Permeation shear test method of rock fracture surface under triaxial stress effect

Abstract

The invention provides a seepage shear test method for a rock fracture surface under the action of triaxial stress, the device can apply confining pressure, bias pressure and pore water pressure at the same time, the device comprises a triaxial chamber and a bias pressure chamber which are arranged from bottom to top and are both hollow, the triaxial chamber comprises a base and a bias axial pressure head right opposite to the upper portion of the base, the bias chamber is provided with a bias piston, the lower end of the bias piston extends out of the bias chamber to be connected with a bias axial pressure head in the triaxial chamber, gas in the triaxial chamber is exhausted through an inlet and an outlet formed in the triaxial chamber, oil is filled into the triaxial chamber, confining pressure is applied, oil is filled into the bias chamber through an inlet formed in the bias chamber, and confining pressure is applied to the bias chamber. A bias piston and a bias shaft are pushed to move downwards towards the pressure head, and bias is applied; and pore water pressure is applied to the sample through a water inlet hole of the triaxial chamber. The device provides a new method for the shear test of the rock fracture surface under the triaxial stress condition, and the equivalent internal friction angle phi, the equivalent cohesive force c and the dilatancy angle alpha of the fracture surface can be obtained based on the Mohr-Coulomb criterion.

Description

Technical Field

[0001] The invention belongs to the field of rock mechanical property testing technology, and specifically relates to a fracture surface penetration shear test method under triaxial stress, which is mainly used to study the shear mechanical properties of rock structure surfaces under multi-field coupling. Background Art

[0002] Rock fracture surfaces are natural interfaces within or between rocks, usually caused by geological processes such as tectonic stress, rock weathering or thermal expansion. They are one of the more common adverse geological conditions encountered in underground projects such as hydropower stations and tunnels. Fracture surfaces can easily become channels for groundwater to flow, and there is often water seepage near the fractures in underground projects. Under the combined influence of groundwater and excavation disturbance, the permeability, strength and stability of the surrounding rock are reduced, and the surrounding rock may slip and tensile damage along the fracture surface, posing a threat to the stability of the underground cavern.

[0003] In order to deeply understand the mechanical properties of rock fracture surfaces, shear tests on fracture surfaces are one of the important methods to study the mechanical properties of rock fracture surfaces. Through shear tests, we can explore the shear response characteristics of rock fracture surfaces under different stress conditions. At present, biaxial stress conditions are usually adopted when shear tests are performed on fracture surfaces, that is, shear force is applied in one direction and normal force is applied in the other direction. For example, patent application number CN201711002741.3, invention name Shear box for rock direct shear strength test, the invented shear box can only achieve biaxial loading. Patent application number CN202211257915.1, invention name Constant normal force and constant normal stiffness combined pressure assembly and rock shear test device, this experimental device can only achieve biaxial loading, and does not consider three-way loading.

[0004] At present, there are mainly the following types of shear equipment on the fracture surface: (1) conventional direct shear tester that only applies normal stress and shear stress, (2) test equipment that superimposes a water tank on the conventional direct shear tester to simulate groundwater, and (3) true triaxial shear equipment that uses two-way rigid loading and one-way hydraulic flexible loading. However, in actual engineering, rock mass is usually subjected to not only single-direction or biaxial stress, but also a more complex triaxial stress environment, which may be accompanied by the action of groundwater. Although the existing biaxial shear test method can obtain the strength of rock fracture surface, it has certain limitations in fully reflecting the mechanical behavior of rock under the action of real environmental fields. For example, conventional biaxial shear test cannot fully simulate the multi-directional stress and deformation of rock mass under triaxial stress conditions, and ignoring the effect of groundwater will ignore the weakening effect of water on the strength of rock fracture surface; biaxial shear test considering seepage field cannot restore the real stress field of rock fracture surface; true triaxial shear test lacks consideration of seepage field of surrounding rock fracture surface. These deficiencies will result in the inability to restore the real environmental field of the rock fracture surface, thus affecting the authenticity and reliability of the research results. Therefore, in order to more accurately reflect the rock mechanical behavior under actual engineering conditions, it is necessary to improve the test equipment, design a test device that can simultaneously consider the triaxial stress field and seepage field of the surrounding rock fracture surface, and conduct a seepage 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 fracture surface penetration shear test device and a test method thereof under triaxial stress conditions. According to the shear test of the rock fracture surface under different confining pressures, the evolution law of the strength and pore mechanical properties of the rock fracture surface is obtained, providing theoretical and technical support for the stability analysis of underground engineering.

