Method for determining anisotropic strength of weakly cemented soft rock

Abstract

The invention provides a method for determining anisotropic strength of weakly cemented soft rock, and relates to the field of rock mass mechanics. The method specifically comprises the following steps: S1, preparing a weakly cemented soft rock hollow cylinder sample by adopting a coaxial double-core drill bit; s2, carrying out weak cemented soft rock directional shear tests in different large principal stress action directions by using a hollow cylinder torsional shear apparatus; s3, according to a directional shear test result, simplifying a damage point strength envelope line into an elliptical model; s4, establishing an anisotropic elliptical failure criterion implicit function mathematical model; and S5, determining parameters of the anisotropy model of the weakly cemented soft rock. Compared with the prior art, the method has the advantages that the defect that the isotropic strength criterion does not consider the main stress direction effect is overcome, and the reliability and accuracy of the anisotropic strength of the soft rock are improved.

Description

Technical Field

[0001] The invention relates to the field of rock mass mechanics, and in particular to a method for determining the anisotropic strength of weakly cemented soft rock. Background Art

[0002] Weakly cemented soft rock is widely found in geotechnical engineering fields such as mining and tunnel construction. Due to its low strength, poor cementation, easy softening when exposed to water, high sensitivity to disturbance and strong rheological properties, weakly cemented soft rock presents complex and significant anisotropic characteristics in engineering practice. When disturbing loads such as tunnel excavation or face mining are applied, the direction of the principal stress rotates, thereby weakening the bearing capacity of the tunnel surrounding rock, shortening its self-stabilization time, and expanding the plastic zone. This leads to the failure of support means such as anchors (cables) and scaffolding, inducing engineering disasters such as roof collapse, floor bulging and uneven deformation of the surrounding rock surface. The current technical methods are mostly based on the isotropic assumption to determine the anisotropic strength of weakly cemented soft rock, which fails to fully consider its anisotropic characteristics and is difficult to accurately reflect its mechanical performance in the actual engineering environment. Therefore, a new analysis method is urgently needed to determine the anisotropic strength characteristics of weakly cemented soft rock, so as to provide a more scientific basis for the design and construction of geotechnical engineering. Summary of the invention

[0003] The present invention aims to provide a method for determining the anisotropic strength of weakly cemented soft rock, which effectively overcomes the limitation of the prior art that the principal stress direction and the anisotropic characteristics of soft rock are ignored in strength determination, and can consider the effect of the principal stress direction and accurately evaluate the anisotropic strength of weakly cemented soft rock.

[0004] To achieve the above object, the present invention provides a method for determining the anisotropic strength of weakly cemented soft rock, the specific steps are as follows:

[0005] S1, hollow cylindrical specimens of weakly cemented soft rock were prepared using a coaxial double-core drill bit;

[0006] S2, using a hollow cylindrical torsion shear apparatus to carry out directional shear tests on weakly cemented soft rock under different directions of major principal stress;

[0007] S3, based on the results of the directional shear test, the strength envelope of the failure point is simplified into an elliptical model;

[0008] S4, establish an implicit function mathematical model of anisotropic elliptical failure criterion;

[0009] S5, determine the anisotropic model parameters of weakly cemented soft rock;

[0010] Furthermore, in step 1, the mined large block of weakly cemented soft rock is cut into standard cubes, and then a coaxial double-core drill is used to obtain hollow cylindrical samples by core sampling. The inner diameter, outer diameter and height of each group of samples are consistent, and the inner diameter, outer diameter and height are 60 mm, 100 mm and 200 mm respectively.

[0011] Furthermore, in step 2, a directional shear test under different principal stress directions is carried out on a hollow cylindrical specimen of weakly cemented soft rock using a hollow cylindrical torsion shear apparatus. The test conditions are set to an average principal stress of 5 MPa, an intermediate principal stress coefficient of 0.5, principal stress action direction angles of 0°, 15°, 22.5°, 30°, 45°, 60°, 67.5°, 75° and 90°, respectively. The deviatoric stress starts from 0 MPa and is continuously loaded at a rate of 0.1 MPa / min until the specimen is damaged.

