A test method for the seepage shear test of a rock fracture surface under triaxial stress

Abstract

The present invention provides a method for testing the permeability and shear of a rock fracture surface under triaxial stress. This device can simultaneously apply confining pressure, deviatoric pressure, and pore water pressure. It includes a triaxial cell and a deviatoric cell that are both hollow inside and arranged from bottom to top. The triaxial cell includes a base and a deviatoric axial press head facing it above. The deviatoric cell is provided with a deviatoric piston. The lower end of the deviatoric piston extends out of the deviatoric cell and is connected to the deviatoric axial press head inside the triaxial cell. Through the inlet and outlet provided in the triaxial cell, the gas in the triaxial cell is removed, and the triaxial cell is filled with oil to apply confining pressure. Through the inlet provided in the deviatoric cell, the deviatoric cell is filled with oil to push the deviatoric piston and the deviatoric axial press head downward to apply deviatoric pressure. Through the water inlet hole in the triaxial cell, pore water pressure is applied to the specimen. This device provides a new method for the shear test of the rock fracture surface under triaxial stress conditions. Based on the Mohr-Coulomb criterion, the equivalent internal friction angle φ, the equivalent cohesion c, and the dilation angle α of the fracture surface can be obtained.

Description

Technical Field

[0001] The present invention belongs to the field of rock mechanics property testing technology, and particularly relates to a method for testing the seepage shear of a fracture surface under triaxial stress, which is mainly used for studying the shear mechanical properties of rock discontinuity surfaces under the coupling action of multiple fields. Background Technique

[0002] The rock fracture surface is a natural interface existing inside or between rocks, usually caused by geological processes such as tectonic stress, rock weathering or thermal expansion, etc. It is one of the unfavorable geological conditions that are more likely to be encountered in underground projects such as hydropower stations and tunnels. The fracture surface is likely to become a flow channel for groundwater, and water seepage often occurs near the fractures in underground projects. Under the combined influence of groundwater and excavation disturbance, the permeability, strength and stability of the surrounding rock decrease, and the surrounding rock may undergo slip and tensile fracture failure along the fracture surface, posing a threat to the stability of underground chambers.

[0003] In order to deeply understand the mechanical properties of rock fracture surfaces, the shear test of fracture surfaces is one of the important methods for studying the mechanical properties of rock fracture surfaces. Through the shear test, we can explore the shear response characteristics of rock fracture surfaces under different stress conditions. At present, when conducting a shear test on a fracture surface, a biaxial stress condition is usually adopted, that is, a shear force is applied in one direction and a normal force is applied in the other direction. For example, in the patent application No. CN201711002741.3, the invention name is a shear box for rock direct shear strength test, and the invented shear box can only achieve biaxial loading. In the patent application No. CN202211257915.1, the invention name is a combined pressurization component with constant normal force and constant normal stiffness and a rock shear test device, and this experimental device can also only achieve biaxial loading and does not consider the triaxial loading situation.

[0004] Currently, the main types of shear devices for fracture surfaces are as follows: (1) Conventional direct shear apparatuses that only apply normal stress and shear stress; (2) Test apparatuses that simulate groundwater by superimposing a water tank on the basis of conventional direct shear apparatuses; (3) True triaxial shear apparatuses that adopt two-way rigid loading and one-way flexible oil pressure loading. However, in actual engineering, rock masses are usually subjected to not only unidirectional or biaxial stresses, but a more complex triaxial stress environment, and may be accompanied by the action of groundwater. Although the existing biaxial shear test methods can obtain the strength of rock fracture surfaces, there are certain limitations in comprehensively reflecting the mechanical behavior of rocks under the action of a real environmental field. For example, conventional biaxial shear tests cannot fully simulate the multi-directional stresses and deformations suffered by rock masses under triaxial stress conditions. Ignoring the action of groundwater will neglect the weakening effect of water on the strength of rock fracture surfaces; biaxial shear tests considering the seepage field cannot restore the real stress field suffered by rock fracture surfaces; true triaxial shear tests lack consideration of the seepage field of surrounding rock fracture surfaces. These deficiencies will lead to the inability of the test to restore the real environmental field where the rock fracture surface is located, thus affecting the authenticity and reliability of research results. Therefore, in order to more accurately reflect the mechanical behavior of rocks under actual engineering conditions, it is necessary to improve the test apparatuses, design a test device that can simultaneously consider the triaxial stress field and seepage field of surrounding rock fracture surfaces, and conduct a permeation shear test that can simulate triaxial stress conditions. Summary of the Invention

[0005] In order to solve the problems existing in the above-mentioned prior art, the present invention provides a permeation shear test device and a test method for a fracture surface under triaxial stress conditions. According to the shear test of a rock fracture surface under different confining pressures, the evolution laws of the strength and pore mechanical properties of the rock fracture surface are obtained, providing theoretical support and technical support for the stability analysis of underground engineering.

