Model test system and method for simulating liquid nitrogen freezing of sand layers in different seepage environments based on different sand layer compactness conditions

Abstract

The invention discloses a sand layer liquid nitrogen freezing model test system and method for simulating different seepage environments based on different sand layer compactness conditions. The system comprises a seepage freezing model device, a sand layer free implementation system, a liquid nitrogen cooling system, a constant-temperature circulating water supply system, a water head difference seepage system and a monitoring system. During freezing simulation, the compactness of different sand layers is determined according to actual application occasions, and filling is conducted through a sand layer free implementation system; seepage of the seepage freezing model device is formed through a constant-temperature circulating water supply system and a water head difference seepage system, and seepage speeds of different sand layers are obtained; and finally, data of temperature field change and displacement change in the liquid nitrogen seepage freezing process are obtained through a liquid nitrogen cooling system and a monitoring system, and then the coupling action relation between the liquid nitrogen freezing temperature field and the seepage field under the conditions of multiple groups of different sand layers and different seepage speeds is obtained. By means of the liquid nitrogen freezing model test system, the coupling action relation between a liquid nitrogen freezing temperature field and a seepage field at different seepage speeds can be obtained through one-time test.

Description

Technical Field

[0001] The invention belongs to the field of liquid nitrogen artificial freezing model test, and in particular relates to a liquid nitrogen freezing model test system and method for sand layers simulating different seepage environments under different sand layer densities. Background Art

[0002] With the continuous rise of mine construction and tunnel excavation projects, underground engineering has entered a new stage. However, there is still an important problem of groundwater seepage in the construction process of underground engineering. With the continuous construction of underground infrastructure along the river and the coast, the groundwater intrusion faced during the construction process is becoming more and more frequent. The large groundwater flow rate often leads to slow development of the freezing curtain, or even failure to close the circle, which will eventually bring serious safety hazards to the project. Therefore, how to effectively and timely deal with the impact of large groundwater flow rates on underground engineering construction has become a top priority.

[0003] At present, the experimental research on the influence of groundwater flow rate on artificial freezing method mainly focuses on the condition of low groundwater flow rate. Usually, salt water is used as the freezing medium for freezing test. Due to the small permeability coefficient of the soil layer, the maximum seepage velocity that can be achieved is usually less than 5m / d. At the same time, the seepage system used in the model test for studying the influence of seepage on freezing construction at this stage cannot provide a seepage environment with a large flow rate. At present, the seepage velocity of more than 10m / d has occurred during the construction of underground projects in many riverside and coastal areas such as Nanjing, Shanghai, and Guangzhou. At this time, salt water freezing can no longer provide the cold amount required to close the freezing curtain, and liquid nitrogen as an ultra-low temperature freezing medium can provide assistance for freezing construction in a high flow rate environment.

[0004] Therefore, it is necessary to develop a model test system that simulates large-scale seepage liquid nitrogen artificial freezing of the formation and propose an effective test method. Summary of the invention

[0005] Purpose of the invention: The technical problem to be solved by the present invention is to provide a liquid nitrogen freezing model test system and method for sand layers under a seepage environment, which can simulate the freezing process of liquid nitrogen strata with different sand layer densities and different seepage environments to obtain the evolution law of the temperature field and seepage field of liquid nitrogen artificial freezing in large seepage strata, that is, the evolution law of the freezing temperature field and seepage field in the process of liquid nitrogen strata freezing under multiple groups of different density conditions and seepage velocity environments can be obtained at one time, thereby improving the test efficiency and reliability and providing a basis for the construction of liquid nitrogen artificial freezing in large seepage strata.

