A device and method for testing the permeability coefficient of a mud film in a saturated silty sand formation

Abstract

The application discloses a kind of saturated fine sand stratum mud film permeability coefficient testing device and testing method, including water supply mechanism and mud permeation mechanism;Water supply mechanism is used for the water supply of mud permeation mechanism in stratum saturation and mud permeability coefficient calibration, including water supply bottle, water pump, water bucket, backwater pipe and first water supply pipe;Mud permeation mechanism is used for carrying out mud permeation film forming, including second- fourth water supply pipe, tee joint, first- fourth valve, first- fifth TDR sensor, three threaded rods, organic glass column, distance measuring sensor, first, second hole pressure gauge, base, flange, electronic scale, water outlet pipe, percolate collection container, first- third gas supply pipe, control monitoring instrument, air compressor and computer.The application is simple in operation, low in cost, and widely applicable.

Description

Technical Field

[0001] This invention belongs to the field of slurry shield tunnel technology. Background Technology

[0002] Since the beginning of the new century, slurry shield tunneling has been widely used in the construction of cross-river and sea tunnels due to its advantages such as environmental friendliness, flood control safety, and minimal disruption to navigation. When the tunnel boring machine (TBM) traverses complex geological formations, the variable geological conditions and the presence of quartz, gravel, and other materials can cause wear on the cutterhead. This necessitates a shutdown for pressurized chamber opening to replace the cutters, repair the cutterhead, and clean it. Pressurized chamber opening primarily involves forming a dense mud film at the excavation face, converting the support force at the excavation face into effective pressure to balance the soil and water pressure behind the excavation face. Then, the slurry level is lowered, and compressed air is used to support the excavation face. Workers then enter the pressure chamber to perform a series of operations. Considering the crucial role of mud film quality in the pressurized chamber opening process... Meanwhile, bentonite mud slurry is significantly alkaline. The alkaline liquid in the mud penetrates into the formation and diffuses within the formation during the film formation process, which has a certain impact on the groundwater environment. Therefore, indoor tests are required before pressurized opening to verify the quality of the mud film. The main verification indicator is the permeability coefficient of the mud film, while monitoring the diffusion behavior of the alkaline slurry in the formation.

[0003] Currently, the formation of mud films is mainly simulated through filtration loss tests. Pressurized mud forms a dense mud film on filter paper, but because the filter paper has fixed dimensions, it cannot reflect the actual formation conditions. Therefore, instruments that better reflect actual working conditions are needed to evaluate mud infiltration film formation. During mud infiltration, it is necessary to monitor the infiltration flow rate and surface pore pressure, and track the diffusion range of alkaline liquid in the mud. Traditional infiltration column tests cannot provide pressure to the formation, which does not match actual excavation conditions. Furthermore, traditional tests require removing the mud film and measuring its thickness and permeability coefficient, making the simulation of mud film formation cumbersome and requiring a huge amount of monitoring. In addition, the leachate is a transparent liquid, making it impossible to track its traces. Summary of the Invention

[0004] Purpose of the invention: In order to solve the problems existing in the prior art, the present invention proposes a device and method for testing the mud film permeability coefficient in saturated fine sand formations.

[0005] Technical Solution: This invention provides a device for testing the mud film permeability coefficient in saturated silty sand formations, including a water supply mechanism and a mud permeation mechanism; the water supply mechanism includes a water supply bottle, a water pump, a water bucket, a return water pipe, and a first water supply pipe; the water bucket is placed on the ground and filled with tap water, the water supply bottle is placed at a certain height, the water pump is placed inside the water bucket, the water pump inputs the tap water in the water bucket into the water supply bottle through the first water supply pipe, and when the water in the water supply bottle reaches a preset height, it flows into the water bucket through the return water pipe;

