A model test device and test method for studying sand piping

Abstract

The present application belongs to the field of soil seepage failure model test, and particularly relates to a model test device and test method for studying sand piping, which comprises a model box 1, a one-way valve 2, a high-pressure water inlet pipe 3, a fixing caliper 4, a faucet 5, a water outlet pipe 6, a water tank 7, a soil sieve 8, a steady flow chamber 9, a first sieve 10, a filter layer 11, a second sieve 12, a saturated sand sample 13, a water pressure gauge 14, a dynamic strain acquisition instrument 15, a computer 16, a freezer 17 and a table saw 18. Compared with the prior art, the present application controls the pressure of water flow in the sand piping process, and carries out frozen cutting and subsequent analysis on the sand sample that has piping phenomenon, so that the critical hydraulic gradient of sand piping, the total amount of particle loss and the particle loss amount of each part of the sand after piping can be obtained in the sand piping model test process.

Description

Technical Field

[0001] This invention belongs to the field of soil seepage failure model testing, specifically relating to a model test device and test method for studying piping in sandy soil. Background Technology

[0002] Piping in sand refers to the phenomenon where fine particles in the soil are carried away or washed out from the pores of coarse particles under the action of seepage. It can cause the foundation of hydraulic structures such as dikes and sluices to be eroded, resulting in serious accidents such as dike breaches, dam collapses, and sluice gate failures.

[0003] Currently, some scholars have conducted model tests on piping failure in sandy soil. For example, patent CN102277850A invented a model test device for the development process of piping failure in sandy soil in embankment engineering, which can simulate the development process of piping failure in river embankments under various hydraulic conditions.

[0004] CN104914232A discloses a foundation pit piping simulation test device, including a water tank and a soil box for placing sand. One end of the soil box is connected to the water tank, and a simulated foundation pit is provided on the top surface of the other end of the soil box. A perforated plate is provided between the simulated foundation pit and the soil box. The piping process in foundation pit engineering can be simulated by simulating the foundation pit.

[0005] However, existing patents mostly provide macroscopic descriptions of piping phenomena, lack specific data calculations, and cannot determine the critical hydraulic gradient of piping in sand, nor can they obtain the total amount of particle loss in the sand and the amount of particle loss in each part of the sand after piping. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the existing technology by providing a model test device and method for studying piping in sand. This addresses the problem that existing devices only provide a macroscopic description of the piping phenomenon without specific data calculations. By controlling the pressure of the water flow during the piping process in sand, and by performing frozen cutting and subsequent analysis on the sand sample where piping occurs, the critical hydraulic gradient of the sand piping, the total amount of particle loss after piping, and the amount of particle loss in each part of the sand are obtained during the sand piping model test.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] One of the technical solutions of the present invention is to provide a model test device for studying piping in sandy soil, including a model box, a flow stabilization chamber, a first sieve, a filter layer, a second sieve, a saturated sandy soil sample, and a water pressure gauge;

[0009] The model box has a partition plate near the bottom. The upper part of the partition plate is the test area, and the lower part of the partition plate is the flow stabilization chamber. The partition plate has water passage holes. The inner wall of the model box has positioning grooves parallel to the partition plate at intervals along the height direction. The model box has a water outlet at a position no lower than the highest positioning groove, and a water inlet is provided inside the flow stabilization chamber.

[0010] The upper surface of the partition plate is sequentially provided with a first screen, a filter layer and a second screen;

[0011] The saturated sand sample is placed on the second sieve, and a water pressure gauge is buried at the bottom and middle of the saturated sand sample.

[0012] Furthermore, the model box is secured to the movable panel and the fixed part by a fixing clamp.

[0013] Furthermore, the water inlet is located near the bottom of the movable panel, and the water outlet is located on the upper part of the side panel of the fixed part, with the water outlet position not lower than the highest positioning groove.

[0014] The inlet is connected in sequence to a one-way valve and a high-pressure inlet pipe. The one-way valve controls the high-pressure inlet pipe to inject water into the inlet. The outlet is connected in sequence to a faucet and an outlet pipe. The faucet controls the drainage and the overflow water is diverted to the water tank through the water pipe.

[0015] Furthermore, the inlet and outlet are sections of outwardly extending pipes assembled after holes are made in the panel, and the assembly method of the pipes and the panel includes threaded connection or adhesive bonding.

[0016] Furthermore, the model box is made of transparent material.

[0017] Furthermore, the model box is a transparent cuboid box.

[0018] Furthermore, the water passages are arranged in an array.

[0019] Furthermore, the cavity area on the lower side of the partition plate is a flow stabilization chamber, and a first screen, a filter layer, a second screen, and a saturated sand sample are sequentially arranged on the upper surface of the partition plate.

[0020] Furthermore, the material of the filter layer is gravel.

[0021] Furthermore, the aperture of the first screen is smaller than the particle size of the filter layer, preferably 2 mm, and the aperture of the second screen is smaller than the particle size of the saturated sand sample, preferably 0.075 mm.

[0022] The main function of the flow stabilizing chamber is to generate a vertically upward water flow with stable pressure and velocity. The filter layer mainly serves as a transition layer, working in conjunction with the flow stabilizing chamber to further form a vertically upward laminar flow. The first screen is used to prevent gravel particles from the filter layer from falling into the flow stabilizing chamber, and the second screen is used to prevent sand particles from the saturated sand sample from falling into the filter layer.

