Model test device and test method for researching sandy soil piping

Abstract

The invention belongs to the field of soil seepage failure model tests, and particularly relates to a model test device and a test method for researching sandy soil piping. Comprising a model box 1, a one-way valve 2, a high-pressure water inlet pipe 3, fixed calipers 4, a water faucet 5, a water outlet pipe 6, a water tank 7, a geotechnical screen 8, a steady flow chamber 9, a first screen 10, an inverted filter 11, a second screen 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 bench saw 18. Compared with the prior art, the pressure of inflow water flow in the sandy soil piping process is controlled, and freezing cutting and follow-up analysis are conducted on the sandy soil sample with the piping phenomenon; the critical hydraulic gradient of sand piping is obtained in the sand piping model test process, and the total particle loss amount of sand after piping and the particle loss amount of all parts in the sand are obtained.

Description

Technical Field

[0001] The invention belongs to the field of soil seepage failure model test, and in particular relates to a model test device and a test method for studying sand soil piping. Background Art

[0002] Sand and soil piping refers to the phenomenon that fine particles in the soil are carried away or flushed out from the pores of coarse particles under the action of seepage, which can cause the foundations of hydraulic structures such as dikes and sluices to be hollowed out, resulting in serious accidents such as dike breach, dam collapse, and gate failure.

[0003] At present, some scholars have carried out sand and soil piping model tests. For example, patent CN102277850A invented a model test device for the development process of sand and soil piping failure in embankment engineering, which can simulate the development process of piping failure under various hydraulic conditions of river embankments.

[0004] CN104914232A discloses a foundation pit piping simulation test device, comprising a water tank and a soil box for placing sand, wherein one end of the soil box is connected to the water tank, and a simulated foundation pit is arranged on the top surface of the other end of the soil box, and a porous plate is arranged between the simulated foundation pit and the soil box, so that the piping process in the foundation pit project can be simulated by simulating the foundation pit.

[0005] However, existing patents mostly provide a macroscopic description of the piping phenomenon, lack specific data calculations, and are unable to obtain the critical hydraulic gradient of sand piping, and are unable to obtain the total amount of sand particle loss after piping and the particle loss of each internal part. Summary of the invention

[0006] The purpose of the present invention is to provide a model test device and test method for studying sand piping in order to overcome the shortcomings of the above-mentioned prior art, so as to solve the problem that the devices in the prior art only provide a macroscopic description of the piping phenomenon but lack specific data measurement. By controlling the pressure of the inlet water flow during the sand piping process, and performing freeze-cutting and subsequent analysis on the sand samples where the piping phenomenon occurs, the critical hydraulic gradient of the sand piping, the total amount of particle loss of the sand after the piping, and the particle loss of each internal part are obtained during the sand piping model test.

[0007] The purpose of the present invention is achieved through the following technical solutions:

[0008] One of the technical solutions of the present invention is to provide a model test device for studying sand pipe burst, comprising a model box, a steady flow chamber, a first screen, a filter layer, a second screen, a saturated sand sample, and a water pressure gauge;

[0009] A partition plate is provided near the bottom of the model box, the upper part of the partition plate is a test area, the lower part of the partition plate is a flow stabilization chamber, a water hole is provided on the partition plate, and positioning grooves parallel to the partition plate are provided at intervals along the height direction on the inner wall of the model box, a water outlet is provided in the model box at a position not 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 provided with a first screen, a reverse filter layer and a second screen in sequence;

[0011] The saturated sand sample is arranged on the second sieve, and water pressure gauges are buried at the bottom and the middle of the saturated sand sample.

[0012] Furthermore, the model box fixes the movable panel and the fixed part by means of a fixing caliper.

[0013] Furthermore, the water inlet is arranged near the bottom of the movable panel, the water outlet is arranged at the upper part of the side panel of the fixed part, and the position of the water outlet is not lower than the highest positioning groove;

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

[0015] Furthermore, the water inlet and the water outlet are a section of outwardly extending pipe assembled after a hole is opened on the panel, and the assembly method of the pipe and the panel includes threaded connection or bonding.