[0006] The mud infiltration mechanism includes second to fourth water supply pipes, a tee connector, first to fourth valves, first to fifth TDR sensors, three threaded rods, an acrylic column, a distance sensor, first and second pore pressure gauges, a base, a flange, an electronic scale, an outlet pipe, a leachate collection container, first to third air supply pipes, a control and monitoring instrument, an air compressor, and a computer. The control and monitoring instrument integrates first and second air pressure control and display devices. The first valve is a two-way valve. The acrylic column is mounted on the base, which has two opposing first and second through holes, both of which are L-shaped, with one end of the L-shaped through hole on the base. The surface is one end, and the other end is on the side of the base; the first through hole at one end of the base is connected to the second valve, the second valve is connected to the first port of the tee connector through the third water supply pipe, and the second port of the tee connector is connected to the bottom of the water supply bottle through the second water supply pipe; the second through hole at one end of the base is connected to the third valve, the third valve is connected to the leachate collection container through the outlet pipe, used to transfer the leachate in the plexiglass to the leachate collection container; the flange edge and the base edge are provided with three corresponding holes, the top of each of the three threaded rods is provided with a nut, and the bottom of each of the three threaded rods is respectively fixed to the base. The tops of three threaded rods pass through the holes in the flange, and tightening the nuts seals the plexiglass column. A first valve is located at the center of the flange; one valve is for water supply, and the other is for air supply. The third port of the tee connector is connected to the water supply end via a fourth water supply pipe for supplying water into the plexiglass column. The air supply end is connected to a first air pressure control and display device via a second air supply pipe. The leachate collection container is mounted on an electronic scale, which is connected to a computer. A fourth valve is located on the top of the leachate collection container, and this fourth valve is connected to the second air pressure control and display device via a third air supply pipe. The control and monitoring instrument is connected to the air compressor via a first air supply pipe; the distance sensor is located at the bottom of the flange, inside the plexiglass column; the first to fifth TDR sensors are evenly spaced from top to bottom on the side of the plexiglass column; the first and second orifice gauges are located on the side of the plexiglass column, with the height of the first orifice gauge matching the height of the first TDR sensor, and the height of the second orifice gauge matching the height of the fifth TDR sensor; the distance sensor, the five TDR sensors, and the two orifice gauges are all connected to the control and monitoring instrument, which transmits the received data to a computer for display.

[0007] Furthermore, the ranging sensor is an ultrasonic ranging sensor.

[0008] Furthermore, it also includes first to fifth data lines; the ranging sensor is connected to the control and monitoring instrument via the first data line, the five TDR sensors are connected to the control and monitoring instrument via the second data line, the two orifice manometers are connected to the control and monitoring instrument via the third data line, the control and monitoring instrument is connected to the computer via the fourth data line, and the electronic scale is connected to the computer via the fifth data line.

[0009] A testing method for a mud film permeability coefficient testing device in saturated silty fine sand formations, specifically including the following steps:

[0010] Step 1: Prepare a formation in an plexiglass column and conduct a constant head test. Determine whether the formation preparation is qualified by the permeability coefficient between two air pressure gauges. If it is qualified, proceed to Step 2; otherwise, prepare the formation again until it is qualified.

[0011] Step 2: Close all four valves and slowly add mud along the inner wall of the plexiglass column using the drainage method, then seal the flange. The control monitor transmits the data measured by the distance sensor to the computer. Turn on the air compressor, open the air supply end of the first valve and the fourth valve. The first air pressure control display device adjusts the pressure provided by the air compressor to a preset value, and then transmits it to the plexiglass column to provide osmotic pressure for the mud. The second air pressure control display device adjusts the pressure provided by the air compressor to a preset value, and then transmits it to the leachate collection container. When the readings of the two pore pressure gauges stabilize, open the third valve. At this time, the pressure transmitted to the leachate collection container provides pore water pressure for the formation. The mud begins to permeate under the action of osmotic pressure.

[0012] Step 3: The experiment begins when the third valve is opened, and ends at a pre-set time. The total experimental time is denoted as X. Time X is divided into n time segments. The last moment of the x-th time segment is used as the node for calculating the permeability coefficient, denoted as T. x x = 1, 2, ..., n; take T x The first y time periods and the last y time periods are used as the time periods for calculating the permeability coefficient, and the length of this time period is denoted as t. After the experiment, a mud film is obtained, and an appropriate amount of mud film is taken to measure its water content and obtain its density. Based on the density of the mud film, the time node T is determined. x The mass ΔM of the leachate measured by the electronic balance and the time from the start of the experiment to the time point T x The mud film thickness H is calculated by measuring the mud level change Δh1 using the time-range sensor.

