Method for acquiring shear brittleness index of rock structural surface and method for evaluating shear brittleness

Abstract

The invention belongs to the field of rock mechanical property evaluation, and particularly relates to a rock structural surface shear brittleness index obtaining method and a shear brittleness evaluation method. And carrying out a direct shear test on the rock structural surface to obtain a shear stress-displacement whole-process curve, calculating according to the shear stress-displacement whole-process curve to obtain a pre-peak brittleness parameter S1 and a post-peak brittleness parameter S2, and multiplying the pre-peak brittleness parameter S1 and the post-peak brittleness parameter S2 to obtain the shear brittleness index SI of the rock structural surface. According to the method, the shear stress rise rate in the pre-peak loading stage, the shear stress drop amplitude in the post-peak stage and the relative displacement in the residual deformation stage are comprehensively considered, the whole-process shear deformation characteristics of the rock structural surface can be comprehensively reflected, and the required parameters are easily measured through a mechanical test.

Description

Technical Field

[0001] The present invention belongs to the field of rock mechanical property evaluation, and particularly relates to a method for obtaining a shear brittleness index of a rock structural surface and a shear brittleness evaluation method. Background Art

[0002] Various structural planes are widely present in deep rock masses. Structural planes with higher brittleness are more likely to induce catastrophes with greater intensity when destroyed. It is crucial to better understand the brittle failure of structural planes and accurately assess their brittleness.

[0003] The shear stress-displacement curve can fully reflect the influence of factors such as morphological characteristics, inclusion degree, normal stress, and groundwater on the tendency of shear instability.

[0004] At present, the quantitative analysis of shear brittleness of structural surfaces mainly includes the following aspects.

[0005] Cui (Cui Y.Effect of joint type on the shear behavior of synthetic rock[J].Bulletin of Engineering Geology and the Environment,2018,78:3395-3412.) refers to the rock brittleness index and uses the stress drop degree from peak shear stress to residual shear stress I b Characterizing the shear brittleness of structural surfaces:

[0006]

[0007] Li Yuzong et al. (Li Yuzong, Gao Sheng, Shen Shuqiang, et al. Experimental study on the influence of immersion time on rock strength and failure characteristics [J]. Chinese Journal of Rock Mechanics and Engineering 2025, 44(1): 99-113.) proposed an index B for describing the shear brittleness of rock joints based on the shear stress-shear displacement curve characteristics of rock joints. S :

[0008]

[0009] In formula (2), σ n is the normal stress applied during the shearing process, θ is the peak point in the shear stress-shear displacement curve (x s ,τ s ) and the point where the shear strength tends to stabilize after the peak strength (x r ,τ d ) and the horizontal direction, the point (x r ,τ d ) is related to the maximum shear displacement of the test.

[0010] The above indicators can reflect the degree of brittleness of the structural surface during the shear process to a certain extent, but the factors considered are not comprehensive, and there are usually some lack of self-consistency, such as misjudgment of some shear processes with obviously different brittleness, or the inability to give objective and reasonable brittleness parameter values ​​under some conditions. Summary of the Invention

[0011] The purpose of the present invention is to provide a method for obtaining the shear brittleness index of rock structural surfaces and a method for evaluating shear brittleness. By comprehensively considering the shear stress rising rate in the pre-peak loading stage, the shear stress drop amplitude in the post-peak stage, and the relative displacement in the residual deformation stage, the method can comprehensively reflect the shear deformation characteristics of the rock structural surface throughout the entire process, and the required parameters can be easily measured through mechanical tests.

[0012] The technical solution for achieving the purpose of the present invention is: a method for obtaining the shear brittleness index of a rock structural surface, performing a direct shear test on the rock structural surface to obtain a shear stress-displacement full-process curve, calculating the pre-peak brittleness parameter S1 and the post-peak brittleness parameter S2 based on the shear stress-displacement full-process curve, and multiplying the pre-peak brittleness parameter S1 and the post-peak brittleness parameter S2 to obtain the shear brittleness index SI of the rock structural surface.

[0013] Furthermore, the pre-peak brittleness parameter S1 is calculated by the following formula:

[0014] S1=lg(E+1)=lg(τ s / x s +1)

[0015] Where E is the secant modulus of the shear stress-displacement curve before the peak, τ s is the peak shear stress, x s is the shear displacement corresponding to the peak shear stress.

