A cyclic loading and unloading stress path method for studying true triaxial creep characteristics of rock

Abstract

The present invention provides a cyclic loading and unloading stress path method for studying the true triaxial creep characteristics of rock, and relates to the field of rock mechanics technology. The method first pre-stresses the rock sample, and then independently loads the true triaxial stress on the pre-stressed rock sample until the true triaxial stress reaches the set minimum principal stress level; then the minimum principal stress is kept unchanged, and the maximum principal stress and the intermediate principal stress are cyclically loaded until the set number of loading cycles is reached, and the cyclic loading of the rock sample is completed; the stress-strain curve data and the strain-time curve data obtained during the test are recorded, and then the creep deformation and creep rate of the rock sample under different stress conditions are determined. By designing a reasonable stress loading path, the method can complete creep tests at different stress levels on one sample; and can simultaneously study the influence of the intermediate principal stress on the long-term mechanical properties of hard rock and the influence of the maximum principal stress on the long-term mechanical properties of hard rock.

Description

Technical Field

[0001] The invention relates to the technical field of rock mechanics, and in particular to a cyclic loading and unloading stress path method for studying true triaxial creep characteristics of rock. Background Art

[0002] Rock mechanics testing is an effective means to study the mechanical properties of rocks and is the basis for promoting the development of the field of rock mechanics. As a natural carrier for underground mining, deep buried tunnels, nuclear waste disposal, carbon dioxide storage, etc., the bearing capacity and long-term stability of rocks directly affect the progress of the project and the safety of workers. After excavation of deep projects, they show continuous strong creep characteristics under high stress, not only with large deformation, but also obvious "time effect". It is generally believed that it is difficult for intact hard rock to produce large rheology. However, many deep engineering practices have shown that hard rock also has obvious time effect in deep environments. High stress in deep places leads to significant rheological phenomena in hard rocks, which brings great difficulties to support work. In addition, quantifying the characteristics of rock deformation over time can also help reveal the evolution of the brittle crust and the complexity of rock dynamics. Therefore, it is of great significance to carry out research on the long-term mechanical properties of hard rocks.

[0003] Under the condition of unchanged external conditions, the phenomenon that the stress or strain of rock changes with time is called rock rheology, which mainly includes creep, relaxation, elastic aftereffect and long-term strength. In the indoor rheological test of rock, there are two different loading methods for creep test, namely separate loading and graded loading.

[0004] Separate loading requires exactly the same rock samples, and requires exactly the same test instruments and test conditions, which is extremely difficult; the data obtained from graded loading needs to be processed according to the Boltzmann linear superposition principle, assuming that the rock is an ideal linear creep body, which is not entirely consistent with the actual situation.

[0005] The separate loading method and graded loading method described above, in addition to being time-consuming, also have two problems that cannot be ignored: First, the rock is affected by the geological conditions in which it is located, resulting in the unevenness of the mineral composition and microstructure of the rock sample, and the large discreteness. Although the acoustic wave detection system can be used to select rocks with similar wave velocities as test rock samples to minimize the distortion of the test results due to excessive discreteness, it is still very difficult to make the rock samples completely identical; second, the data obtained from a creep test is too little, and usually a series of tests are required to meet the requirements, which undoubtedly increases the test cost to a large extent. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a cyclic loading and unloading stress path method for studying the true triaxial creep characteristics of rock in view of the deficiencies of the above-mentioned prior art, so as to realize the rapid study of the long-term mechanical properties of hard rock.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is: a cyclic loading and unloading stress path method for studying the true triaxial creep characteristics of rock,

[0008] Pre-compressing the rock specimen;

[0009] The pre-stressed rock specimens are independently loaded with true triaxial stress until the true triaxial stresses all reach the set minimum principal stress level;

[0010] Keep the minimum principal stress unchanged, and cyclically load the maximum principal stress and the intermediate principal stress until the set number of loading cycles is reached, completing the cyclic loading of the rock sample;

[0011] The stress-strain curve data and strain-time curve data obtained during the test are recorded to determine the creep deformation and creep rate of the rock specimen under different stress conditions.

