Method for calculating long-term shear strength of clay

Abstract

A method for calculating the long-term shear strength of clay includes the following steps: S1: Obtain the clay sample to be tested; S2: Conduct a conventional triaxial shear test to obtain the deviatoric stress q of the sample at failure. f S3: Conduct a triaxial creep test and obtain the creep test curve of clay using the obtained loading time t, axial strain ε1, and loading deviatoric stress q from the triaxial creep test; S4: Calculate the logarithmic strain and stress ratio based on the data obtained in S3, and plot the isochronous stress ratio-logarithmic strain curve; S5: Establish a mathematical model based on the plotted isochronous stress ratio-logarithmic strain curve; S6: Based on the mathematical model of the isochronous stress ratio-logarithmic strain curve obtained in S5, plot the isochronous stress ratio-logarithmic strain curve, and obtain the characteristic points in the coordinate system of the isochronous stress ratio-logarithmic strain curve; S7: Obtain the long-term shear strength of clay based on the coordinates of the characteristic points obtained in S6.

Description

Technical Field

[0001] This invention relates to the field of geotechnical engineering technology, specifically a method for calculating the long-term shear strength of clay. Background Technology

[0002] Clay often exhibits highly complex engineering mechanical properties due to the influence of factors such as its mineral composition, gradation, structure, and moisture content. Time-dependent properties are one extremely important aspect of these properties. In most geotechnical engineering projects, the shear strength of clay is directly determined by conventional direct shear tests or conventional triaxial shear tests; in time-dependent properties, this is understood as instantaneous shear strength. Because clay exhibits significant time-dependent properties, long-term shear strength is a crucial manifestation of this property. When instantaneous shear strength or its parameters are used to calculate the stability of semi-permanent or permanent geotechnical engineering projects, it can introduce significant safety risks to the stability of these projects. Therefore, using long-term shear strength and its parameters to calculate the stability of related projects is more scientific, reasonable, safe, and reliable.

[0003] Currently, some mainstream methods for determining long-term shear strength, such as the isochronous stress-strain curve inflection point method, creep strain rate method, transition creep method, and empirical model method, suffer from several drawbacks. These include heavy reliance on creep or relaxation test load grading, unclear curve inflection points, poor regularity, and the inability to directly obtain long-term shear strength within certain value ranges. Empirical model methods also suffer from a lack of rigorous logic, numerous parameters, complex forms, and poor applicability. Consequently, it is difficult to accurately obtain the long-term shear strength of clay and to provide scientifically sound, accurate, and easily applicable long-term shear strength and its parameters for calculating the stability of related engineering projects.

[0004] Therefore, given the engineering mechanical property of clay's aging properties and the safety risks, and even accidents, that it poses to related construction projects, there is an urgent need for a scientific, reasonable, accurate, effective, simple, and practical calculation method to obtain the long-term shear strength and parameters of clay, effectively reducing engineering safety risks and improving the overall quality of the project. Summary of the Invention

[0005] Given the current deficiencies in the calculation of long-term shear strength of clay, the purpose of this invention is to provide a method for calculating the long-term shear strength of clay, so as to obtain scientific, reasonable, accurate, effective, simple and practical long-term shear strength and parameters of clay, provide scientific, accurate and practical parameters for the calculation of stability in related geotechnical engineering, meet the needs of actual engineering design and related technical problems, and fill the gaps in the standards.

[0006] To address the problems raised in the background section above.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A method for calculating the long-term shear strength of clay includes the following steps: S1: Obtain the clay sample to be tested; S2: Conduct a conventional triaxial shear test on the soil sample obtained in S1 to obtain the deviatoric stress at failure of the sample. q f ; S3: Conduct a triaxial creep test and obtain the loading time of the triaxial creep test. t Axial strain e 1. Applying eccentric stress q Using the obtained loading time from the creep triaxial test t Axial strain e 1. Applying eccentric stress q Obtain the creep test curve of clay ( t , e 1, q ); S4: Based on the data obtained in S3, calculate the logarithmic strain and stress ratio, and plot the isochronous stress ratio-logarithmic strain curve based on the calculated data; S5: Establish a mathematical model of the law of the isochronous stress ratio-logarithmic strain curve based on the isochronous stress ratio-logarithmic strain curve drawn in S4; S6: Based on the mathematical model of the isochronous stress ratio-logarithmic strain curve law obtained in S5, draw the isochronous stress ratio-logarithmic strain curve, and obtain the characteristic points in the isochronous stress ratio-logarithmic strain curve coordinate system based on the drawn isochronous stress ratio-logarithmic strain curve. S7: Obtain the long-term shear strength of clay based on the coordinates of the feature points obtained in S6.