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

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

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

[0019] Furthermore, the cavity area at the lower side of the partition plate is a steady flow chamber, and the upper surface of the partition plate is sequentially provided with a first screen, a filter layer, a second screen and a saturated sand sample.

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

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

[0022] The main function of the steady flow chamber is to generate a vertical upward water flow with stable pressure and flow rate. The filter layer mainly plays a transitional role, and cooperates with the steady flow chamber to further form a vertical upward laminar flow. The first screen is used to prevent the gravel particles in the filter layer from falling into the steady flow chamber, and the second screen is used to prevent the sand particles of the saturated sand sample from falling into the filter layer.

[0023] Furthermore, a plurality of water inlets are arranged at intervals along the length direction, and a plurality of water outlets are arranged at intervals along the height direction.

[0024] Furthermore, a water pump with a pressure gauge is provided at the water inlet to control the pressure of the water flowing through the water inlet.

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

[0026] Furthermore, the model box, one-way valve, high-pressure water inlet pipe, fixed caliper, faucet, water outlet pipe, sink, steady flow chamber, first screen, filter layer, second screen, saturated sand sample, and water pressure gauge are all located in a refrigerator, so as to quickly freeze the saturated sand sample after the pipe burst test, forcing the saturated sand sample to form a frozen sample and forming a positioning convex strip at the positioning groove.

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

[0028] Furthermore, the test device also includes a table saw, which includes a base, a table top, a circular saw and a limit bar. The distance between the circular saw and the limit bar is equal to the distance between the circular saw and the positioning groove, so as to cut the frozen sample into blocks along the positioning convex strip.

[0029] Furthermore, the test device also includes a geosieve, which includes 6 sub-sieves with apertures of 0.75 mm, 0.1 mm, 0.25 mm, 0.5 mm, 1 mm and 2 mm, respectively, for performing particle screening tests on saturated sand samples, obtaining the sand particle content in each particle size range, and drawing a particle size distribution curve.

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

[0031] The second technical solution of the present invention is to provide a test method for a model test device for studying sand piping, and the specific steps are as follows:

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

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

[0034] S3: Observe whether there is a sand dune with fine particles on the top of the saturated sand sample. If there is, piping occurs. Continue to observe the sand dune. When the size and shape of the sand dune no longer change, stop water inflow and outflow.

[0035] S4: Freezing model box, the frozen sand sample is cut according to the traces formed by the positioning groove. The cut sand sample is melted and dried to determine the total amount of sand particle loss and the sand particle loss in each block area.

[0036] Furthermore, in step S1, the movable panel and the fixed part are first fixed and the model box is sealed, and the first screen, the filter layer and the second screen are sequentially installed 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, a saturated sand sample is prepared by a falling sand method in water.

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

[0040] Furthermore, in step S1, the water layer height at the top of the saturated sand sample is stabilized by slowly opening the faucet connected to the water outlet, and the height of the water at the top of the saturated sand sample is kept the same as that of the pre-experiment.

[0041] Furthermore, in step S4, the freezer is started to freeze the model box.

[0042] Furthermore, in step S4, a positioning convex strip is formed on the frozen sample at the positioning groove.

[0043] Furthermore, the sand sample frozen in step S4 is cut according to the marks formed by the positioning grooves as follows: loosen the fixed caliper, remove the movable panel, take out the frozen sample, place the frozen sample on a table saw, and cut the frozen sample into n blocks with reference to the positioning convex strips, which are numbered 1-n from bottom to top, wherein the first (n-1) blocks are the parts of the area where the original saturated sand sample is located, and the nth block is the area above the original saturated sand sample, including the sand dunes produced by piping.

[0044] Furthermore, the frozen sand sample blocks cut in step S4 are melted and dried, the mass of each sand particle is obtained, and the total amount of sand particle loss and the amount of sand particle loss in each block area are measured.

[0045] Furthermore, a particle size distribution curve of each sand sample can be drawn by using a geosieve to perform a particle screening test on the particles of each part of the sand sample.

