A method for analyzing axial friction in triaxial testing

Abstract

The present invention relates to a method for analyzing axial friction force during triaxial testing, comprising the following steps: filling a triaxial pressure chamber with degassed water; gradually increasing the confining pressure p c , and record the load sensor reading F ext , and obtain a series of data points (p c , F ext ); the relationship between the axial friction force f is: f = F ext ‑p c ﹒ A+G; The function of axial friction force with respect to confining pressure is obtained by numerical fitting: f=f(p c ); During the formal test, the axial deflection force F is obtained by static balance in The relationship formula: F in =F ext ‑p c ﹒ A+G‑f(p c The triaxial friction analysis method described in this invention allows for the selection of different functions to meet varying accuracy requirements, resolving the issue of excessive error in prior art axial friction analysis. This method can be programmed into the triaxial tester's results analysis system, significantly improving the accuracy of axial pressure and deviatoric stress analysis of soil specimens.

Description

Technical Field

[0001] The invention relates to the technical field of rock and soil testing, in particular to an analysis method for axial friction force of a soil sample during a triaxial test. Background Art

[0002] The deformation and strength of soil seriously affect the safety and economy of geotechnical engineering, and the main method for measuring the deformation and strength parameters of soil is the triaxial test of soil samples. When conducting a triaxial test, the soil sample is placed in a pressure chamber, and confining pressure is usually applied to the sample by water pressure. After the sample is consolidated and stabilized, an axial force or axial displacement is applied to the sample through an axial force rod to cause shear failure of the sample. A load sensor is installed on the axial force rod to measure the magnitude of the axial force. Since the external load sensor has a simple principle and is easy to assemble, compared with the built-in load sensor, it does not require waterproofing and is not affected by horizontal water pressure. At present, most triaxial instruments use external load sensors to measure axial pressure.

[0003] However, the friction generated by the axial force rod and the pressure chamber cover during the relative movement is also recorded by the external sensor and reflected in the test results. The magnitude of this axial friction force affects the test results. The relevant explanation of this axial friction force is as follows: Water is usually used in the pressure chamber to apply confining pressure to the specimen, so the sealing of the entire pressure chamber must be ensured, including the place where the axial force rod and the pressure chamber cover interact. Usually, an O-ring is set between the pressure chamber cover and the axial force rod to ensure this sealing. In this case, there is a certain pressure between the O-ring and the axial force rod. When the axial force rod needs to apply axial force to the specimen and produce relative movement with the pressure chamber cover, friction is generated between the axial force rod and the O-ring; in addition, there may also be slight friction between the axial force rod and other parts of the pressure chamber cover.

[0004] The above-mentioned axial friction force is not easy to measure. The current national geotechnical test standard (Geotechnical test method standard, GB / T50123-2019) ignores this friction force and assumes that the recorded value of the external sensor is the deflection force value of the specimen. The current relevant American geotechnical test standard (Standard test method for consolidated undrained triaxial compression test for cohesive soils, D4767-11, reapproved 2020) records the load sensor readings before and after the axial force rod contacts the specimen for the case of isotropic consolidation triaxial test, and then calculates the pressure difference between the two as the basis for the deflection force applied to the specimen, thereby eliminating the influence of axial friction force. However, this method is not applicable to general situations, such as when the axial force rod is always in contact with the specimen during non-isotropic consolidation triaxial test.

[0005] In their monograph on triaxial testing of soils (Lade PV, 2016, Wiley Balckwell), Lade (2016) proposed a method for estimating axial friction. When no specimen is placed in the triaxial pressure chamber, the axial friction is assumed to be the difference between the pressure exerted on the axial compression rod by the external load cell and the confining pressure. The axial friction is then fitted to a straight line that increases monotonically with the confining pressure. However, axial friction varies not only with the confining pressure but is also affected by complex factors such as the condition of the O-ring. Therefore, it is not a linear function that increases monotonically with confining pressure. Therefore, the method described by Lade (2016) suffers from significant errors. Summary of the Invention

[0006] Therefore, the technical problem to be solved by the present invention is to overcome the problem that the existing technology cannot be applied to general triaxial test situations (such as D4767-11), or the estimation error of the axial friction force is large (such as Lade, 2016), which affects the accuracy of the triaxial test analysis and causes unnecessary errors.