[0006] The technical solution adopted to achieve the above-mentioned purpose of the present invention is:

[0007] Implementation of a fracture surface penetration shear test device under triaxial stress conditions:

[0008] A semi-cylindrical metal pressure head is added and fixed on the left side of the axial pressure head of the test instrument, and a semi-cylindrical sponge placeholder pressure head is added and fixed on the right side. At the same time, a semi-cylindrical sponge placeholder pressure head is added and fixed on the left side of the test instrument base, and an upper semi-cylindrical metal pressure head is added and fixed on the right side. The pressure heads, metal pressure heads on the base, and sponge placeholder pressure heads are required to be aligned. In the test, the upper and lower plates of the rock containing the fracture surface are aligned and installed between the pressure head and the base of the test instrument. After the rock fracture surface is facing the edge of the metal pressure head, it is wrapped and fixed with a thermoplastic casing. The confining pressure is used to realize the bidirectional loading of the rock, the bias pressure is used to realize the tangential loading of the rock fracture surface, and the pore pressure is used to realize the seepage field loading of the fracture surface. The permeability shear test of the rock fracture surface under the coupling of three-dimensional stress and multi-field can be realized.

[0009] A method for testing fracture surface penetration shear under triaxial stress conditions comprises the following steps:

[0010] 1. Sample preparation: artificially split the standard cylindrical sample from the middle of the top surface of the cylinder, and make the top surface offset of the upper and lower plates of the crack surface not exceed 2mm, or do not affect the dislocation of the semicircular pressure head. Use wire cutting to cut the sample into a cylindrical shape, and then use a rock engraving machine to carve the structural surface to make the upper and lower plates of the semi-cylindrical crack surface.

[0011] 2. Sample loading: 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 the metal indenter and sponge spacer indenter below the axial indenter and above the base. Then, load the sample into the rock triaxial chamber and fix it with a sealing sleeve. Install the circumferential displacement sensor in the middle of the sample, and install the axial displacement sensor between the indenter and the base.

[0012] 3. Filling with oil: Open the confining pressure pump and the corresponding pipelines to fill the triaxial chamber with oil. At the same time, open the bias pressure pump and the corresponding pipelines to fill the bias pressure chamber with oil.

[0013] 4. Add confining pressure to reset the annular and axial displacement meters to zero, and use the confining pressure pump to fill the triaxial chamber with oil and pressurize it. After the confining pressure reaches the predetermined value, keep it constant.

[0014] 5. Add pore water pressure. After the confining pressure stabilizes, record the fluid volume V1 in the pore pressure loading pump. Add pore pressure to the sample at a flow rate of 0.1cm3 / min through the pore pressure pump. When the data of the axial displacement meter and the annular displacement meter change significantly, stop the pore pressure pump. At this time, the fluid has been fully saturated in the sample. Record the fluid volume V2 in the pore pressure loading pump again; start the pore pressure pump again and set the pore pressure to the preset value pMPa.

[0015] 6. Apply bias pressure, and after the confining pressure and pore pressure are stable, record the annular displacement meter reading D C1 and axial displacement meter reading D A1 , and then the bias pump pressurizes the oil in the triaxial chamber at a fixed loading rate A. When the axial displacement meter reading suddenly increases rapidly (increases by more than 0.5 mm), stop applying the bias.

