Rock long-term strength determination method for uniaxial compression test based on multistage stress

Abstract

The invention relates to the technical field of rock engineering, in particular to a rock long-term strength determination method based on a uniaxial compression creep test of multistage stress. Comprising the following steps: S1, carrying out multi-stage stress uniaxial compression creep test on a rock sample, and drawing a rock creep curve based on a test result; s2, carrying out reverse extension on a steady-state creep stage curve in the rock creep curve; and S3, approximately fitting the relationship between the initial deformation of the rock and the stress level under the low stress level and the high stress level by using the two linear functions respectively, and taking the stress level corresponding to the intersection point of the two linear functions as the long-term strength of the rock. Through the uniaxial compression test, different stress level loading tests can be carried out, the problem that the inflection point of a creep isochronous curve is not obvious is solved, and a basis is provided for determining the long-term strength of rock and the evolution law of strength parameters along with time; compared with the method for determining the long-term strength through an isochronous curve method, the parameter value obtained through the method is higher in precision and reliability.

Description

Technical Field

[0001] The invention relates to the technical field of rock engineering, and in particular to a method for determining the long-term strength of rock based on a uniaxial compression test with multi-level stress. Background Art

[0002] Long-term strength refers to the strength of rock to resist destruction under long-term load. Rock will produce creep effect under long-term load, so creep strength is taken as the long-term strength of rock. Rock can undergo stable creep or unstable creep, which depends on the magnitude of rock stress. When it exceeds a certain critical stress, creep develops into unstable creep. When it is less than this critical stress, creep develops as stable creep. This critical stress is usually called the long-term strength of rock.

[0003] In the prior art, the long-term strength of rock and the evolution of its strength parameters over time are mostly obtained based on creep test results, using creep isochronous curve method or steady creep rate method. The inflection point method of isochronous stress-strain curve is used to determine the long-term strength of salt rock. Sufficient salt rock creep data are required to ensure the accuracy of the obtained long-term strength value. If the creep load level is small, the inflection point of the isochronous stress-strain curve is not obvious enough, and the long-term strength is difficult to accurately determine, and the reliability of the result is poor.

[0004] To solve this problem, the existing patent number is "CN104849134B" "Method for determining the long-term strength of rock by multi-level stress graded loading creep mechanics test". By analyzing the strain rate of the steady-state stage of axial strain, circumferential strain and volume strain, the long-term strength of rock is determined by the inflection point of its steady-state strain rate and the intersection of the above three.

[0005] This method still has the following problems:

[0006] (1) It is necessary to monitor the changes of axial compression strain and annular expansion strain over time under different stresses and calculate the changes of specimen volume strain over time. This requires high standards for experimental instruments and operations, and requires a lot of data to be tested and calculated.

[0007] (2) This invention needs to calculate the steady-state creep rate of axial strain, hoop strain and volume strain at each stress level, which requires a lot of data to be processed and the operation is complicated;

[0008] (3) In this invention, “the relationship curves of axial, circumferential and volume steady-state creep rates versus stress levels are plotted, and their intersection points are determined. The stress corresponding to the intersection points is the long-term strength of the rock.” The method for determining the long-term strength here is the intersection point of the three curves. As is known to all, two lines intersect at an intersection point, and the third curve cannot be determined to pass through the exact intersection point of the first two curves. Summary of the invention

[0009] In view of the above-mentioned problems, the present invention proposes a method for determining the long-term strength of rock based on a uniaxial compression test with multi-level stress, which solves the problem of difficulty and inaccuracy in obtaining long-term strength in the prior art and improves the accuracy and reliability of determining the long-term strength of rock.

[0010] In order to achieve the above object, the technical solution of the present invention is as follows:

[0011] A method for determining the long-term strength of rock based on a uniaxial compression test with multi-level stress, comprising:

[0012] S1: Conduct uniaxial compression creep tests at multiple stress levels on rock samples, and draw rock creep curves at different stress levels based on the test results;

[0013] S2: The steady-state creep curve in the rock creep curve of S1 is extended in the reverse direction and intersects with the ordinate axis. The intersection value is the sum of the instantaneous elastic deformation and the decay creep. It is assumed that the rock directly enters the steady-state creep stage after an instantaneous initial deformation.

[0014] S3: Extract the initial deformation values ​​under different stress levels, and use two linear functions to approximate the relationship between the initial deformation of the rock and the stress level under lower and higher stress levels. The stress value corresponding to the intersection of the two linear functions is the long-term strength of the rock.

[0015] Preferably, in the uniaxial compression creep test of S1 multi-level stress, the loading adopts strain control mode, the loading rate is ≥5 to 10e-4 mm / s, and after the strain is loaded to the design value, the strain is kept unchanged, and the attenuation law of stress over time is monitored.

[0016] Preferably, in the S1 multi-level stress uniaxial compression creep test, the rock uniaxial compression test stress level is not less than level 4, wherein the low stress level and the high stress level are not less than level 2.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] (1) The present invention has low test requirements and only requires a uniaxial compression creep test, which provides convenience for construction environments with simple test conditions;

[0019] (2) Compared with the prior art, the present invention requires much less data to be processed, and the reverse extension line of steady-state creep is simpler and more intuitive;

[0020] (3) The method of the present invention to ultimately determine the long-term strength is the intersection of two lines, which has a unique definite value (the two straight lines cannot be parallel if the test is correct);

[0021] (4) The present invention overcomes the problem of unclear inflection point of creep isochronal curve by loading test at different stress levels through uniaxial compression test, and provides a basis for determining the long-term strength of rock and the evolution law of strength parameters over time. Compared with the long-term strength determined by creep test, the parameter value obtained by the method of the present invention has higher accuracy and reliability, which improves the understanding of rock creep characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. They are used to explain the present invention together with the embodiments of the present invention and do not constitute a limitation of the present invention.