[0006] The technical solutions adopted to achieve the above objects of the present invention are as follows:

[0007] Realization of a permeation shear test device for a fracture surface under triaxial stress conditions:

[0008] Add and fix a semi-cylindrical metal indenter on the left side of the axial indenter of the test instrument, add and fix a semi-cylindrical sponge placeholder indenter on the right side, and at the same time add and fix a semi-cylindrical sponge placeholder indenter on the left side of the test instrument base and add and fix an upper semi-cylindrical metal indenter on the right side. Requirements for the metal indenters and sponge placeholder indenters on its indenter and base. During the test, align the upper and lower plates of the rock containing the fracture surface, install it between the indenter and the base of the test instrument, and after aligning the rock fracture surface with the edge of the metal indenter, wrap and fix it with a thermoplastic sleeve. Use confining pressure to achieve two-way loading of the rock and use deviatoric pressure to achieve tangential loading of the rock fracture surface. At the same time, use pore pressure to achieve seepage field loading of the fracture surface, and then the permeation shear test of the rock fracture surface under the coupling action of triaxial stress and multi-fields can be realized.

[0009] A method for testing the seepage shear of a fracture surface under triaxial stress conditions, and the implementation includes the following steps:

[0010] 1. Sample preparation: Manually split the standard cylindrical sample from the middle part of the top surface of the cylinder, and at the same time, ensure that the offset of the top surface of the fracture surface of the upper and lower plates does not exceed 2 mm, or it does not affect the misalignment of the semi-circular indenter. Use wire cutting to cut the sample into a cylindrical shape, and then use a rock engraving machine to engrave the structural surface to make the upper and lower plates of the semi-cylindrical fracture surface.

[0011] 2. Sample installation: Align the upper and lower semi-cylindrical samples with a diameter of 50 mm and a height of 100 mm. At the same time, arrange a metal indenter and a sponge placeholder indenter below the axial indenter and above the base, then install the sample into the rock triaxial chamber and fix it with a sealing sleeve. Install a circumferential displacement sensor in the middle of the sample, and install an axial displacement sensor between the indenter and the base.

[0012] 3. Oil filling: Open the confining pressure pump and the corresponding pipelines to fill the triaxial chamber with oil, and at the same time open the deviatoric pressure pump and the corresponding pipelines to fill the deviatoric pressure chamber with oil.

[0013] 4. Confining pressure application: Zero the circumferential and axial displacement gauges, and fill the triaxial chamber with oil and pressurize it through the confining pressure pump. After the confining pressure reaches the predetermined value, keep it constant.

[0014] 5. Pore water pressure application: After the confining pressure is stable, record the fluid volume V1 in the pore pressure loading pump. Apply pore pressure to the sample through the pore pressure pump at a flow rate of 0.1 cm3 / min. When the data of the axial displacement gauge and the circumferential displacement gauge change significantly, stop the operation of the pore pressure pump. At this time, the fluid is fully saturated in the sample, and then record the fluid volume V2 in the pore pressure loading pump; then start the pore pressure pump and set the pore pressure to the predetermined value p MPa.

[0015] 6. Deviatoric pressure application: After the confining pressure and the pore pressure are stable, record the circumferential displacement gauge reading D C1 and the axial displacement gauge reading D A1 , and then pressurize the triaxial chamber with oil through the deviatoric pressure pump at a fixed loading rate A. When the axial displacement gauge reading suddenly increases rapidly (the increase exceeds 0.5 mm), stop applying the deviatoric pressure.

[0016] 7. Pressure relief and sample removal: After reducing the deviatoric pressure to zero through the deviatoric pressure pump, and then reducing the confining pressure to zero through the confining pressure pump, drain the oil in the triaxial chamber and the deviatoric pressure chamber into the fuel tank, open the triaxial chamber, and take out the rock sample to complete the test.

