In-situ testing device and method for seepage characteristics of fault zone under different effective stress states

Abstract

The application discloses a kind of different effective stress state under fault zone seepage characteristic in-situ test device and method, comprising: water supply system, osmotic pressure control system, field in-situ sample, data measurement system and permeate collection system;The field in-situ sample is connected with water supply system, osmotic pressure control system, data measurement system, permeate collection system respectively;The application realizes the double regulation of sample pore water pressure, osmotic pressure by osmotic pressure control system, to obtain the coupling constitutive relation between the permeability coefficient-effective stress in fault zone, the critical hydraulic gradient and the failure hydraulic gradient under a certain effective stress state determined according to the specific circumstances of project.

Description

Technical Field

[0001] This invention belongs to the field of water conservancy and hydropower technology, specifically relating to an in-situ test device and method for the seepage characteristics of fault zones under different effective stress states; in particular, it relates to a test device and method for determining the changes in seepage characteristics (permeability coefficient, critical hydraulic gradient and destructive hydraulic gradient) of fault zones caused by changes in pore water pressure. Background Technology

[0002] Fault zones generally consist of a main fault plane, flanking fractured rock blocks, and several secondary faults or fracture surfaces. Based on rock mass properties, they can be further divided into fault cores and fracture zones. Western my country has a complex geological structure with numerous large-scale, crisscrossing fault zones and well-developed secondary faults. Many large hydropower projects have to traverse these challenging geological structures. After the reservoirs are in normal operation, the high head pressure and large annual fluctuations in reservoir water level, coupled with the poor physical and mechanical properties of the fault zone rock mass, alter the seepage characteristics and impermeability of the fault zone under the influence of high head and significant water level changes. This negatively impacts seepage control and safety in engineering projects. Therefore, studying the influence of effective stress changes on the seepage characteristics of fault zones is crucial for the design of seepage control systems in water conservancy and hydropower projects and for ensuring the long-term safe operation of these projects.

[0003] Currently, the seepage characteristics of fault zones can generally be determined through laboratory tests, borehole water pressure tests, or in-situ tests. However, laboratory tests often fail to reflect the in-situ seepage characteristics of fault zones due to the unrepresentative size of the test samples and the secondary disturbances that affect borehole water pressure tests. Therefore, conducting in-situ tests in adits can better preserve the actual rock mass structure and stress state, resulting in test results that more accurately reflect the real situation. Studies have shown that the seepage characteristics of rock masses are greatly affected by the effective stress state. When the reservoir water level rises and the pore water pressure increases, the effective stress of the fault zone decreases, leading to changes in the permeability coefficient and impermeability strength of the fault zone. Traditional in-situ tests typically employ a "gradual pressurization upstream, free outflow downstream" seepage pressure loading method, which can only control the permeability gradient of the sample but cannot control the magnitude of the pore water pressure inside the sample. Consequently, the measured permeability coefficient values ​​do not reflect the impact of different effective stress states on the seepage characteristics of the fault zone. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides an in-situ testing device and method for the seepage characteristics of fault zones under different effective stress states. This invention achieves dual adjustment of pore water pressure and osmotic pressure in the sample through an osmotic pressure control system, thereby obtaining the coupled constitutive relationship between the permeability coefficient and effective stress within the fault zone, as well as the critical hydraulic gradient and failure hydraulic gradient under a certain effective stress state determined according to the specific engineering conditions.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] The present invention discloses an in-situ testing device for the seepage characteristics of a fault zone under different effective stress states, comprising: a water supply system, an osmotic pressure control system, an in-situ sample, a data measurement system, and a permeate collection system; wherein the in-situ sample is connected to the water supply system, the osmotic pressure control system, the data measurement system, and the permeate collection system respectively.

[0007] The water supply system includes: a water tank, a single-suction multi-stage centrifugal pump, a pressure regulating valve, and a pressure stabilizing tank, which are connected in sequence.