[0016] Furthermore, the calculation formula of the post-peak brittleness parameter S2 is:

[0017]

[0018] Where, τ d is the residual shear stress, x d is the residual shear stress corresponding to the shear displacement.

[0019] Furthermore, when the curve is of low strength and high ductility type, the structural surface undergoes plastic slip after reaching the peak shear stress. In extreme cases, there is no stress drop after the peak. At this time, the shear displacement corresponding to the participating stress is considered infinite, and x s / x d =0, the calculated value of the post-peak brittleness parameter S2 is 0, and the brittleness index SI is 0;

[0020] When the curve has an obvious inflection point in the post-peak stage, after reaching the peak shear stress, the shear stress gradually decreases with the increase of shear deformation. After reaching the residual shear stress, the shear stress no longer decreases. In extreme cases, the post-peak stress drops suddenly. At this time, x s =x d , x s / x d =1, the calculated value of the post-peak brittleness parameter S2 is τ s / τ d .

[0021] A method for evaluating the shear brittleness of a rock structural surface uses the shear brittleness index obtained by the above-mentioned acquisition method to evaluate the shear brittleness of a rock structural surface.

[0022] Compared with the prior art, the present invention has the following significant advantages:

[0023] The peak brittleness parameter constructed by the present invention after appropriate mathematical processing of the pre-peak secant modulus E can more comprehensively characterize the brittle characteristics of the pre-peak shear deformation stage; the peak shear stress τ s and residual shear stress τ d Ratio τ s / τ d Displacement x corresponding to peak shear stress s Displacement x corresponding to residual shear stress d Ratio x s / x d The constructed post-peak brittleness parameters can comprehensively reflect the decrease amplitude and decrease rate of the shear bearing capacity of the structural surface in the post-peak stage; the pre-peak brittleness parameters and the post-peak brittleness parameters are coupled in the form of a product, which can comprehensively reflect the shear brittleness of the structural surface shear process; since the calculation parameters used in this method are easy to measure and obtain in direct shear experiments, they are highly objective and convenient for data comparison in different strata and regions; the shear brittleness evaluation method of rock structural surfaces uses the shear brittleness index obtained according to the above calculation method to evaluate the shear brittleness of rock structural surfaces, which can relatively comprehensively and objectively characterize the shear brittleness degree of rock structural surfaces. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a typical shear stress-shear displacement curve.

[0025] Figure 2 Schematic diagram of the calculation principle of the shear brittleness index of the present invention.

[0026] Figure 3 It is the calculated value of shear brittleness index SI.

[0027] Figure 4 Shear brittleness index I d Calculated value.

[0028] Figure 5 is the shear brittleness index B s Calculated value.

[0029] Figure 6 This is a comparison chart of three indicators. DETAILED DESCRIPTION

[0030] To make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are described clearly and completely below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer are used. Where the manufacturer of the reagents or instruments is not specified, all are conventional products that can be purchased commercially.

[0031] The following is a detailed description of the shear brittleness evaluation method for the shear brittleness index of a rock structure surface according to an embodiment of the present invention.

[0032] A method for obtaining a shear brittleness index of a rock structure surface comprises the following steps:

[0033] Direct shear tests on rock structural surfaces were carried out to obtain the shear stress-shear displacement curve of the rock structural surface.

[0034] According to the shear stress-shear displacement curve of the rock structure surface, the pre-peak secant modulus E and peak shear stress τ are obtained. s , peak shear stress corresponds to displacement x s , residual shear stress τ d , the residual shear stress corresponds to the displacement x d .

[0035] The pre-peak brittleness parameter S1 is calculated based on the pre-peak secant modulus E.

[0036] The calculation formula of the pre-peak brittleness parameter S1 is:

[0037] S1=lg(E+1)=lg(τ s / x s +1)

[0038] Where E is the secant modulus of the pre-peak deformation of the shear stress-shear displacement curve, that is, the ratio of the peak shear stress of the shear stress-shear displacement curve to the corresponding displacement τ of the rock structure surface s / x s .