[0017] 8. Correct the displacement and stress of the fracture surface through the recorded circumferential deformation data and axial deformation data. Assume that the pump pressure of the deviatoric pressure motor servo pump at a certain moment when applying the deviatoric pressure is F A , and the pump pressure of the confining pressure motor servo pump is F C, the tangential stress and normal stress on the fracture surface are as follows:

[0018]

[0019] D C is the reading of the circumferential deformeter at a certain moment when the bias voltage is applied, R i The sample radius is 0.5D (mm), c0 = πD is the original circumference of the sample (mm), L c The chain length is 169 mm, r The radius of the chain roller is 0.5 mm, Δl = Axial Circumferential deformation measurement value (mm), then the normal displacement ΔD C ,:

[0020] Δl = D C -D C1 (3)

[0021]

[0022] D A is the reading of the axial deformeter at a certain moment when the bias voltage is applied, and the reading of the axial displacement meter after the confining pressure is loaded is D A1 , the length of the fracture surface is H, then the tangential displacement is:

[0023] ΔD A = D A -D A1 (7)

[0024] From the changes in axial stress and displacement, tangential stress and displacement within a certain period during the bias voltage loading, the normal stiffness and tangential stiffness of the fracture surface can be calculated as follows:

[0025]

[0026] The normal stiffness Kn and tangential stiffness Ks of the said fracture surface are the slopes within the elastic straight-line segment in the curve graph based on the normal stress σ1 and normal displacement ΔD A , tangential stress σ3 and tangential displacement ΔD C in the curve graph.

[0027] Meanwhile, the equivalent dilation angle of the irregular undulating fracture surface can be obtained through the following formula:

[0028]

[0029] Assume that the structural plane undergoes shear failure. According to the Mohr-Coulomb criterion, there is:

[0030] τ = σ'tanφ + c (10)

[0031] where

[0032] τ = σ1 (11)

[0033] σ' = σ3 - p (12)

[0034] Under two groups of different normal stresses, while keeping the pore water pressure unchanged, repeat steps 1 - 7, and two groups of rock failure stresses can be obtained. Then, by simultaneously solving equations (10), (11), and (12), the equivalent internal friction angle φ and equivalent cohesion c of the fracture surface can be obtained.

[0035]

[0036] τ a and τ b are the shear stresses under the stress conditions corresponding to different pressure change rates in two groups of step 8. According to formulas (11) and (1), σ a ’ and σ b ’ are the effective stress values corresponding to two groups of different confining pressures in step 6, which can be obtained from formulas (12) and (2).

[0037] Compared with the prior art, the beneficial effects and advantages of the present invention are as follows:

[0038] 1. Invented a shear test device under triaxial stress conditions for a fracture surface, providing a new test method for the shear test of a rock fracture surface under triaxial stress conditions;

[0039] 2. Based on the Mohr - Coulomb criterion, under triaxial stress conditions, the tangential stiffness Kn, normal stiffness Ks, equivalent internal friction angle φ, equivalent cohesion c, and dilation angle α of the fracture surface under seepage action can be obtained. The above parameters are all important parameters for describing the mechanical behavior of rock fractures or crack surfaces, and are used to understand and predict the behavior of rocks under various engineering and natural conditions, and for the evaluation of engineering stability. Description of the Drawings

[0040] Figure 1 is a schematic structural diagram of a rock triaxial rheometer.

[0041] Figure 2 is a schematic diagram of specimen loading.

[0042] Figure 3 is a schematic diagram of a stress - deformation curve.

[0043] Among them, 1 - bias axial indenter, 2 - test base, 3 - axial displacement sensors (axial displacement LVDT Ⅰ and axial displacement LVDT Ⅱ), 4 - bias piston, 5 - valve, 6 - air pump, 7 - three-way pump valve Ⅰ, 8 - confining pressure pump valve Ⅰ, 9 - confining pressure servo motor pump, 10 - oil tank, 11 - bias valve Ⅰ, 12 - bias valve Ⅱ, 13 - bias servo motor pump, 14 - three-way valve Ⅱ, 15 - pore pressure valve, 16 - pore pressure servo motor pump, 17 - water tank, 18 - rock sample, 19 - triaxial cell, 20 - bias cell, 21 - bias cell pipeline, 22 - triaxial cell pipeline Ⅰ, 23 - air pump pipeline, 24 - triaxial cell pipeline Ⅱ, 25 - triaxial cell pipeline Ⅲ, 26 - pore pressure inlet pipeline, 32 - pore pressure outlet pipeline, 27 - metal indenter, 28 - sponge placeholder indenter, 29 - rock fracture surface sample, 30 - circumferential displacement sensor (circumferential displacement gauge LVDT), 31 - thermoplastic sleeve, 41 - piston rod, 42 - extension rod, 43 - operating rod. Detailed implementation mode

[0044] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0045] Taking altered rock as a case object, a seepage shear test of the altered fracture surface is carried out under the action of confining pressure, bias pressure (axial pressure) and pore water pressure. This test is carried out at room temperature. The selected confining pressure is 10 MPa and the pore water pressure is 2 MPa.