[0008] The osmotic pressure control system includes: a pressure regulating device and a reverse pressure control device; both consist of pressure regulating and control equipment and a high-precision pressure gauge; the pressure regulating and control equipment includes: a speed controller and a stop valve, the input end of the speed controller is connected to the output end of the stop valve; the output end of the speed controller is connected to the high-precision pressure gauge; the pressure regulating device is connected to the water supply system and the on-site in-situ sample respectively; the reverse pressure control device is connected to the permeate collection tank and the on-site in-situ sample respectively; high-precision flow meters are installed between the pressure regulating device and the on-site in-situ sample, and between the reverse pressure control device and the on-site in-situ sample;

[0009] The on-site in-situ sample is enclosed by reinforced concrete, and consists of an in-situ sample, an inlet chamber, and an outlet chamber. The inlet and outlet chambers are located at opposite ends of the in-situ sample, and both are filled with pebbles. The inlet chamber is equipped with an inlet pipe connected to a pressure regulating device. The outlet chamber is equipped with an outlet pipe, an vent, and a funnel pipe. The outlet pipe is connected to a reverse pressure control device. One end of the funnel pipe is embedded in the bottom of the outlet chamber.

[0010] The data measurement system includes: pressure measuring tubes, a paperless recorder, and a high-precision flow meter; multiple pressure measuring tubes are evenly buried in the in-situ sample and pebbles; the paperless recorder is connected to multiple pressure measuring tubes respectively; and the high-precision flow meter is arranged on both sides of the inlet pipe and the outlet pipe respectively.

[0011] The permeate collection system includes a particle collection tank and a permeate collection bucket; the particle collection tank is connected to the other end of a funnel tube.

[0012] Furthermore, the on-site in-situ samples are connected to the water supply system, the osmotic pressure control system, the data measurement system, and the permeate collection system via pipelines.

[0013] Furthermore, the high-precision pressure gauge has a range of 6 MPa and an accuracy of 0.001 MPa, and the high-precision flow meter has a range of 1.5 m. 3 / h, with an accuracy of 0.01m 3 / h.

[0014] Furthermore, the in-situ sample is prepared by selecting a suitable location on the wall of the main cavity, opening branch holes on both sides of the sample, and then connecting the branch holes to form a connecting cavity.

[0015] Furthermore, the appropriate location refers to an in-situ sample thickness of 0.5–0.8 m in the fault zone, with a fault core thickness of approximately 0.1–0.3 m and a fracture zone thickness of 0.4–0.5 m, and with a certain thickness of intact bedrock retained on both sides, forming an in-situ sample with a seepage diameter of 2 m, a width of 1.5 m, and a height of 2 m.

[0016] Furthermore, the pebble has a diameter of 2 cm.

[0017] Furthermore, the pressure regulating device and the reverse pressure control device in the osmotic pressure control system have the same nature and function.

[0018] An in-situ experimental method for detecting the seepage characteristics of fault zones under different effective stress states, based on the aforementioned apparatus, comprises the following steps:

[0019] Step 1: Locate the fault zone within the main tunnel and analyze the degree of fracture of the fault zone and the properties of its infill material and the content of fine particles;

[0020] Step 2: Except for the principal stress direction where no unloading occurs, two branch tunnels and one connecting tunnel are opened on each other face of the fault zone to cut off it from the surrounding area. In-situ specimens are prepared. The initial normal stress σ is determined based on engineering geostress data, and the maximum pore water pressure P used in the test is determined based on the actual engineering conditions. max ;

[0021] Step 3: After verifying that all equipment and valves in the in-situ sample are operating normally, install the test device and saturate the sample. After the sample is saturated, ensure that all pipelines are filled with water.

[0022] Step 4: Turn off the reverse pressure control device, adjust the pressure regulating valve in the water supply system to the maximum, and adjust the pressure regulating device in the osmotic pressure control system to a small pressure value (less than 0.02MPa). Observe the corresponding readings of each pressure measuring tube in the paperless recorder. After all readings are consistent, confirm that the test device is airtight and start the test.

[0023] Step 5: Keep the pressure of the pressure regulating device constant, and adjust the pressure of the reverse pressure control device to be slightly lower than the pressure of the pressure regulating device, so as to ensure that the pressure difference between the two (i.e., the osmotic pressure) is less than 0.005 MPa. After stabilization, at the same time, increase the pore water pressure inside the sample to P by adjusting the osmotic pressure control system.

[0024] Step 6: Gradually increase the osmotic pressure. After each stage of pressure increase, record the outflow rate Q and the readings h of each pressure measuring tube every 10-30 minutes. iOnce the two readings remain unchanged and the seepage reaches a stable state, the next level of osmotic pressure can be applied.