[0012] Furthermore, in step 3, a stress plane is established with the horizontal coordinate being the difference between the axial stress component and the hoop stress component and the vertical coordinate being twice the shear stress component. According to the results of the directional shear test, a stress path is drawn in the stress plane, and the strength envelope is drawn by connecting the peak points of the stress path. The strength envelope is simplified into an ellipse with a center offset in the stress plane. The difference in the deviatoric stress in the directional shear test when the principal stress direction angle α=0° and 90° is taken as the major axis AC of the ellipse, and the shear strength τ in the pure torsion shear test when α=45° is taken as the shear strength τ zθ The line OD connecting the point O is extended by offset ζ to the ellipse trajectory line as the minor semi-axis BE of the ellipse; the angle β between the line connecting any point F on the ellipse trajectory line and the point O and the positive direction of the horizontal axis is twice the principal stress direction angle α.

[0013] Furthermore, in step 4, an implicit function formula of anisotropic elliptical failure criterion is established;

[0014] The ellipse intensity relationship is constructed according to the function mapping relationship, as shown in formula (1).

[0015] Take the difference in stress ratio when the major principal stress direction angle is 0° and 90° (σ f,0 / p'-σ f,90 / p') is the major semi-axis a, as shown in formula (2),

[0016] Assuming that the major principal stress direction angle is 0°, the interpolation of the stress ratio and the major semi-axis a is the horizontal axis offset ζ, as shown in formula (3),

[0017] Substituting the stress ratio when the major principal stress direction angle is 45° into formula (1), and combining formula (2) with formula (3), the functional expression of the minor semi-axis b is obtained as shown in formula (4):

[0018] The vertical axis offset η can be determined by the difference between the shear strength corresponding to the center point of the ellipse and the minor semi-axis, as shown in formula (5): μ=τ f -b (5)

[0019] Substituting formula (2) to formula (5) into formula (1), we get formula (6):

[0020] Shear stress ratio τ when the direction of any large principal stress acts f,α / p' and normal stress ratio σ f,α / p' has the functional relationship shown in formula (7), τ f,a / p'=(σ f,a / p')tan2α (7)

[0021] Combining equations (6) and (7) can obtain the implicit function formula of the anisotropic elliptical failure criterion of shear strength of weakly cemented soft rock, as shown in equation (8):

[0022] Where: α is the principal stress direction angle, σ f.α is the peak deviatoric stress of weakly cemented soft rock under different α, τf is the peak shear stress of weakly cemented soft rock under different α, τ f.α is the peak shear stress of weakly cemented soft rock under α, a is the major semi-axis of the ellipse, b is the minor semi-axis of the ellipse, ζ is the distance of horizontal axis offset, and η is the vertical axis offset.

[0023] Furthermore, in step 5, triaxial compression test, triaxial tension test and pure torsion shear test are carried out to measure the compressive strength, tensile strength and shear strength of weakly cemented soft rock, which are 2.32, 0.74 and 1.66 respectively; the ratio of the obtained compressive strength, tensile strength and shear strength parameters to the average principal stress is substituted into the implicit function formula of the anisotropic elliptical failure criterion obtained in step 4, and finally the strength model is obtained.

[0024] Compared with the prior art, the technical solution provided by the present invention has the following beneficial technical effects: a method for determining the anisotropic strength of weakly cemented soft rock is proposed, which overcomes the limitation of the prior art that the strength is inaccurately determined based on the isotropic assumption, and effectively improves the reliability of the calculation of the anisotropic strength results of weakly cemented soft rock. Compared with the traditional isotropic method, the error between the strength value predicted by this method and the actual measured value is reduced by more than 60%. Through scientific experiments and theoretical analysis, the present invention has shown significant advantages in terms of technical, economic and social benefits. The strength determination method provided provides a more scientific basis for engineering designs such as mining and tunnel construction, reduces engineering disasters such as roof collapse, floor bulging and surrounding rock deformation caused by unreasonable design, and provides reliable technical support and scientific basis for geotechnical engineering fields such as mining and tunnel construction. Figures and their descriptions

[0025] The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0026] Figure 1 A technical flow chart for determining the anisotropic strength of weakly cemented soft rock according to the present invention;

[0027] Figure 2 The weakly cemented soft rock hollow cylindrical specimen provided in the embodiment of the present invention;

[0028] Figure 3 is the stress path of the directional shear test in the embodiment of the present invention;

[0029] Figure 4 A simplified elliptical mechanics model provided in an embodiment of the present invention;

[0030] Figure 5 This is the elliptical anisotropic strength model provided in the embodiment of the present invention. DETAILED DESCRIPTION

[0031] In order to enable professionals in the technical field to better understand and implement the scheme of the present invention, the specific embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings. The embodiments are only descriptions of a part of the specific implementation of the present invention and do not cover all possible implementation methods of the present invention. Based on the embodiments of the present invention, all other embodiments that can be thought of by ordinary technicians in this field without creative work should belong to the protection scope of the present invention.

