Method for correcting shear stress in large-scale direct shear test of cylindrical specimens with different sizes

Abstract

The present application belongs to the field of geotechnical test, and provides a shear stress correction method for large-scale direct shear test of different size cylindrical samples, which is based on a large-scale direct shear apparatus for different size cylindrical samples, according to the structural features of a shear box and the test principle of the direct shear test, the stress condition of the shear box is determined, the horizontal static equilibrium equation of the shear surface is determined by using the relevant theory of statics, the change condition of the actual shear area of the shear box is derived, the shear stress correction formula for the large-scale direct shear test of different size samples is further derived, the test error is eliminated, and the relationship curve between the corrected shear displacement and the shear stress is established, so that the shear strength of the soil body is accurately determined.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of geotechnical test, and particularly relates to a correction method for shear stress of large-scale direct shear test suitable for different sizes of cylindrical samples. BACKGROUND

[0002] The shear strength of soil is an important soil mechanics parameter in geotechnical engineering, and therefore how to accurately and quickly determine the shear strength of soil has always been an important research content in geotechnical engineering. For example, the method for detecting the shear strength of cohesive soil by using a large-scale direct shear test device on site is disclosed in Chinese patent CN201410335875.7. The direct shear test becomes a commonly used test method for testing the shear strength of soil because of its simple instrument structure and easy operation.

[0003] The conventional direct shear test is difficult to be applied to the research of coarse-grained soil due to the size limitation. Therefore, in recent years, many scholars have paid attention to the research of large-scale direct shear test, and the shear strength of geotechnological bodies such as coarse-grained soil and soil-rock mixture is researched by the large-scale direct shear test.

[0004] The size of the conventional direct shear test is small, and the shear displacement required by the specification is generally about 6mm, and the influence of the external force on the shear strength can be ignored. However, unlike the conventional direct shear test, there is no clear specification for the large-scale direct shear test in the current specification and research. Because the large-scale direct shear test instrument is large and the shear displacement is large, the influence of the external force and the change of the sample area on the shear strength is large. If the influence of the two on the shear strength is directly ignored, there will be a large error between the determined shear strength and the actual shear strength, which directly affects the safety of engineering construction.

[0005] In view of the above, it is necessary to correct the calculation method of the shear strength of the existing large-scale direct shear test, and therefore a shear stress correction method for large-scale direct shear test of different sizes of cylindrical samples is proposed. SUMMARY

[0006] The purpose of the present application is to correct the shear stress calculation method of the existing large-scale direct shear test, and to propose a more reliable and comprehensive calculation method for the shear stress of large-scale direct shear test of different sizes of samples.

[0007] The shear stress is defined as the shear force on the unit area, and the direction of the force is orthogonal to the normal direction of the force surface. In soil mechanics, the shear stress of the direct shear test is the ratio of the shear force of the sample to the shear area of the sample.

[0008] In the existing large-scale direct shear test research, the test data is not processed, the stress condition of the external structure of the shear box and the change condition of the shear area in the shearing process are ignored, the influence of both on the actual shear strength is not considered, the horizontal thrust in the shearing process is directly considered as the shear stress and the shear area is always considered as unchanged, which leads to a large error between the test shear strength and the actual shear strength.

[0009] In the derivation process, the existing calculation method is modified according to the structure of the actual shear box and the test principle, and the error between the test value and the true value is minimized to the greatest extent. The key of the present application lies in: the test principle of the direct shear test is clear, the overall structural characteristics of the shear box of the large-scale direct shear instrument are determined, and the stress conditions of each component of the instrument are clear.

[0010] The shear stress correction method for large-scale direct shear test of different size cylindrical samples disclosed by the present application comprises the following steps:

[0011] (1) According to the overall structural characteristics of the shear box of the large-scale direct shear instrument, the test principle of the direct shear test is clear, and the stress condition of the direct shear test of different size samples is analyzed;

[0012] (2) Based on the related theory of statics, the shear surface is analyzed separately, the shear surface stress diagram in the horizontal direction is drawn, and the horizontal static equilibrium equation of the shear surface is derived;

[0013] (3) The change condition of the actual shear area is derived according to the geometric analysis method;

[0014] (4) The large-scale direct shear test of different size samples is carried out, the uncorrected shear displacement and shear stress relationship curve is preliminarily established, and the test parameters of each period are obtained;

[0015] (5) According to the stress condition, the horizontal static equilibrium equation and the test parameters, the shear stress of the test is corrected, the corrected shear displacement and shear stress relationship curve is established, and the shear strength of the soil is determined.