[0013] Step 4: Calculate the mud film permeability coefficient corresponding to the time length t based on the formation pore pressure measured by the pore pressure gauge and the mud film thickness H.

[0014] Further, step 1 specifically involves: laying a layer of geotextile at the bottom of the plexiglass column; laying a layer of coarse sand as a filter layer on top of the geotextile; laying a soil layer on top of the filter layer, with the upper surface of the soil layer flush with the first TDR sensor; closing the first and third valves, keeping the fourth valve open and setting the air pressure displayed by the second air pressure control display device to 0; opening the water pump and the second valve to saturate the soil layer from bottom to top; closing the second valve after saturation and injecting water along the inner wall of the plexiglass column to the top of the plexiglass column using a drainage method; tightening the threaded rod to seal the plexiglass column with the flange; opening the water supply ends of the third and first valves to conduct a constant head test, recording the readings of the two pore pressure gauges and the mass of the leachate; calculating the permeability coefficient of the soil layer between the two pore pressure gauges according to Darcy's law; if the calculated permeability coefficient is within the preset range, it indicates that the soil layer preparation is qualified.

[0015] Furthermore, in step 3, the thickness H of the mud film is calculated according to the following formula:

[0016]

[0017] A is the cross-sectional area inside the plexiglass column, ρ' is the mud density, and ρ1 is the mud film density.

[0018] Furthermore, the mud film permeability coefficient k over time length t is calculated using the following formula:

[0019]

[0020] Where Δm is the mass change of leachate within time length t, g is the gravitational constant, P1 is the surface pore pressure measured by the first air pressure gauge, P2 is the air pressure input into the plexiglass column by the first air pressure control and display device, and h is the mud height before mud infiltration.

[0021] Furthermore, the method also includes repeating steps 1 to 4 several times to obtain the diffusion law of alkaline solution over time and the diffusion law of mud film permeability coefficient over time; the diffusion law of alkaline liquid over time is specifically obtained as follows: after mud infiltration, the water in the formation gradually becomes alkaline liquid, and the conductivity of the liquid in the formation is measured by five TDR sensors to obtain the diffusion law of alkaline liquid over time.

[0022] Beneficial effects:

[0023] 1. This invention uses a water supply pipe to flow water from a fixed height into an acrylic column, overcoming the time-consuming and labor-intensive drawbacks of traditional manual saturation. Furthermore, the water supply speed can be adjusted by regulating the height of the water supply bottle. Compared to directly using a water pump, its slower supply speed ensures complete removal of air bubbles from the formation while preventing excessive water pressure at the bottom that could cause the formation to float. On the other hand, verifying the formation permeability coefficient better demonstrates the applicability of the test results to actual construction formations.

[0024] 2. This invention transmits data to a computer in a timely manner through an electronic scale and an automatic reading system for a borehole manometer, thereby automating the testing process. While ensuring accurate readings, it can significantly increase the reading frequency, thus capturing relevant data changes more deeply during the testing process.

[0025] 3. By providing pressure to the formation, this invention can ensure that the formation pore pressure conditions in the laboratory test are consistent with the actual conditions, making the test results closer to the actual conditions.

[0026] 4. This invention uses indoor experiments to simulate mud permeation and film formation. Based on the system mass conservation, the mud film permeability coefficient at each moment of mud permeation and film formation is dynamically tested to evaluate the permeability performance of the mud film. This allows for a preliminary prediction of mud ratio and film formation conditions, as well as a preliminary understanding of the feasibility of the scheme. The scheme is simple to operate, low in cost, and highly applicable.

[0027] 5. This invention uses a TDR sensor to monitor the changes in the electrical conductivity of soil in different strata, thereby indirectly tracking the diffusion pattern of mud slurry in the strata and comprehensively evaluating the impact of mud slurry infiltration and film formation on the groundwater environment. Attached Figure Description

[0028] Figure 1 This is an overall device diagram of the present invention;

[0029] Figure 2 This is a top view of the base of the present invention;

[0030] Figure 3 This is a cross-sectional view at the height of the first borehole manometer.