[0032] The present invention provides a method for determining the anisotropic strength of weakly cemented soft rock, comprising the following steps:

[0033] S1. Use coaxial double-core drill bit to prepare weakly cemented soft rock hollow cylindrical specimens;

[0034] S2. Use a hollow cylindrical torsion shear apparatus to carry out directional shear tests on weakly cemented soft rock under different directions of major principal stress;

[0035] S3. According to the results of the directional shear test, the strength envelope of the failure point is simplified into an elliptical model;

[0036] S4. Establish an implicit function mathematical model of anisotropic elliptical failure criterion;

[0037] S5. Determine the anisotropic model parameters of weakly cemented soft rock;

[0038] In step 1, a hollow cylindrical sample of weakly cemented soft rock is prepared. The mined large piece of weakly cemented soft rock is cut into a standard cube, and a coaxial double-core drill is used to prepare a hollow cylindrical sample of weakly cemented soft rock by core sampling. In order to ensure the consistency of the sample size, the inner diameter, outer diameter and height of the sample are 60 mm, 100 mm and 200 mm respectively, to ensure the uniformity of the material and avoid the influence of processing errors on the experimental results.

[0039] In step 2, directional shear tests with different major principal stress direction angles are carried out to determine the shear strength of weakly cemented soft rock under different principal stress directions. A hollow cylindrical torsion shear apparatus is used to carry out directional shear tests on the prepared hollow cylindrical specimens under different major principal stress directions. During the experiment, the average principal stress is set to 5MPa, the intermediate principal stress coefficient is 0.5, the principal stress direction angles are 0°, 15°, 22.5°, 30°, 45°, 60°, 67.5°, 75° and 90°, respectively. The deviatoric stress is continuously loaded at a rate of 0.1MPa / min from 0MPa until the specimen is destroyed. The shear stress is 0 when the principal stress direction angle is 0° and 90°. Finally, the shear strengths of 7 groups of different principal stress directions from 15° to 75° are obtained through the test results, which are 2.8MPa, 3.6MPa, 4.05MPa, 3.7MPa, 3.45MPa, 2.85MPa and 0.21MPa, respectively.

[0040] In step 3, the elliptical mechanical model is simplified according to the directional shear stress path; a stress plane is established with the horizontal coordinate being the difference between the axial stress component and the annular stress component and the vertical coordinate being twice the shear stress component, and the peak shear stress ratio under different principal stress directions is calculated based on the directional shear test data. The peak shear stress ratio is defined as the ratio of the shear strength to the average principal stress under different principal stress directions, and the calculated results are 0.56, 0.72, 0.81, 0.74, 0.69, 0.57 and 0.42, respectively; a stress path is drawn in the stress plane according to the peak shear stress ratio data, and the shear strength envelope in each direction is drawn by connecting the peak points of the stress path in the stress plane to provide an intuitive display of the anisotropic strength characteristics; according to the characteristics of the shear strength envelope, the strength envelope is simplified into an ellipse with a center offset in the stress plane; the difference in the deviatoric stress in the directional shear test when the principal stress direction angle α=0° and 90° is taken as the major axis AC of the ellipse, and the shear strength τ in the pure torsional shear test when α=45° is taken as the major axis AC of the ellipse. zθ The line OD connecting the point O is extended by offset ζ to the ellipse trajectory line as the minor semi-axis BE of the ellipse; the angle β between the line connecting any point F on the ellipse trajectory line and the point O and the positive direction of the horizontal axis is twice the principal stress direction angle α.

[0041] In step 4, an implicit function formula of the anisotropic elliptical failure criterion is established; and an elliptical strength relationship is constructed according to the function mapping relationship, as shown in formula (1).