[0023] Furthermore, multiple inlets are spaced apart along the length direction, and multiple outlets are spaced apart along the height direction.

[0024] Furthermore, the pressure of the water flow through the inlet is controlled by installing a water pump equipped with a pressure gauge at the inlet.

[0025] Furthermore, the model box is located inside the refrigerator.

[0026] Furthermore, the model box, one-way valve, high-pressure inlet pipe, fixing clamp, faucet, outlet pipe, water tank, flow stabilizing chamber, first screen, reverse filter layer, second screen, saturated sand sample, and water pressure gauge are all located inside the refrigerator. The purpose is to quickly freeze the sample after the piping test, forcing the saturated sand sample to form a frozen sample and forming a positioning protrusion at the positioning groove.

[0027] Furthermore, the water pressure gauge is connected to a dynamic strain gauge and a computer to measure the water pressure at the corresponding location.

[0028] Furthermore, the testing apparatus also includes a table saw, which includes a base, a table, a circular saw, and a limiting strip. The distance between the circular saw and the limiting strip is equal to the distance between the circular saw and the positioning groove, with the purpose of cutting the frozen sample into sections along the positioning protrusion.

[0029] Furthermore, the test apparatus also includes a geotextile sieve, which comprises six sieves with apertures of 0.75 mm, 0.1 mm, 0.25 mm, 0.5 mm, 1 mm, and 2 mm, respectively. This sieve is used to perform particle size distribution tests on saturated sand samples, obtain the sand particle content within each particle size range, and plot particle size distribution curves.

[0030] Furthermore, the inlet can be supplied with water flows of different pressures to control the hydraulic gradient i.

[0031] The second technical solution of the present invention provides a test method for a model test device for studying piping in sandy soil, the specific steps of which are as follows:

[0032] S1: Prepare saturated sand samples in the model box. During the preparation process, a water pressure gauge is buried at a predetermined position to control the water outlet and keep the water level at the top of the saturated sand sample in the model box stable.

[0033] S2: Inject water into the model box through the inlet, adjust the water pressure according to the water pressure gauge to stabilize the seepage, and conduct a sand piping test under the selected pressure;

[0034] S3: Observe whether fine-particle sand dunes appear at the top of the saturated sand sample. If so, piping has occurred. Continue to observe the sand dunes. When the size and shape of the sand dunes no longer change, stop the water inflow and outflow.

[0035] S4: Frozen model box, where frozen sand samples are cut along the marks formed by the positioning groove. After the cut sand samples are thawed and dried, they are used to determine the total amount of sand particles lost and the amount of sand particles lost in each block.

[0036] Furthermore, in step S1, the movable panel and the fixed part are first fixed and the model box is sealed. The first screen, the filter layer and the second screen are then installed in sequence on the partition plate.

[0037] Furthermore, in step S1, a saturated sand sample is prepared by slowly filling the model box with water.

[0038] Furthermore, in step S1, saturated sand samples are prepared by the water drop method.

[0039] Furthermore, in step S1, the hydraulic gauge is embedded in the center of the saturated sand sample at a predetermined height.

[0040] Furthermore, in step S1, the water level at the top of the saturated sand sample is stabilized by slowly opening the tap connected to the water outlet, and the water level at the top of the saturated sand sample is kept the same as in the preliminary experiment.

[0041] Furthermore, in step S4, the freezer model box is activated.

[0042] Furthermore, in step S4, the frozen sample at the positioning groove forms a positioning protrusion.

[0043] Furthermore, the specific steps for cutting the frozen sand sample in step S4 according to the marks formed by the positioning groove are as follows: loosen the fixing clamp, remove the movable panel, take out the frozen sample, place the frozen sample on the table saw, and cut the frozen sample into n pieces according to the positioning protrusion, numbered 1-n from bottom to top. The first (n-1) pieces are the part of the original saturated sand sample area, and the nth piece is the area above the original saturated sand sample, including the sand dunes generated by piping.

[0044] Furthermore, the frozen sand sample blocks cut in step S4 are melted and dried, and the mass of sand particles in each block is obtained. The total amount of sand particles lost and the amount of sand particles lost in each block are determined.

[0045] Furthermore, by using a geotextile sieve to perform particle sieving tests on the particles of each sand sample, particle size distribution curves for each sand sample can be plotted.

[0046] Furthermore, the critical hydraulic gradient i cr The determination method is as follows:

[0047] T1: Obtain the theoretical critical hydraulic gradient i based on the properties of the sand sample used. cr,the ;

[0048] T2: Set the critical hydraulic gradient i cr The value range is 0.1i cr,the -2i cr,the The theoretical bottom pressure head h is calculated using the relationship between the hydraulic gradient i and the bottom pressure head h1. 1,the By controlling the inlet water pressure, the model tank is kept at the corresponding theoretical bottom pressure head h. 1,the Conduct a piping test on sandy soil;

[0049] T3: Based on whether piping occurred in each group of sand piping tests, further determine the accurate critical hydraulic gradient i. cr .

[0050] Furthermore, in step T1, the theoretical critical hydraulic gradient i is calculated. cr,the The specific steps are as follows:

[0051] The theoretical critical hydraulic gradient i was calculated using existing technology and the properties of the selected saturated sand sample. cr,the :

[0052] M1: Dry the saturated sand sample and weigh its soil particles (m). s (g); Specific gravity G was obtained through indoor geotechnical tests. s Particle size distribution tests were conducted using a geotextile sieve, and particle size distribution curves were plotted. The particle size distribution d was then obtained from these curves. 20 d5;

[0053] d 20 d5 and d5 are particle sizes on the particle size distribution curve, and the sand content smaller than these particle sizes accounts for 20% and 5% of the total mass, respectively.