[0031] Explanation of reference numerals in the attached diagram: 1. First valve; 2. Second valve; 3. Third valve; 4. TDR sensor; 5. Threaded rod; 6. Acrylic glass column; 7. Distance sensor; 8. Mud; 9. Mud film; 10. First pore pressure gauge; 11. Formation; 12. Second pore pressure gauge; 13. Filter layer; 14. Geotextile; 15. Base; 16. Flange; 17. Water supply bottle; 18. Water pump; 19. Water bucket; 20. Return water pipe; 21. First water supply pipe; 22. Second water supply pipe; 2 3. T-connector; 24. Electronic scale; 25. Water outlet pipe; 26. Leachate collection container; 27. Fourth valve; 28. First air supply pipe; 29. ​​Second air supply pipe; 30. Third air supply pipe; 31. Control and monitoring instrument; 32. First air pressure control and display device; 33. Second air pressure control and display device; 34. First data cable; 35. Second data cable; 36. Third data cable; 37. Fourth data cable; 38. Fifth data cable; 39. Air compressor; 40. Computer. Detailed Implementation

[0032] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0033] like Figure 1 and Figure 2 As shown, the present invention provides a device and method for testing the mud film permeability coefficient in saturated silty sand formations. The device includes a water supply mechanism and a mud permeation mechanism.

[0034] The water supply mechanism includes a water supply bottle 17, a water pump 18, a water bucket 19, a return water pipe 20, and a first water supply pipe 21. The water bucket 19 is placed on the ground and filled with an appropriate amount of tap water. The water supply bottle 17 is placed at a certain height and connected to the return water pipe 20 and the first water supply pipe 21 respectively. The water pump 18 is connected to the first water supply pipe 21 and pumps the water from the water bucket into the water supply bottle 17. After the water in the water supply bottle 17 reaches the height of the return water pipe 19 interface, it flows back from the return water pipe to the water bucket 19.

[0035] The mud infiltration mechanism includes a first valve 1, a second valve 2, a third valve 3, five TDR sensors 4, three threaded rods 5, an acrylic column 6, a distance sensor 7 (in this embodiment, the distance sensor is an ultrasonic distance sensor), mud 8, mud film 9, a first pore pressure gauge 10, a formation 11, a first pore pressure gauge 12, a filter layer 13, geotextile 14, a base 15, a flange 16, a second water supply pipe 22, a tee connector 23, an electronic scale 24, a water outlet pipe 25, a leachate collection container 26, a fourth valve 27, a first air supply pipe 28, a second air supply pipe 29, a third air supply pipe 30, a control and monitoring instrument 31, a first data cable 34, a second data cable 35, a third data cable 36, a fourth data cable 37, a fifth data cable 38, an air compressor 39, and a computer 40; the control and monitoring instrument 31 integrates a first air pressure control display device 32 and a second air pressure control display device 33.

