CN104777047A - Testing method for measuring unloading strength of fractured rock - Google Patents

Abstract

A test method for measuring the unloading strength of cracked rocks, comprising the following steps: (1) core sampling is carried out to the rock mass, a deformation measuring device is installed on the test piece, and it is placed in a triaxial test cylinder; (2) keeping Keep the axial displacement of the specimen constant, apply the confining pressure to the design value, then keep the confining pressure constant, apply the axial pressure to the design value; (3) keep the axial displacement of the specimen constant, and gradually decrease according to the design unloading rate Confining pressure until the axial pressure mutation control point, record the axial pressure and confining pressure of the specimen corresponding to the mutation point; (4) Keep the axial displacement of the specimen unchanged, restore the confining pressure to the initial design value according to the design loading rate, and then maintain The confining pressure remains unchanged, and the axial displacement of the specimen is increased to the next design value according to the design loading rate; (5) Repeat the above process of unloading rupture, restoring the confining pressure, and increasing the axial pressure to obtain the axial pressure of the specimen corresponding to different confining pressures Unloading fracture strength at the sudden change point; (6) Obtain the mechanical parameters of unloading fracture strength through data fitting.

Description

A test method for measuring unloading strength of cracked rock

technical field

The invention relates to a test method for rock unloading strength, in particular to a test method for measuring the unloading strength of cracked rocks.

Background technique

With the vigorous development of national infrastructure construction, the excavation depth of rock engineering continues to increase. For example, the diversion tunnel of Jinping Hydropower Station has a buried depth of 2525m, and the mining depth of Huainan Coal Mine has reached 1200m. The increase in buried depth has significantly increased the stress of the surrounding rock , and rock bursts, landslides, large deformations and other engineering geological disasters often occur, and many damaged tunnel (roadway) surrounding rocks will still be damaged after repair and reinforcement, which brings difficulties and difficulties to the safe construction of rock engineering. challenge. The deep rock engineering is obviously different from the shallow rock engineering: the engineering surrounding rock is firstly in a state of extrusion loading, and after the excavation of the underground roadway, the surrounding surrounding rock changes from a three-dimensional stress state to a quasi-two-dimensional stress state. Under the action of the stress field, rocks exceeding the maximum unloading bearing strength of the rock will break. Due to the existence of natural defects in the rock medium and the influence of the mechanical properties of compression and tension, the fracture strength of the rock under unloading conditions is different from the loading conditions. Significant difference.

The current test method for determining rock unloading strength is to load a series of cylindrical specimens to different stress levels according to the hydrostatic pressure, then gradually reduce the confining pressure, record the axial pressure and confining pressure when the rock breaks, and obtain Rock unloading strength and mechanical parameters. However, during the excavation of underground engineering, the radial stress of the surrounding rock mass decreases due to excavation and unloading, but the circumferential and axial directions of the surrounding rock are displacement constraints, which is different from the constraints of constant axial pressure in conventional unloading tests different. In addition, due to rock defects and human factors, unloading test data often have singular or even wrong values, and supplementary tests are required, which increases the workload of the experiment, and the unloading strength obtained by this series of experiments is only the peak strength of the intact rock. Unloading and re-destruction strength of fractured rock cannot be correctly characterized, and there is an essential difference between the two.

Therefore, how to measure the unloading and re-destruction strength of fractured rock and its process, and provide technically correct surrounding rock strength parameters for deep underground engineering well roadway support, has become a hot research direction in deep underground engineering.

Contents of the invention

The purpose of this invention is to provide a test method for measuring the unloading strength of cracked rocks, which is used to measure the mechanical properties of engineering surrounding rock mass after unloading and then destroying, and to analyze the development law of unloading and cracking of surrounding rocks in deep roadways and support reinforcement. Parametric design provides research methods and technical guidance.

Test method step of the present invention is as follows:

(1) Sampling and sample preparation of the rock mass to be tested to obtain a cylindrical test piece;

(2) Place the cylindrical test piece in the triaxial pressure cylinder of the servo testing machine, and install the deformation measurement sensor to ensure that the end of the testing machine is in close contact with the end of the test piece;

(3) Apply confining pressure and axial pressure to the cylindrical specimen, so that the initial confining pressure and initial axial pressure of the specimen are loaded to the set value, wherein the initial axial pressure must exceed its uniaxial compressive strength;

(4) Keep the axial displacement constant, gradually reduce the confining pressure of the specimen, monitor the axial pressure value in the process of unloading the confining pressure, stop unloading the confining pressure when there is a sudden change, and record the corresponding axial pressure and confining pressure of the specimen at the time of the sudden change. pressure;

(5) Keep the axial displacement constant, restore the confining pressure of the ruptured specimen to the initial value, then keep the confining pressure constant, and increase the axial pressure to the next design value;

(6) Keep the axial displacement constant, gradually reduce the confining pressure of the specimen to cause secondary rupture of the specimen, and record the confining pressure and axial pressure of the specimen corresponding to the control point of the sudden change of axial pressure;

(7) Repeat steps (5) and (6) to obtain data values of confining pressure and axial pressure of a series of ruptured specimens after failure and unloading;

(8) Through least square fitting, the unloading strength corresponding to the confining pressure of the fractured rock based on the selected strength criterion is obtained.