[0007] To solve the above technical problems, the present invention provides a method for analyzing axial friction force in a triaxial test, comprising the following steps:

[0008] S1. When no specimen is placed in the triaxial pressure chamber, the chamber is filled with degassed water.

[0009] S2. Gradually increase the confining pressure p c , and record the load sensor reading F ext , and obtain a series of data points (p c , F ext );

[0010] S3. According to the force balance of the axial force rod in each state, the relationship between the axial friction force f is:

[0011] f=F ext -p c ﹒ A+G,

[0012] Where A is the cross-sectional area of ​​the axial force rod, and G is the weight of the axial force rod. Here, the friction force f is positive when it is upward, F ext and p c Taking pressure as positive, a series of data points (p c , f);

[0013] S4, for the above series of data points (p c , f) The function of axial friction force with respect to confining pressure is obtained by curve fitting: f = f(p c );

[0014] S5. When the soil sample is tested later, the axial deflection force F under the corresponding working condition is obtained according to the force balance of the axial force rod. in The relationship formula: F in =F ext -p c ﹒ A+Gf(p c ).

[0015] The above axial friction force will change with time and environment. After a period of time, the axial friction force f=f(p c ).

[0016] In one embodiment of the present invention, in the above step S4, the above series of data points (p c , f), the B-spline function is used to fit the function of axial friction force with respect to confining pressure: f = f(p c ).

[0017] In one embodiment of the present invention, in the above step S4, the above series of data points (p c , f), linear interpolation is used to obtain the axial friction force function: f = f(p c ); Specifically, for any given p c , judge its value between (p c ) i and (p c ) i+1 The corresponding data points are [(p c ) i ,f i ] and [(p c ) i+1 ,f i+1 ], then: f(p c )=f i +[p c -(p c ) i ] / [(p c ) i+1 -(p c ) i ]×(f i+1 -f i ).

[0018] The above technical solution of the present invention has the following beneficial effects compared with the prior art:

[0019] The proposed method for analyzing axial friction during triaxial testing accurately accounts for axial friction and is applicable to a variety of triaxial testing conditions. This method is simple and practical, allowing for the selection of different functions to meet varying accuracy requirements. It addresses the issue of excessive error in axial friction analysis in existing techniques. This method can be programmed into the triaxial tester's results analysis system, significantly improving the accuracy of axial pressure and deviatoric stress analyses of soil samples. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below based on specific embodiments of the present invention in conjunction with the accompanying drawings, wherein

[0021] Figure 1 Schematic diagram of a triaxial test with a soil sample in a preferred embodiment of the present invention;

[0022] Figure 2 This is a schematic diagram of the calibration of the correlation coefficient of the axial friction force without a soil sample in a preferred embodiment of the present invention;

[0023] Figure 3 This is a force diagram of the axial force rod when a soil sample is installed in a preferred embodiment of the present invention;

[0024] Figure 4 This is a force diagram of the axial force rod when no soil sample is installed in the preferred embodiment of the present invention;

[0025] Figure 5 The curve fitting of the axial friction force changing with the confining pressure in the preferred embodiment of the present invention is Figure 1 ;

[0026] Figure 6 The curve fitting of the axial friction force changing with the confining pressure in the preferred embodiment of the present invention is Figure 2 .

[0027] Explanation of the reference numerals in the accompanying drawings in the specification: 1-load sensor; 2-axial force rod; 3-stainless steel pulley sleeve; 4-O-ring at the upper cover of the pressure chamber; 5-sample cap; 6-permeable stone; 7-filter paper; 8-soil sample; 9-sample membrane; 10-base; 11-upper cover of the pressure chamber; 12-pressure chamber; 13-sample O-ring; 14-confined water inlet. DETAILED DESCRIPTION

[0028] The present invention will be further described below with reference to the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it. However, the embodiments are not intended to limit the present invention.