[0025] Step 7: Based on the cross-sectional area A and seepage path length L of the in-situ sample, calculate the average seepage gradient i = (h1-h5) / L and the average flow velocity v = Q / A of the sample when the seepage is stable under each level of osmotic pressure. Plot the lgv-lgi curve and use the formula lgv = lgk + lgi to fit and obtain the permeability coefficient k under the pore water pressure P.

[0026] Step 8: Gradually increase the pore water pressure P inside the sample to P' using the method described in Step 5. max The permeability coefficient under different pore water pressures is obtained using steps 6 and 7.

[0027] Step 9: Calculate the effective stress σ corresponding to the pore water pressure P. e =σ-P, plot k~σ e The curve is calculated using the exponential empirical formula k = k0 · exp(-b · σ). e The initial permeability coefficient k0 and stress sensitivity coefficient b are obtained by fitting the data.

[0028] Step 10: The pore water pressure in step 8 is P max Based on the corresponding osmotic pressure, continue to gradually increase the osmotic pressure, while also observing the particle collection tank and the effluent: when fine particles appear in the particle collection tank or the effluent becomes turbid, record the effluent flow rate Q promptly. c Readings of each pressure measuring tube (h) ic When the sample is damaged or the water head cannot be increased, record the outflow rate Q. f Readings of each pressure gauge (h) if The experiment was terminated;

[0029] Step 11: Process the data measured in Step 10 using the same method as in Step 7 and plot the lgv-lgi curve. Combine this with the experimental phenomena observed in Step 10 to determine the minimum effective stress σ. emin =σ-P max Lower critical hydraulic gradient i c =(h 1c -h 5c ) / L、Destructive hydraulic gradient i f =(h 1f -h 5f ) / L.

[0030] Furthermore, in steps 5 and 8, the pore water pressure P inside the sample is gradually increased to P through an osmotic pressure control system. maxThe specific method is as follows: Simultaneously adjust the pressure regulating device and the reverse pressure control device in the osmotic pressure control system to increase the pore water pressure P in the same direction and with the same pressure difference, increasing it step by step by 0.02MPa, 0.05MPa, 0.1MPa, 0.2MPa, 0.4MPa, and then each increment by 0.2MPa until P is reached. max .

[0031] Furthermore, the specific method for progressively increasing the osmotic pressure in step 6 is as follows: simultaneously adjust the pressure regulating device and the reverse pressure control device in the osmotic pressure control system to increase the osmotic pressure in opposite directions with the same pressure difference, progressively increasing it by 0.005MPa, 0.01MPa, 0.02MPa, 0.03MPa, and 0.05MPa.

[0032] Furthermore, the specific method for continuing to gradually increase the osmotic pressure in step 10 is as follows: simultaneously adjust the pressure regulating device and the reverse pressure control device in the osmotic pressure control system to increase the osmotic pressure in opposite directions with the same pressure difference, increasing it step by step by 0.1MPa, 0.15MPa, 0.2MPa, 0.3MPa, 0.4MPa, 0.6MPa, and then by an increment of 0.3MPa each time, until the sample is destroyed.

[0033] Furthermore, the allowable range of the equal pressure difference shall not exceed 0.1% of the pressure value in the reverse pressure control device. If the pressure value is small, its allowable range shall be 0.001 MPa.

[0034] The beneficial effects of this invention are as follows:

[0035] (1) The present invention is equipped with an osmotic pressure control system, which can not only realize the dual adjustment of pore water pressure and osmotic pressure; moreover, the test device of the present invention is simple to operate and can be directly improved on the existing in-situ test device. Compared with the "upstream step-by-step pressurization and downstream free flow" method commonly used in existing devices, it has wide applicability and low modification cost, and has significant advantages.

[0036] (2) The method of the present invention uses a step-by-step cyclic adjustment of pore water pressure and osmotic pressure. During the pore water pressure adjustment process, the osmotic pressure is relatively small, and during the osmotic pressure loading process, the pore water pressure remains constant. Under the premise of ensuring that the sample does not suffer osmotic damage, different effective stresses σ are measured. e The permeability coefficient k of the lower fault zone accurately reflects the coupled constitutive relationship between the permeability coefficient and effective stress of the in-situ sample.