[0016] 7. Depressurize and remove the sample. After reducing the bias pressure to zero through the bias pump, and then reducing the confining pressure to zero through the confining pressure pump, discharge the oil in the triaxial chamber and the bias chamber into the oil tank, open the triaxial chamber, take out the rock sample, and the test is completed.

[0017] 8. The displacement and stress of the crack surface are corrected by the recorded annular deformation data and axial deformation data. Assuming that the pump pressure of the bias motor servo pump at a certain moment when the bias is applied is F A , the pump pressure of the confining pressure motor servo pump is F C, then the tangential stress and normal stress on the crack surface are:

[0018]

[0019] D C is the reading of the annular strain gauge at a certain moment when bias pressure is applied, R i Sample radius 0.5D (mm), c0 = πD Sample original circumference (mm), L c Chain length 169mm, r Chain roller radius 0.5mm, 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 axial strain gauge reading at a certain moment when the bias pressure is applied, and the axial displacement gauge reading after the confining pressure is loaded. A1 , the length of the crack surface is H, then the tangential displacement is:

[0023] ΔD A =D A -D A1 (7)

[0024] Based on the changes in axial stress and displacement, tangential stress and displacement over a period of time during bias loading, the normal stiffness and tangential stiffness of the crack surface can be calculated as follows:

[0025]

[0026] 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.

[0027] At the same time, the equivalent shear dilation angle of the irregular undulating crack surface can be obtained by the following formula:

[0028]

[0029] Assuming that shear failure occurs on the structural surface, the Mohr-Coulomb criterion is:

[0030] τ=σ'tanφ+c (10)

[0031] in

[0032] τ=σ1 (11)

[0033] σ'=σ3-p (12)

[0034] Under the action of two different sets of normal stresses, keeping the pore water pressure unchanged, repeating steps 1-7, we can get two sets of rock failure stresses, and then combining equations (10), (11), and (12) to calculate the equivalent internal friction angle φ and equivalent cohesion c of the fracture surface.

[0035]

[0036] 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).

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

[0038] 1. A shear test device under triaxial stress conditions on the fracture surface was invented, which provides a new test method for shear tests under triaxial stress conditions on rock fracture surfaces;

[0039] 2. Based on the Mohr-Coulomb criterion, under triaxial stress conditions, the tangential stiffness Kn and normal stiffness Ks of the fracture surface under penetration, as well as the equivalent internal friction angle φ, equivalent cohesion c and shear dilation angle α can be obtained. The above parameters are important parameters for describing the mechanical behavior of rock cracks or fracture surfaces, and are used to understand and predict the behavior of rocks under various engineering and natural conditions, and for evaluating engineering stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 Schematic diagram of the structure of the rock triaxial rheometer.

[0041] Figure 2 Schematic diagram of sample loading.

[0042] Figure 3 Schematic diagram of stress-strain curve.

[0043] Among them, 1-bias axial pressure head, 2-test base, 3-axial displacement sensor (axial displacement LVDTⅠ and axial displacement LVDTⅡ), 4-bias piston, 5-valve, 6-air pump, 7-three-way pump valve 1, 8-confining pressure pump valve 1, 9-confining pressure servo motor pump, 10-oil tank, 11-bias valve 1, 12-bias valve 2, 13-bias servo motor pump, 14-three-way valve 2, 15-pore pressure valve, 16-pore pressure servo motor pump, 17-water tank, 1 8- rock sample, 19- triaxial chamber, 20- bias chamber, 21- bias chamber pipeline, 22- triaxial chamber pipeline one, 23- air pump pipeline, 24- triaxial chamber pipeline two, 25- triaxial chamber pipeline three, 26- orifice pressure water inlet pipeline, 32- orifice pressure water outlet pipeline, 27- metal pressure head, 28- sponge occupying pressure head, 29- rock fracture surface sample, 30- annular displacement sensor (annular displacement meter LVDT), 31- thermoplastic sleeve, 41- piston rod, 42- extension rod, 43- operating rod. DETAILED DESCRIPTION