[0046] Such as Figure 1As shown in the figure, confining pressure and deviatoric pressure are applied through a rock triaxial rheometer. The rock triaxial rheometer includes a triaxial cell 19 and a deviatoric pressure cell 20 vertically arranged above the triaxial cell 19. The triaxial cell 19 is hollow, and a base 2 and a deviatoric axial pressure head 1 facing each other are arranged inside. The specimen to be tested can be placed between the deviatoric axial pressure head 1 and the base 2. A plurality of axial displacement sensors 3 are also vertically arranged between the facing base 2 and deviatoric axial pressure head 1; the deviatoric pressure cell 20 is also hollow, and a deviatoric piston 4 is arranged inside. The lower end of the deviatoric piston 4 extends out of the deviatoric pressure cell 20 and is connected to the deviatoric axial pressure head 1 inside the triaxial cell 19. The deviatoric piston 4 can move back and forth vertically in the deviatoric pressure cell 20, and at the same time drive the deviatoric axial pressure head 1 fixed below the deviatoric piston 4 to move together, conducting pressure to the specimen. A jack is arranged below the base 2 of the triaxial cell 19, and controlling the jack can adjust the tightening degree between the base 2 and the circumferential periphery of the triaxial cell 19, so that the triaxial cell 19 forms a sealed space. The triaxial cell 19 is provided with an inlet and an outlet. Through this outlet, it is connected to an exhaust pump 6 for exhausting the gas in the triaxial cell 19. Through this inlet, it is connected to an oil tank 10 for filling the triaxial cell 19 with oil liquid, so as to apply confining pressure. An inlet is arranged in the deviatoric pressure cell 20. Through this inlet, it is connected to the oil tank 10 for filling the deviatoric pressure cell 20 with oil liquid, so as to push the deviatoric piston 4 to move downward and apply deviatoric pressure, that is, axial pressure, to the deviatoric piston 4.

[0047] The application of pore water pressure includes the following device. An inlet hole is also arranged on the triaxial cell 19. It is connected to a water tank 17 through this inlet hole, and the water in the water tank is transported to the central positions of the deviatoric axial pressure head 1 and the base 2 for transporting the water flow into the fracture surfaces of the specimen to be tested, so as to apply pore water pressure to the specimen.

[0048] For the application of the above confining pressure, deviatoric pressure (axial pressure) and pore water pressure, the specific test includes the following implementation steps:

[0049] 1. Specimen preparation: Artificially split the standard cylindrical specimen of altered rock from the middle part of the cylindrical top surface, so that the offset of the cracks on the upper and lower plate fracture surfaces in the vertical direction does not exceed 2 mm, and two upper and lower plate semi-cylindrical specimens with a diameter of 50 mm and a height of 100 mm are obtained. Prepare semi-circular pressure heads including a semi-circular metal pressure head 27 and a semi-circular sponge placeholder pressure head 28. The two semi-circular pressure heads are combined to form an integral circular pressure head.

[0050] 2. Specimen loading: Align the upper and lower plate semi-cylindrical specimens of altered rock with a diameter of 50 mm and a height of 100 mm prepared in step 1, so that the upper and lower plates of the rock fracture surfaces are well aligned. Arrange a pair of metal pressure heads 27 and sponge placeholder pressure heads 28 below the deviatoric axial pressure head 1. After arranging a pair of metal pressure heads 27 and sponge placeholder pressure heads 28 with opposite positions above the base 2, as Figure 2As shown in (a) of [reference], place the well-fitted specimen between two pairs of metal indenters 27 and sponge placeholder indenters 28. Align the fracture surfaces of the well-fitted specimen with the midlines of the upper and lower pairs of semi-circular indenters respectively, so that the fracture surfaces of the well-fitted specimen are close to the boundary surfaces of the two semi-circular indenters. The specific placement method is as shown in Figure 2 (b) of [reference]; make the pore pressure outlet pipe 32 buried in the bias axial indenter 1 and the pore pressure inlet pipe 26 buried in the test base 2 face the fractures at the upper and lower ends of the specimen; as shown in Figure 2 (c) of [reference], encapsulate the specimen and the two pairs of semi-circular indenters above and below it in a sealed thermoplastic sleeve 31, and then install it between the bias axial indenter 1 and the test base 2 in the rock triaxial cell 19. After placing and aligning, heat it with a heat gun, so that the thermoplastic sleeve 31 tightly wraps the rock specimen and the two pairs of semi-circular indenters arranged up and down, and is jointly fixed between the bias axial indenter 1 and the test base 2; install a circumferential displacement sensor 30, i.e., a circumferential displacement gauge LVDT, outside the specimen, and install two axial displacement sensors 3, i.e., axial displacement gauges LVDTⅠ and axial displacement LVDTⅡ, between the bias axial indenter 1 and the test base 2, for collecting the tangential displacement of the rock fracture surface.