[0008] As a further aspect of the present invention: the calculation formulas for logarithmic strain and stress ratio in step S4 are as follows: ; In the formula: q f —The deviatoric stress that causes failure in conventional triaxial tests; e q , q r —Logarithmic strain, stress ratio, 0 < q r ≤1.

[0009] As a further aspect of the present invention: the mathematical model for establishing the law of the isochronous stress ratio-logarithmic strain curve in S5 is as follows: ; In the formula: k , n , m —Model fitting parameters, and m =1-1 / n ; Will( e q , q r The series of data were fitted with the proposed mathematical model to obtain the fitting parameters. k , n , m。

[0010] As a further aspect of the present invention: the isochronous stress ratio-logarithmic strain curve is composed of a convex curve and a concave curve, and the connection point between the convex curve and the concave curve is a feature point. B , B Point Tangent and q r The intersection of the lines =1 is the characteristic point. D , D The intersection of the vertical line at a point and the isochronous stress ratio-logarithmic strain curve is the characteristic point. E , E Point Tangent and B The intersection of the tangents of a point is a feature point. C .

[0011] As a further aspect of the present invention: the feature points B The coordinates and slope are: ; In the formula: ( e qB , q rB )for B Point coordinates, K B for B The slope of the tangent line at the point.

[0012] As a further aspect of the present invention: the feature points E The coordinates and slope are: ; In the formula: ( e qE , q rE )for E Point coordinates, K E for E Slope of the tangent line at the point, .

[0013] As a further aspect of the present invention: the feature points C The coordinates are: ; In the formula: ( eqC , q rC )for C Point coordinates.

[0014] As a further aspect of the present invention: the formula for calculating the long-term shear strength of the clay is: ; In the formula: q rC for C Stress ratio at point q f This represents the deviatoric stress that causes clay failure.

[0015] Compared with the prior art, the beneficial effects of the present invention are: This invention is based on conventional triaxial shear tests and triaxial creep tests of clay. Through the processing of creep isochronous stress-strain curve data and coordinate axis transformations, it delves into the inherent laws governing creep testing, proposes a scientifically sound mathematical model to describe these laws, and ultimately solves for the long-term shear strength of clay and its parameters within this mathematical model. The long-term shear strength of clay determined by this invention features a simple calculation method, few parameters, and high accuracy. It can provide scientifically accurate calculation parameters for the stability calculation of semi-permanent and permanent construction projects based on clay, effectively ensuring project safety and quality, controlling project safety risks, and filling gaps in standards. Attached Figure Description

[0016] Figure 1 In a semi-logarithmic coordinate system q r - e q Line graph; Figure 2 The curves are from a standard triaxial test under a confining pressure of 100 kPa. Figure 3 The creep triaxial test curves are shown under a confining pressure of 100 kPa. Figure 4 The isochronous stress ratio-logarithmic strain test curves are shown in a semi-logarithmic coordinate system. Figure 5 for t =10 min isochronous stress ratio-strain fitting curve and experimental data; Figure 6 for t =Isochronous stress ratio-strain fitting curve and experimental data under 60min condition; Figure 7 for t Isochronous stress ratio-strain fitting curve and experimental data under the condition of 1440 min. Detailed Implementation

[0017] The technical solution of this patent will be further described in detail below with reference to specific embodiments.

[0018] Please see Figure 1-7 A method for calculating the long-term shear strength of clay includes the following steps: (1) Take clay samples on site at the construction site; (2) Indoor test; Conventional triaxial shear test was carried out according to the "Standard for Geotechnical Testing Methods" (GB / T 50123-2019) to obtain the failure deviatoric stress of the specimen. q f Then, triaxial creep tests were conducted to obtain the creep test curves of the clay. t , e 1, q ), t , e 1. q These represent the loading time, axial strain, and deviatoric stress in the creep triaxial test, respectively. (3) Plotting the isochronous stress-strain ratio-logarithmic strain curve for creep. The stress and strain of the isochronous stress-strain curve are changed as follows: ; In the formula: q f —The deviatoric stress that causes failure in conventional triaxial tests; e q , q r —Logarithmic strain, stress ratio, 0 < q r ≤1.