[0039] Based on rock properties, the pre-peak secant modulus E is typically estimated to be between 0 and 30 MPa / mm. By using a base-10 logarithmic form to process the pre-peak secant modulus E and converting the true number in the pre-peak brittleness parameter calculation formula to E+1, we can avoid the calculated pre-peak brittleness parameter S1 being negative when the value is less than 1.

[0040] According to the peak shear stress τ s , peak shear stress corresponds to displacement x s , residual shear stress τ d , the residual shear stress corresponds to the displacement x d Calculate the post-peak brittleness parameter S2.

[0041] The calculation formula of post-peak brittleness parameter S2 is:

[0042]

[0043] Where τ s / τ d is the relative magnitude of the post-peak stress drop, x s / x d It is the ratio of the displacement corresponding to the peak stress to the displacement when the residual shear stress is reached, reflecting the rate of decrease of the structural surface bearing capacity in the post-peak stage. The post-peak brittleness parameter S2 can be simplified as Figure 2 The area ratio S of the red box to the blue box Redbox / S Bluebox .

[0044] When the curve is of low strength and high ductility type, the structural surface undergoes plastic slip after reaching the peak shear stress. In extreme cases, there is no stress drop after the peak. At this time, the shear displacement corresponding to the participating stress can be regarded as infinite. s / x d =0, the calculated value of the post-peak brittleness parameter S2 is 0, and the brittleness index SI is also 0.

[0045] When the curve has an obvious inflection point in the post-peak stage, after reaching the peak shear stress, the shear stress gradually decreases with the increase of shear deformation. After reaching the residual shear stress, the shear stress no longer decreases. In extreme cases, the post-peak stress drops suddenly. At this time, x s =x d , x s / x d =1, the calculated value of the post-peak brittleness parameter S2 is τ s / τ d In the residual shear stress stage, due to the existence of normal stress, even extremely brittle rock structures can maintain a certain value of residual friction, τ s / τ d Usually it does not exceed 3, but in some special cases it can reach 5 or more.

[0046] In the intermediate transition stage, the post-peak brittleness parameter S2 is determined by the post-peak stress drop amplitude and deformation relationship.

[0047] The shear brittleness index SI of the rock structural surface is calculated by multiplying the pre-peak brittleness parameter S1 and the post-peak brittleness parameter S2.

[0048] SI=S1·S2

[0049] SI is the brittleness index, S1 is the pre-peak brittleness parameter, and S2 is the post-peak brittleness parameter.

[0050] In the shear brittleness index SI, the larger the pre-peak secant modulus, the less likely it is to undergo elastic deformation and the higher the rigidity; the faster the stress drop rate in the post-peak stage, the higher the stress drop amplitude, the more intense the energy release, and the stronger the shear brittleness.

[0051] A method for evaluating the shear brittleness of a rock structural surface comprises: evaluating the shear brittleness of the rock structural surface according to the shear brittleness index obtained by the above-mentioned acquisition method.

[0052] The shear brittleness index SI of the rock structure surface can be obtained by the above method. The size of the shear brittleness index SI can be used to judge the shear brittleness of the rock structure surface. The larger the shear brittleness index SI, the greater the shear brittleness of the rock structure surface.

[0053] In this embodiment, three kinds of regular jagged red sandstone structural surface samples were selected.

[0054] The features and performance of the present invention are further described in detail below with reference to the embodiments.

[0055] Example

[0056] Experimental object: Take the dense red sandstone from Jining, Shandong as an example, with a density of 2540kg / m 3 The average uniaxial compressive strength (UCS) is 49.30 MPa, and the average cohesion is 9.75 MPa. The prepared specimen has a size of length × width × height = 100 mm × 100 mm × 100 mm, consisting of two completely closed parts, the upper and lower parts. The structural surface consists of five continuous isosceles triangular saw teeth, the bottom length of the saw teeth is 20 mm, and the undulation angles are set at three levels: 15°, 30°, and 45°.

[0057] Experimental Procedure: Direct shear tests were conducted using a ZW-30B series microcomputer-controlled electronic rock direct shear apparatus. Normal loads were controlled by force, while tangential loads were controlled by displacement. Normal stress values ​​were set to 1.5 MPa, 2.5 MPa, 5 MPa, 10 MPa, 15 MPa, and 25 MPa. Normal stress was applied first. Once the normal load reached the target value and stabilized, tangential loading was applied at a rate of 0.02 mm / s.