[0054] M2: Calculate the dry density ρ of the saturated sand sample. d (g / cm 3 ), void ratio e, porosity n:

[0055] ρ d =m s / v

[0056] e = (G s ρw / ρ d )-1

[0057] n = e / (1+e)

[0058] Where v is the volume of the saturated sand sample, v = 60 * 30 * 50 = 90000 cm³ 3 ;

[0059] M3: Referring to Appendix G of the "Code for Geological Investigation of Water Conservancy and Hydropower Projects" (GB50487-2008), the theoretical critical hydraulic gradient i for piping is calculated using the following formula. cr,the :

[0060] i cr,the =2.2(G s -1)(1-n) 2 d5 / d 20

[0061] Furthermore, in step T2, the relationship between the hydraulic gradient i and the bottom pressure head h1 is calculated using the experimental apparatus of this invention. The experimental process is the same as in steps S1-S3, but the freezing treatment in S4 is not required.

[0062] N1: Determine the pressure head h1 (m) at the bottom of the saturated sand sample:

[0063] h1=p1 / ρ w g

[0064] Where p1 is the average pressure measured by the two water pressure gauges at the bottom of the saturated sand sample, in kPa; g is the acceleration due to gravity, taken as 9.8 m / s². 2 ;ρ w The density of water is taken as 1 g / cm³. 3 ;

[0065] N2: Determine the water head at the bottom of the saturated sand sample, z1 (m), the pressure water head at the top, h2 (m), and the water head at the top position, z2 (m).

[0066] Taking the bottom of the saturated sand sample as the reference surface, z1 = 0m, z2 = 50cm = 0.5m, and h2 is the water height at the top of the saturated sand sample, which is obtained based on the usage of the faucet.

[0067] N3: Calculate the hydraulic gradient i (unit: 1):

[0068] i = [(z1+h1)-(z2+h2)] / L

[0069] Where L is the height of the saturated sand sample, which is 0.5m.

[0070] Furthermore, the method for determining the amount of sand and soil particle loss is as follows:

[0071] U1: Cut the frozen sand sample along the mark formed by the positioning groove. The part above the mark formed by the highest positioning groove is the sand particles lost after piping occurred, and its mass is the total amount of sand particles lost after piping occurred.

[0072] U2: Calculate the difference between the cut positions of the remaining sand samples and the corresponding cut positions of the sand samples that did not experience piping. The difference is the amount of sand particles lost in each block.

[0073] Furthermore, in step U2, the cut frozen sand sample blocks are melted and dried, and a geotextile sieve is used to perform particle sieving tests on the sand sample particles of each part. The particle size distribution curve of each sand sample block can be plotted. By analyzing the particle size distribution curve of each sand sample block, the total lost sand particle size distribution can be obtained. By comparing the sand particle size distribution curve of the part that does not exceed the highest positioning groove with the sand particle size distribution curve of the part that has not experienced piping, the particle size distribution of lost sand particles in each block area can be further obtained.

[0074] Compared with the prior art, the present invention has the following advantages:

[0075] 1. In the experimental apparatus of this invention, a water pressure gauge is embedded to increase the pressure of the water flow into the inlet, thereby controlling the inlet pressure during the piping process in sandy soil. The theoretical critical hydraulic gradient is set to 0.1. icr,the -2 icr,the A series of tests were conducted to further obtain the accurate critical hydraulic gradient of the sand in this experiment by examining whether the top dune exists and whether there is any loss of internal mass.

[0076] 2. The present invention places the experimental device in a refrigerator and quickly freezes and cuts the saturated sand sample after piping occurs. The saturated sand sample is processed in batches, and the total amount of sand particles lost after piping and the amount of particles lost in each part of the sample can be calculated.

[0077] 3. This invention provides a new device and method for simulating piping, which allows for free selection of sand samples for simulating piping in sandy soil, free adjustment of the properties of the sand, and simulation of piping failure phenomena in various foundations, thus having a wider range of applications.

[0078] 4. The device of the present invention has a simple structure, is easy to implement, and is easy to operate. Attached Figure Description

[0079] Figure 1 This is an overall schematic diagram of the device when the refrigerator is not displayed.

[0080] Figure 2 This is a schematic diagram of the structure of the model box of this device.

[0081] Figure 3This is a perspective view of the model box of this device.

[0082] Figure 4 This is a schematic diagram of the structure of the refrigerator when the device is in use.

[0083] Figure 5 This is a schematic diagram of the table saw structure of this device.