[0023] In the attached picture:

[0024] Figure 1 Flow chart of the method for determining long-term strength of rock;

[0025] Figure 2 is the rock creep strain curve;

[0026] Figure 3 It is the reverse extension line of the creep curve in the steady state stage;

[0027] Figure 4 Diagram for the process of determining long-term strength of rock. DETAILED DESCRIPTION

[0028] The following combination Figure 1-Figure 4 The preferred embodiments of the present invention are described. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0029] Example:

[0030] A method for determining the long-term strength of rock based on a uniaxial compression test with multi-level stress, comprising:

[0031] S1: Conduct uniaxial compression creep tests at multiple stress levels on rock samples, and draw rock creep curves at different stress levels based on the test results; the stress level of the rock uniaxial compression test is not less than 4, and the low stress level and high stress level are not less than 2.

[0032] Among them, the loading adopts strain control method, the loading rate is ≥5~10e-4mm / s, and after the strain is loaded to the design value, the strain is kept unchanged and the attenuation law of stress over time is monitored.

[0033] S2: The steady-state creep curve in the rock creep curve of S1 is extended in the reverse direction and intersects with the ordinate axis. The intersection value is the sum of the instantaneous elastic deformation and the decay creep. It is assumed that the rock directly enters the steady-state creep stage after an instantaneous initial deformation.

[0034] S3: Extract the instantaneous initial deformation values ​​under different stress levels, and use two linear functions to approximate the relationship between the initial deformation of the rock and the stress level under lower and higher stress levels. The stress value corresponding to the intersection of the two linear functions is the long-term strength of the rock.

[0035] Test verification:

[0036] In this experiment, RLW-2000 rock rheology testing machine was used to carry out uniaxial compression creep test.

[0037] Table 1 Initial data of uniaxial compression creep test (unit: %)

[0038]

[0039] See also Figure 2 , taken from the uniaxial compression test, the uniaxial compression test curves with axial stresses of 6.50MPa, 9.50MPa, 12.50MPa, 14.00MPa, 17.50MPa and 21.00MPa.

[0040] Table 2 Intersection value of the reverse extension line of the creep curve in the steady state stage and the ordinate axis

[0041] Stress / MPa 6.5 9.5 12.5 14.0 17.5 21.0 Initial strain / % 0.186 0.323 0.374 0.517 1.721 3.035

[0042] See also Figure 3 , according to the uniaxial compression curve, draw the reverse extension line of the rock steady-state creep stage curve, and intersect it with the vertical axis.

[0043] See also Figure 4 , a scatter plot is drawn based on the obtained intersection value, and two linear functions are used to approximate the relationship between the initial deformation of the rock and the stress level at lower stress levels and higher stress levels. The stress level corresponding to the intersection of the linear functions is the long-term strength of the rock.

[0044] Line l1: y = -0.00307 + 0.03129x (1)

[0045] Line l2: y = -4.53826 + 0.16149x (2)

[0046] Finally, the stress level at the intersection of the two straight lines was 13.8 MPa, that is, the long-term strength of the salt rock used in this test determined by this method was 13.8 MPa, which is close to the long-term strength value of 14 MPa of salt rock determined by the inflection point method of the isochronal stress-strain curve.

[0047] The method for determining the long-term strength of rock based on uniaxial compression test provided in the embodiment of the present application can overcome the problem of unclear inflection point of creep isochronal curve by loading test at different stress levels through uniaxial compression test, and provide a basis for determining the long-term strength of rock and the law of evolution of strength parameters over time; compared with the long-term strength determined by isochronal curve method, the parameter value obtained by the method of the present invention has higher accuracy and reliability, and improves the understanding of rock creep characteristics. Compared with the isochronal curve method in the existing literature, the long-term strength determined by the existing literature is generally a stress interval, and the determination method is highly subjective. The long-term strength determined by the present invention is a determined stress value, and the determination method is the intersection of two fitting curves. In comparison, the subjectivity is much smaller, and the parameter value accuracy and reliability are higher.

[0048] The above shows and describes the basic principles, main features and advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for determining the long-term strength of rock based on a uniaxial compression test with multi-level stress, characterized in that: include: S1: Conduct uniaxial compression creep tests at multiple stress levels on rock samples, and draw rock creep curves at different stress levels based on the test results; S2: The steady-state creep curve in the rock creep curve of S1 is extended in the reverse direction and intersects with the ordinate axis. The intersection value is the sum of the instantaneous elastic deformation and the decay creep. It is assumed that the rock directly enters the steady-state creep stage after an instantaneous initial deformation. S3: Extract the initial deformation values ​​under different stress levels, and use two linear functions to approximately fit the relationship between the initial deformation of the rock and the stress level under low stress level and high stress level. The stress value corresponding to the intersection of the two linear functions is the long-term strength of the rock.

2. The method for determining the long-term strength of rock based on a uniaxial compression test with multi-level stress according to claim 1, characterized in that: In the uniaxial compression creep test of S1 multi-level stress, the loading adopts strain control mode, and the loading rate is ≥5~10e-4mm / s. After the strain is loaded to the design value, the strain is kept unchanged and the attenuation law of stress with time is monitored.

3. The method for determining the long-term strength of rock based on a uniaxial compression test with multi-level stress according to claim 2, characterized in that: In the S1 multi-level stress uniaxial compression creep test, the rock uniaxial compression test stress level is not less than level 4, among which the low stress level and high stress level are not less than level 2.

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