[0051] 3. Fill with oil. After the specimen, axial displacement sensors, and circumferential displacement sensors are installed, use a jack to lift the test base 2 in the triaxial cell 19 (the jack is located below the base 2 and is not shown in the figure), so that the test base 2 is tightly combined with the circumferential periphery of the triaxial cell 19, and then tighten the screws to make the triaxial cell 19 form a sealed space; open the valve 5 and the three-way pump valve 1-7, so that the first triaxial cell pipe 22 is connected to the air pump pipe 23, and then open the exhaust pump 6. Through the first triaxial cell pipe 22 and the air pump pipe 23, pump out the air in the triaxial cell 19. The pressure in the triaxial cell 19 gradually decreases. Keep the valve 5 open, and transfer the oil in the oil tank 10 into the triaxial cell 19 through the third triaxial cell pipe 25. After the triaxial cell 19 is filled with oil, first close the three-way pump valve 1-7. After the pressure in the triaxial cell 19, i.e., the confining pressure, is stable, then close the valve 5 and the air pump 6 to complete the oil filling process of the triaxial cell 19, so that the oil fills the triaxial cell 19.

[0052] 4. Before applying the confining pressure, fill the confining pressure servo motor pump 9 with oil. Open the connection between the oil tank 10 and the confining pressure pump valve 1-8, keep the three-way pump valve 1-7 closed, fill the confining pressure servo motor pump 9 with oil, and finally close the confining pressure pump valve 1-8.

[0053] 5. Before applying the bias pressure, fill the bias chamber 20 and the bias pressure servo motor pump 13 with oil. Open the first bias pressure valve 11 and the second bias pressure valve 12, so that the bias chamber 20 is connected to the bias pressure pump servo motor 13 and the oil tank 10. At this time, fill the bias chamber 20 with oil. After filling, close the first bias pressure valve 11. After filling the bias pressure servo motor pump 13 with oil, close the second bias pressure valve 12.

[0054] 6. Before applying pore water pressure, fill the pore pressure servo motor pump 16 with water. Open the three-way valve II 14 until the water tank 17 is connected to the pore pressure servo motor pump 16 while the port of the pore pressure outlet pipe 32 is closed. Open the pore pressure valve 15 to connect the pore pressure servo motor pump 16 to the water tank 17 and fill the pore pressure servo motor pump 16 with water. After filling with water, close the pore pressure three-way valve II 14 and the pore pressure valve 15.

[0055] 7. Apply confining pressure to the set pressure value. After zeroing the circumferential displacement gauge LVDT, the axial displacement gauge LVDT Ⅰ, and the axial displacement LVDT Ⅱ, open the three-way pump valve I 7 to connect the first triaxial chamber pipe 22 and the second triaxial chamber pipe 24 to the confining pressure servo motor pump 9. Keep the confining pressure valve I 8 and the valve 5 closed. Set the value of the confining pressure servo motor pump 9 to p1 = 10 MPa, start the confining pressure servo motor pump 9, continue to fill the triaxial chamber 19 with oil and pressurize it, load the confining pressure to the set value and keep the confining pressure value constant.

[0056] 8. Apply pore water pressure value to the set pressure value. After the confining pressure reaches the set value and stabilizes, set the value of the pore pressure servo motor pump to p2 = 2 MPa, close the three-way valve I 7, start the pore pressure servo motor pump 16, connect the pore pressure inlet pipe 26 through the pore pressure servo motor pump 16, and add pore pressure to the specimen at a flow rate of 0.1 cm 3 / min in the pore pressure inlet pipe 26. Record the fluid volume V1 in the pump. Load the pore pressure to the set value and keep the pore pressure value constant. After the pore pressure stabilizes, record the readings of the circumferential displacement gauge LVDT, the axial displacement gauge LVDT Ⅰ, and the axial displacement gauge LVDT Ⅱ as D C1 、D 1 A1 、D 2 A1 。