[0016] The large-scale direct shear instrument comprises an upper shear box and a lower shear box, the upper and lower shear boxes generate relative displacement during the shear test, so that the shear sample is the shear surface, according to the static equilibrium condition of the stress state of the lower shear box, the corresponding horizontal static equilibrium equation of the shear surface is determined:

[0017] ∑F x =0, F x =τS i +f

[0018] In the formula, Fx is the horizontal thrust, f is the friction, and Si is the actual shear area at i moment.

[0019] Wherein, the actual shear area Si decreases with the increase of shear displacement amount xi, and its equation is:

[0020]

[0021] In the formula, R is the sample size, i.e., the sample radius.

[0022] Wherein, the corrected shear displacement amount and shear stress relationship curve is, the corrected shear stress at corresponding moment is calculated by the shear displacement amount and the horizontal thrust size at different i moments, so as to establish the relationship curve of the corrected shear stress and the shear displacement amount, to determine the shear strength of the soil body, including the following steps:

[0023] 1) When the shear displacement is xi, the corrected shear stress is:

[0024]

[0025] Wherein, the friction force f is:

[0026] f=f1+f2+f3

[0027] In the formula, f1 is the friction force of the upper shear box to the lower shear box; f2 is the friction force between the sample and the shear box; the lower shear box is arranged on the slide rail, and f3 is the friction force between the lower shear box and the slide rail;

[0028] Wherein, the friction force is related to the friction coefficient of the contact surface and the upper load, and the specific relationship is as follows:

[0029]

[0030] In the formula, m0 is the total mass of the sample; m1 is the mass of the upper shear box; f 21 is the friction force between the upper sample on the shear surface and the lower shear box; f 22 is the friction force between the lower sample on the shear surface and the upper shear box; m i2 is the mass of the upper sample on the shear surface acting on the lower shear box at i moment; m i3 is the mass of the lower sample on the shear surface acting on the upper shear box at i moment; m3 is the total mass of the shear box; μ1 is the friction coefficient between the shear boxes; μ2 is the friction coefficient between the sample and the shear box; μ3 is the friction coefficient between the shear box and the slide rail; P is the vertical pressure;

[0031] 2) According to the large-scale direct shear test of different sizes, the mass of each parameter is determined, and the corrected shear stress is finally derived as:

[0032]

[0033] 3) According to the above relationship and test parameters, the corrected shear stress at different i moments is calculated, the relationship curve between the corrected shear displacement and the shear stress is established, so as to determine the shear strength of the soil.

[0034] Compared with the prior art, the present application has the following beneficial effects: the present application provides a correction method for the shear stress of large-scale direct shear test of different size cylindrical samples, the method corrects the existing shear stress calculation, for cylindrical samples of different sizes, the influence of the actual shear area and the friction force on the shear stress is comprehensively considered, the relationship curve between the corrected shear displacement and the shear stress is established, the shear strength of the soil is determined, the error between the test value and the true value is minimized, and more accurate soil shear strength is obtained, and more safe and reliable test parameters are provided for engineering construction. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 is a space force diagram of a shear box of a large-scale direct shear instrument;

[0036] Figure 2 is a force system diagram of the shear surface in the horizontal direction;

[0037] Figure 3 is a diagram of the change of the sample area of the direct shear test.

[0038] Marked in the figure: 1, upper shear box; 2, lower shear box. DETAILED DESCRIPTION

[0039] The present application will be further described below in combination with the drawings and examples.

[0040] It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as generally understood by those skilled in the art to which the present application belongs.