[0058] The parameters required for the shear brittleness index SI can be obtained through experiments, and the calculation results are recorded in Table 1. The main parameters include the pre-peak secant modulus E, the peak shear stress τ s , the peak shear stress corresponds to the shear displacement x s , residual shear stress τd , the residual shear stress corresponds to the shear displacement x d .

[0059] The specific value of the shear brittleness index SI of the rock structure surface is calculated according to the formula SI = S1·S2 (see Table 1), where the pre-peak brittleness parameter S1 = lg(E+1) = lg(τ s / x s +1), post-peak brittleness parameter

[0060] Comparative Example 1

[0061] The stress drop degree I from peak shear stress to residual shear stress is used b Characterizing the shear brittleness of the structural surface, the calculation model is as follows: Where τ s is the peak shear stress, τ d is the residual shear stress, and its calculation results are recorded in Table 1.

[0062] Comparative Example 2

[0063] An index B is proposed based on the shear stress-shear displacement curve characteristics of rock joints to describe the shear brittleness of rock joints. S The shear brittleness of the structural surface is evaluated, and the calculation model is as follows: Where σ n is the normal stress applied during the shearing process, θ is the peak point in the shear stress-shear displacement curve (x s ,τ s ) and the point where the shear strength tends to stabilize after the peak strength (x r ,τ d ) and the horizontal direction, the point (x r ,τ d The selection of ) is related to the maximum shear displacement of the test. The calculation results are recorded in Table 1.

[0064] Table 1 shows the direct shear test results and shear brittleness index of three red sandstone serrated surface specimens

[0065] Table 1 Test parameters and calculation results of shear brittleness index

[0066]

[0067]

[0068] According to the calculation results of shear brittleness index in Table 1, index SI and index I b 、Indicator B S The relationship between Figure 3-Figure 6 .

[0069] from Figure 3 As can be seen, the shear brittleness index (SI) shows a positive correlation between the shear brittleness of the structural surface and the undulation angle. The larger the undulation angle, the greater the shear brittleness of the structural surface. As the normal stress increases, the SI shows that the shear brittleness of the structural surfaces with different undulation angles first increases and then decreases, and can be well fitted by a quadratic polynomial. The SI can well reflect the influence of the undulation angle and normal stress level on the shear brittleness of the structural surface.

[0070] Indicator I b 、Indicator B S The changing pattern of is slightly chaotic and does not completely match the changing pattern of the indicator SI. Figure 4 、 Figure 5 As shown in the figure, the brittleness indexes of rock structural surfaces with different relief angles are somewhat intertwined and there is no uniform rule; although under low normal stress conditions, index I b 、Indicator B S The changes vary, but overall they decrease with increasing normal stress, indicating that the shear brittleness of the structural plane decreases with increasing normal stress. This conflicts with the existing academic understanding of shear brittleness of structural planes.

[0071] Figure 6 The shear stress-shear deformation curves of the structural surface with an undulation angle of 30° are shown when the normal stress is 1.5MPa, 2.5MPa, and 5MPa. In the pre-peak stage, as the normal stress increases, the pre-peak stress magnitude and rising rate continue to increase, and the corresponding secant modulus E 30-3 >E 30-2 >E 30-1 , sample 30-3 (representing a structural surface with a 30° undulation angle under 5MPa normal stress conditions) has the strongest ability to resist deformation when subjected to stress, while sample 30-1 (representing a structural surface with a 30° undulation angle under 1.5MPa normal stress conditions) has the weakest ability to resist deformation. In the post-peak stage, the stress drop amplitude is observed. As the normal stress increases, the stress drop amplitude increases from 1.77MPa to 3.62MPa. The post-peak stress drop rate also shows k d30-3 >k d30-2 >k d30-1 , combined with the stress drop amplitude and stress drop rate, sample 30-3 exhibits stronger shear brittleness in the post-peak stage; among them, sample 30-2 represents a structural surface with a ripple angle of 30° under a positive stress of 2.5 MPa.

[0072] Indicator I bOnly focusing on the magnitude of stress drop, while ignoring the effect of stress drop rate, the calculated results (0.51, 0.50, 0.4 respectively) show that shear brittleness decreases with the increase of normal stress, which is inconsistent with the analysis of the curve based on the physical meaning of brittleness. For example, under the condition of 5MPa normal stress, the stress drop develops quickly after the shear peak and the energy released is large, but the index I b The brittleness represented is less than that of the two working conditions with low normal stress.