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

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

[0046] like Figure 1As shown, confining pressure and bias pressure are applied through the rock triaxial rheometer. The rock triaxial rheometer includes a triaxial chamber 19 and a bias chamber 20 vertically arranged above the triaxial chamber 19. The triaxial chamber 19 is hollow, and a base 2 and a bias axial pressure head 1 are arranged inside. The sample to be tested can be placed between the bias axial pressure head 1 and the base 2. A plurality of axial displacement sensors 3 are also vertically arranged between the base 2 and the bias axial pressure head 1. The bias chamber 20 is also hollow, and a bias piston 4 is arranged therein. The lower end of the bias piston 4 extends out of the bias chamber 20 and is connected to the bias axial pressure head 1 inside the triaxial chamber 19. The bias piston 4 can move back and forth in the bias chamber 20 along the vertical direction, and at the same time drive the bias axial pressure head 1 fixed under the bias piston 4 to move together, so as to transmit pressure to the sample. A jack is provided below the base 2 of the triaxial chamber 19, and the control jack can adjust the degree of tightness of the base 2 and the triaxial chamber 19 in the circumferential direction, so that the triaxial chamber 19 forms a closed space. The triaxial chamber 19 is provided with an inlet and an outlet, and is connected to the exhaust pump 6 through the outlet to exhaust the gas in the triaxial chamber 19, and is connected to the oil tank 10 through the inlet to fill the triaxial chamber 19 with oil liquid, thereby applying confining pressure. The bias chamber 20 is provided with an inlet, and is connected to the oil tank 10 through the inlet to fill the bias chamber 20 with oil liquid, thereby pushing the bias piston 4 to move downward, and applying bias pressure, i.e., axial pressure, to the bias piston 4.

[0047] The application of pore water pressure includes the following devices: a water inlet hole is also provided on the triaxial chamber 19, which is connected to the water tank 17 through the water inlet hole, and the water in the water tank is transported to the center position of the biased axial pressure head 1 and the base 2, so as to transport the water flow to the fracture surface of the sample to be tested, thereby applying pore water pressure to the sample.

[0048] The application of the above confining pressure, bias pressure (axial pressure) and pore water pressure includes the following implementation steps:

[0049] 1. Sample preparation: artificially split the standard cylindrical sample of altered rock from the middle of the top surface of the cylinder so that the vertical displacement of the cracks on the upper and lower plates does not exceed 2 mm, and obtain two upper and lower plate semi-cylindrical samples with a diameter of 50 mm and a height of 100 mm. Prepare semi-circular indenters including a semi-circular metal indenter 27 and a semi-circular sponge placeholder indenter 28. The two semi-circular indenters are combined to form an integral circular indenter.

[0050] 2. Sample loading: align the two semi-cylindrical samples of the upper and lower plates of the 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 surface are well matched. Arrange a pair of metal indenters 27 and sponge occupiers 28 below the bias axial indenter 1, and arrange a pair of metal indenters 27 and sponge occupiers 28 in opposite positions above the base 2. Figure 2As shown in (a) of FIG. 1 , the matched sample is placed between two pairs of metal indenters 27 and sponge spacer indenters 28, and the crack surface of the matched sample is aligned with the midline of each pair of upper and lower semicircular indenters, so that the crack surface of the matched sample is close to the boundary surface of the two semicircular indenters. The specific placement method is as follows: Figure 2 As shown in (b); the hole pressure outlet pipe 32 buried in the biased axial pressure head 1 and the hole pressure inlet pipe 26 buried in the test base 2 are directly opposite to the cracks on the upper and lower end surfaces of the specimen; Figure 2 As shown in (c), the sample and the two pairs of semicircular pressure heads above and below it are encapsulated in a sealed thermoplastic sleeve 31, and then installed between the biased axial pressure head 1 and the test base 2 in the rock triaxial chamber 19. After being aligned and heated with a drying gun, the thermoplastic sleeve 31 tightly wraps the rock sample and the two pairs of semicircular pressure heads arranged above and below, and fixes them together between the biased axial pressure head 1 and the test base 2; an annular displacement sensor 30, namely an annular displacement meter LVDT, is added to the outside of the sample, and two axial displacement sensors 3, namely axial displacement meter LVDTⅠ and axial displacement LVDTⅡ, are installed between the biased axial pressure head 1 and the test base 2 to collect the tangential displacement of the rock fracture surface.