[0029] Reference Figure 1-6As shown, the test system for the above-mentioned triaxial test axial friction analysis method includes: a base 10, which is arranged at the lower end for support; a pressure chamber 12, which is arranged on the base 10, and the upper and lower ends of the pressure chamber 12 are both open structures. The base 10 is provided with a confined pressure water inlet 14, one end of which is connected to the pressure chamber 12, and the other end of the confined pressure water inlet 14 extends out of the base 10; a pressure chamber cover 11, which is provided at the upper end opening of the pressure chamber 12; a stainless steel pulley sleeve 3, which is provided on the pressure chamber cover 11; an axial force rod 2, which passes through the stainless steel pulley sleeve 3 and is coaxially arranged with the stainless steel pulley sleeve 3; and a load sensor 1, which is provided at the upper end of the axial force rod 2 extending from the stainless steel pulley sleeve 3. The stainless steel pulley sleeve 3 is provided with an O-ring 4, which is sleeved on the outside of the axial force rod 2 and is used to seal between the axial force rod 2 and the stainless steel pulley sleeve 3. A sample membrane 9 is provided within the pressure chamber 12 and is placed on a base 10. A sample cap 5 is provided at the upper end of the sample membrane 9, and the lower end of the axial force rod 2 is disposed within the sample cap 5. Sample O-rings 13 are sleeved on the upper and lower ends of the outer wall of the sample membrane 9. Permeable stones 6 are provided at the upper and lower ends of the interior of the sample membrane 9, with the upper layer of permeable stones 6 located below the sample cap 5. A soil sample 8 is provided between the upper and lower layers of permeable stones 6, and filter paper 7 is provided between the upper and lower ends of the soil sample 8 and the permeable stones 6.

[0030] The soil sample 8 is located between the base 10 and the sample cap 5, and is sealed on all sides by the sample membrane 9 and the sample O-ring 13. During the triaxial test, the confining water applies confining pressure to the soil sample 8 through the confining water inlet 14. When it is necessary to apply axial force to the soil sample 8, the axial force rod 2 can be fixed and the base 10 can be raised to make the soil sample 8 rise relative to the axial force rod 2, thereby applying axial pressure to the soil sample 8. Figure 3 As shown, the axial force rod is taken as the analysis object, and the load sensor reading is F ext (pressure is positive), the force between the specimen cap and the axial force rod is F in (with pressure as positive), there is an axial friction force f (with upward as positive) between the upper cover of the pressure chamber and the axial force rod, and the axial force rod is simultaneously subjected to gravity G (with downward as positive) and the pressure p of the confining water c ﹒ A (positive with upward), where p c is the confining pressure, A is the cross-sectional area of ​​the axial force rod. The axial force actually transmitted to the soil sample is F in Instead of F ext .Depend on Figure 3 , axial force F in The expression is: F in =F ext -p c﹒ A+Gf; When there is no soil sample in the pressure chamber, the relevant status is as follows Figure 2 As shown, the force analysis of the axial force rod is as follows Figure 4 As shown, F in =0, so the expression of axial friction force f can be obtained as: f=F ext -p c ﹒ A+G.

[0031] Example 1

[0032] The analysis method of the axial friction force of the triaxial test of the present invention has the following specific steps:

[0033] S1. The triaxial pressure chamber is not filled with a specimen but is filled with degassed water (degassed water is a kind of treated water required for this test, that is, the air in the water is removed by vacuuming). Figure 2 As shown;

[0034] S2. Gradually increase the confining pressure p c , and record the value F of the load sensor at the same time ext , and obtain a series of data points (p c , F ext );

[0035] S3. According to the force balance of the axial force rod in each state, the relationship between the axial friction force f is obtained:

[0036] f=F ext -p c ﹒ A+G;

[0037] Where A is the cross-sectional area of ​​the axial force rod, and G is the weight of the axial force rod, both of which are known quantities that have been measured. Here, the friction force f is positive in the upward direction, F ext and p c The pressure is considered positive, that is, the pressure on a component is considered positive (+). Thus, a series of data points (p c , f);

[0038] S4, for the above series of data points (p c , f), where the B-spline function is used to fit the function of axial friction force with respect to confining pressure:

[0039] f=f(p c );

[0040] The schematic diagram of the fitting curve is shown in Figure 5 . Subsequent given any p c The value of friction force f(p c ), which can be achieved by using numerical software such as Matlab, or directly programming according to the principle of related functions;