[0041] In the derivation process, the existing calculation method is corrected according to the structure of the actual shear box and the test principle, and the error between the test value and the true value is minimized to the greatest extent. The key of the present application lies in: the test principle of the direct shear test is clear, the overall structural features of the shear box of the large-scale direct shear instrument are determined, and the force conditions of each component of the instrument are clear.

[0042] Please refer to Figure 1The shear box structure of the large direct shear apparatus disclosed in this embodiment includes an upper shear box 1 and a lower shear box 2. The lower shear box 2 is mounted on a slide rail. Both the upper and lower shear boxes are square solid structures. Both the upper shear box 1 and the lower shear box 2 have circular through holes of various sizes. An annular shear sleeve is nested in each circular through hole, with the annular shear sleeve in the upper shear box 1 corresponding one-to-one with the annular shear sleeve in the lower shear box 2, and they are paired for shearing operations. The annular shear sleeves can be made of a material with good rigidity to prevent deformation and damage during the direct shear test, which would affect the accuracy of the data. It should be understood that if the upper and lower shear boxes are made of durable materials, the annular shear sleeves may not be necessary, and the sample can be directly placed into the circular through hole for testing.

[0043] The principle of the direct shear test is as follows: When conducting the direct shear test, the upper shear box 1 is fixed, the sample is placed in the annular shear sleeve of the upper shear box 1, and a vertical pressure P is applied to the top of the sample. After the vertical pressure P stabilizes, the lower shear box 2 is pushed at a constant speed, so that shear deformation occurs on the horizontal contact surface between the upper and lower shear boxes until the sample is destroyed.

[0044] This embodiment discloses a method for correcting shear stress in large-scale direct shear tests of cylindrical specimens of different sizes, including the following steps:

[0045] (1) Based on the overall structural characteristics of the shear box of the large direct shear apparatus, clarify the test principle of the direct shear test, analyze the stress conditions of specimens of different sizes undergoing the direct shear test, and refer to... Figure 1 As shown;

[0046] (2) Based on relevant statics theories, the lower shear box 2 is separated, and the shear surface is analyzed separately. The force diagram of the shear surface in the horizontal direction is drawn, referring to... Figure 2 As shown, the horizontal static equilibrium equation of the shear plane is derived;

[0047] (3) The variation of the actual shear area was derived based on geometric analysis, referring to... Figure 3 As shown.

[0048] (4) Conduct large-scale direct shear tests on specimens of different sizes, initially establish the relationship curve between uncorrected shear displacement and shear stress, and obtain test parameters such as horizontal thrust, vertical pressure, horizontal displacement, vertical displacement, and shear displacement at each time period.

[0049] (5) Based on the stress conditions, equilibrium equations and test parameters, the shear stress of the test is corrected, and the relationship curve between the corrected shear displacement and shear stress is established.

[0050] Figure 1The drawing is based on relevant statics theories and the overall structural characteristics of the shear box of a large direct shear apparatus. The forces acting on the lower shear box 2 include: the horizontal thrust, vertical pressure, friction between the upper shear box and the lower shear box 2, friction between the soil and the lower shear box 2, friction between the lower shear box 2 and the slide rail, and shear force between the soil.

[0051] The influence of specimen size on the shear stress in the direct shear test is manifested in the following ways: different specimen sizes require different test termination conditions; larger specimens require a larger shear displacement, directly affecting the actual shear area; and different specimen sizes indirectly change the weight of the shear box, affecting the magnitude of friction. The horizontal static equilibrium equation of the shear surface is determined based on the static equilibrium conditions of the lower shear box 2, resulting in the corresponding horizontal static static equilibrium equation ∑F. x =0:

[0052] F x =τS i +f (0.1)

[0053] In the formula: F x is the horizontal thrust; f is the frictional force; S i Let be the actual shear area at time i.

[0054] Actual shear area S i It decreases as the shear displacement increases, referencing Figure 3 As shown. The centers of the upper and lower shear boxes are O1 and O2, respectively. During the direct shear test, the shear area of ​​the cylindrical specimen continuously decreases with the shear displacement. The geometric relationships in the image are:

[0055]

[0056] In the formula: R is the sample size, i.e., the sample radius; x i Let i be the shear displacement at time i; other parameters refer to Figure 3 As shown.