[0057] 9. Apply the bias pressure after the confining pressure and pore pressure stabilize. Open the bias pressure valve I 11, keep the bias pressure valve II 12 closed, open the bias pressure servo motor pump 13, and slowly fill the bias chamber 20 with oil in a flow rate loading manner through the bias chamber pipe 21 to apply pressure, so that the bias piston 4 moves downward, thereby transmitting the pressure to the bias axial pressure head 1, and the bias axial pressure head 1 applies pressure to the metal pressure head 27 and the sponge placeholder pressure head 28. Stop loading when the readings of the axial displacement gauge LVDT Ⅰ and the axial displacement gauge LVDT Ⅱ change. Close the bias pressure valve I 11, start automatically recording the readings of the circumferential displacement gauge LVDT, the axial displacement gauge LVDT Ⅰ, and the axial displacement gauge LVDT Ⅱ, and at the same time change to loading in a stress control manner (loading pressure at a predetermined pressure change rate, such as 0.5 mpa / s), and load until the rock fracture surface where the upper and lower discs fit well is damaged.

[0058] Assume that when bias is applied, the pump pressure of the bias servo motor pump at the mth moment is F A , the pump pressure of the confining pressure servo motor pump is F C , then the tangential stress σ3 and normal stress σ1 on the crack surface are:

[0059]

[0060] In the above formula, D is the sample diameter (mm), σ1 is the normal stress, τ is the shear stress, and σ3 is the tangential stress. In the above formula, the pump pressure of the bias motor servo pump at the mth moment is F A , the pump pressure of the confining pressure motor servo pump is F C , which are read from the pressure gauges on the bias pressure servo motor pump and the confining pressure servo motor pump respectively.

[0061] L c The chain length of the circumferential strain gauge, i.e. the circumferential displacement gauge LVDT, is 169 mm, the radius of the chain roller is 0.5 mm, Δl = axial circumferential deformation measurement value (mm), then the tangential displacement ΔD C :

[0062] Δl=D C -D C1 (3)

[0063]

[0064] In the above formula, D C is the reading of the circumferential displacement meter at the mth moment when the bias voltage is applied, D C1 is the reading of the annular displacement meter after the confining pressure is loaded, R i is the sample radius (mm), Lc is the chain length, r is the chain roller radius 0.5mm, θ i is the deviation angle, ΔC is the correction value of the change value of the annular strain gauge, and c0=πD is the original circumference of the specimen (mm).

[0065]

[0066] ΔD A =D A -D A1 (7)

[0067] In the above formula, D A D is the average value of the axial displacement meter reading at a certain moment when bias pressure is applied. 1 A D is the reading of the axial displacement meter LVDTⅠ during bias loading, 2 A D is the reading of the axial displacement meter LVDTⅡ during bias loading. A1is the average reading of the axial displacement gauge after confining pressure loading, D 1 A1 is the reading of axial displacement gauge LVDTⅠ after confining pressure loading, D 2 A1 is the reading of axial displacement gauge LVDTⅡ after confining pressure loading, ΔD A is the normal displacement.

[0068] From the changes in axial stress and deformation and tangential stress and deformation over a period of time during bias loading, the normal stiffness Kn and tangential stiffness Ks of the fracture surface can be calculated as follows:

[0069]

[0070] In the above formula, △σ3 is the change in tangential stress within the elastic straight line segment and △σ1 is the change in normal stress within the elastic straight line segment (as Figure 3 ), after the confining pressure and pore pressure loading are completed, read the circumferential displacement gauge reading D C1 and the average reading of the axial displacement gauge D A1 , during a period of time in the bias loading test, read the readings of the pressure gauges on the confining pressure motor servo pump 9 and the bias pressure motor servo pump 13, obtain the values of the normal stress σ1 and tangential stress σ3 through formulas (1) - (2), and calculate the normal displacement ΔD A and tangential displacement ΔD C using the normal stress σ1 and normal displacement ΔD A , tangential stress σ3 and tangential displacement ΔD C to plot curves, and take the slopes within the elastic straight line segments of the two curves as the normal stiffness Kn and tangential stiffness Ks respectively.

[0071] Meanwhile, the equivalent dilation angle α of the irregular undulating fracture surface can be obtained through the following formula:

[0072]

[0073] Through the above calculations, the normal stiffness Kn and tangential stiffness Ks of the fracture surface and the equivalent dilation angle α of the fracture surface are obtained. They are all important parameters describing the mechanical behavior of rock fractures or crack surfaces, used to understand and predict the behavior of rocks under various engineering and natural conditions, and for the evaluation of engineering stability.