[0019] Will( e q , q r The series of data is plotted in the following semi-logarithmic coordinate system ( q r The vertical axis represents natural numbers. e q The isochronous stress ratio-logarithmic strain curve can be obtained by using a semi-logarithmic abscissa. q r - e q ; like Figure 1 As shown, in the semi-logarithmic coordinate system q r - e q Curves have the following characteristics: ① Curves BEF The segment has the property of a "convex" curve, and whenq r Approaching 1, e q Approaching infinitesimal e 1. Approaching infinity indicates that the clay has reached a state of complete plasticity or failure; ② Curve BA The segment has the property of a "concave" curve, and when q r Approaching a minimum value q r0 hour, e q Approaching infinity e A value close to zero indicates that the clay is in a perfectly elastic state. Therefore, it can be concluded that... q r - e q The inflection point of the curve is at B Point; (4) The following mathematical model is proposed to describe the law of the isochronous stress ratio-logarithmic strain curve. ; In the formula: k , n , m —Model fitting parameters, and m =1-1 / n ; Will( e q , q r By fitting the series of data with the proposed mathematical model, the fitting parameters can be obtained. k , n , m； (5) Calculation of isochronous stress ratio-logarithmic strain characteristic points. In a semi-logarithmic coordinate system, q r - e q On the curve B , C , E Point coordinates and B , E Point tangent slope K B , K E The specific calculation method is as follows: ① B Method for determining point coordinates: by... q r - e q The mathematical model of the curve can be determined by taking the second derivative and setting it to zero. BPoint coordinates, curve in B The first derivative of a point is the slope of its tangent line; ② E Method for determining point coordinates: First, determine... B Point Tangent and q r =1 intersection of lines D The coordinates, then through D The intersection of the vertical line at point A with the isochronous stress ratio-logarithmic strain curve is... E Points, curves E The first derivative of a point is the slope of its tangent; ③ C Methods for determining point coordinates: q r - e q The curve is E Tangent to the point and B The intersection of the tangents of a point is C point. B , C , E Point coordinates and B , E Point tangent slope K B , K E The calculation methods are as follows: B The coordinates of the point and the slope are: ; In the formula: ( e qB , q rB )for B Point coordinates, K B for B Slope of the point tangent; E The coordinates of the point and the slope are: ; In the formula: ( e qE , q rE )for E Point coordinates, K E for E Slope of the tangent line at the point, ; C The coordinates of the point are: ; In the formula: ( e qC , qrC ( ) represents the coordinates of point C; (6) Calculation of long-term shear strength of clay. The calculated results... C Stress ratio at point q rC Deviatoric stress in clay failure q f The product of these two values ​​is the long-term shear strength of clay. q L ; ; Under different confining pressures q L The long-term shear strength parameter of clay, i.e., long-term cohesion, can then be calculated. c L Long-term internal friction angle f L .

[0020] Based on conventional triaxial shear tests and triaxial creep tests of clay, this invention delves into the intrinsic laws governing creep by processing isochronous stress-strain curve data and coordinate axis transformations. A scientifically sound mathematical model is proposed to describe these laws, and ultimately, the long-term shear strength and its parameters of clay are solved within this mathematical model. The long-term shear strength of clay determined by this invention is characterized by its simple calculation method, few parameters, and high accuracy. It can provide scientifically accurate calculation parameters for the stability calculation of semi-permanent and permanent construction projects primarily based on clay, effectively ensuring project safety and quality, controlling project safety risks, and filling gaps in relevant standards.

[0021] Example: The implementation steps of a method for calculating the long-term shear strength of clay are as follows: (1) Taking the clay from the Chengdu Universiade Village as an example, the physical and mechanical properties of the clay are shown in Table 1: Table 1 Physical and mechanical properties of clay in Chengdu Universiade Village .

[0022] (2) Indoor conventional triaxial test curves are as follows Figure 2 As shown, by Figure 2 The deviatoric stress at which clay fails under a confining pressure of 100 kPa can be determined. q f =154kPa.

[0023] (3) Creep triaxial test curves are as follows Figure 3 As shown.