[0057] Calculate the area of ​​the shaded region for:

[0058]

[0059] The actual shear area of ​​the sample is 4 times the area of ​​the shaded region. Calculate the actual shear area S at time i. i for:

[0060]

[0061] The uncorrected shear stress is directly calculated by the ratio of the horizontal thrust on the lower shear box 2 and the shear area of the sample; the test parameters of each period can be directly obtained by the real-time data acquisition system of the direct shear apparatus.

[0062] The corrected shear displacement and shear stress relationship curve is: the corrected shear stress at different i time is calculated by the shear displacement and the size of the horizontal thrust at different i time, the relationship curve of the corrected shear stress and the shear displacement is established, and the shear strength of the soil body is determined, including the following steps:

[0063] When the shear displacement is x i , the corrected shear stress is:

[0064]

[0065] Among them, the friction force f is:

[0066] f=f1+f2+f3 (0.6)

[0067] In the formula: f1 is the friction force of the upper shear box on the lower shear box 2; f2 is the friction force between the sample and the shear box; f3 is the friction force between the shear box and the slide rail. Among them, the friction force f2 between the sample and the shear box includes the friction force between the upper sample on the shear surface and the lower shear box 2 and the friction force between the lower sample on the shear surface and the upper shear box 1.

[0068] The friction force is related to the friction coefficient of the contact surface and the upper load, and the specific relationship is as follows:

[0069]

[0070] In the formula: m0 is the total mass of the sample; m1 is the mass of the upper shear box 1; f 21 is the friction force between the upper sample on the shear surface and the lower shear box 2; f 22 is the friction force between the lower sample on the shear surface and the upper shear box 1; m i2 is the mass of the upper sample on the shear surface acting on the lower shear box 2 at i time; m i3 is the mass of the lower sample on the shear surface acting on the upper shear box 1 at i time; m3 is the total mass of the shear box; μ1 is the friction coefficient between the shear boxes; μ2 is the friction coefficient between the sample and the shear box; μ3 is the friction coefficient between the shear box and the slide rail; P is the vertical pressure.

[0071] If the friction coefficient of the shear box is unknown, the large direct shear apparatus of this invention can directly determine the magnitude of the friction force using a direct shear test in an empty box state. Then, the friction coefficients μ1 and μ3 can be calculated based on the known mass m1 of the upper shear box 1 and the total mass m3 of the shear box. Alternatively, the magnitudes of the friction forces f1 and f3 can be directly measured using a force gauge. At the same time, the friction coefficients μ1 and μ3 can be calculated based on the known mass m1 of the upper shear box 1 and the total mass m3 of the shear box. The value of μ3 can be obtained by placing the sample directly on the surface of the shear box, using a force gauge to pull the sample at a uniform speed, measuring the friction force between the sample and the shear box, and then calculating the friction coefficient μ2 between the sample and the shear box based on the known sample mass.

[0072] Among them, the frictional force f between the upper sample on the shear surface and the lower shear box 2 21 The mass m of the upper sample acting on the lower shear box 2 at time i i2 Related, m i2 It varies with the amount of shear displacement, and its derivation process is as follows:

[0073] Assuming the sample density remains constant during shearing, the relationship between sample mass and area can be established as follows:

[0074]

[0075] Where: m s S is the total mass of the sample in the upper shear box 1; S is the initial shear area of ​​the sample; S i2 This is the difference between the total area of ​​the sample and the actual shear area.

[0076] Reference Figure 3 As shown, based on geometric relationships, we can obtain:

[0077]

[0078] According to the relationship between mass and area (1.8), the mass m of the upper sample acting on the lower shear box 2 at time i can be obtained. i2 for:

[0079]

[0080] Therefore, the frictional force f between the upper sample on the shear surface and the lower shear box 2 21 for:

[0081]

[0082] Frictional force f between the lower sample and the upper shear box 22 It is related to the shear displacement at time i, and its derivation process is as follows:

[0083] Establish the mass m of the sample at the lower part of the shear plane. i3 The relationship between area and region is:

[0084]