[0037] (3) This invention applies only to the maximum pore water pressure P determined in actual engineering practice. maxThe in-situ sample impermeability strength was measured under the condition of (i.e., the corresponding minimum effective stress σemin). The measured critical and failure hydraulic gradient corresponded to the most unfavorable situation of the project, which not only avoided the excessive cost of multiple in-situ tests, but also effectively met the needs of the project's seepage safety evaluation. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the principle of the device of the present invention;

[0039] Figure 2 This is a schematic diagram of the in-situ test site of the fault zone in an embodiment of the present invention;

[0040] Figure 3 for Figure 2 Section I-I;

[0041] Figure 4 This is a schematic diagram illustrating the determination of the permeability coefficient k under a certain pore water pressure P according to an embodiment of the present invention;

[0042] Figure 5 To illustrate different effective stresses σ in embodiments of the present invention e A schematic diagram illustrating the variation of the permeability coefficient k in the lower fault zone;

[0043] Figure 6 To determine the minimum effective stress state σ under actual engineering conditions according to embodiments of the present invention emin Schematic diagram of critical hydraulic gradient and failure hydraulic gradient in the lower fault zone;

[0044] In the diagram: 1. Water supply system; 2. Osmotic pressure control system; 3. On-site sample; 4. Data measurement system; 5. Permeate collection system; 6. On-site sample; 7. Reinforced concrete; 8. Water supply tank; 9. Single-suction multi-stage centrifugal pump; 10. Pressure regulating valve; 11. Pressure stabilizing tank; 12. Pressure regulating and control equipment; 13. High-precision pressure gauge; 14. High-precision flow meter; 15. Pebbles; 16. Inlet chamber; 17. Pressure measuring tube; 18. Paperless recorder; 19. Vent; 20. Outlet chamber; 21. Funnel pipe; 22. Permeate collection tank; 23. Main tunnel; 24. Branch tunnel; 25. Connecting tunnel; 26. Inlet pipe; 27. Outlet pipe; 28. Speed ​​controller; 29. ​​Stop valve; 30. Particle collection tank. Detailed Implementation

[0045] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to embodiments and accompanying drawings. The content mentioned in the embodiments is not intended to limit the present invention.

[0046] Reference Figures 1-3As shown, an in-situ testing device for the seepage characteristics of a fault zone under different effective stress states according to the present invention includes: a water supply system 1, an osmotic pressure control system 2, an in-situ sample 3, a data measurement system 4, and a permeate collection system 5; the in-situ sample 3 is connected to the water supply system 1, the osmotic pressure control system 2, the data measurement system 4, and the permeate collection system 5 respectively through pipelines.

[0047] The water supply system 1 includes: a water supply tank 8, a single-suction multi-stage centrifugal pump 9, a pressure regulating valve 10, and a pressure stabilizing tank 11, which are connected in sequence.

[0048] The osmotic pressure control system 2 includes: a pressure regulating device 2-1 and a reverse pressure control device 2-2; both are composed of a pressure regulating and control device 12 and a high-precision pressure gauge 13; the pressure regulating and control device 12 includes: a speed regulator 28 and a stop valve 29, the input end of the speed regulator 28 is connected to the output end of the stop valve 29; the output end of the speed regulator 28 is connected to the high-precision pressure gauge 13; the pressure regulating device 2-1 is connected to the water supply system 1 and the on-site sample 3 respectively; the reverse pressure control device 2-2 is connected to the permeate collection tank 22 and the on-site sample 3 respectively; high-precision flow meters 14 are evenly distributed between the pressure regulating device 2-1 and the on-site sample 3, and between the reverse pressure control device 2-2 and the on-site sample 3;

[0049] The on-site in-situ sample 3 is enclosed by reinforced concrete 7, and consists of an in-situ sample 6, a water inlet chamber 16, and a water outlet chamber 20. The water inlet chamber and the water outlet chamber are located at opposite ends of the in-situ sample 6, and both contain pebbles 10 with a particle diameter of 2 mm. The water inlet chamber 16 is equipped with a water inlet pipe 26, which is connected to a pressure regulating device 2-1. The water outlet chamber 20 is equipped with a water outlet pipe 27, a vent 19, and a funnel pipe 21. The water outlet pipe 27 is connected to the pressure control device 2-2. One end of the funnel pipe 21 is embedded in the bottom of the water outlet chamber 20.