[0073] Indicator B S During the calculation of x r We take 8mm as the normal stress increases, and cosθ 30-1 =0.96, cosθ 30-2 =0.93, cosθ 30-3 =0.81, which to some extent reflects the increasing brittleness of the post-peak stage of the curve. However, due to the neglect of the effects of pre-peak deformability and post-peak stress drop rate, the overall calculated index results (2.12, 1.92, and 1.98, respectively) show a decreasing trend, which is inconsistent with the direct analysis of the curve brittleness. Based on the brittleness index calculation formula proposed in this paper, the calculated SI values ​​are 0.41, 0.59, and 0.75, respectively, which can better reflect the trend of increasing shear brittleness of the structural surface with increasing normal stress.

[0074] The shear brittleness evaluation method of the rock structural surface uses the shear brittleness index calculated according to the above method to evaluate the shear brittleness of the rock structural surface, which can more comprehensively characterize the shear brittle characteristics of the rock structural surface.

[0075] In summary, the method for obtaining the shear brittleness index of a rock structural surface and the method for evaluating the shear brittleness of a rock structural surface according to the embodiment of the present invention construct the pre-peak brittleness parameter after appropriate mathematical processing of the pre-peak secant modulus E, which can more comprehensively characterize the brittle characteristics of the pre-peak shear deformation stage; using the peak shear stress τ s and residual shear stress τ d Ratio τ s / τ d Displacement x corresponding to peak shear stress s Displacement x corresponding to residual shear stress d Ratio x s / x d Constructing a post-peak brittleness parameter can reflect the magnitude and rate of decline in the bearing capacity of a structural surface during the post-peak phase. The pre-peak and post-peak brittleness parameters are multiplied to comprehensively reflect the shear brittleness of the structural surface under different conditions. This method utilizes parameters that are readily available in direct shear tests, resulting in high objectivity and ease of operation, facilitating data comparison across different strata and regions.

[0076] The embodiments described above are some, but not all, of the embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the invention as claimed, but rather merely represents selected embodiments of the present invention. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are also within the scope of protection of the present invention.

Claims

1. A method for obtaining a shear brittleness index of a rock structure surface, characterized in that: A direct shear test on the rock structural surface is carried out to obtain the shear stress-displacement full process curve. The pre-peak brittleness parameter S1 and the post-peak brittleness parameter S2 are calculated according to the shear stress-displacement full process curve. The shear brittleness index SI of the rock structural surface is obtained by multiplying the pre-peak brittleness parameter S1 and the post-peak brittleness parameter S2.

2. The acquisition method according to claim 1, characterized in that The pre-peak brittleness parameter S1 is calculated by the following formula: S1=lg(E+1)=lg(τ s / x s +1) Where E is the secant modulus of the shear stress-displacement curve before the peak, τ s is the peak shear stress, x s is the shear displacement corresponding to the peak shear stress.

3. The acquisition method according to claim 2, characterized in that The calculation formula of the post-peak brittleness parameter S2 is: Where, τ d is the residual shear stress, x d is the residual shear stress corresponding to the shear displacement.

4. The acquisition method according to claim 3, characterized in that When the curve is of low strength and high ductility type, the structural surface undergoes plastic slip after reaching the peak shear stress. In extreme cases, there is no stress drop after the peak. At this time, the shear displacement corresponding to the participating stress is considered infinite, and x s / x d =0, the calculated value of the post-peak brittleness parameter S2 is 0, and the brittleness index SI is 0; When the curve has an obvious inflection point in the post-peak stage, after reaching the peak shear stress, the shear stress gradually decreases with the increase of shear deformation. After reaching the residual shear stress, the shear stress no longer decreases. In extreme cases, the post-peak stress drops suddenly. At this time, x s =x d , x s / x d =1, the calculated value of the post-peak brittleness parameter S2 is τ s / τ d .

5. A method for evaluating shear brittleness of rock structural surfaces, characterized in that: The shear brittleness of the rock structure surface is evaluated using the shear brittleness index obtained by the acquisition method according to any one of claims 1 to 4.

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