[0036] An acrylic column 6 is mounted on a base 15. The base has two opposing first and second through holes, both L-shaped. One end of each L-shaped through hole is on the surface of the base, and the other end is on the side of the base. A second valve 2 is located at the side of the first through hole, and a third valve 3 is located at the side of the second through hole. The third valve 3 is connected to a water outlet pipe 25 to collect leachate into a leachate collection container 26. The second valve is connected to the first port of a three-way connector 23 via a third water supply pipe. The second port of the three-way connector is connected to the bottom of a water supply bottle via a second water supply pipe 22. Water from the water bottle 17 flows into the second water supply pipe 22 due to gravity, and thus into the acrylic column. Five TDR sensors 4 are evenly spaced from top to bottom on the side wall of the acrylic column 6. A first orifice gauge 10 and a second orifice gauge 12 are also located on the side wall of the acrylic column 6. Figure 3As shown, the height of the first borehole manometer is flush with the top TDR sensor, and the height of the second borehole manometer is flush with the bottom TDR sensor. During testing, geotextile 14, filter layer 13, stratum 10, mud film 9, and mud 8 are respectively arranged from bottom to top on the plexiglass column. Above the plexiglass column 6 is a flange 16, with three corresponding holes on the edge of the flange and the edge of the base. Nuts are provided on the top of the three threaded rods, and the bottoms of the three threaded rods are respectively fixed in the holes of the base. The tops of the three threaded rods pass through the holes on the flange. Tightening the nuts can seal the plexiglass column. The distance sensor is located at the bottom of the flange, inside the plexiglass column. A first valve 1 is provided on the upper part of the flange. The first valve is a two-way valve. One valve of the first valve is a water supply end, and the other valve is an air supply end. The water supply end is connected to the third port of the tee connector through the fourth water supply pipe. The system connects the second water supply pipe to the fourth water supply pipe; the air supply end is connected to the first air pressure control display device 32 on the control monitor 31 via the second air supply pipe 29; the distance sensor 7 is connected to the control monitor 31 via the first data line 34; the five TDR sensors 4 are connected to the control monitor 31 via the second data line 35; the first orifice gauge 10 and the second orifice gauge 12 are connected to the control monitor 31 via the third data line 36; the air compressor 39 is connected to the control monitor 31 via the first air supply pipe 28; the computer 40 is connected to the control monitor 31 via the fourth data line 37 and the electronic scale 24 via the fifth data line 38; a sealable leachate collection container 26 is placed on the electronic scale 24, the lower part of which is connected to the water outlet pipe 25, and the upper part is equipped with a fourth valve 27 to control the back pressure input; the fourth valve is connected to the second air pressure control display device 33 on the control monitor 31 via the third air supply pipe 30.

[0037] In this embodiment, the internal diameter of the plexiglass column 6 is 15cm, the spacing between the five TDR sensors 4 is 10cm, the distance from the bottom pore pressure gauge to the base 15 is 15cm, the height of the filter layer 13 is 10cm, and the height of the ground layer 11 is 45cm.

[0038] A testing method for a mud film permeability coefficient testing device in saturated silty sand formations, specifically comprising:

[0039] Step 1, Formation Preparation: Lay a layer of geotextile on the plexiglass column formation to prevent soil particles from clogging the outlet; lay a layer of coarse sand on top of the geotextile as a filter layer; based on the actual soil density of the construction formation, lay a formation of the same density on top of the filter layer; the upper surface of the formation should be at the same height as the top TDR sensor and the first pore pressure gauge; open the water pump and the second valve, close the first and third valves, keep the fourth valve open and set the air pressure displayed by the second air pressure control display device to 0, so that the formation is saturated from bottom to top; after saturation, close the second valve and use the drainage method to inject water along the inner wall of the plexiglass column to the top of the plexiglass column; tighten the threaded rod to seal the plexiglass column with the flange, open the water supply end of the first valve and the third valve, conduct a constant head test, and record the reading of the first pore pressure gauge and the mass of the leachate; calculate the permeability coefficient of the formation between the two pore pressure gauges according to Darcy's law. If the calculated value is close to the actual formation, the formation preparation is qualified; otherwise, repeat Step 1 to re-prepare the formation until it is qualified.

[0040] The difference ΔP between the two pore gauge readings is extracted. The distance between the pore gauges is L. The permeability coefficient k1 of the formation between the two pore gauges is shown below:

[0041]

[0042]

[0043]

[0044] In the formula, k1 is the permeability coefficient, in cm / s; t1 is the measurement time period, in seconds; and Q is the volume of leachate within t1, in cm³. 3 Δh represents the total hydraulic head difference between the formations between the pore pressure gauges, in cm; A is the cross-sectional area inside the plexiglass column, in cm². 2 Δm represents the change in leachate mass within t1, i.e., the change in the electronic scale reading; ρ represents the density of the leachate, in g / cm³. 3 L is the distance between the two manometers, in cm; g is the gravitational constant, in N / g.