[0085] The lower sample mass m of the shear plane is derived i3 is:

[0086]

[0087] Therefore, the friction force f between the lower sample of the shear plane and the upper shear box 22 is:

[0088]

[0089] The friction force f between the upper sample of the shear plane and the lower shear box 2 and the friction force f between the lower sample of the shear box and the upper shear box are integrated 21 The friction force f between the lower sample of the shear plane and the upper shear box 22 The friction force f2 of the sample acting on the shear box can be calculated as:

[0090]

[0091] According to the above, the revised shear stress is derived by substituting the parameters into formula (1.5):

[0092]

[0093] where S i As shown in formula (1.3), the remaining parameters are known.

[0094] According to the above relationship and test parameters, the revised shear stress at different i moments is calculated, the relationship curve between the revised shear displacement and the shear stress is established, and the shear strength of the soil body is determined.

Claims

1. A method for correcting the shear stress in a large-scale direct shear test of cylindrical specimens of different sizes, characterized in that, It comprises the following steps: (1) According to the overall structural features of the shear box of the large-scale direct shear apparatus, the test principle of the direct shear test is determined, and the stress conditions of the direct shear test on different sizes of samples are analyzed; (2) Based on the related theory of statics, the shear surface is separately analyzed, the stress diagram of the shear surface in the horizontal direction is drawn, and the horizontal static equilibrium equation of the shear surface is derived; (3) According to the geometric analysis method, the change of the actual shear area is derived; (4) The large-scale direct shear test on different sizes of samples is carried out, the uncorrected shear displacement and shear stress relationship curve is preliminarily established, and the test parameters at each period are obtained; (5) According to the stress condition, the horizontal static equilibrium equation and the test parameters, the shear stress of the test is corrected, the corrected shear displacement and shear stress relationship curve is established, and the shear strength of the soil is determined; The large-scale direct shear apparatus comprises an upper shear box (1) and a lower shear box (2), the horizontal contact surface between the upper and lower shear boxes is the shear surface, and the horizontal static equilibrium equation of the corresponding shear surface is determined according to the static equilibrium condition of the stress state of the lower shear box (2): ， ； wherein: is the horizontal thrust, is the friction, is i the actual shear area at the moment; The modified shear displacement amount and shear stress relationship curve is: wherein the modified shear displacement amount and shear stress relationship curve is calculated by the shear displacement amount and the size of the horizontal thrust at different time points. i The modified shear stress corresponding to the time point is calculated from the shear displacement amount and the size of the horizontal thrust at different time points, and a modified shear stress and shear displacement amount relationship curve is established to determine the shear strength of the soil body, including the following steps: 1) When the shear displacement is the corrected shear stress is: wherein the friction force f is: wherein: F1 is the friction force of the upper shear box (1) against the lower shear box (2); F2 is the friction force between the sample and the shear box; the lower shear box (2) is arranged on a slide rail, F3 is the friction force between the lower shear box (2) and the slide rail; Wherein, the friction force is related to the friction coefficient of the contact surface and the upper load, and the specific relationship is as follows: In the formula: This refers to the total mass of the sample. For the mass of the upper shear box (1); The frictional force between the upper sample and the lower shear box (2) on the shear surface; The frictional force between the lower part of the shear surface sample and the upper shear box (1); for i The mass of the upper sample on the shear surface acting on the lower shear box (2) at any given moment; for i The mass of the sample at the lower part of the shear plane acting on the upper shear box (1) at any given moment; This represents the total mass of the shear box. The coefficient of friction between the shear boxes; The coefficient of friction between the sample and the shear box; The coefficient of friction between the shear box and the slide rail; P For vertical pressure; 2) According to the large-scale direct shear test of different sizes, the size of each parameter is determined, and the corrected shear stress is finally derived as: ； In the formula: R r is the radius of the sample; 3) According to the above relationship and test parameters, the corrected shear stress at different time is calculated, the relationship curve between the corrected shear displacement and shear stress is established to determine the shear strength of soil. i The corrected shear stress at different time is calculated, the relationship curve between the corrected shear displacement and shear stress is established to determine the shear strength of soil.

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