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

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

[0048] T2: Set critical hydraulic gradient i cr The value range is 0.1i cr,the -2i cr,the The corresponding theoretical bottom pressure head h is calculated by the relationship between the hydraulic gradient i and the bottom pressure head h1. 1,the By controlling the water pressure at the water inlet, the model box is set at the corresponding theoretical bottom pressure head h 1,the The pipe burst test of sand soil was carried out under the following conditions;

[0049] T3: Further determine the accurate critical hydraulic gradient i according to whether piping occurs in each group of sand piping tests cr .

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

[0051] Theoretical critical hydraulic gradient i is calculated by 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 particle mass m s (g); Obtain specific gravity G through indoor geotechnical test s ; Carry out particle screening test through geotextile sieve, draw particle size distribution curve, and obtain d according to particle size distribution curve 20 , d5;

[0053] d 20 and d5 are particle sizes on the particle size distribution curve, and the content of sand smaller than this particle size 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 ), porosity 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 saturated sand sample, v = 60 * 30 * 50 = 90000 cm 3 ;

[0059] M3: Refer to Appendix G of the Code for Geological Investigation of Water Conservancy and Hydropower Engineering (GB50487-2008) and use the following formula to calculate the theoretical critical hydraulic gradient i of piping 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 by the test device of the present invention. The experimental process is the same as steps S1-S3, but there is no need to perform the freezing treatment in S4:

[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 two water pressure gauges at the bottom of the saturated sand sample, unit: kPa; g is the acceleration of gravity, which is 9.8m / s 2 ρ w is the density of water, take 1g / cm 3 ;

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

[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 use of the faucet;

[0067] N3: Calculate 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 particle loss is:

[0071] U1: The frozen sand sample is cut according to the traces formed by the positioning grooves. The part above the trace formed by the highest positioning groove is the sand particles lost after the piping occurs, and its mass is the total amount of sand particles lost after the piping occurs;

[0072] U2: Calculate the difference between the remaining sand samples obtained after cutting and the corresponding cutting positions of the sand samples without piping, and the difference is the amount of sand particle loss in each block area.

[0073] Furthermore, in step U2, the cut frozen sand sample blocks are melted and dried, and a geosieve is used to perform a particle screening test on the particles of each part of the sand sample. The particle size distribution curve of each piece of sand sample can be drawn, and the particle size distribution of each piece of sand can be obtained by analyzing the particle size distribution curve of each piece of sand. The particle size distribution curve of the sand that does not exceed the highest positioning groove is compared with the particle size distribution curve of the sand where no pipe burst occurs, so as to further obtain the particle size distribution of the lost sand particles in each block area.

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

[0075] 1. In the test device of the present invention, a water pressure gauge is buried to increase the pressure of the water flow injected into the water inlet, control the water inlet pressure during the sand pipe burst, and set the theoretical critical hydraulic gradient to 0.1. icr,the -2 icr,the A series of tests were conducted to further obtain the accurate critical hydraulic gradient of the test sand by examining whether the top sand dune exists and whether the internal mass is lost.

[0076] 2. The present invention places the experimental device in a refrigerator, quickly freezes and cuts the saturated sand sample after the piping phenomenon occurs, and processes the saturated sand sample in batches. The total amount of sand particle loss after the piping phenomenon and the particle loss of each internal part can be calculated.

[0077] 3. The present invention provides a new device and method for simulating piping, which can freely select sand samples for simulating sand piping, freely adjust the properties of sand, simulate piping damage phenomena of various foundations, and has 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0079] Figure 1 This is an overall schematic diagram of the device without displaying a refrigerator.

[0080] Figure 2 It is the structural schematic diagram of the model box of this device.

[0081] Figure 3It 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 it is activated.

[0083] Figure 5 It is the structural schematic diagram of the table saw of this device.