[0041] S5. After the calibration of the above axial friction force f is completed, the soil sample can be placed in the pressure chamber for triaxial testing. Figure 1 , the force analysis reference of the axial force rod Figure 3 , and the soil specimen is subjected to the corresponding axial deflection force F in Calculation is performed according to the following formula:

[0042] F in =F ext -p c ﹒ A+Gf(p c );

[0043] The axial friction force f=f(p c ), thereby accurately calculating the axial pressure and the associated deviatoric stress on the soil sample. The above data processing method can be incorporated into the data analysis system of the triaxial apparatus to improve convenience;

[0044] S6. As time and environment change, the axial friction force will also change; therefore, after a period of time, the axial friction force f=f(p c ).

[0045] Example 2

[0046] The analysis method of the axial friction force of the triaxial test of the present invention has the following specific steps:

[0047] S1. The triaxial pressure chamber is not filled with a specimen but is filled with degassed water. Figure 2 As shown;

[0048] S2. Gradually increase the confining pressure p c , and record the value F of the load sensor at the same time ext , and obtain a series of data points (p c , F ext );

[0049] S3. According to the force balance of the axial force rod in each state, the relationship between the axial friction force f is obtained:

[0050] f=F ext -p c ﹒ A+G;

[0051] Where A is the cross-sectional area of ​​the axial force rod, and G is the weight of the axial force rod, both of which are known quantities that have been measured. Here, the friction force f is positive in the upward direction, F ext and p c The pressure is positive. Thus, a series of data points (p c , f);

[0052] S4, for the above series of data points (p c , f), linear interpolation is used to obtain the axial friction force function f = f(p c ), the fitting function at this time is shown in Figure 6 . Subsequently, for any given p c , determine which two adjacent known p c Between, for example, between (p c ) i and (p c ) i+1 Between, corresponding Figure 6 The data point is [(p c ) i ,f i ] and [(p c ) i+1 ,f i+1 ],but:

[0053] f(p c )=f i +[p c -(p c ) i ] / [(p c ) i+1 -(p c ) i ]×(f i+1 -f i );

[0054] S5. After the calibration of the above axial friction force f is completed, the soil sample can be placed in the pressure chamber for triaxial testing. Figure 1 , the force analysis reference of the axial force rod Figure 3 , and the soil specimen is subjected to the corresponding axial deflection force F in Calculation is performed according to the following formula:

[0055] F in =F ext -p c ﹒ A+Gf(p c );

[0056] The axial friction force f=f(p c ), thereby accurately calculating the axial pressure and the associated deviatoric stress on the soil sample. The above data processing method can be incorporated into the data analysis system of the triaxial apparatus to improve convenience;

[0057] S6. As time and environment change, the axial friction force will also change; therefore, after a period of time, the axial friction force f=f(p c ).

[0058] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A method for analyzing axial friction force in a triaxial test, characterized by: The steps include: S1. When no specimen is placed in the triaxial pressure chamber, the chamber is filled with degassed water. S2. Gradually increase the confining pressure p c , and record the load sensor reading F ext , and obtain a series of data points (p c , F ext ); S3. According to the force balance of the axial force rod in each state, the relationship between the axial friction force f is: f=F ext -p c ﹒A+G, Where A is the cross-sectional area of ​​the axial force rod, and G is the weight of the axial force rod. Here, the friction force f is positive when it is upward, F ext and p c Taking pressure as positive, a series of data points (p c , f); S4, for the above series of data points (p c , f) linear interpolation is used to obtain the axial friction force function: f = f(p c ), for any given p c , judge its value between (p c ) i and (p c ) i+1 The corresponding data points are [(p c ) i ,f i ] and [(p c ) i+1 ,f i+1 ], then: f(p c )=f i +[p c -(p c ) i ] / [(p c ) i+1 -(p c ) i ]×(f i+1 -f i ); S5. When the soil sample is tested later, the axial deflection force F under the corresponding working condition is obtained according to the force balance of the axial force rod. in The relationship formula: F in =F ext -p c ﹒ A+Gf(p c ).

2. The method for analyzing axial friction force in triaxial testing according to claim 1, characterized in that: The above axial friction force will change with time and environment. After a period of time, the axial friction force f=f(p c ).

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