[0084] In the diagram: 1-Model box; 1-1-Modible panel; 1-1-1-Inlet; 1-2-Fixing part; 1-2-1-Divider plate; 1-2-1-1-Water passage hole; 1-2-2-Positioning groove; 1-2-3-Outlet; 2-One-way valve; 3-High-pressure water inlet pipe; 4-Fixing clamp; 5-Faucet; 6-Outlet pipe; 7-Water tank; 8-Geometry screen; 9-Flow stabilizing chamber; 10-First screen; 11-Filter layer; 12-Second screen; 13-Saturated sand sample; 13-1-Frozen sample; 13-1-Positioning protrusion; 14-Water pressure gauge; 15-Dynamic strain acquisition instrument; 16-Computer; 17-Freezer; 18-Table saw; 18-1-Base; 18-2-Tabletop; 18-3-Circular saw; 18-4-Limiting strip. Detailed Implementation

[0085] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0086] Example 1

[0087] See Figures 1-5 The following describes a specific embodiment of the present invention: a model test device for studying piping in sandy soil, comprising a model box 1, a one-way valve 2, a high-pressure water inlet pipe 3, a fixing clamp 4, a faucet 5, a water outlet pipe 6, a water tank 7, a geotextile sieve 8, a flow stabilizing chamber 9, a first sieve 10, a filter layer 11, a second sieve 12, a saturated sandy soil sample 13, a water pressure gauge 14, a dynamic strain acquisition instrument 15, a computer 16, a freezer 17, and a table saw 18.

[0088] The model box 1 has a horizontal partition plate 1-2-1 near the bottom inside. The upper part of the partition plate 1-2-1 is the test area, and the lower part of the partition plate 1-2-1 is the flow stabilization chamber 9. The partition plate 1-2-1 has water passage holes 1-2-1-1. The inner wall of the model box 1 is provided with positioning grooves 1-2-2 parallel to the partition plate 1-2-1 at intervals along the height direction. The model box 1 has a water outlet 1-2-3 at a position not lower than the highest positioning groove 1-2-2. The flow stabilization chamber 9 has a water inlet 1-1-1 inside.

[0089] The upper surface of the partition plate 1-2-1 is sequentially provided with a first screen 10, a filter layer 11 and a second screen 12;

[0090] The saturated sand sample 13 is placed on the second sieve 12, and a water pressure gauge 14 is buried at the bottom and middle of the saturated sand sample 13.

[0091] The model box 1 is fixed to the movable panel 1-1 and the fixed part 1-2 by the fixing clamp 4. The water inlet 1-1-1 is located near the bottom of the movable panel 1-1, and the water outlet 1-2-3 is located on the upper part of the side plate of the fixed part 1-2. The position of the water outlet 1-2-3 is not lower than the highest positioning groove 1-2-2.

[0092] The inlet 1-1-1 is connected in sequence to the one-way valve 2 and the high-pressure inlet pipe 3. The one-way valve 2 controls the high-pressure inlet pipe 3 to inject water into the inlet 1-1-1. The outlet 1-2-3 is connected in sequence to the faucet 5 and the outlet pipe 6. The faucet 5 controls the drainage and the overflow water is diverted to the water tank 7 through the outlet pipe 6.

[0093] The model box 1 is located inside the refrigerator;

[0094] The water pressure gauge 14 is connected to the dynamic strain acquisition instrument 15 and the computer 16, and is used to measure the water pressure at the location where the water pressure gauge 14 is installed.

[0095] The test apparatus also includes a table saw 18, which includes a base 18-1, a table 18-2, a circular saw 18-3, and a limiting strip 18-4. The distance between the circular saw 18-3 and the limiting strip 18-4 is equal to the distance between the circular saw and the positioning groove 1-2-2. The purpose is to cut the frozen sample into sections along the positioning protrusion 13-1-1.

[0096] The inlet 1-1-1 and outlet 1-2-3 are sections of outward-extending pipes that are installed after holes are made in the panel. The pipes are attached to the panel by adhesive bonding.

[0097] The model box 1 is a transparent cuboid box.

[0098] The model box 1 has an internal length, width, and height of 60cm, 30cm, and 102cm, respectively, and is made of highly transparent acrylic glass with a thickness of 2cm.

[0099] The movable panel 1-1 and the fixed part 1-2 are fixed with 6 fixing clamps 4, and the joint is sealed with glass glue.

[0100] The fixing part 1-2 has a 2cm thick partition plate 1-2-1 at a height of 10cm from the bottom. The partition plate 1-2-1 has water passage holes 1-2-1-1 with a diameter of 2cm. The water passage holes 1-2-1-1 are arranged in an array with a spacing of 5cm between them.

[0101] The cavity area on the lower side of the partition plate 1-2-1 is the flow stabilizing chamber 9. The main function of the flow stabilizing chamber 9 is to generate a vertically upward water flow with stable pressure and velocity.

[0102] The inner wall of the model box 1 is provided with positioning grooves 1-2-2 parallel to the partition plate 1-2-1 at intervals along the height direction; the inner wall of the fixing part 1-2 of the model box 1 is engraved with 5 positioning grooves 1-2-2, 0.5cm wide and 0.5cm deep, parallel to the partition plate 1-2-1, at intervals along the height direction.

[0103] The positioning grooves 1-2-2 are spaced 10cm apart, and the lowest positioning groove 1-2-2 is 10cm away from the partition plate 1-2-1.

[0104] The filter layer 11 is made of gravel and is 10cm thick. It mainly serves as a transition layer and, together with the flow stabilizing chamber 9, further forms a vertically upward laminar flow. A first screen 10 is laid between the filter layer 11 and the flow stabilizing chamber 9, and a second screen 12 is laid between the saturated sand sample 13 and the filter layer 11.

[0105] The aperture of the first screen 10 is smaller than the particle size of the filter layer, and the aperture of the second screen 12 is smaller than the particle size of the saturated sand sample.