[0084] In the figure: 1-model box; 1-1-movable panel; 1-1-1-water inlet; 1-2-fixed part; 1-2-1-partition plate; 1-2-1-1-water hole; 1-2-2-positioning groove; 1-2-3-water outlet; 2-check valve; 3-high-pressure water inlet pipe; 4-fixed caliper; 5-faucet; 6-water outlet pipe; 7-water tank; 8-geoscreen; 9-steady flow chamber; 10-first screen; 11-filter layer; 12-second screen; 13-saturated sand sample; 13-1-frozen sample; 13-1-1-positioning convex strip; 14-water pressure gauge; 15-dynamic strain collector; 16-computer; 17-freezer; 18-table saw; 18-1-base; 18-2-table top; 18-3-circular saw; 18-4-limiting strip. DETAILED DESCRIPTION

[0085] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0086] Example 1

[0087] See also Figure 1-Figure 5 The specific implementation mode of the present invention is described, a model test device for studying sand pipe burst, comprising a model box 1, a one-way valve 2, a high-pressure water inlet pipe 3, a fixed caliper 4, a faucet 5, a water outlet pipe 6, a water tank 7, a geosieve 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 collector 15, a computer 16, a freezer 17, and a table saw 18;

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

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

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

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

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

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

[0094] The water pressure gauge 14 is connected to the dynamic strain collector 15 and the computer 16 to measure the water pressure at the location where the water pressure gauge 14 is arranged;

[0095] The test device also includes a table saw 18, which includes a base 18-1, a table top 18-2, a circular saw 18-3 and a limit bar 18-4. The distance between the circular saw 18-3 and the limit bar 18-4 is equal to the distance between the circular saw 18-3 and the limit bar 18-4, so as to cut the frozen sample into blocks along the positioning convex strip 13-1-1.

[0096] The water inlet 1-1-1 and the water outlet 1-2-3 are a section of outwardly extending pipe assembled after a hole is opened on the panel, and the pipe and the panel are assembled by bonding.

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

[0098] The interior length, width and height of the model box 1 are 60 cm, 30 cm and 102 cm respectively, and it is made of highly transparent organic glass with a thickness of 2 cm.

[0099] The movable panel 1 - 1 and the fixed part 1 - 2 are fixed by six fixing clamps 4 , and the joints are sealed by glass glue.

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

[0101] The hollow area at the lower side of the partition plate 1-2-1 is a flow stabilization chamber 9, and the main function of the flow stabilization chamber 9 is to generate a vertically upward water flow with stable pressure and flow rate.

[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 with a width of 0.5 cm and a depth of 0.5 cm at intervals along the height direction and parallel to the partition plate 1-2-1.

[0103] The spacing between the positioning grooves 1-2-2 is 10 cm, and the distance between the lowest positioning groove 1-2-2 and the partition plate 1-2-1 is 10 cm.

[0104] The filter layer 11 is made of round gravel and is 10 cm thick. It mainly plays a transition role and cooperates with the flow stabilization chamber 9 to further form a vertical upward laminar flow. A first screen 10 is laid between the filter layer 11 and the flow stabilization 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 sieve 10 is smaller than the particle size of the filter layer particles, and the aperture of the second sieve 12 is smaller than the particle size of the saturated sand sample particles.

[0106] The aperture of the first sieve 10 is 2 mm, which can prevent the gravel particles in the filter layer 11 from falling into the steady flow chamber 9 . The aperture of the second sieve 12 is 0.075 mm, which can prevent the sand particles of the saturated sand sample 13 from falling into the filter layer 11 .

[0107] The main function of the flow stabilization chamber is to generate a vertical upward water flow with stable pressure and flow rate. The filter layer mainly plays a transition role, and cooperates with the flow stabilization chamber to further form a vertical upward laminar flow. The first screen is used to prevent the gravel particles in the filter layer from falling into the flow stabilization chamber, and the second screen is used to prevent the sand particles of the saturated sand sample from falling into the filter layer.

[0108] A plurality of water inlets 1-1-1 are arranged at intervals along the length direction, and a plurality of water outlets 1-2-3 are arranged at intervals along the height direction.

[0109] The model box is provided with three water inlets 1-1-1 at a height of 5 cm from the bottom, and 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] By setting a water pump with a pressure gauge at the water inlet 1-1-1, the pressure of the water flowing through the water inlet 1-1-1 is controlled.

[0111] The fixed part 1-2 is provided with three water outlets 1-2-3 at 10 cm, 20 cm and 30 cm away from the top, and each water outlet 1-2-3 is connected to three faucets 5 respectively. The water outlet 1-2-3 can control the water head height at the top of the saturated sand sample 13, specifically, by opening the faucet 5, the water is discharged into the sink 7 through the outlet pipe 6.