[0074] Assume that the structural plane of the sample undergoes shear failure. According to the Mohr - Coulomb criterion:

[0075] τ=σ'tanφ + c (10)

[0076] Among them, as can be seen from formula (1),

[0077] τ = σ1 (11)

[0078] σ' = σ3 - p (12)

[0079] In the above equations, τ is the shear stress, c is the equivalent cohesion, φ is the equivalent internal friction angle, p is the pore water pressure, and σ′ is the effective tangential stress.

[0080] Under the action of two sets of different normal stresses and tangential stresses, that is, set different values for the setting value of the confining pressure servo motor pump 9 in step 7, control the pressure change rate in step 9 to be different values, keep the pore water pressure unchanged, repeat steps 1 - 7, then two sets of rock failure stresses can be obtained. By combining equations (10), (11), and (12), the equivalent internal friction angle φ and equivalent cohesion c of the fracture surface can be calculated.

[0081]

[0082] Among them, τ a and τ b are the shear stresses under the stress conditions corresponding to different pressure change rates in step 8 for two sets. From equations (11) and (1), σ a ’ and σ b ’ are the effective stress values corresponding to two sets of different confining pressures in step 6, which can be obtained from equations (12) and (2).

[0083] The normal stiffness Kn is the ratio of stress to strain when the rock is under the action of normal pressure. It reflects the elastic properties of the rock in the compression state and is a key parameter for evaluating the bearing capacity of the rock in underground structures.

[0084] The tangential stiffness Ks is the ratio of shear stress to shear strain when the rock is under shear action. It describes the elastic properties of the rock in the shear state and is very important for analyzing the shear deformation and failure mode of the rock.

[0085] The equivalent dilation angle α is the ratio of the dilation amount to the shear displacement when the rock is under shear. It characterizes the volume change characteristics of the rock during shear and is of great significance for understanding the dilation behavior of the rock and designing the support structure of underground engineering.

[0086] The equivalent internal friction angle φ is the angle corresponding to the ratio of frictional force to normal pressure when the fracture surface is under shear action. It reflects the frictional properties of the fracture surface during shear and is an important parameter for evaluating the stability of fractured rock masses and designing reinforcement measures.

[0087] The equivalent cohesion c is the shear strength that does not depend on the normal pressure when the rock material undergoes shear failure. It characterizes the cohesive properties of the rock material and is crucial for evaluating the shear strength of the rock and designing the stability of rock engineering structures.