The ratio of height to diameter of the cylindrical test piece is 2:1.

The loading rate controlled by the confining pressure and the axial pressure is 0.5-1.0 MPa/s, the unloading rate is 0.05-0.1 MPa/s, and the loading rate controlled by the displacement is 0.001-0.002 mm/s.

The unloading strength described above is the stress state of the specimen corresponding to the sudden decrease point of the axial pressure during the process of unloading the confining pressure while keeping the axial displacement constant.

Compared with the existing testing technology, the present invention has the following beneficial effects:

(1) Break the specimen by unloading the confining pressure, then increase the confining pressure and axial pressure to the design value, then keep the axial displacement unchanged, gradually unload the confining pressure to obtain the specimen corresponding to the control point of the sudden change of axial pressure, and then rupture and unload The strength truly reflects the radial stress reduction of the surrounding rock during excavation and unloading of the engineering rock mass, the fracture under the constraints of axial displacement, and the mechanical behavior of deformation and rupture after support reinforcement. The stress path of the test method reflects the real stress conditions of the engineering rock mass excavation process, and it is a stress path test method in line with the actual engineering site, which is helpful for the stability and safety control of the surrounding rock of the underground engineering.

(2) The loading method is simple and easy to operate, and has wide practicability.

(3) It is used to study the mechanical characteristics of post-peak unloading of rocks in mining, energy, hydropower, transportation and other fields in high stress areas.

Description of drawings:

Fig. 1 is a schematic diagram of the unloading stress path of the test specimen of the present invention.

Fig. 2 is an experimental curve diagram of the unloading path of the present invention.

Detailed ways

Below in conjunction with accompanying drawing 1, the present invention will be further described. The test method steps are as follows:

(1) Sampling and sample preparation of the rock mass to be tested to obtain a cylindrical test piece, the method is as follows:

① Drill and core the rock blocks sampled on site to obtain cylindrical long rock blocks;

② Use a rock cutting machine to cut the cylindrical rock block, and grind the two ends to ensure that the unevenness deviation of the two ends does not exceed 0.05mm, and obtain a cylindrical test piece with a height-to-diameter ratio of 2:1.

(2) Place the cylindrical test piece in the triaxial pressure cylinder of the servo testing machine, install the deformation measurement sensor, and ensure that the end of the testing machine is in close contact with the end of the test piece. The method is as follows:

① Place a cylindrical steel spacer with the same diameter as the test piece on both ends of the cylindrical test piece to ensure that the spacer is coaxial with the test piece;

② Cover the test piece and the steel gasket with a heat-shrinkable tube of appropriate diameter to ensure that the heat-shrinkable tube is completely shrunk and adheres closely to the test piece and the gasket;

③Axial strain sensors are installed at the steel gaskets at both ends to measure the axial strain, and strain gauges are installed in the middle of the specimen to measure the circumferential strain of the specimen;

④ Place the test piece with the measuring sensor installed in the triaxial pressure cylinder, adjust the position of the pressure head of the testing machine to ensure that it is in close contact with the end of the test piece;

(3) Apply confining pressure and axial pressure to the cylindrical specimen, so that the initial confining pressure and initial axial pressure of the specimen are loaded to the set value, and the initial axial pressure must exceed its uniaxial compressive strength. The method is as follows:

① Ensure that the axial displacement of the specimen remains unchanged, and apply an initial confining pressure σ ₃₀ to the specimen at a loading rate of 0.1-0.5MPa/s, and σ ₃₀ is not less than 30Mpa (section oa);

② Keep the confining pressure constant, increase the axial pressure of the specimen to the initial value σ ₁₀ , σ ₁₀ can be approximately 1.5 times the uniaxial compressive strength of the specimen (paragraph ab);

(4) Keep the axial displacement constant, gradually reduce the confining pressure of the specimen, monitor the axial pressure value in the process of unloading the confining pressure, stop unloading the confining pressure when there is a sudden change, and record the corresponding axial pressure and confining pressure of the specimen at the time of the sudden change. pressure, as follows:

① Keep the axial displacement of the specimen constant, gradually reduce the confining pressure of the specimen according to the unloading rate of 0.05-0.1MPa/s, and monitor the change of the axial pressure of the specimen during the confining pressure unloading process (bcd section);

② When the axial pressure suddenly decreases, stop unloading the confining pressure, and record the confining pressure σ ₃₁ and axial pressure σ ₁₁ of the specimen when the axial pressure suddenly decreases (point c);

(5) Keep the axial displacement constant, restore the confining pressure of the ruptured specimen to the initial value, then keep the confining pressure constant, and increase the axial pressure to the next design value, the method is as follows:

① Keep the axial displacement of the specimen unchanged, and restore the confining pressure of the specimen to the initial value σ ₃₀ according to the loading rate of 0.5-1.0MPa/s (section de);

② Keep the confining pressure constant, increase the axial pressure of the specimen according to the displacement or stress control method, the axial displacement control increment is 10% of the axial strain corresponding to the peak point of the uniaxial compressive strength of the specimen, and the stress control increment is uniaxial 20% of the compressive strength (section ef);