[0112] The saturated sand sample 13 is prepared by using Chinese ISO standard sand and has a thickness of 50 cm. Two water pressure gauges 14 are buried at the bottom and the middle of the saturated sand sample 13 .

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

[0114] The model box 1, the one-way valve 2, the high-pressure water inlet pipe 3, the fixed caliper 4, the faucet 5, the water outlet pipe 6, the water tank 7, the steady flow chamber 9, the first screen 10, the filter layer 11, the second screen 12, the saturated sand sample 13, and the water pressure gauge 14 are all located in the refrigerator 17, the purpose is to quickly freeze the saturated sand sample 13 after the pipe burst test is completed, forcing the saturated sand sample 13 to form a frozen sample 13-1, and forming a positioning convex strip 13-1-1 at the positioning groove 1-2-2.

[0115] The geosieve 8 includes 6 sub-sieves with apertures of 0.75 mm, 0.1 mm, 0.25 mm, 0.5 mm, 1 mm and 2 mm, respectively, and is used to perform particle screening tests on the saturated sand sample 13, obtain the sand particle content in each particle size range, and draw a particle size distribution curve.

[0116] The water inlet 1-1-1 can input water flows of different pressures to control the hydraulic gradient i.

[0117] Example 2

[0118] See also Figure 1-Figure 5 The specific implementation of the present invention is described as follows: a test method for a model test device for studying sand pipe bursts, the specific steps are as follows:

[0119] S1: prepare a saturated sand sample 13 in a model box 1, bury a water pressure gauge 14 at a predetermined position during the preparation process, and 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 the model box 1 through the water inlet 1-1-1, adjust the water pressure according to the water pressure gauge 14, stabilize the seepage, and conduct a sand pipe burst experiment at the selected pressure;

[0121] S3: Observe whether there is a sand dune with fine particles accumulated on the top of the saturated sand sample 13. If there is, piping occurs. Continue to observe the sand dune. When the size and shape of the sand dune no longer change, stop water inflow and outflow;

[0122] S4: Freeze the model box 1, cut the frozen saturated sand sample 13 according to the traces formed by the positioning grooves 1-2-2, and melt and dry the cut sand sample 13 to determine the total amount of sand particle loss and the sand particle loss in each block area.

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

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

[0125] In step S1, a saturated sand sample is prepared by a falling sand method in water.

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

[0127] In step S1, the faucet 5 connected to the water outlet 1-2-3 is slowly opened to stabilize the height of the water layer at the top of the saturated sand sample 13, and the height of the water at the top of the saturated sand sample 13 is kept the same as that of the pre-experiment.

[0128] In step S4, the model box 1 is frozen by starting the freezer 17.

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

[0130] The saturated sand sample 13 after being frozen in step S4 is cut according to the trace formed by the positioning groove 1-2-2 as follows: loosen the fixed caliper 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 blocks with reference to the positioning ridge 13-1-1, which are numbered 1-n from bottom to top, wherein the first (n-1) blocks are the parts of the area where the original saturated sand sample 13 is located, and the nth block is the area above the original saturated sand sample 13, including the sand dunes produced by piping.

[0131] The frozen sand sample 13 - 1 cut in step S4 is melted and dried, the mass of each sand particle is obtained, and the total amount of sand particle loss and the amount of sand particle loss in each block area are measured.

[0132] The particle size distribution curve of each sand sample can be drawn by using geosieve 8 to carry out particle screening test on the particles of each part of the sand sample.

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

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

[0135] T2: Set critical hydraulic gradient i cr The value range is 0.1i cr,the -2i cr,the The corresponding theoretical bottom pressure head h is calculated by the relationship between the hydraulic gradient i and the bottom pressure head h1. 1,the By controlling the water pressure at the water inlet, the model box is set at the corresponding theoretical bottom pressure head h 1,the The pipe burst test of sand soil was carried out under the following conditions;

[0136] T3: Further determine the accurate critical hydraulic gradient i according to whether piping occurs in each group of sand piping tests cr .