[0045] Step 2, Mud Infiltration Film Formation: After the constant head test, drain the water above the formation and close the four valves; slowly add mud along the inner wall of the plexiglass column using the drainage method, taking care to avoid disturbing the formation and controlling the mud level at approximately 20cm; measure the actual mud height through the plexiglass column; seal the flange; connect the ultrasonic ranging sensor channel and transmit the data to the computer in real time; turn on the air compressor; open the air supply end of the first valve and the fourth valve; the first air pressure control display device adjusts the air compressor pressure to the preset value and then transmits it into the plexiglass column; the second air pressure control display... The device adjusts the air compressor pressure to a preset value and then transmits it to a leachate collection container. After the readings of the two pore pressure gauges stabilize, the five TDR sensor channels are connected, and their initial readings are recorded as the initial formation value. The third valve is opened, and after the third valve opens, the pressure inside the plexiglass column is transmitted, causing the mud to begin to permeate. The pressure transmitted to the leachate collection container applies pore water pressure to the formation. The mass of the leachate is weighed using an electronic scale, the formation pore pressure is measured using pore pressure gauges, the conductivity of different formations is measured using TDR sensors, and the mud level change is measured using an ultrasonic ranging sensor, all of which are transmitted to a computer. In this embodiment, the opening of the third valve is taken as the start time of the experiment, and the end time is preset by the user. The total time taken for the entire experiment is X. Time X is divided into n time periods, and the last moment of the xth time period is taken as the permeability coefficient calculation node and recorded as T. x x = 1, 2, ..., n; take T x The first y time periods and the next y time periods (y is a pre-set value) are used as the time periods for calculating the permeability coefficient, and the length of these time periods is denoted as t. After the experiment, a mud film is obtained, and an appropriate amount of mud film is taken to measure its water content, thus obtaining the density of the mud film. Based on the density of the mud film, the time node T is determined. x Calculate the thickness H of the mud film by taking the mass ΔM and Δh1 of the leachate weighed by the electronic scale.

[0046] Step 4: Calculate the mud film permeability coefficient corresponding to the time length t based on the formation pore pressure measured by the pore pressure gauge and the mud film thickness H.

[0047] Step 3: End the experiment and take an appropriate amount of mud film to measure the moisture content.

[0048] Step 4: Calculation of the permeability coefficient of the mud film at a certain moment: Due to the small pore structure of saturated silty sand formations, water in the mud penetrates the formation and enters the leachate collection container, while bentonite particles remain on the formation surface to form a mud film. Therefore, the mud components after infiltration exist in the form of mud film and leachate. The mud film thickness is calculated based on the conservation of system mass.

[0049] A(Δh1+H)ρ'=ΔM+ρ1AH

[0050]

[0051] In the formula, A is the cross-sectional area of ​​the plexiglass column, in cm². 2 Δh1 represents the time from the start of the experiment to time node T. x The time-displacement sensor measures the total change in mud level in cm; H is the mud film thickness in cm; ΔM is the time node T. x The mass of leachate measured by the electronic scale, i.e., the change in the scale reading, is expressed in grams (g); ρ1 is the density of the mud film, in g / cm³. 3 ρ' represents the density of the mud, in g / cm³. 3 w represents the mud film moisture content, in %; d s Here is the specific gravity of bentonite. The left side of the equation represents the mass of mud consumed, and the right side represents the total mass of mud film and leachate.

[0052] After further simplification, we get:

[0053]

[0054] The mud film permeability coefficient k during time period t is calculated using the following formula:

[0055]

[0056]

[0057]

[0058] P3=ρ'g(h-Δh1-H)

[0059] In the formula, k is the mud film permeability coefficient, in cm / s; Q is the volume of leachate in t, in cm³. 3 Δh represents the total head difference across the mud film, in cm; A is the cross-sectional area of ​​the plexiglass column, in cm². 2 Δm represents the change in leachate mass within time t, i.e., the change in the electronic scale reading, in grams; ρ represents the density of the leachate, in g / cm³. 3 P1 is the reading of the first orifice pressure gauge (if the reading of the first orifice pressure gauge does not change within the time period t, then P1 is the reading of the first orifice pressure gauge; if it changes, then P1 is the average reading of the first orifice pressure gauge), in Pa; P2 is the air pressure, that is, the air pressure input into the plexiglass column by the first air pressure control and display device, in Pa; P3 is the self-weight pressure of the mud, in Pa; H is the mud film thickness, in cm; g is the gravitational constant, in N / g; ρ' is the mud density.