[0051] 3. Fill with oil. After the specimen, axial displacement sensor and circumferential displacement sensor are installed, use a jack to lift the test base 2 in the triaxial chamber 19 (the jack is located below the base 2, not shown in the figure), so that the test base 2 and the triaxial chamber 19 are tightly closed around the periphery, and then tighten the screws to form a closed space in the triaxial chamber 19; open valve 5 and open three-way pump valve 7 to connect the triaxial chamber pipeline 22 with the air pump pipeline 23, and then open the exhaust pump 6 to pass the three-way pump valve 7. The air in the three-axis chamber 19 is extracted through the axis chamber pipeline 122 and the air pump pipeline 23, and the pressure in the three-axis chamber 19 gradually decreases. The valve 5 is kept in the open state, and the oil in the oil tank 10 is transferred to the three-axis chamber 19 through the three-axis chamber pipeline 3 25. After the three-axis chamber 19 is filled with oil, the three-way pump valve 7 is closed first. After the pressure in the three-axis chamber 19, i.e. the surrounding pressure, is stable, the valve 5 and the air pump 6 are closed to complete the oil filling process of the three-axis chamber 19, so that the oil is filled in the three-axis chamber 19.

[0052] 4. Before adding 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 8, keep the three-way pump valve 7 closed, fill the confining pressure servo motor pump 9 with oil, and finally close the confining pressure pump valve 8.

[0053] 5. Before adding bias pressure, fill the bias chamber 20 and the bias servo motor pump 13 with oil, open the bias valve 11 and the bias valve 2 12, so that the bias chamber 20 is connected to the bias pump servo motor 13 and the oil tank 10. At this time, fill the bias chamber 20 with oil. After it is full, close the bias valve 1 11. After the bias servo motor pump 13 is full of oil, close the bias valve 2 12.

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

[0055] 7. Apply confining pressure to the set pressure value, clear the annular displacement meter LVDT, axial displacement meter LVDTⅠ and axial displacement LVDTⅡ, open the three-way pump valve 7, so that the three-axis chamber pipeline 1 22 and the three-axis chamber pipeline 24 are connected to the confining pressure servo motor pump 9, keep the confining pressure valve 1 8 and valve 5 closed, set the value p1 of the confining pressure servo motor pump 9 to 10MPa, start the confining pressure servo motor pump 9, continue to fill the three-axis 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 the 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 p2 = 2MPa, close the three-way valve 7, start the pore pressure servo motor pump 16, connect the pore pressure water inlet pipe 26 through the pore pressure servo motor pump 16, and add 0.1cm 3 / min flow rate to add pore pressure to the sample, record the fluid volume V1 in the pump, load the pore pressure to reach the set value and keep the pore pressure constant, after the pore pressure stabilizes, record the readings of the annular displacement meter LVDT, axial displacement meter LVDTⅠ and axial displacement meter LVDTⅡ C1 , D 1 A1 , D 2 A1 .