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

[0138] Theoretical critical hydraulic gradient i is calculated by 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 particle mass m s (g); Obtain specific gravity G through indoor geotechnical test s ; Perform particle screening test by geosieve 8, draw particle size distribution curve, and obtain d according to particle size distribution curve 20 , d5;

[0140] d 20 and d5 are particle sizes on the particle size distribution curve, and the content of sand smaller than this particle size 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 ), porosity 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 saturated sand sample, v = 60 * 30 * 50 = 90000 cm 3 ;

[0146] M3: Refer to Appendix G of the Code for Geological Investigation of Water Conservancy and Hydropower Engineering (GB50487-2008) and use the following formula to calculate the theoretical critical hydraulic gradient i of piping 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 by the test device of the present invention. The experimental process is the same as steps S1-S3, but there is no need to perform the freezing treatment in S4:

[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 two water pressure gauges at the bottom of the saturated sand sample, unit: kPa; g is the acceleration of gravity, which is 9.8m / s 2 ρ w is the density of water, take 1g / cm 3 ;

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

[0153] Taking the bottom of the saturated sand sample as the reference surface, z1 = 0m, z2 = 50cm = 0.5m, h2 is the water height at the top of the saturated sand sample, obtained according to the use of tap 5;

[0154] N3: Calculate 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 measuring the loss of sand particles is as follows:

[0158] U1: The frozen sand sample is cut according to the traces formed by the positioning grooves 1-2-2. The part above the traces formed by the highest positioning grooves 1-2-2 is the sand particles lost after the piping occurs, and its mass is the total amount of sand particles lost after the piping occurs;

[0159] U2: Calculate the difference between the remaining sand samples obtained after cutting and the corresponding cutting positions of the sand samples without piping, and the difference is the amount of sand particle loss in each block area.

[0160] In step U2, the cut frozen sand sample blocks are melted and dried, and the geosieve 8 is used to perform a particle screening test on the sand sample particles in each part. The particle size distribution curve of each sand sample can be drawn, and the particle size distribution of each sand sample can be obtained by analyzing the particle size distribution curve of each sand sample. The particle size distribution curve of the total lost sand particles can be compared with the particle size distribution curve of the sand that does not exceed the highest positioning groove, and the particle size distribution curve of the sand without pipe burst can be further obtained.

[0161] Example 3

[0162] See also Figure 1-Figure 5 Describe the specific implementation of the present invention.

[0163] Firstly, the theoretical critical hydraulic gradient i is calculated by using the 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 particle mass m s (g); Obtain specific gravity G through indoor geotechnical test s ; Perform particle screening test by geosieve 8, draw particle size distribution curve, and obtain d according to particle size distribution curve 20 , d5;

[0165] d 20 and d5 are particle sizes on the particle size distribution curve, and the content of sand smaller than this particle size 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 ), porosity 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 cm 3 ;

[0171] M3: Refer to Appendix G of the Code for Geological Investigation of Water Conservancy and Hydropower Engineering (GB50487-2008) and use the following formula to calculate the theoretical critical hydraulic gradient i of piping cr,the :

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

[0173] The water hole 1-2-1-1 can input water flows of different pressures to control the hydraulic gradient i.

[0174] Then, a preliminary experiment is performed using the test device of the present invention to calculate the relationship between the hydraulic gradient i and the bottom pressure head h1. The experimental process is the same as steps S1-S3, but there is no need to perform the freezing treatment in S4:

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

[0176] h1=p1 / ρ w g

[0177] Where p1 is the average of the pressures measured by the two water pressure gauges 14 at the bottom of the saturated sand sample 13, unit: kPa; g is the acceleration of gravity, which is 9.8 m / s 2 ρ w is the density of water, take 1g / cm 3 ;

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

[0179] Taking the bottom of the saturated sand sample 13 as the reference surface, z1 = 0m, z2 = 50cm = 0.5m, h2 is the water height at the top of the saturated sand sample 13, obtained according to the use of the faucet 5;