[0050] The data measurement system 4 includes: pressure measuring tubes 17, paperless recorder 18, and high-precision flow meter 14; multiple pressure measuring tubes 17 are evenly buried in the in-situ sample 6 and pebbles 15; the paperless recorder 18 is connected to multiple pressure measuring tubes 17 respectively; the high-precision flow meter 14 is arranged on both sides of the inlet pipe 26 and the outlet pipe 27 respectively.

[0051] The permeate collection system 5 includes a particle collection tank 30 and a permeate collection bucket 22; the particle collection tank 30 is connected to the other end of the funnel tube 21.

[0052] The in-situ sample 6 is prepared by selecting a suitable position on the wall of the main hole 23, opening branch holes 24 on both sides of the sample, and then connecting the branch holes 24 to form a connecting hole 25.

[0053] Specifically, the suitable location refers to an in-situ sample thickness of 0.5 to 0.8 m in the fault zone, with a fault core thickness of approximately 0.1 to 0.3 m and a fracture zone thickness of 0.4 to 0.5 m, and with a certain thickness of intact bedrock retained on both sides, forming an in-situ sample with a seepage diameter of 2 m, a width of 1.5 m, and a height of 2 m.

[0054] In the osmotic pressure control system 2, the pressure regulating device 2-1 and the reverse pressure control device 2-2 have the same properties and functions. By adjusting the two in different combinations, the function of changing the pore water pressure and osmotic pressure can be achieved.

[0055] (1) The method of changing the pore water pressure is as follows: the initial pore water pressure is the average of the sum of the upstream and downstream pressure values. Under the condition that the upstream and downstream permeation pressure difference is always constant, the pressure is adjusted and changed by simultaneously increasing or decreasing the pressure in the same direction and with the same pressure difference through pressure regulating equipment and reverse pressure control equipment.

[0056] (2) The method of changing the osmotic pressure is as follows: the initial pressure difference is the pressure difference between the upstream and downstream. Under the condition that the internal pore water pressure is always constant, the pressure is adjusted and changed by simultaneously increasing or decreasing the pressure in the opposite direction and with the same pressure difference through pressure regulating equipment and reverse pressure control equipment.

[0057] The water inlet chamber 16 is located inside the connecting hole 25, and the water outlet chamber 20 is located inside the main hole 24. During the saturated sample test in the field, the water inlet chamber 16 is located inside the main hole 24, the vent 19 is located at the upper end of the water inlet chamber 16, and the water outlet chamber 20 is located inside the connecting hole 25, which is exactly the opposite of the position at the beginning of the test.

[0058] An in-situ experimental method for detecting the seepage characteristics of fault zones under different effective stress states, based on the aforementioned apparatus, comprises the following steps:

[0059] Step 1: Locate the fault zone within the main tunnel and analyze the degree of fracture of the fault zone and the properties of its infill material and the content of fine particles;

[0060] Step 2: Except for the principal stress direction where no unloading occurs, two branch tunnels and one connecting tunnel are opened on each other face of the fault zone to cut off it from the surrounding area. In-situ specimens are prepared. The initial normal stress σ is determined based on engineering geostress data, and the maximum pore water pressure P used in the test is determined based on the actual engineering conditions. max ;

[0061] The thickness of the in-situ sample in the fault zone is 0.5 to 0.8 m, of which the thickness of the fault fracture zone is about 0.1 to 0.3 m and the thickness of the fault influence zone is 0.4 to 0.5 m. A certain thickness of intact bedrock is retained at both ends to form an in-situ sample with a seepage diameter of 2 m, a width of 1.5 m and a height of 2 m.

[0062] Step 3: After verifying that all equipment and valves in the in-situ sample are operating normally, install the test device and saturate the sample. After the sample is saturated, ensure that all pipelines are filled with water.

[0063] Step 4: Turn off the reverse pressure control device, adjust the pressure regulating valve in the water supply system to the maximum, and adjust the pressure regulating device in the osmotic pressure control system to a small pressure value (less than 0.02MPa). Observe the corresponding readings of each pressure measuring tube in the paperless recorder. After all readings are consistent, confirm that the test device is airtight and start the test.