[0012] The following steps are involved:

[0013] Step 1: Install the square column hard rock sample taken from the project site into the sealing cylinder of the testing machine for creep test, and pre-press the square column rock sample; the pre-pressing pressure is 1kN~5kN;

[0014] Step 2: The true triaxial stress is independently applied to the square cylindrical hard rock specimen until the true triaxial stress reaches the set minimum principal stress level, and then remains unchanged throughout the test until the end of the test;

[0015] Using stress control method, the true triaxial stress is loaded on the square cylindrical hard rock specimen simultaneously and independently at the loading rate recommended by the "Rock True Triaxial Test Code" until the three stresses reach the set minimum principal stress level;

[0016] Step 3: Keep the intermediate principal stress unchanged and load the maximum principal stress until the maximum principal stress reaches the set first-level maximum principal stress level;

[0017] The maximum principal stress is loaded at the same rate as step 2 by using a stress control method until the set first-level maximum principal stress level is reached; wherein the set first-level maximum principal stress level is not less than 60% of the conventional triaxial compression test failure strength of the hard rock specimen under the same confining pressure;

[0018] Step 4: Keep the maximum principal stress unchanged, and load the intermediate principal stress step by step to the set stress level of each intermediate principal stress level;

[0019] The stress control method is adopted to load the intermediate principal stress stepwise at the same rate as step 2 to each set intermediate principal stress level, and when each stress level is reached, the intermediate principal stress remains unchanged for a period of time, the difference between the intermediate principal stress levels of two adjacent levels cannot exceed the set threshold, and the time for each intermediate principal stress to be maintained and the difference between the intermediate principal stress levels of two adjacent levels are equal;

[0020] Step 5: Unload the intermediate principal stress from the highest stress level to the minimum principal stress level, and then load the maximum principal stress until the set next maximum principal stress level is reached;

[0021] Using stress control, unload the intermediate principal stress to the minimum principal stress level at the same rate as step 2, and then load the maximum principal stress at the same rate until the next maximum principal stress level is reached. The difference between the two maximum principal stress levels is no more than 10% of the failure strength of the specimen under the same confining pressure in a conventional triaxial compression test.

[0022] Step 6: Repeat steps 4-5 until the set number of loading cycles is reached, completing the cyclic loading of the square cylinder rock sample;

[0023] The above steps 4-5 constitute a loading cycle. Steps 4-5 are repeated until the set number of loading cycles is reached and the test is finished. The number of loading cycles in the same test is determined according to the test plan.

[0024] Step 7: Record the stress-strain curve data and strain-time curve data obtained during the test, and obtain the creep deformation and creep rate in the direction of the maximum principal stress under different intermediate principal stress conditions, as well as the creep deformation and creep rate in the direction of the intermediate principal stress under different maximum principal stress conditions from the two curves.

[0025] The beneficial effects of the above technical solution are: the cyclic loading and unloading stress path method for studying the true triaxial creep characteristics of rock provided by the present invention can quickly study the influence of intermediate principal stress and maximum principal stress on the long-term mechanical properties of hard rock. By designing a reasonable stress loading path, creep tests at different stress levels can be completed on one specimen, which can save a lot of time; by conducting the test on only one specimen, the problem of excessive discreteness of the test results due to the inhomogeneity of the rock specimen structure is better solved; in the same true triaxial creep test, the influence of intermediate principal stress on the long-term mechanical properties of hard rock is studied, and the influence of maximum principal stress on the long-term mechanical properties of hard rock is also studied. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1A flow chart of a cyclic loading and unloading stress path method for studying true triaxial creep characteristics of rock provided by an embodiment of the present invention;

[0027] Figure 2 A schematic diagram of a loading method for pre-compressing a square cylindrical rock sample provided in an embodiment of the present invention;

[0028] Figure 3 A schematic diagram of a stress loading path provided by an embodiment of the present invention;

[0029] Figure 4 A schematic diagram of a stress path and a corresponding schematic diagram of a strain-time curve provided in an embodiment of the present invention, wherein (a) is a schematic diagram of a stress path; and (b) is a schematic diagram of a strain-time curve. DETAILED DESCRIPTION

[0030] The specific implementation of the present invention is further described in detail below in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0031] This embodiment takes a rock servo-controlled true triaxial testing machine as an example, and adopts the cyclic loading and unloading stress path method for studying the true triaxial creep characteristics of rock of the present invention, cyclically loading and unloading true triaxial stress on the rock sample to determine the true triaxial creep characteristics of the rock sample.

[0032] In this embodiment, a cyclic loading and unloading stress path method for studying the true triaxial creep characteristics of rock is provided.

[0033] Pre-compressing the rock specimen;

[0034] The pre-stressed rock specimens are independently loaded with true triaxial stresses σ1, σ2, and σ3 until the true triaxial stresses all reach the set minimum principal stress level;

[0035] Keep the minimum principal stress σ3 unchanged, and cyclically load the maximum principal stress σ1 and the intermediate principal stress σ2 until the set number of loading cycles is reached, completing the cyclic loading of the rock sample;

[0036] The stress-strain curve data and strain-time curve data obtained during the test are recorded to determine the creep deformation and creep rate of the rock specimen under different stress conditions.