[0042] Take the difference in stress ratio when the major principal stress direction angle is 0° and 90° (σ f,0 / p'-σ f,90 / p') is the major semi-axis a, as shown in formula (2),

[0043] Assuming that the major principal stress direction angle is 0°, the interpolation of the stress ratio and the major semi-axis a is the horizontal axis offset ζ, as shown in formula (3),

[0044] Substituting the stress ratio when the major principal stress direction angle is 45° into formula (1), and combining formula (2) with formula (3), the functional expression of the minor semi-axis b is obtained as shown in formula (4):

[0045] The vertical axis offset η can be determined by the difference between the shear strength corresponding to the center point of the ellipse and the minor semi-axis, as shown in formula (5): μ=τ f -b (5)

[0046] Substituting formula (2) to formula (5) into formula (1), we get formula (6):

[0047] Shear stress ratio τ when the direction of any large principal stress acts f,α / p' and normal stress ratio σ f,α / p' has the functional relationship shown in formula (7), τ f,a / p'=(σ f,a / P')tan2α (7)

[0048] Combining equations (6) and (7) can obtain the implicit function formula of the anisotropic elliptical failure criterion of shear strength of weakly cemented soft rock, as shown in equation (8):

[0049] Where: α is the principal stress direction angle, σ f.α is the peak deviatoric stress of weakly cemented soft rock under different α, τ f is the peak shear stress of weakly cemented soft rock under different α, τ f.α is the peak shear stress of weakly cemented soft rock under α, a is the major semi-axis of the ellipse, b is the minor semi-axis of the ellipse, ζ is the distance of horizontal axis offset, and η is the vertical axis offset.

[0050] In step 5, the anisotropic model parameters of weakly cemented soft rock are determined; triaxial compression test, triaxial tension test and pure torsion test are carried out to determine the ratios of compressive strength, tensile strength and shear strength to the average principal stress of weakly cemented soft rock, which are 2.32, 0.74 and 1.66 respectively; the ratios of the obtained compressive strength, tensile strength and shear strength parameters to the average principal stress are substituted into the implicit function formula of the anisotropic elliptical failure criterion obtained in step 4, and finally the strength model is obtained.

[0051] The present invention proposes a method for determining the anisotropic strength of weakly cemented soft rock, which relates to the field of rock mechanics. The method prepares hollow cylindrical specimens of weakly cemented soft rock, conducts directional shear tests at different major principal stress direction angles, simplifies the directional shear stress path into an elliptical mechanics model, establishes an anisotropic elliptical failure criterion implicit function formula, and determines the model parameters through triaxial compression test, triaxial tension test and pure torsion shear test. Compared with the traditional method, the method overcomes the error caused by the prior art based on the isotropic assumption, improves the reliability of the calculation of the anisotropic strength results of weakly cemented soft rock, and has significant technical and economic benefits.