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

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

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

[0183] Set up 5 parallel test groups, control the hydraulic gradient of saturated sand sample 13 in each group 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 Determination of critical hydraulic gradient i cr Specific steps for calculating the total amount of sand and soil particle loss after pipe burst and the particle loss of each internal part:

[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 arrange the reverse filter layer 11 thereon. 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 60 cm, and prepare a saturated sand sample 13 in the model box 1 by the sand falling method in water. During the preparation process, a water pressure gauge 14 is buried at a predetermined position, and the water pressure gauge 14 is buried at the center of the saturated sand sample 13 at a predetermined height. Slowly open the faucet 5 connected to the water outlet 1-2-3 according to actual needs to stabilize the height of the water on the top of the saturated sand sample 13 and keep the height of the water on the top of the saturated sand sample 13 the same as that of the pre-experiment.

[0187] S3: Open the one-way valve 2, inject water into the high-pressure water inlet pipe 3, and slowly increase the water pressure to the pressure h1 corresponding to the hydraulic gradient set in the 5 groups of tests, stabilize the seepage, and observe whether there is a sand dune with fine particles accumulated on the top of the saturated sand sample 13. If there is, pipe burst occurs. Continue to observe the sand dune. When the size and shape of the sand dune no longer change, close the one-way valve 2 and the faucet 5. In the test group where the pipe burst occurs, open the freezer 17 to apply freezing.

[0188] S4: After the saturated sand sample 13 is frozen to form the frozen sample 13-1, the freezing is stopped. After freezing, a positioning convex strip 13-1-1 is formed at the positioning groove 1-2-2 of the frozen sample 13-1. The fixed clamp 4 is loosened, the movable panel 1-1 is removed, the frozen sample 13-1 is taken out, and the frozen sample 13-1 is placed on the table saw 18. With reference to the five positioning convex strips 13-1-1, the frozen sample 13-1 is cut into six pieces, which are numbered ①-⑥ from bottom to top, wherein the height of the blocks ①-⑤ is 10 cm, which are the five parts of the area where the original saturated sand sample 13 is located, and the sixth piece is the area above the original saturated sand sample 13, including the sand dunes produced by piping.

[0189] S5: Melt and dry the samples ①-⑥ in step 4, and obtain the mass of each sand particle. When no pipe burst occurs, the mass of the sand particles in the samples ①-⑤ is 0.2m s, a particle screening test was carried out on the sand particles of blocks ①-⑥ using geosieve 8, and the particle size distribution curves of blocks ①-⑥ were plotted. When no piping occurred, the sand particle size distribution curves of blocks ①-⑤ were consistent with the sand particle size distribution curve of saturated sand sample 13 without piping.

[0190] S6: According to whether piping occurs in the five groups of tests, the critical hydraulic gradient can be further obtained. After obtaining the mass of sand particles in blocks ① to ⑥, the mass of sand particles in block ⑥ is the total amount of sand particles lost after piping occurs. 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 law of sand particle loss along the seepage direction can be obtained. By analyzing the sand particle size distribution curve of block ⑥, the particle size distribution of total lost sand particles can be obtained. By comparing the sand particle size distribution curves of samples ①-⑤ with the sand particle size distribution curve of saturated sand sample 13, the particle size distribution of lost sand particles in each block can be further obtained.

[0191] The above description of the embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and 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 present invention should be within the scope of protection of the present invention.

Claims

1. A model test device for studying pipe bursts in sand, characterized in that: It comprises a model box (1), a steady flow 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); A partition plate (1-2-1) is provided near the bottom of the model box (1); the upper part of the partition plate (1-2-1) is a test area; the lower part of the partition plate (1-2-1) is a flow stabilization chamber (9); a water hole (1-2-1-1) is provided on the partition plate (1-2-1); positioning grooves (1-2-2) parallel to the partition plate (1-2-1) are provided at intervals along the height direction on the inner wall of the model box (1); a water outlet (1-2-3) is provided on the model box (1) at a position not lower than the highest positioning groove (1-2-2); and a water inlet (1-1-1) is provided inside the flow stabilization chamber (9); The upper surface of the partition plate (1-2-1) is provided with a first screen (10), a reverse filter layer (11) and a second screen (12) in sequence; The saturated sand sample (13) is arranged on the second screen (12), and a water pressure gauge (14) is buried at the bottom and the middle of the saturated sand sample (13).