[0037] like Figure 1 As shown, the specific steps include:

[0038] Step 1: Install the square column hard rock sample taken from the project site into the sealing cylinder of the rock servo-controlled true triaxial testing machine for creep test, and pre-compress the square column rock sample; the loading method is as follows: Figure 2As shown in the figure, the rock sample must be a regular square cylinder to ensure that it can be subjected to true triaxial test. The square cylinder rock sample is preloaded, and the preload applied to the sample is 1kN to 5kN.

[0039] Step 2: independently load the true triaxial stresses σ1, σ2, and σ3 on the square cylindrical hard rock specimen, where σ1 is the maximum principal stress, σ2 is the intermediate principal stress, and σ3 is the minimum principal stress, until the true triaxial stresses all reach the set minimum principal stress level. After that, σ3 remains unchanged during the entire test until the end of the test;

[0040] Using stress control method, the true triaxial stresses σ1, σ2, and σ3 are simultaneously and independently loaded on the square cylindrical hard rock specimen at the loading rate recommended by the "Rock True Triaxial Test Code" until these three stresses reach the set minimum principal stress level;

[0041] Step 3: Keep the intermediate principal stress σ2 unchanged and load the maximum principal stress σ1 until the maximum principal stress σ1 reaches the set first-level maximum principal stress level;

[0042] Using a stress control method, the maximum principal stress σ1 is loaded at the same rate as step 2 until the set first-level maximum principal stress level is reached; wherein the set first-level maximum principal stress level is not less than 60% of the conventional triaxial compression test failure strength of the hard rock specimen under the same confining pressure;

[0043] Step 4: Keep the maximum principal stress σ1 unchanged, and step-load the intermediate principal stress σ2 to the set stress level of each intermediate principal stress level;

[0044] The stress control method is adopted to load the intermediate principal stress σ2 stepwise at the same rate as step 2 to each set intermediate principal stress level, and when each stress level is reached, the intermediate principal stress σ2 remains unchanged for a period of time, the difference between the intermediate principal stress levels of two adjacent levels cannot exceed the set threshold, and the time for each intermediate principal stress to be maintained and the difference between the intermediate principal stress levels of two adjacent levels are equal;

[0045] Step 5: Unload the intermediate principal stress σ2 from the highest stress level to the minimum principal stress level, and then load the maximum principal stress σ1 until the set next maximum principal stress level is reached;

[0046] Using the stress control method, the intermediate principal stress σ2 is unloaded to the minimum principal stress level at the same rate as step 2, and then the maximum principal stress σ1 is loaded at the same rate until the set next level of maximum principal stress level is reached. The difference between the two levels of maximum principal stress levels is not greater than 10% of the failure strength of the specimen under the same confining pressure in the conventional triaxial compression test;

[0047] Step 6: Repeat steps 4-5 until the set number of loading cycles is reached, completing the cyclic loading of the square cylinder rock sample;

[0048] The above steps 4-5 constitute a loading cycle. Steps 4-5 are repeated until the set number of loading cycles is reached and the test is finished. The number of loading cycles in the same test is determined according to the test plan.

[0049] In this embodiment, the stress path of the above steps 3-6 cyclic loading and unloading is as follows Figure 3 shown.

[0050] Step 7: Record the stress-strain curve data and strain-time curve data obtained during the test, and obtain the creep deformation and creep rate in the direction of the maximum principal stress σ1 under different intermediate principal stress σ2 conditions, as well as the creep deformation and creep rate in the direction of the intermediate principal stress σ2 under different maximum principal stress σ1 conditions from the two curves.

[0051] In this embodiment, the stress path of cyclic loading and unloading and the corresponding strain-time curve are as follows: Figure 4 shown.

[0052] The method of the present invention can quickly study the influence of the intermediate principal stress σ2 and the maximum principal stress σ1 on the long-term mechanical properties of hard rock. The specific reasons are as follows:

[0053] In rock creep tests, different test conditions and the inherent microstructure of rock samples often lead to very different test results.

[0054] By the method of the present invention, the maximum principal stress σ1 is firstly reached at a constant loading rate, and then the intermediate principal stress σ2 is loaded so that the specimen can enter steady-state creep. When the creep deformation reaches a sufficient level, the intermediate principal stress σ2 is increased, and a new creep deformation can be determined. By repeating the cycle in this way, more creep data of the specimen under different intermediate principal stress σ2 conditions can be obtained. At the same time, by step-by-step increasing the maximum principal stress σ1 and the cyclic loading of the intermediate principal stress σ2, it is ensured that the creep data of the specimen under different maximum principal stress σ1 conditions can be obtained until the end of the test.