[0106] The first screen 10 has a pore size of 2 mm to prevent gravel particles from falling into the flow stabilization chamber 9 in the filter layer 11. The second screen 12 has a pore size of 0.075 mm to prevent sand particles from falling into the filter layer 11 in the saturated sand sample 13.

[0107] The main function of the flow stabilizing chamber is to generate a vertically upward water flow with stable pressure and velocity. The filter layer mainly serves as a transition layer, working in conjunction with the flow stabilizing chamber to further form a vertically upward laminar flow. The first screen is used to prevent gravel particles from the filter layer from falling into the flow stabilizing chamber, and the second screen is used to prevent sand particles from the saturated sand sample from falling into the filter layer.

[0108] Multiple inlets 1-1-1 are spaced apart along the length direction, and multiple outlets 1-2-3 are spaced apart along the height direction.

[0109] The model box has three water inlets 1-1-1 at a height of 5cm from the bottom. Each water inlet 1-1-1 is connected to three one-way valves 2 and three high-pressure water inlet pipes 3 in sequence.

[0110] The pressure of the water flow through inlet 1-1-1 is controlled by installing a water pump with a pressure gauge at inlet 1-1-1.

[0111] The fixing part 1-2 has three water outlets 1-2-3 at distances of 10cm, 20cm and 30cm from the top. Each water outlet 1-2-3 is connected to three faucets 5. The water outlets 1-2-3 can control the water head height at the top of the saturated sand sample 13. Specifically, the faucets 5 are turned on and the water is discharged into the water tank 7 through the water outlet pipe 6.

[0112] The saturated sand sample 13 was prepared using Chinese ISO standard sand, with a thickness of 50 cm. Two water pressure gauges 14 were buried at the bottom and middle of the saturated sand sample 13.

[0113] The model box 1 is placed in the freezer 17 and frozen after the piping test, forcing the saturated sand sample 13 to form a frozen sample 13-1, and forming a positioning protrusion 13-1-1 at the positioning groove 1-2-2.

[0114] The model box 1, one-way valve 2, high-pressure water inlet pipe 3, fixing clamp 4, faucet 5, water outlet pipe 6, water tank 7, flow stabilizing chamber 9, first screen 10, reverse filter layer 11, second screen 12, saturated sand sample 13, and water pressure gauge 14 are all located inside the refrigerator 17. The purpose is to freeze them quickly after the piping test, so that the saturated sand sample 13 becomes a frozen sample 13-1, and a positioning protrusion 13-1-1 is formed at the positioning groove 1-2-2.

[0115] The geotextile sieve 8 includes 6 sieves with apertures of 0.75 mm, 0.1 mm, 0.25 mm, 0.5 mm, 1 mm and 2 mm, which are used to perform particle sieving tests on saturated sand sample 13, obtain the sand particle content in each particle size range, and plot the particle size distribution curve.

[0116] The inlet 1-1-1 can be used to input water flow of different pressures, thereby controlling the hydraulic gradient i.

[0117] Example 2

[0118] See Figures 1-5 The specific embodiments of the present invention are described below, including a test method for a model test device for studying piping in sandy soil. The specific steps are as follows:

[0119] S1: Prepare a saturated sand sample 13 in the model box 1. During the preparation process, a water pressure gauge 14 is buried at a predetermined position to control the water outlet 1-2-3 to keep the water layer height at the top of the saturated sand sample 13 in the model box stable.

[0120] S2: Inject water into model box 1 through inlet 1-1-1, adjust the water pressure according to water pressure gauge 14 to stabilize seepage, and conduct sand piping test under the selected pressure;

[0121] S3: Observe whether fine particles accumulate on the top of saturated sand sample 13 to form sand dunes. If so, piping has occurred. Continue to observe the sand dunes. When the size and shape of the sand dunes no longer change, stop the water inflow and outflow.

[0122] S4: Frozen model box 1, the frozen saturated sand sample 13 is cut according to the mark formed by positioning groove 1-2-2. After the cut sand sample 13 is melted and dried, it is used to determine the total amount of sand particles lost and the amount of sand particles lost in each block.

[0123] In step S1, the movable panel 1-1 and the fixed part 1-2 are fixed and the model box 1 is sealed. The first screen 10, the filter layer 11 and the second screen 12 are installed in sequence on the partition plate 1-2-1.

[0124] In step S1, saturated sand sample 13 is prepared by slowly filling the model box with water.

[0125] In step S1, saturated sand samples are prepared by the water drop method.

[0126] In step S1, the hydraulic gauge is buried at the center of the saturated sand sample at a predetermined height.

[0127] In step S1, the water faucet 5 connected to the water outlet 1-2-3 is slowly opened to stabilize the water level at the top of the saturated sand sample 13 and keep the water level at the top of the saturated sand sample 13 the same as in the preliminary experiment.

[0128] In step S4, the freezer 17 freezes the model box 1.

[0129] In step S4, a positioning protrusion 13-1-1 is formed on the frozen sample 13-1 at the positioning groove 1-2-2.

[0130] The specific steps for cutting the frozen saturated sand sample 13 in step S4 according to the marks formed by the positioning groove 1-2-2 are as follows: loosen the fixing clamp 4, remove the movable panel 1-1, take out the frozen sample 13-1, place the frozen sample 13-1 on the table saw 18, and cut the frozen sample 13-1 into n pieces according to the positioning protrusion 13-1-1, numbered 1-n from bottom to top. The first (n-1) pieces are the part of the original saturated sand sample 13, and the nth piece is the area above the original saturated sand sample 13, including the sand dunes generated by piping.