2. A model test device for studying pipe bursting in sand soil according to claim 1, characterized in that: The water inlet (1-1-1) is connected to a one-way valve (2) and a high-pressure water inlet pipe (3) in sequence, and the one-way valve (2) controls the high-pressure water inlet pipe (3) to inject water into the water inlet; the water outlet (1-2-3) is connected to a faucet (5) and a water outlet pipe (6) in sequence, and the faucet (5) controls drainage and drains overflow water to a water tank (7) through the water outlet pipe (6).

3. A model test device for studying pipe bursting in sand soil according to claim 1, characterized in that: A plurality of water inlets (1-1-1) are arranged at intervals along the length direction, and a plurality of water outlets (1-2-3) are arranged at intervals along the height direction.

4. A model test device for studying pipe bursting in sand 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 particles, and the aperture of the second screen (12) is smaller than the particle size of the saturated sand sample particles.

5. A model test device for studying pipe bursting in sand soil according to claim 1, characterized in that: The model box (1) is located in a refrigerator (17).

6. A model test device for studying pipe bursting in sand soil according to claim 1, characterized in that: The water pressure gauge (14) is connected to the dynamic strain collector (15) and the computer (16) and is used to measure the water pressure at the corresponding position.

7. A model test device for studying pipe bursting in sand soil according to claim 1, characterized in that: The test device also includes a table saw (18), wherein the table saw (18) includes a circular saw (18-3) and a limiting bar (18-4), wherein the distance between the circular saw (18-3) and the limiting bar (18-4) is equal to the distance between the circular saw (18-3) and the limiting bar (18-4) and the positioning groove (1-2-2).

8. A test method for the model test device for studying pipe bursting in sand as claimed in claim 1, characterized in that: The specific steps are as follows: S1: preparing a saturated sand sample (13) in a model box (1), burying a water pressure gauge (14) at a predetermined position during the preparation process, and controlling the water outlet (1-2-3) to maintain a stable height of the water layer at the top of the saturated sand sample (13) in the model box (1); S2: injecting water into the model box (1) through the water inlet (1-1-1), adjusting the water flow pressure according to the water pressure gauge (14), stabilizing the seepage, and conducting a sand pipe burst experiment at a selected pressure; S3: Observe whether there is a sand dune with fine particles on the top of the saturated sand sample (13). If there is, piping occurs. Continue to observe the sand dune. When the size and shape of the sand dune no longer change, stop water inflow and outflow. S4: A freezing model box (1), wherein the frozen sand sample is cut according to the traces formed by the positioning grooves (1-2-2), and the cut sand sample is melted and dried to measure the total amount of sand particle loss and the sand particle loss in each block area.

9. The test method of the model test device for studying pipe burst in sand soil according to claim 8, characterized in that: The critical hydraulic gradient is determined by: T1: Obtain the theoretical critical hydraulic gradient i based on the properties of the sand sample used cr,the ; T2: Set critical hydraulic gradient i cr The value range is 0.1i cr,the -2i cr,the The corresponding theoretical bottom pressure head h is calculated by the relationship between hydraulic gradient i and pressure head h 1,the By controlling the water pressure at the water inlet, the model box is set at the corresponding theoretical bottom pressure head h 1,the The pipe burst test of sand soil was carried out under the following conditions; T3: Further determine the accurate critical hydraulic gradient i according to whether piping occurs in each group of sand piping tests cr .

10. The test method of the model test device for studying pipe burst in sand soil according to claim 8, characterized in that: The method for measuring the loss of sand particles is as follows: U1: The frozen sand sample is cut according to the traces formed by the positioning grooves (1-2-2). The part above the trace formed by the highest positioning groove (1-2-2) is the sand particles lost after the piping occurs, and its mass is the total amount of sand particles lost after the piping occurs; U2: Calculate the difference between the remaining sand samples obtained after cutting and the corresponding cutting positions of the sand samples without piping, and the difference is the amount of sand particle loss in each block area.

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