[0055] Of course, the brittle creep of hard rock is driven by irreversible crack growth, and the microstructural state of the specimen is constantly evolving throughout the entire creep test, so hard rock creep has always been an unstable state process. The generation of cracks usually extends in the direction parallel to the maximum principal stress σ1 and along the direction of the minimum principal stress σ3. The gradual increase of the intermediate principal stress σ2 inhibits the generation of damage, and the unloading of the intermediate principal stress σ2 increases the generation of damage. The strain caused by an intermediate principal stress σ2 loading cycle is very small, far less than the microstructural difference between two different specimens. Therefore, the degree of damage before and after an intermediate principal stress σ2 loading cycle can be regarded as the same. This provides a theoretical basis for studying the long-term mechanical properties of hard rock under the influence of intermediate principal stress σ2 and maximum principal stress σ1 in the same experiment.

[0056] Based on this, the method of the present invention can quickly study the influence of the intermediate principal stress σ2 and the maximum principal stress σ1 on the long-term mechanical properties of hard rock.

[0057] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope defined by the claims of the present invention.

Claims

1. A cyclic loading and unloading stress path method for studying true triaxial creep characteristics of rock, characterized by: Pre-compressing the rock specimen; The pre-stressed rock specimens are independently loaded with true triaxial stress until the true triaxial stresses all reach the set minimum principal stress level; Keep the minimum principal stress unchanged, and cyclically load the maximum principal stress and the intermediate principal stress until the set number of loading cycles is reached, completing the cyclic loading of the rock sample; Record the stress-strain curve data and strain-time curve data obtained during the test to determine the creep deformation and creep rate of the rock sample under different stress conditions; The method comprises the following steps: Step 1: Install the square column hard rock sample taken from the project site into the sealing cylinder of the testing machine for creep test, and pre-press the square column rock sample; Step 2: The true triaxial stress is independently applied to the square cylindrical hard rock specimen until the true triaxial stress reaches the set minimum principal stress level. After that, the minimum principal stress level remains unchanged during the entire test until the end of the test; Step 3: Keep the intermediate principal stress unchanged and load the maximum principal stress until the maximum principal stress reaches the set first-level maximum principal stress level; Step 4: Keep the maximum principal stress unchanged, and load the intermediate principal stress step by step to the set stress level of each intermediate principal stress level; Step 5: Unload the intermediate principal stress from the highest stress level to the minimum principal stress level, and then load the maximum principal stress until the set next maximum principal stress level is reached; Step 6: Repeat steps 4-5 until the set number of loading cycles is reached, completing the cyclic loading of the square cylinder rock sample; Step 7: Record the stress-strain curve data and strain-time curve data obtained during the test, and obtain the creep deformation and creep rate in the direction of the maximum principal stress under different intermediate principal stress conditions, as well as the creep deformation and creep rate in the direction of the intermediate principal stress under different maximum principal stress conditions from the two curves.

2. The cyclic loading and unloading stress path method for studying true triaxial creep characteristics of rock according to claim 1 is characterized by: The pressure for pre-pressing the square cylindrical rock sample in step 1 is 1 kN to 5 kN.

3. The cyclic loading and unloading stress path method for studying true triaxial creep characteristics of rock according to claim 2 is characterized by: The step 2 adopts a stress control method to synchronously and independently load true triaxial stresses on the square cylindrical hard rock specimen at the loading rate recommended by the "Rock True Triaxial Test Procedure" until the three stresses all reach the set minimum principal stress level.

4. The cyclic loading and unloading stress path method for studying true triaxial creep characteristics of rock according to claim 3 is characterized by: The step 3 adopts a stress control method to load the maximum principal stress at the same rate as step 2 until the set first-level maximum principal stress level is reached; wherein the set first-level maximum principal stress level is not less than 60% of the destructive strength of a conventional triaxial compression test of a hard rock sample under the same confining pressure.

5. The cyclic loading and unloading stress path method for studying true triaxial creep characteristics of rock according to claim 4 is characterized by: The step 4 adopts a stress control method, and loads the intermediate principal stress stepwise at the same rate as step 2 to each set intermediate principal stress level. When each stress level is reached, the intermediate principal stress remains unchanged for a period of time, and the difference between the intermediate principal stress levels of two adjacent levels cannot exceed the set threshold value. The time for each intermediate principal stress to be maintained and the difference between the intermediate principal stress levels of two adjacent levels are equal.

6. The cyclic loading and unloading stress path method for studying true triaxial creep characteristics of rock according to claim 5, characterized in that: The step 5 adopts a stress control method to unload the intermediate principal stress to the minimum principal stress level at the same rate as in step 2, and then load the maximum principal stress at the same rate until the set next level of maximum principal stress level is reached, and the difference between the two levels of maximum principal stress levels is not greater than 10% of the destructive strength of the sample carried out a conventional triaxial compression test under the same confining pressure.

7. The cyclic loading and unloading stress path method for studying true triaxial creep characteristics of rock according to claim 6 is characterized by: The number of loading cycles described in step 6 is determined according to the test plan.

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