[0060] Based on the above formulas, the mud film permeability coefficient is finally determined as follows:

[0061]

[0062] After repeating steps 1 to 4 several times in this embodiment, the variation law of alkaline solution over time and the variation law of mud film permeability coefficient over time can also be obtained; the specific variation law of alkaline liquid diffusion over time is as follows: after mud infiltration, the water in the formation gradually becomes alkaline liquid, and the conductivity of the liquid in the formation is measured by five TDR sensors to obtain the variation law of alkaline liquid diffusion over time.

[0063] It should also be noted that the various specific technical features described in the above embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

Claims

1. A method for testing the permeability coefficient of a mud film in a saturated fine-grained sand formation, characterized in that, The device used in the test method includes a water supply mechanism and a mud infiltration mechanism; the water supply mechanism includes a water supply bottle, a water pump, a water bucket, a return water pipe, and a first water supply pipe; the water bucket is placed on the ground and filled with tap water, the water supply bottle is placed at a certain height, the water pump is placed inside the water bucket, the water pump inputs the tap water in the water bucket into the water supply bottle through the first water supply pipe, and when the water in the water supply bottle reaches a preset height, it flows into the water bucket through the return water pipe; The mud infiltration mechanism includes second to fourth water supply pipes, a tee connector, first to fourth valves, first to fifth TDR sensors, three threaded rods, an acrylic column, a distance sensor, first and second pore pressure gauges, a base, a flange, an electronic scale, an outlet pipe, a leachate collection container, first to third air supply pipes, a control and monitoring instrument, an air compressor, and a computer. The control and monitoring instrument integrates first and second air pressure control and display devices. The first valve is a bidirectional valve. The acrylic column is mounted on the base, which has opposing first and second through holes, both of which are L-shaped, with one end of the L-shaped through hole on the base. The surface is one end, and the other end is on the side of the base; the first through hole at one end of the base is connected to the second valve, the second valve is connected to the first port of the tee connector through the third water supply pipe, and the second port of the tee connector is connected to the bottom of the water supply bottle through the second water supply pipe; the second through hole at one end of the base is connected to the third valve, the third valve is connected to the leachate collection container through the outlet pipe, used to transfer the leachate in the plexiglass to the leachate collection container; the flange edge and the base edge are provided with three corresponding holes, the top of each of the three threaded rods is provided with a nut, and the bottom of each of the three threaded rods is respectively fixed to the base. The tops of three threaded rods pass through the holes in the flange, and tightening the nuts seals the plexiglass column. A first valve is located at the center of the flange; one valve is for water supply, and the other is for air supply. The third port of the tee connector is connected to the water supply end via a fourth water supply pipe for supplying water into the plexiglass column. The air supply end is connected to a first air pressure control and display device via a second air supply pipe. The leachate collection container is mounted on an electronic scale, which is connected to a computer. A fourth valve is located on the top of the leachate collection container, and this fourth valve is connected to the second air pressure control and display device via a third air supply pipe. The control and monitoring instrument is connected to the air compressor via a first air supply pipe. The distance sensor is located at the bottom of the flange, inside the plexiglass column. The first to fifth TDR sensors are evenly spaced from top to bottom on the side of the plexiglass column. The first and second orifice gauges are located on the side of the plexiglass column, with the height of the first orifice gauge matching the height of the first TDR sensor, and the height of the second orifice gauge matching the height of the fifth TDR sensor. The distance sensor, the five TDR sensors, and the two orifice gauges are all connected to the control and monitoring instrument, which transmits the received data to a computer for display. The testing method specifically includes the following steps: Step 1: Prepare a formation in an plexiglass column and conduct a constant head test. Determine whether the formation preparation is qualified by the permeability coefficient between two air pressure gauges. If it is qualified, proceed to Step 2; otherwise, prepare the formation again until it is qualified. Step 2: Close the four valves, slowly add mud along the inner wall of the plexiglass column using the drainage method, and seal the flange; the control monitor transmits the data measured by the distance sensor to the computer; turn on the air compressor, open the air supply end of the first valve and the fourth valve, and the first air pressure control display device adjusts the pressure provided by the air compressor to the preset value, and then transmits it to the plexiglass column to provide permeation pressure for the mud; The second air pressure control and display device adjusts the pressure provided by the air compressor to a preset value and then transmits it to the leachate collection container; when the readings of the two pore pressure gauges stabilize, the third valve is opened, and the pressure transmitted to the leachate collection container at this time provides pore water pressure to the formation; the mud begins to permeate under the action of osmotic pressure; Step 3: The experiment begins when the third valve is opened, and ends at a pre-set time. Let the total experiment time be X. Divide time X into n time segments. The last moment within each time period is taken as the node for calculating the permeability coefficient, denoted as . ; ;Pick The first y time periods and the last y time periods are used as the time periods for calculating the permeability coefficient, and the length of this time period is denoted as t. After the experiment, a mud film is obtained, and an appropriate amount of mud film is taken to measure its water content, thus obtaining the density of the mud film. Based on the density of the mud film, the time nodes are determined. The mass ΔM of the leachate measured by the electronic balance and the time from the start of the experiment to the time point. The mud film thickness H is calculated by measuring the mud level change Δh1 using the time-range sensor. Step 4: Calculate the mud film permeability coefficient corresponding to the time length t based on the formation pore pressure measured by the pore pressure gauge and the mud film thickness H.