[0131] The frozen sand sample 13-1 cut in step S4 was melted and dried. The mass of sand particles in each sample was obtained, and the total amount of sand particles lost and the amount of sand particles lost in each area were determined.

[0132] By using a geotextile sieve 8 to perform particle sieving tests on the particles of each sand sample, particle size distribution curves for each sand sample can be plotted.

[0133] The critical hydraulic gradient i cr The determination method is as follows:

[0134] T1: Obtain the theoretical critical hydraulic gradient i based on the properties of the sand sample used. cr,the ;

[0135] T2: Set the critical hydraulic gradient i cr The value range is 0.1i cr,the -2i cr,the The theoretical bottom pressure head h is calculated using the relationship between the hydraulic gradient i and the bottom pressure head h1. 1,the By controlling the inlet water pressure, the model tank is kept at the corresponding theoretical bottom pressure head h. 1,the Conduct a piping test on sandy soil;

[0136] T3: Based on whether piping occurred in each group of sand piping tests, further determine the accurate critical hydraulic gradient i. cr .

[0137] In step T1, the theoretical critical hydraulic gradient i is calculated. cr,the The specific steps are as follows:

[0138] The theoretical critical hydraulic gradient i was calculated using existing technology and the properties of the selected saturated sand sample. cr,the :

[0139] M1: Dry the saturated sand sample 13 and weigh its soil particles m. s (g); Specific gravity G was obtained through indoor geotechnical tests. s Particle size distribution tests were conducted using a geotextile sieve (8), and particle size distribution curves were plotted. The particle size distribution d was then obtained from these curves. 20 d5;

[0140] d 20 d5 and d5 are particle sizes on the particle size distribution curve, and the sand content smaller than these particle sizes accounts for 20% and 5% of the total mass, respectively.

[0141] M2: Calculate the dry density ρ of the saturated sand sample. d (g / cm 3 ), void ratio e, porosity n:

[0142] ρ d =m s / v

[0143] e = (G s ρ w / ρ d )-1

[0144] n = e / (1+e)

[0145] Where v is the volume of the saturated sand sample, v = 60 * 30 * 50 = 90000 cm³ 3 ;

[0146] M3: Referring to Appendix G of the "Code for Geological Investigation of Water Conservancy and Hydropower Projects" (GB50487-2008), the theoretical critical hydraulic gradient i for piping is calculated using the following formula. cr,the :

[0147] i cr,the =2.2(G s -1)(1-n) 2 d5 / d 20

[0148] In step T2, the relationship between the hydraulic gradient i and the bottom pressure head h1 is calculated using the experimental apparatus of this invention. The experimental process is the same as in steps S1-S3, but the freezing treatment in step S4 is not required.

[0149] N1: Determine the pressure head h1 (m) at the bottom of the saturated sand sample:

[0150] h1=p1 / ρ w g

[0151] Where p1 is the average pressure measured by the two water pressure gauges at the bottom of the saturated sand sample, in kPa; g is the acceleration due to gravity, taken as 9.8 m / s². 2 ;ρ w The density of water is taken as 1 g / cm³. 3 ;

[0152] N2: Determine the water head at the bottom of the saturated sand sample, z1 (m), the pressure water head at the top, h2 (m), and the water head at the top position, z2 (m).

[0153] Taking the bottom of the saturated sand sample as the reference surface, z1 = 0m, z2 = 50cm = 0.5m, and h2 is the water height at the top of the saturated sand sample, which is obtained based on the usage of faucet 5.

[0154] N3: Calculate the hydraulic gradient i (unit: 1):

[0155] i = [(z1+h1)-(z2+h2)] / L

[0156] Where L is the height of the saturated sand sample, which is 0.5m.

[0157] The method for determining the amount of sand and soil particle loss is as follows:

[0158] U1: Cut the frozen sand sample according to the mark formed by positioning groove 1-2-2. The part above the mark formed by the highest positioning groove 1-2-2 is the sand particles lost after piping occurred, and its mass is the total amount of sand particles lost after piping occurred.

[0159] U2: Calculate the difference between the cut positions of the remaining sand samples and the corresponding cut positions of the sand samples that did not experience piping. The difference is the amount of sand particles lost in each block.

[0160] In step U2, the cut frozen sand sample blocks are thawed and dried. A geotextile sieve 8 is used to perform particle sieving tests on the sand sample particles in each part. The particle size distribution curve of each sand sample block can be plotted. By analyzing the particle size distribution curve of each sand sample block, the particle size distribution of the total lost sand particles can be obtained. By comparing the particle size distribution curve of the sand particles that does not exceed the highest positioning groove with the particle size distribution curve of the sand particles that did not experience piping, the particle size distribution of the lost sand particles in each block area can be further obtained.

[0161] Example 3

[0162] See Figures 1-5 This invention describes specific embodiments.

[0163] First, the theoretical critical hydraulic gradient i is calculated using existing technology and the properties of the selected saturated sand sample. cr,the :

[0164] M1: Dry the saturated sand sample 13 and weigh its soil particles m. s (g); Specific gravity G was obtained through indoor geotechnical tests. s Particle size distribution tests were conducted using a geotextile sieve (8), and particle size distribution curves were plotted. The particle size distribution d was then obtained from these curves. 20 d5;

[0165] d 20 d5 and d5 are particle sizes on the particle size distribution curve, and the sand content smaller than these particle sizes accounts for 20% and 5% of the total mass, respectively.