[0024] (4) q r - e qThe curve data were fitted to the proposed mathematical model, and the results are shown in Table 2. Table 2. Fitting parameters for isochronous stress ratio-logarithmic strain curves ; draw t =Isochronous stress ratio-logarithmic strain test curves at 10min, 60min, and 1440min, such as Figure 5 , Figure 6 , Figure 7 As shown.

[0025] (5) Calculate the long-term shear strength of clay, and the stress ratio-logarithmic strain curve. B , C , E Point coordinates and B , E The results of the point tangent slope calculation are shown in Table 3: Table 3 Calculation results of characteristic points of the qr-εq curve .

[0026] As can be seen from Table 3, C Point coordinate stress ratio q rC The stress ratio eventually stabilizes at 0.707 over time, indicating that the long-term shear strength stress ratio of clay is 0.707, demonstrating the significant time-dependent nature of clay strength. The calculated stress ratio and failure deviatoric stress are then compared. q f The results of calculating the long-term shear strength of clay are shown in Table 4: Table 4 Long-term shear strength of clay in Chengdu Universiade Village .

[0027] The preferred embodiments of this patent have been described in detail above. However, this patent is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of this patent.

Claims

1. A method for calculating the long-term shear strength of clay, characterized in that, Includes the following steps: S1: Obtain the clay sample to be tested; S2: Conduct a conventional triaxial shear test on the soil sample obtained in S1 to obtain the deviatoric stress at failure of the sample. q f ; S3: Conduct a triaxial creep test and obtain the loading time of the triaxial creep test. t Axial strain ε 1. Applying eccentric stress q Using the obtained loading time from the creep triaxial test t Axial strain ε 1. Applying eccentric stress q Obtain the creep test curve of clay; S4: Based on the data obtained in S3, calculate the logarithmic strain and stress ratio, and plot the isochronous stress ratio-logarithmic strain curve based on the calculated data; S5: Establish a mathematical model of the law of the isochronous stress ratio-logarithmic strain curve based on the isochronous stress ratio-logarithmic strain curve drawn in S4; S6: Based on the mathematical model of the isochronous stress ratio-logarithmic strain curve law obtained in S5, draw the isochronous stress ratio-logarithmic strain curve, and obtain the characteristic points in the isochronous stress ratio-logarithmic strain curve coordinate system based on the drawn isochronous stress ratio-logarithmic strain curve. S7: Obtain the long-term shear strength of clay based on the coordinates of the feature points obtained in S6; The mathematical model for establishing the isochronous stress ratio-logarithmic strain curve law in S5 is as follows: ； In the formula: k , n , m —Model fitting parameters, and m =1-1 / n ; ε q , q r —Logarithmic strain, stress ratio, 0 < q r ≤1; Will( ε q , q r The series of data were fitted with the proposed mathematical model to obtain the fitting parameters. k , n , m； The isochronous stress ratio-logarithmic strain curve consists of a convex curve and a concave curve, with the junction of the convex and concave curves being the characteristic points. B , B Point Tangent and q r The intersection of the lines =1 is the characteristic point. D , D The intersection of the vertical line at a point and the isochronous stress ratio-logarithmic strain curve is the characteristic point. E , E Point Tangent and B The intersection of the tangents of a point is a feature point. C； The formula for calculating the long-term shear strength of the clay is: ； In the formula: q rC for C Stress ratio at point q f This represents the deviatoric stress that causes clay failure.

2. The method for calculating the long-term shear strength of clay according to claim 1, characterized in that, The formulas for calculating logarithmic strain and stress ratio in step S4 are as follows: ； In the formula: q f —The deviatoric stress that causes failure of a specimen in a conventional triaxial test.

3. The method for calculating the long-term shear strength of clay according to claim 1, characterized in that, The feature points B The coordinates and slope are: ； In the formula: ( ε qB , q rB )for B Point coordinates, K B for B The slope of the tangent line at the point.

4. The method for calculating the long-term shear strength of clay according to claim 3, characterized in that, The feature points E The coordinates and slope are: ； In the formula: ( ε qE , q rE )for E Point coordinates, K E for E Slope of the tangent line at the point, .

5. The method for calculating the long-term shear strength of clay according to claim 4, characterized in that, The feature points C The coordinates are: ； In the formula: ( ε qC , q rC )for C Point coordinates.

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