2. The method according to claim 1, characterized in that, The ranging sensor is an ultrasonic ranging sensor.

3. The method according to claim 1, characterized in that, It also includes data lines one through five; the ranging sensor is connected to the control and monitoring instrument via the first data line, the five TDR sensors are connected to the control and monitoring instrument via the second data line, the two orifice manometers are connected to the control and monitoring instrument via the third data line, the control and monitoring instrument is connected to the computer via the fourth data line, and the electronic scale is connected to the computer via the fifth data line.

4. The method for testing the mud film permeability coefficient in saturated silty sand formations according to claim 1, characterized in that, Step 1 specifically involves: laying a layer of geotextile at the bottom of the plexiglass column; laying a layer of coarse sand as a filter layer on top of the geotextile; laying a soil layer on top of the filter layer, with the upper surface of the soil layer flush with the first TDR sensor; closing the first and third valves, keeping the fourth valve open and setting the air pressure displayed by the second air pressure control display device to 0; opening the water pump and the second valve to saturate the soil layer from bottom to top; closing the second valve after saturation and using a drainage method to inject water along the inner wall of the plexiglass column to the top of the plexiglass column; tightening the threaded rod to seal the plexiglass column with the flange; opening the water supply ends of the third and first valves to conduct a constant head test, recording the readings of the two pore pressure gauges and the mass of the leachate; calculating the permeability coefficient of the soil layer between the two pore pressure gauges according to Darcy's law; if the calculated permeability coefficient is within the preset range, it indicates that the soil layer preparation is qualified.

5. The method for testing the mud film permeability coefficient in saturated silty sand formations according to claim 1, characterized in that, In step 3, the thickness H of the mud film is calculated according to the following formula: ； A is the cross-sectional area of ​​the plexiglass column. ρ is the density of the mud slurry, and ρ1 is the density of the mud film.

6. The method for testing the mud film permeability coefficient in saturated silty sand formations according to claim 1, characterized in that, The mud film permeability coefficient k over time length t is calculated using the following formula: ； in, Let g be the mass change of the leachate over a time period t, and g be the gravitational constant. The surface pore pressure was measured by the first air pressure gauge. The first air pressure control display device inputs the air pressure into the plexiglass column, where h is the mud height before mud penetration.

7. The method for testing the mud film permeability coefficient in saturated silty sand formations according to claim 1, characterized in that, The method also includes repeating steps 1 to 4 several times to obtain the diffusion law of alkaline liquid over time and the diffusion law of mud film permeability coefficient over time; the diffusion law of alkaline liquid over time is specifically obtained as follows: after mud infiltration, the water in the formation gradually becomes alkaline liquid, and the conductivity of the liquid in the formation is measured by five TDR sensors to obtain the diffusion law of alkaline liquid over time.

Images