[0057] 9. Apply bias pressure after the confining pressure and pore pressure are stable, open bias valve 11, keep bias valve 2 12 closed, open bias servo motor pump 13, and slowly fill oil into the bias chamber 20 through the bias chamber pipe 21 in a flow loading manner to load pressure, so that the bias piston 4 moves downward, thereby transmitting pressure to the bias axial pressure head 1. The bias axial pressure head 1 applies pressure to the metal pressure head 27 and the sponge occupying pressure head 28. Stop loading when the readings of the axial displacement meter LVDTⅠ and the axial displacement meter LVDTⅡ change, close bias valve 11, and start automatically recording the readings of the annular displacement meter LVDT, axial displacement meter LVDTⅠ and axial displacement meter LVDTⅡ. At the same time, change to stress control loading (load pressure according to a predetermined pressure change rate, such as 0.5 MPa / s) until the rock fracture surface that matches the upper and lower plates is destroyed.

[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 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. A1D is the average value of the axial displacement meter reading after confining pressure loading. 1 A1 D is the reading of the axial displacement meter LVDTⅠ after the confining pressure is loaded. 2 A1 ΔD is the reading of the axial displacement meter LVDTⅡ after the confining pressure is loaded. A is the normal displacement.

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

[0069]

[0070] In the above formula, △σ3 is the change of tangential stress in the elastic straight line segment and △σ1 is the change of normal stress in the elastic straight line segment (such as Figure 3 ), read the annular displacement meter reading D after the confining pressure and pore pressure are loaded C1 and the average value of the axial displacement meter reading D A1 During the bias loading test, the pressure gauges on the confining pressure motor servo pump 9 and the bias pressure motor servo pump 13 are read. The normal stress σ1 and the tangential stress σ3 are obtained by equations (1) to (2). The normal displacement ΔD is calculated by equations (3) to (7). A and tangential displacement ΔD C , using the normal stress σ1 and normal displacement ΔD A , tangential stress σ3 and tangential displacement ΔD C Draw a graph and take the slopes of the elastic straight line segments in the two graphs as the normal stiffness Kn and the tangential stiffness Ks respectively.

[0071] At the same time, the equivalent dilatancy angle α of the irregular undulating crack surface can be obtained by the following formula:

[0072]

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

[0074] Assuming that shear failure occurs on the structural surface of the sample, the Mohr-Coulomb criterion is:

[0075] τ=σ'tanφ+c (10)

[0076] Among them, from formula (1), we can know that

[0077] τ=σ1 (11)

[0078] σ'=σ3-p (12)

[0079] In the above formula, 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 different groups of normal stresses and tangential stresses, that is, the setting value of the confining pressure servo motor pump 9 in step 7 is set to different values, the pressure change rate in step 9 is controlled to different values, the pore water pressure is kept unchanged, and steps 1-7 are repeated to obtain two groups of rock failure stresses. Then, the equivalent internal friction angle φ and equivalent cohesion c of the fracture surface can be obtained by combining equations (10), (11), and (12).

[0081]

[0082] Among them, 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).

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

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

[0085] The equivalent dilatancy angle α is the ratio of dilatancy to shear displacement during the shear process of rock. Characterizing the volume change of rock during shear is of great significance for understanding the dilatancy behavior of rock and designing support structures for underground engineering.

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

[0087] The equivalent cohesion c is the shear strength of rock materials that does not depend on normal pressure when they fail in shear. Characterizing the cohesive properties of rock materials is crucial for evaluating the shear strength of rocks 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 sample, c0=πD is the original circumference of the sample; Δl=D C -D C1 (3) 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, i.e., under the action of two different groups of normal stress and tangential stress, the setting value of the confining pressure servo motor pump (9) in step 6 is controlled to be different, the pressure change rate when the bias pressure is applied in step 8 is controlled to be different, the pore water pressure p is kept constant, and steps 1-6 are repeated to obtain two groups of rock failure stresses. The equivalent internal friction angle φ and equivalent cohesion c of the fracture surface can be calculated by the following formula: τ=σ'tanφ+c (10) τ=σ1 (11) σ'=σ3-p (12) 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).

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