(6) Using the same method as step (4) (section fgh), obtain the confining pressure σ ₃₂ and axial pressure σ ₁₂ (point g) corresponding to the secondary unloading rupture of the specimen;

(7) Repeat steps (5) and (6) to obtain the unloading re-destruction strength (σ _1i ) of the ruptured specimen under different confining pressures (σ _3i ), such as point k (σ ₃₃ , σ ₁₃ ) and point p (σ ₃₄ , σ ₁₄ );

(8) The least square fitting method is used to obtain the unloading strength parameters of the fractured rock based on the selected strength criterion, and the rock unloading strength σ ₁ corresponding to any confining pressure σ ₃ can be obtained accordingly.

Specifically: Figure 2 shows the test curve corresponding to the initial confining pressure σ ₃₀ =35 MPa in the unloading test of weathered granite. First, put the Φ50×100mm granite cylindrical specimen in the three-axis airtight pressure cylinder of the servo testing machine, and install the sensor, first make the pressure head of the testing machine closely contact with the upper and lower rigid loading ends of the specimen, and then press the The sealed pressure cylinder is filled with hydraulic oil until it is completely full, and the confining pressure is applied to the granite cylindrical specimen at a loading rate of 0.5MPa/s, so that the confining pressure of the specimen is loaded to the initial confining pressure σ ₃₀ =35MPa (Oa section); Keep the confining pressure constant, apply axial pressure to the granite cylindrical specimen to 125MPa (ab section) at a loading rate of 0.5MPa/s; keep the axial displacement of the specimen constant, gradually reduce the confining pressure at an unloading rate of 0.05MPa/s Press down to the control point of abrupt decrease in axial pressure (section bcd), and obtain the specimen axial pressure σ ₁₁ =87.9MPa and confining pressure σ ₃₁ =11.7MPa corresponding to the abrupt change point c; keep the axial displacement unchanged, according to 0.5MPa/s The loading rate restores the confining pressure to σ ₃₀ ＝35MPa (de section); keep the confining pressure unchanged, and load the axial pressure to 160MPa (ef section) at a loading speed of 0.5MPa/s; then keep the axial displacement unchanged, according to 0.05MPa/ The unloading rate of s gradually reduces the confining pressure to the control point of abrupt decrease of axial pressure (fgh section), and obtains the axial pressure σ ₁₂ = 152.5MPa and confining pressure σ ₃₂ = 21.6MPa corresponding to the abrupt change point g; repeat the above loading and unloading process Obtain other series of unloading strengths, such as k-point σ ₁₃ =183.3Mpa, σ ₃₃ =26.9Mpa; through data least square fitting, obtain strength parameters based on Mohr-Coulomb criterion cohesion c=14.7MPa, friction angle According to the formula σ ₁ =6.3σ ₃ +14.7, the unloading strength σ ₁ corresponding to any value of confining pressure σ ₃ can be obtained.

Claims (4)

1. measure a test method for fractured rock loosening strength, it is characterized in that: test method step is as follows:

(1) treat the sampling of testing engineering rock mass, sample preparation, obtain cylinder specimen;

(2) columniform test specimen is placed in the triaxial pressure cylinder of servo testing machine, deformation measurement sensor is installed, warranty test machine termination and test piece end close contact;

(3) apply confined pressure and axial compression to cylinder specimen, confined pressure and axial compression are respectively loaded on design load, and initial axial compression exceedes its uniaxial compressive strength;

(4) retainer shaft is to shift invariant, reduces test specimen confined pressure gradually, and confined pressure process Axial Force is unloaded in monitoring, stops unloading confined pressure when it occurs sudden change, the test specimen axial compression that the record sudden change moment is corresponding and confined pressure;

(5) retainer shaft is to shift invariant, recovers to break test specimen confined pressure to initial value, then keeps confined pressure constant, increases axle pressure to design load;

(6) retainer shaft is to shift invariant, reduces test specimen confined pressure gradually, makes test specimen produce secondary and breaks, the confined pressure that record test specimen axial compression catastrophe point is corresponding and axial compression;

(7) repeat step (5) and (6) series can be obtained to break confined pressure, the axial compression data of test specimen off-load breakdown point again;

(8) by least square fitting, calculate the intensity expression formula based on selected criterion of strength, the loosening strength that any confined pressure of fractured rock value is corresponding can be obtained.

2. a kind of test method measuring fractured rock loosening strength according to claim 1, is characterized in that: the height of described cylinder specimen and diameter ratio are 2:1.

3. a kind of test method measuring fractured rock loosening strength according to claim 1, it is characterized in that: described confined pressure and axial compression controlled loading speed are 0.5 ~ 1.0MPa/s, rate of debarkation is 0.05 ~ 0.1MPa/s, and Bit andits control loading speed is 0.001 ~ 0.002mm/s.

4. a kind of test method measuring fractured rock loosening strength according to claim 1, is characterized in that: described loosening strength is the test specimen stress state of unloading in confined pressure process axial compression sudden change reduction point correspondence of retainer shaft to shift invariant.