[0064] Step 5: Keep the pressure of the pressure regulating device constant, and adjust the pressure of the reverse pressure control device to be slightly lower than the pressure of the pressure regulating device, so as to ensure that the pressure difference between the two (i.e., the osmotic pressure) is less than 0.005 MPa. After stabilization, at the same time, increase the pore water pressure inside the sample to P by adjusting the osmotic pressure control system.

[0065] Step 6: Gradually increase the osmotic pressure. After each stage of pressure increase, record the outflow rate Q and the readings h of each pressure measuring tube every 10-30 minutes. i Once the two readings remain unchanged and the seepage reaches a stable state, the next level of osmotic pressure can be applied.

[0066] Specifically, the method for progressively increasing the osmotic pressure in step 6 is as follows: simultaneously adjust the pressure regulating device and the reverse pressure control device in the osmotic pressure control system to increase the osmotic pressure in opposite directions with the same pressure difference, increasing it progressively by 0.005MPa, 0.01MPa, 0.02MPa, 0.03MPa, and 0.05MPa.

[0067] Step 7: Based on the cross-sectional area A and seepage path length L of the in-situ sample, calculate the average seepage gradient i = (h1-h5) / L and the average flow velocity v = Q / A of the sample when seepage is stable under each osmotic pressure. Plot the lgv-lgi curve, and use the formula lgv = lgk + lgi to fit and obtain the permeability coefficient k under the pore water pressure P, as shown in the attached figure. Figure 4 As shown;

[0068] Step 8: Gradually increase the pore water pressure P inside the sample to P' using the method described in Step 5. max The permeability coefficient under different pore water pressures is obtained using steps 6 and 7.

[0069] Specifically, in steps 5 and 8, the pore water pressure P inside the sample is gradually increased to P through an osmotic pressure control system. maxThe specific method is as follows: Simultaneously adjust the pressure regulating device and the reverse pressure control device in the osmotic pressure control system to increase the pore water pressure P in the same direction and with the same pressure difference, increasing it step by step by 0.02MPa, 0.05MPa, 0.1MPa, 0.2MPa, 0.4MPa, and then each increment by 0.2MPa until P is reached. max .

[0070] Step 9: Calculate the effective stress σ corresponding to the pore water pressure P. e =σ-P, plot k~σ e The curve is calculated using the exponential empirical formula k = k0 · exp(-b · σ). e The initial permeability coefficient k0 and stress sensitivity coefficient b are obtained through fitting, as shown in the attached figure. Figure 5 As shown;

[0071] Step 10: The pore water pressure in step 8 is P max Based on the corresponding osmotic pressure, continue to gradually increase the osmotic pressure, while also observing the particle collection tank and the effluent: when fine particles appear in the particle collection tank or the effluent becomes turbid, record the effluent flow rate Q promptly. c Readings of each pressure measuring tube (h) ic When the sample is damaged or the water head cannot be increased, record the outflow rate Q. f Readings of each pressure gauge (h) if The experiment was terminated;

[0072] Specifically, the method for continuing to apply osmotic pressure in step 10 is as follows: simultaneously adjust the pressure regulating device and the reverse pressure control device in the osmotic pressure control system to increase the osmotic pressure in opposite directions with the same pressure difference, increasing it step by step by 0.1MPa, 0.15MPa, 0.2MPa, 0.3MPa, 0.4MPa, 0.6MPa, and then by an increment of 0.3MPa each time, until the sample is destroyed.

[0073] Step 11: Process the data measured in Step 10 using the same method as in Step 7 and plot the lgv-lgi curve. Combine this with the experimental phenomena observed in Step 10 to determine the minimum effective stress σ. emin =σ-P max Lower critical hydraulic gradient i c =(h 1c -h 5c ) / L、Destructive hydraulic gradient i f =(h 1f -h 5f ) / L, as attached Figure 6 As shown.

[0074] Specifically, the allowable range of the equal pressure difference shall not exceed 0.1% of the pressure value in the reverse pressure control device. If the pressure value is small, its allowable range shall be 0.001 MPa.

[0075] This invention has many specific applications. The above description is only a preferred embodiment of this invention. It should be noted that for those skilled in the art, several improvements can be made without departing from the principle of this invention, and these improvements should also be considered within the scope of protection of this invention.