Claims

1. A method for testing the penetration shear of rock fracture surfaces under triaxial stress, characterized in that: The test method comprises the following steps, Step 1: Sample preparation: split the standard cylindrical sample of the rock mass to be tested from the middle of the top surface of the cylinder to obtain two upper and lower semi-cylindrical samples; Step 2, loading the sample, aligning the upper and lower semi-cylindrical samples of the altered rock prepared in step 1, so that the upper and lower plates of the rock fracture surfaces are matched to form a sample, placing a pair of semi-circular metal indenters (27) and sponge indenters (28) below the sample, placing a pair of metal indenters (27) and sponge indenters (28) in opposite positions above the sample, so that the fracture surface of the matched sample is close to the connecting surface of the straight edges of the two metal indenters (27), encapsulating the sample and the two pairs of metal indenters (27) and sponge indenters (28) above and below in a sealed thermoplastic sleeve (31), and then loading it into the biased axial pressure chamber (19) of the rock triaxial chamber. The axial pressure head (1) and the base (2) are arranged between the biased axial pressure head (1) and the axial pressure head (26) buried in the base (2) so that the upper hole pressure tube (26) buried in the base (2) is directly opposite to the cracks on the upper and lower end surfaces of the sample. After being aligned and heated with a drying gun, the thermoplastic sleeve (31) tightly wraps the sample and two pairs of semicircular metal pressure heads (27) and sponge spacer pressure heads (28) arranged in the upper and lower parts. An annular displacement meter LVDT is arranged around the thermoplastic sleeve (31). Two axial displacement meters LVDTⅠ and LVDTⅡ are evenly installed around the thermoplastic sleeve (31) between the biased axial pressure head (1) and the base (2) to collect the tangential displacement of the rock crack surface. Step 3: After the biased axial pressure head (1) and the base (2) are tightly closed, the oil and air are extracted from the rock triaxial chamber (19) by using a vacuum pump (6), and the vacuum degree is maintained, and then the rock triaxial chamber (19) is filled with oil, and the confining pressure motor servo pump (9) capable of filling the rock triaxial chamber (19) with oil is filled with oil; Step 4, filling the bias chamber (20) for applying pressure to the upper end of the bias piston (4) above the bias axial pressure head (1) with oil, and filling the bias motor servo pump (13) capable of filling the bias chamber (20) with oil; Step 5, filling the pore pressure servo motor pump (16) capable of filling water from the biased axial pressure head (1) and the base (2) to the upper and lower end surfaces of the sample with water; Step 6: After the circumferential displacement meter LVDT, the axial displacement meter LVDTⅠ and the axial displacement LVDTⅡ are reset, oil is filled into the rock triaxial chamber (19) through the confining pressure motor servo pump (9), and the confining pressure is loaded to the set value p1 and maintained constant; Step 7: Fill the upper and lower end surfaces of the sample with water through the pore pressure servo motor pump (16), load the pore pressure to the set value p2 and maintain it constant, record the fluid volume V1 in the pore pressure servo motor pump (16), and record the readings D of the annular displacement meter LVDT, the axial displacement meter LVDTⅠ and the axial displacement meter LVDTⅡ. C1 , D 1 A1 , D 2 A1 ; Step 8, filling the bias chamber (20) with oil at a set flow rate through the bias motor servo pump (13), and when the readings of the axial displacement meter LVDTⅠ and the axial displacement meter LVDTⅡ change, stop controlling the loading in a flow mode, and instead load the pressure at a predetermined pressure change rate, and start automatically recording the readings of the circumferential displacement meter LVDT, the axial displacement meter LVDTⅠ and the axial displacement meter LVDTⅡ, until the rock fracture surface where the upper and lower plates are well matched is destroyed; When bias voltage is applied, read the circumferential displacement meter reading D C , read the pump pressure F of the bias motor servo pump A , the pump pressure F of the confining pressure motor servo pump C , according to formula (1) and formula (2), it is converted into the tangential stress σ3 and normal stress σ1 on the crack surface. According to the reading of the annular displacement meter after the confining pressure is loaded, D C1 , and the sample radius Ri, the chain length Lc of the circumferential displacement meter LVDT, the chain roller radius r of the circumferential displacement meter LVDT, the original circumference of the sample c0 = πD, the sample diameter D, calculate the tangential displacement ΔD C : In the above formula, D is the diameter of the specimen, c0 = πD, and C0 is the original circumference of the specimen; Δl=D C -D C1 (3) Where τ is the shear stress, Δl is the deformation measurement, and θ i is the deviation angle, R i is the radius of the specimen, in mm, and ΔC is the correction value of the change in the annular strain gauge; When bias is applied, the average value of the axial displacement meter reading is also read at the same time. A , and according to the axial displacement meter LVDTⅠ reading D after bias loading 1 A , axial displacement meter LVDTⅡ reading D 2 A , the average value of the axial displacement meter reading after confining pressure loading D A1 , axial displacement meter LVDTⅠ reading D 1 A1 , axial displacement meter LVDTⅡ reading D 2 A1 , calculate the normal displacement ΔD A ; ΔD A =D A -D A1 (7) Calculate the normal stiffness Kn, tangential stiffness Ks and equivalent dilatancy angle α:

2. The method according to claim 1, characterized in that The normal stiffness Kn and tangential stiffness Ks of the crack surface are based on the normal stress σ1 and the normal displacement ΔD A , tangential stress σ3 and tangential displacement ΔD C Plot the slope of the elastic straight line segment in the graph.

3. The method according to claim 1, characterized in that The method further comprises step 9, namely, under the action of two different groups of normal stress and tangential stress, controlling the setting value of the confining pressure servo motor pump (9) in step 6 to be different values, controlling the pressure change rate when applying the bias pressure in step 8 to be different values, keeping the pore water pressure p unchanged, repeating steps 1-6, and obtaining two groups of rock failure stresses. The equivalent internal friction angle φ and equivalent cohesion c of the fracture surface can be obtained by the following formula: τ=σ'tanφ+c (10) τ=σ1 (11) σ'=σ3-p (12) Among them, τ is the shear stress, σ′ is the effective tangential stress, φ is the equivalent internal friction angle, and c is the equivalent cohesion; τ a and τ b is the shear stress under the stress conditions corresponding to the different pressure change rates in the two groups of step 8. From formula (11) and formula (1), we can get: a ' and σ b ' is the effective stress value corresponding to the two different confining pressures in step 6, which can be obtained by formula (12) and formula (2).

Images