[0166] M2: Calculate the dry density ρ of saturated sand sample 13. d (g / cm 3 ), void ratio e, porosity n:

[0167] ρ d =m s / v

[0168] e = (G s ρ w / ρ d )-1

[0169] n = e / (1+e)

[0170] Where v is the volume of saturated sand sample 13, v = 60 * 30 * 50 = 90000 cm³ 3 ;

[0171] M3: Referring to Appendix G of the "Code for Geological Investigation of Water Conservancy and Hydropower Projects" (GB50487-2008), the theoretical critical hydraulic gradient i for piping is calculated using the following formula. cr,the :

[0172] i cr,the =2.2(G s -1)(1-n) 2 d5 / d 20

[0173] The water passage 1-2-1-1 can be used to input water flow of different pressures, thereby controlling the hydraulic gradient i.

[0174] Then, a preliminary experiment was conducted using the experimental apparatus of this invention to calculate the relationship between the hydraulic gradient i and the bottom pressure head h1. The experimental process was the same as steps S1-S3, but the freezing treatment in S4 was not required.

[0175] N1: Determine the bottom pressure head h1 (m) of saturated sand sample 13:

[0176] h1=p1 / ρ w g

[0177] Where p1 is the average pressure measured by the two water pressure gauges 14 at the bottom of the saturated sand sample 13, in kPa; g is the acceleration due to gravity, taken as 9.8 m / s². 2 ;ρ w The density of water is taken as 1 g / cm³. 3 ;

[0178] N2: Determine the water head at the bottom position z1 (m), the pressure water head at the top h2 (m), and the water head at the top position z2 (m) of the saturated sand sample 13:

[0179] Taking the bottom of saturated sand sample 13 as the reference surface, z1 = 0m, z2 = 50cm = 0.5m, and h2 is the water height at the top of saturated sand sample 13, which is obtained based on the usage of faucet 5.

[0180] N3: Calculate the hydraulic gradient i (unit: 1):

[0181] i = [(z1+h1)-(z2+h2)] / L

[0182] Where L is the height of saturated sand sample 13, which is 0.5m.

[0183] Five parallel test groups were set up, and the hydraulic gradient of the saturated sand sample 13 in each group was controlled to be 0.5i. cr,the 0.75i cr,the i cr,the 1.25i cr,the 1.5i cr,the .

[0184] Combination Figure 1-5 Explanation of the determination of the critical hydraulic gradient i cr Specific steps for determining the total particle loss of sand and the particle loss in different parts of the sand after piping:

[0185] S1: Close the one-way valve 2 and the faucet 5. Install the first screen 10 on the partition plate 1-2-1, and lay the reverse filter layer 11 on it. Install the second screen 12 on the reverse filter layer 11.

[0186] S2: Slowly fill the model box 1 with water to a height of 60cm. Prepare a saturated sand sample 13 in the model box 1 using the water drop method. During the preparation process, a water pressure gauge 14 is buried at a predetermined position, with the water pressure gauge 14 positioned at the center of the saturated sand sample 13 at the predetermined height. Slowly open the faucet 5 connected to the water outlet 1-2-3 as needed to stabilize the water level at the top of the saturated sand sample 13 and maintain the water level at the top of the saturated sand sample 13 at the same level as in the preliminary experiment.

[0187] S3: Open the one-way valve 2 and inject water into the high-pressure inlet pipe 3. Slowly increase the water pressure to the pressure h1 corresponding to the hydraulic gradient set in the 5th test group, stabilize the seepage, and observe whether a sand dune with fine particles accumulates on the top of the saturated sand sample 13. If so, piping has occurred. Continue to observe the sand dunes. When the size and shape of the sand dunes no longer change, close the one-way valve 2 and the water tap 5. In the test group where piping has occurred, turn on the freezer 17 to apply freezing.

[0188] S4: After freezing saturated sand sample 13 to form frozen sample 13-1, freezing is stopped. After freezing, a positioning protrusion 13-1-1 is formed on frozen sample 13-1 at positioning groove 1-2-2. Loosen the fixing clamp 4, remove the movable panel 1-1, take out frozen sample 13-1, place frozen sample 13-1 on table saw 18, and cut frozen sample 13-1 into 6 pieces according to the 5 positioning protrusions 13-1-1, numbered ①-⑥ from bottom to top. Among them, pieces ①-⑤ are 10cm high and are the 5 parts of the original saturated sand sample 13. Piece ⑥ is the area above the original saturated sand sample 13, including the sand dunes generated by piping.

[0189] S5: Melt and dry the samples ①-⑥ from step 4, and obtain the mass of sand particles in each sample. In the absence of piping, the mass of sand particles in samples ①-⑤ is 0.2 m³. sParticle size distribution tests were conducted on sand samples ①-⑥ using geotextile sieve 8, and particle size distribution curves of samples ①-⑥ were plotted. When piping did not occur, the particle size distribution curves of sand samples ①-⑤ were consistent with the particle size distribution curve of saturated sand sample 13, which did not experience piping.