Claims

1. An in-situ test method for the seepage characteristics of fault zones under different effective stress states, based on an in-situ test apparatus for the seepage characteristics of fault zones under different effective stress states, the apparatus comprising: Water supply system, osmotic pressure control system, on-site in-situ sampling system, data measurement system and permeate collection system; The on-site in-situ samples are connected to the water supply system, the osmotic pressure control system, the data measurement system, and the permeate collection system, respectively. The water supply system includes: a water tank, a single-suction multi-stage centrifugal pump, a pressure regulating valve, and a pressure stabilizing tank, which are connected in sequence. The osmotic pressure control system includes: a pressure regulating device and a reverse pressure control device; both consist of pressure regulating and control equipment and a high-precision pressure gauge; the pressure regulating and control equipment includes: a speed controller and a stop valve, the input end of the speed controller is connected to the output end of the stop valve, and the output end of the speed controller is connected to the high-precision pressure gauge; the pressure regulating device is connected to the water supply system and the on-site in-situ sample respectively; the reverse pressure control device is connected to the permeate collection tank and the on-site in-situ sample respectively; high-precision flow meters are installed between the pressure regulating device and the on-site in-situ sample, and between the reverse pressure control device and the on-site in-situ sample respectively. The on-site in-situ sample is enclosed by reinforced concrete, and consists of an in-situ sample, an inlet chamber, and an outlet chamber. The inlet and outlet chambers are located at opposite ends of the in-situ sample, and both are filled with pebbles. The inlet chamber is equipped with an inlet pipe connected to a pressure regulating device. The outlet chamber is equipped with an outlet pipe, an vent, and a funnel pipe. The outlet pipe is connected to a reverse pressure control device. One end of the funnel pipe is embedded in the bottom of the outlet chamber. The data measurement system includes: pressure measuring tubes, a paperless recorder, and a high-precision flow meter; multiple pressure measuring tubes are evenly buried in the in-situ sample and pebbles; the paperless recorder is connected to multiple pressure measuring tubes respectively; and the high-precision flow meter is arranged on both sides of the inlet pipe and the outlet pipe respectively. The permeate collection system includes: a particle collection tank and a permeate collection bucket; the particle collection tank is connected to the other end of a funnel tube; The method is characterized by the following steps: Step 1: Locate the fault zone within the main tunnel and analyze the degree of fracture of the fault zone and the properties of its infill material and the content of fine particles; Step 2: Except for the principal stress direction where no unloading occurs, two branch tunnels and one connecting tunnel are opened on each other face of the fault zone to cut off it from the surrounding area. In-situ specimens are prepared. The initial normal stress σ is determined based on engineering geostress data, and the maximum pore water pressure P used in the test is determined based on the actual engineering conditions. max ; Step 3: After verifying that all equipment and valves in the in-situ sample are operating normally, install the test device and saturate the sample. After the sample is saturated, ensure that all pipelines are filled with water. Step 4: Turn off the reverse pressure control device, adjust the pressure regulating valve in the water supply system to the maximum, and adjust the pressure regulating device in the osmotic pressure control system to a certain low pressure value. Observe the corresponding readings of each pressure measuring tube in the paperless recorder. After all readings are consistent, confirm that the test device is airtight and start the test. Step 5: Keep the pressure of the pressure regulating device constant, and adjust the pressure of the reverse pressure control device to be slightly lower than the pressure of the pressure regulating device, ensuring that the pressure difference between the two is less than 0.005MPa. After stabilization, simultaneously increase the pore water pressure inside the sample to P by adjusting the osmotic pressure control system. Step 6: Gradually increase the osmotic pressure. After each stage of pressure increase, record the outflow rate Q and the readings h of each pressure measuring tube every 10-30 minutes. i Once the two readings remain unchanged, the seepage reaches a stable state, and the next level of seepage pressure is applied. Step 7: Based on the cross-sectional area A and seepage path length L of the in-situ sample, calculate the average seepage gradient i = (h1-h5) / L and the average flow velocity v = Q / A of the sample when the seepage is stable under each level of osmotic pressure. Plot the lgv-lgi curve and use the formula lgv = lgk + lgi to fit and obtain the permeability coefficient k under the pore water pressure P. Step 8: Gradually increase the pore water pressure P inside the sample to P' using the method described in Step 5. max The permeability coefficient under different pore water pressures is obtained using steps 6 and 7. Step 9: Calculate the effective stress σ corresponding to the pore water pressure P. e =σ-P, plot k~σ e The curve is calculated using the exponential empirical formula k = k0 · exp(-b · σ). e The initial permeability coefficient k0 and stress sensitivity coefficient b are obtained by fitting the data. Step 10: The pore water pressure in step 8 is P max Based on the corresponding osmotic pressure, continue to gradually increase the osmotic pressure while observing the particle collection tank and the effluent: when fine particles appear in the particle collection tank or the effluent becomes turbid, record the effluent flow rate Q in a timely manner. c Readings of each pressure measuring tube (h) ic When the sample is damaged or the water head cannot be increased, record the outflow rate Q. f Readings of each pressure gauge (h) if The experiment was terminated; Step 11: Process the data measured in Step 10 using the same method as in Step 7 and plot the lgv-lgi curve. Combine this with the experimental phenomena observed in Step 10 to determine the minimum effective stress σ. emin =σ-P max Lower critical hydraulic gradient i c =(h 1c -h 5c ) / L、Destructive hydraulic gradient i f =(h 1f -h 5f ) / L.