[0052] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modification, equivalent replacement, improvement, etc. made within the concept and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for determining the anisotropic strength of weakly cemented soft rock, characterized in that: The method The following steps are involved: S1, a coaxial double-core drill bit was used to prepare a weakly cemented soft rock hollow cylindrical sample with an inner diameter of 60 mm, an outer diameter of 100 mm, and a height of 200 mm; S2, using a hollow cylinder torsion shear apparatus to carry out directional shear tests on weakly cemented soft rock hollow cylinder specimens in different directions of major principal stress; during the test, the average principal stress p and the intermediate principal stress coefficient b were set to constant values; Then, after rotating the principal stress to the target action direction angle, the deviatoric stress is applied in a manner of controlling the stress rate until the specimen fails; S3, collating the directional shear test data, at 2τ zθ / p'-(σ z -σ θ ) / p' plane to draw the strength envelope of weakly cemented soft rock; S4, based on the results of the directional shear test, the implicit function formula of the anisotropic elliptical failure criterion is established, and the elliptical relationship is constructed according to the function mapping relationship as shown in formula (1): Take the difference in stress ratio when the major principal stress direction angle is 0° and 90° (σ f,0 / p'-σ f,90 / p') is the major semi-axis a, as shown in formula (2), Assuming that the major principal stress direction angle is 0°, the interpolation of the stress ratio and the major semi-axis a is the horizontal axis offset ζ, as shown in formula (3), Substituting the stress ratio when the major principal stress direction angle is 45° into formula (1), and combining formula (2) with formula (3), the functional expression of the minor semi-axis b is obtained as shown in formula (4): The vertical axis offset η can be determined by the difference between the shear strength corresponding to the center point of the ellipse and the minor semi-axis, as shown in formula (5): μ = τ f -d (5) Substituting formula (2) to formula (5) into formula (1), we get formula (6): Shear stress ratio τ when the direction of any large principal stress acts f,α / p' and normal stress ratio σ f,α / p' has the functional relationship shown in formula (7), t f,a / p ′ =(s f,a / p ′ )tan2a (7) Combining equations (6) and (7) can obtain the implicit function formula of the anisotropic elliptical failure criterion of shear strength of weakly cemented soft rock, as shown in equation (8): Where: α is the principal stress direction angle, σ f.α is the peak deviatoric stress of weakly cemented soft rock under α, τ f is the peak shear stress of weakly cemented soft rock under different α, τ f.α is the peak shear stress of weakly cemented soft rock under α, a is the major semi-axis of the ellipse, b is the minor semi-axis of the ellipse, ζ is the distance of horizontal axis offset, and η is the vertical axis offset; S5. Conduct indoor tests to measure the compressive strength, tensile strength and shear strength of weakly cemented soft rock, and substitute the compressive strength, tensile strength and shear strength parameters obtained in the test into the implicit function formula of the anisotropic elliptical failure criterion obtained in step 4.

2. A method for determining the anisotropic strength of weakly cemented soft rock according to claim 1, characterized in that: The weakly cemented soft rock in step 1 is a Cretaceous and Jurassic soft rock with poor cementation, which has the characteristics of low strength, easy weathering, easy softening, mudification and disintegration when exposed to water; the bulk density γ of the weakly cemented soft rock is 24.08 kN / m 3 , moisture content ω is between 14.01% and 18.87%, and the average uniaxial compressive strength σ c It is 9.44MPa, the cohesion is 0.48MPa~3.09MPa, the internal friction angle is 30.01°~31.02°, and shear failure is prone to occur.

3. A method for determining the anisotropic strength of weakly cemented soft rock according to claim 1, characterized in that: The weakly cemented soft rock sample described in step 1 is a hollow cylindrical sample with an inner diameter of 60 mm, an outer diameter of 100 mm and a height of 200 mm.

4. A method for determining the anisotropic strength of weakly cemented soft rock according to claim 1, characterized in that: In the directional shear test described in step 2, the average principal stress is set to 5 MPa, the intermediate principal stress coefficient is set to 0.5, and the direction angles of the major principal stresses are 0°, 15°, 22.5°, 30°, 45°, 60°, 67.5°, 75°, and 90°, respectively. There are 9 groups in total, and the deviatoric stress is continuously loaded from 0 MPa at a rate of 0.1 MPa / min until the sample is damaged.

5. The method for determining the anisotropic strength of weakly cemented soft rock according to claim 1, characterized in that: The strength envelope of the directional shear test described in step 3 is based on the axial stress component σ z and the hoop stress component σ θ The ratio of the difference between the average effective principal stress p' (σ z -σ θ ) / p' is the horizontal coordinate, and the ratio of the 2 times shear stress component to the average effective principal stress p' is 2τ zθ / p' is the vertical axis for drawing.

6. A method for determining the anisotropic strength of weakly cemented soft rock according to claim 1, characterized in that Step 4: Set the strength envelope of the anisotropic elliptical failure criterion to 2τ zθ / p'-(σ z -σ θ ) / p' is an ellipse with the center offset in the stress plane, where the difference in the deviatoric stress in the directional shear test when the major principal stress direction angle α = 0° and 90° is taken as the major axis AC of the ellipse, and the shear strength τ in the pure torsion shear test when α = 45° is taken as zθ The line OD connecting the point O is extended by offset ζ to the ellipse trajectory line as the minor semi-axis BE of the ellipse. The angle between the line connecting any point F on the ellipse trajectory line and the point O and the positive direction of the horizontal axis is 2α.

7. A method for determining the anisotropic strength of weakly cemented soft rock according to claim 1, characterized in that: The indoor test described in step 5 includes a triaxial compression test, a triaxial tension test and a pure torsional shear test.

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