[0190] S6: Based on whether piping occurred in the 5 sets of tests, the accurate critical hydraulic gradient can be obtained. After obtaining the mass of sand particles in blocks ①-⑥, the mass of the sand particle in block ⑥ is the total amount of sand particles lost after piping occurred, 0.2m. s The difference between the mass of sand particles in blocks ①-⑤ is the amount of sand particles lost in each block. By comparing their sizes, the pattern of sand particle loss along the seepage direction can be obtained. Analyzing the size distribution curve of sand particles in block ⑥ can obtain the total size distribution of lost sand particles. By comparing and analyzing the size distribution curves of sand particles in blocks ①-⑤ with the size distribution curve of sand particles in saturated sand sample 13, the size distribution of lost sand particles in each block can be further obtained.

[0191] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A model test apparatus for studying piping in sandy soil, characterized in that, Includes a model box (1), a flow stabilizing chamber (9), a first screen (10), a filter layer (11), a second screen (12), a saturated sand sample (13), and a water pressure gauge (14). The model box (1) has a partition plate (1-2-1) near the bottom inside. The upper part of the partition plate (1-2-1) is the test area, and the lower part of the partition plate (1-2-1) is the flow stabilization chamber (9). The partition plate (1-2-1) has water passage holes (1-2-1-1). The inner wall of the model box (1) is provided with positioning grooves (1-2-2) parallel to the partition plate (1-2-1) at intervals along the height direction. The model box (1) has an outlet (1-2-3) at a position not lower than the highest positioning groove (1-2-2). The flow stabilization chamber (9) has an inlet (1-1-1) inside. Multiple inlets (1-1-1) are provided at intervals along the length direction, and multiple outlets (1-2-3) are provided at intervals along the height direction. The upper surface of the partition plate (1-2-1) is sequentially provided with a first screen (10), a filter layer (11) and a second screen (12). The saturated sand sample (13) is placed on the second sieve (12), and a water pressure gauge (14) is buried at the bottom and middle of the saturated sand sample (13). The water pressure gauge (14) is buried at the center of the saturated sand sample (13). The specific steps of the test method using the aforementioned device are as follows: S1: Prepare a saturated sand sample (13) in the model box (1). During the preparation process, a water pressure gauge (14) is buried at a predetermined position to control the water outlet (1-2-3) to keep the water level at the top of the saturated sand sample (13) in the model box (1) stable. S2: Inject water into the model box (1) through the inlet (1-1-1), adjust the water pressure according to the water pressure gauge (14) to stabilize the seepage, and conduct a sand piping test under the selected pressure; S3: Observe whether fine particles accumulate on the top of the saturated sand sample (13). If so, piping occurs. Continue to observe the sand dunes. When the size and shape of the sand dunes no longer change, stop water inflow and outflow. S4: Frozen model box (1), cut the frozen sand sample according to the mark formed by the positioning groove (1-2-2), and use the cut sand sample to determine the total amount of sand particles lost and the amount of sand particles lost in each block after melting and drying. The method for determining the total amount of sand and soil particles lost is as follows: U1: Cut the frozen sand sample according to the mark formed by the positioning groove (1-2-2). The part above the mark formed by the highest positioning groove (1-2-2) is the sand particles lost after the piping occurred, and its mass is the total amount of sand particles lost after the piping occurred. U2: Calculate the difference between the cut positions of the remaining sand samples and the corresponding cut positions of the sand samples that did not experience piping. The difference is the amount of sand particles lost in each block.

2. The model test apparatus for studying piping in sandy soil according to claim 1, characterized in that, The inlet (1-1-1) is connected in sequence to the one-way valve (2) and the high-pressure water inlet pipe (3). The one-way valve (2) controls the high-pressure water inlet pipe (3) to inject water into the inlet. The outlet (1-2-3) is connected in sequence to the faucet (5) and the outlet pipe (6). The faucet (5) controls the drainage and guides the overflow water to the water tank (7) through the outlet pipe (6).

3. The model test apparatus for studying piping in sandy soil according to claim 1, characterized in that, The aperture of the first screen (10) is smaller than the particle size of the filter layer, and the aperture of the second screen (12) is smaller than the particle size of the saturated sand sample.

4. The model test apparatus for studying piping in sandy soil according to claim 1, characterized in that, The model box (1) is located inside the refrigerator (17).

5. The model test apparatus for studying piping in sandy soil according to claim 1, characterized in that, The water pressure gauge (14) is connected to the dynamic strain acquisition instrument (15) and the computer (16) to measure the water pressure at the corresponding location.

6. The model test apparatus for studying piping in sandy soil according to claim 1, characterized in that, The test apparatus also includes a table saw (18), which includes a circular saw (18-3) and a limiting strip (18-4). The distance between the circular saw (18-3) and the limiting strip (18-4) is equal to the distance between them and the positioning groove (1-2-2).

7. The model test apparatus for studying piping in sandy soil according to claim 1, characterized in that, The method for determining the critical hydraulic gradient in the sand piping experiment is as follows: T1: Obtain the theoretical critical hydraulic gradient i based on the properties of the sand sample used. cr,the ; T2: Set the critical hydraulic gradient i cr The value range is 0.1i cr,the -2i cr,the The theoretical bottom pressure head h is calculated using the relationship between the hydraulic gradient i and the pressure head h. 1,the By controlling the inlet water pressure, the model tank is kept at the corresponding theoretical bottom pressure head h. 1,the Conduct a piping test on sandy soil; T3: Based on whether piping occurred in each group of sand piping tests, further determine the accurate critical hydraulic gradient i. cr .

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