2. The in-situ test method for seepage characteristics of fault zones under different effective stress states according to claim 1, characterized in that, The in-situ sample is prepared by selecting a suitable location on the wall of the main cavity, opening branch holes on both sides of the sample, and then connecting the branch holes to form a connecting cavity.

3. The in-situ test method for seepage characteristics of fault zones under different effective stress states according to claim 2, characterized in that, The appropriate location refers to an in-situ sample thickness of 0.5–0.8 m in the fault zone, with a fault core thickness of approximately 0.1–0.3 m and a fracture zone thickness of 0.4–0.5 m, and with a certain thickness of intact bedrock retained on both sides, forming an in-situ sample with a seepage diameter of 2 m, a width of 1.5 m, and a height of 2 m.

4. The in-situ test method for seepage characteristics of fault zones under different effective stress states according to claim 1, characterized in that, In the osmotic pressure control system, the pressure regulating device and the reverse pressure control device have the same nature and function.

5. The in-situ test method for seepage characteristics of fault zones under different effective stress states according to claim 1, characterized in that, The high-precision pressure gauge has a range of 6 MPa and an accuracy of 0.001 MPa, and the high-precision flow meter has a range of 1.5 m. 3 / h, with an accuracy of 0.01m 3 / h.

6. The in-situ test method for seepage characteristics of fault zones under different effective stress states according to claim 1, characterized in that, In steps 5 and 8, the pore water pressure P inside the sample is gradually increased to P through an osmotic pressure control system. max The specific method is as follows: Simultaneously adjust the pressure regulating device and the reverse pressure control device in the osmotic pressure control system to increase the pore water pressure P in the same direction and with the same pressure difference, increasing it step by step by 0.02MPa, 0.05MPa, 0.1MPa, 0.2MPa, 0.4MPa, and then each increment by 0.2MPa until P is reached. max .

7. The in-situ test method for seepage characteristics of fault zones under different effective stress states according to claim 1, characterized in that, The specific method for gradually increasing the osmotic pressure in step 6 is as follows: simultaneously adjust the pressure regulating device and the reverse pressure control device in the osmotic pressure control system to increase the osmotic pressure in opposite directions with the same pressure difference, increasing it step by step by 0.005MPa, 0.01MPa, 0.02MPa, 0.03MPa, and 0.05MPa.

8. The in-situ test method for seepage characteristics of fault zones under different effective stress states according to claim 1, characterized in that, The specific method for continuing to gradually increase the osmotic pressure in step 10 is as follows: simultaneously adjust the pressure regulating device and the reverse pressure control device in the osmotic pressure control system to increase the osmotic pressure in opposite directions with the same pressure difference, increasing it step by step by 0.1MPa, 0.15MPa, 0.2MPa, 0.3MPa, 0.4MPa, 0.6MPa, and then by an increment of 0.3MPa each time, until the sample is destroyed.

9. The in-situ test method for seepage characteristics of fault zones under different effective stress states according to claim 6, 7, or 8, characterized in that, The allowable range of the equal pressure difference shall not exceed 0.1% of the pressure value in the reverse pressure control device. If the pressure value is small, its allowable range shall be 0.001 MPa.

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