The parameter values ​​are the expected pre-consolidated cone tip factors: k1, k2, k3, k4, b, m; The overconsolidation ratio (OCR) of cohesive soil can be calculated using the following formula: Using the above CPTU prediction calculation formula, the soil overconsolidation ratio OCR is initially calculated. Combined with the soil consolidation test in step (1), the soil overconsolidation ratio OCR is obtained and a curve is plotted. (3) Select evaluation factors to analyze the prediction formula, evaluate the prediction accuracy and effectiveness of each initial formula, and determine the range of values ​​for the empirical parameters in the formula. Error analysis is performed using the root mean square deviation (RMSD) factor, the Nash efficiency coefficient (NSE) factor, and the coefficient of determination (R²) factor, and the formulas are as follows: Root mean square deviation , Nash efficiency coefficient , Coefficient of determination , Correlation coefficient , In the formula x i The value is from the geotechnical test, y i These are the fitted values ​​of the empirical formula; (4) By using EXCEL spreadsheet software to solve the evaluation factors in a better way, the CPTU calculation OCR formula is modified, and the modified estimated overconsolidation ratio relationship is obtained as follows. The evaluation factors before and after the modification are compared, and the calculation of the overconsolidation ratio of cohesive soil based on the pore pressure static penetration test is completed. The above steps set the target value for fitting to the consolidation test value, and by changing the empirical coefficients in the empirical formula, a corrected prediction model that is closer to the geotechnical test results is finally obtained; The steps to correct the CPTU OCR calculation formula using Excel spreadsheet software are as follows: Import the data obtained in steps (1) and (2) into Excel; 1) In the Excel add-ins, call the Solver add-in menu; 2) In the top data pane of Excel, click Solver to construct the cell containing the objective function, and select Set Target to the maximum value of 0; 3) Select the variable cell and choose the cell containing the coefficient of the undetermined expected pre-consolidation cone tip factor; 4) Based on the expected pre-consolidated cone tip factor coefficients k1, k2, k3, k4, b, m in step 2), set the initial constraints of the variable cells using default values; 5) Set the solution method, adopt the nonlinear GRG solution method, and set it to use multi-initial-point optimization; 6) Once the desired preconsolidated cone tip factor value is obtained, stop solving and record it as the first solution value; 7) Repeat sub-steps 1 to 3, reduce the upper limit value of the constraint condition in sub-step 4 by 0.05 and solve it again, recording it as the second solution value; 8) Repeat the above sub-steps, changing the constraints according to sub-step 7), and solve multiple times. When sub-step 7 is repeated for the nth time, the expected solution value of the preconsolidated cone tip factor is equal to the solution value when sub-step 7 is repeated for the (n-1)th time. At this time, the expected solution value of the preconsolidated cone tip factor is the corrected exact solution. The default value sets the initial constraint condition for the variable cell as follows: the constraint condition is 0 < k1, k2, k3, m < 1; ; 0 < k4 < 15; The constraint condition is changed in each solution: reduced by 0.05 based on the previous constraint condition.

2. The method for calculating the overconsolidation ratio of cohesive soil based on pore pressure static cone penetration test according to claim 1, characterized in that: The evaluation principles in step (3) are as follows: The root mean square deviation (RMSD) factor ranges from (0, +∞). The calculation results are selected when the root mean square deviation (RMSD) factor is close to 0. Correlation coefficient r xy The range of values ​​for the factor is: , If the correlation coefficient r xy A value close to 0 indicates that the empirical formula fit value has a low correlation with the geotechnical test report value, and the empirical formula has low reliability. If r xy The calculation result is close to 1, indicating that the empirical formula fit value is highly correlated with the geotechnical test value, and the empirical formula has high reliability. The range of values ​​for the Nash efficiency coefficient (NSE factor) is as follows: , If the Nash efficiency coefficient (NSE factor) is close to 1, the model has high credibility. If the Nash efficiency coefficient (NSE) is close to 0, the empirical formula results are close to the average level of geotechnical tests, meaning the overall results are reliable, but the process simulation error may be too large.

3. The method for calculating the overconsolidation ratio of cohesive soil based on pore pressure static cone penetration test according to claim 2, characterized in that: The evaluation is based on the Nash efficiency coefficient (NSE factor), and is assessed using the root mean square deviation (RMSD factor) and the coefficient of determination (R²). 2 Factors, relation numbers r xy To assist in the evaluation.

4. The method for calculating the overconsolidation ratio of cohesive soil based on pore pressure static cone penetration test according to claim 1, characterized in that: In step (1), the overconsolidation ratio OCR of the soil is the preconsolidation pressure P of the soil. c With overlying effective stress The ratio, in which the preconsolidation pressure of the soil is measured by consolidation tests.

Images