[0006] Technical solution: The present invention is based on a sand layer liquid nitrogen freezing model test system that simulates different seepage environments under different sand layer densities. The test system includes:

[0007] A seepage freezing model device, used to store sand for model tests and conduct seepage freezing tests;

[0008] The sand layer free implementation system is used to fill the seepage freezing model device with test sand of different densities;

[0009] Liquid nitrogen cooling system provides low-temperature cooling for freezing the seepage sand layer in the seepage freezing model device;

[0010] The constant temperature circulating water supply system provides constant temperature boundary conditions and thermal insulation effect for the seepage environment in the seepage freezing model device;

[0011] The water head difference seepage system provides the seepage conditions required for the test of the seepage freezing model device;

[0012] A monitoring system is installed in the sand layer within the seepage freezing model device to obtain temperature data and frost heave displacement;

[0013] Among them, the seepage freezing model device includes a box body for accommodating a test sand cavity, a constant temperature water area structure circumferentially nested inside the box body, and an upstream permeable plate and a downstream permeable plate located between the constant temperature water area structure and the box body, and a plurality of permeable holes arranged horizontally and vertically are provided on the upstream permeable plate and the downstream permeable plate, a through hole is provided on the surface of the constant temperature water area structure connected to the upstream permeable plate, and a plurality of water inlet holes and water outlet holes are provided on the box body.

[0014] Furthermore, the constant temperature water area structure of the test system is a frame-type structure with a cavity structure, on which a liquid inlet and a liquid outlet are respectively opened; the seepage freezing model device also includes a cover plate located on the top of the box body, and the cover plate is provided with an exhaust hole.

[0015] Furthermore, the sand layer free implementation system of the test system includes a sand box frame, a sand storage box connected to the sand box frame through a lifting mechanism, a sand outlet is opened at the bottom of the sand storage box, and a screening plate is provided at the sand outlet.

[0016] Furthermore, the liquid nitrogen cooling system of the test system includes a liquid nitrogen tank, and a plurality of liquid nitrogen freezing tubes connected to the liquid nitrogen tank. The plurality of liquid nitrogen freezing tubes are inserted into the sand layer of the seepage freezing model device, and the liquid nitrogen freezing tubes are provided with a liquid nitrogen flowmeter and a control valve.

[0017] Furthermore, the constant temperature circulating water supply system of the test system includes a constant temperature circulating water tank, which is provided with a water inlet pipe and a water outlet pipe, and the water inlet pipe and the water outlet pipe are respectively connected to the liquid inlet and the liquid outlet on the constant temperature water area structure.

[0018] Furthermore, the head difference seepage system of the test system includes an upstream water supply tank and a downstream water supply tank that form a height difference, and the upstream water supply tank is connected with a water inlet pipe and a water outlet pipe, the water outlet pipe is connected with a plurality of water inlet holes on the seepage freezing model device box, and the water inlet pipe is connected with a constant temperature circulating water tank; the downstream water supply tank is connected with a water inlet pipe and a water outlet pipe, the water outlet pipe is connected with the constant temperature circulating water tank, and the water inlet pipe is connected with the water outlet holes on the seepage freezing model device box.

[0019] Furthermore, the monitoring system of the test system includes a displacement sensor located above the sand layer in the seepage freezing model device and a temperature sensor located in the sand layer.

[0020] The method for simulating the freezing of sand layers in different seepage environments based on the above test system comprises the following steps:

[0021] (1) According to the actual application occasions, the density of different sand layers is determined in combination with the following formula, and the filling is carried out through the sand layer free filling system;

[0022]

[0023] Where: ρ(h) is the density of the sand layer; γ d is the dry density of sand (kg / m 3 ); γ s is the particle density of sand (kg / m 3 );v s is the falling speed of sand particles (m / s); h is the height of the sand outlet from the sand filling box; E k is the kinetic energy of the sand; m is the mass of the sand; ρ min is the minimum density, corresponding to zero height (m); ρ max is the maximum density, corresponding to infinite height (m); κ is the kinetic energy influence coefficient, among which medium-coarse sand: 0.67, fine sand: 1.15, silt sand: 0.79; β is the compaction coefficient, among which medium-coarse sand: 0.03, fine sand: 0.02, silt sand: 0.01; Δh is the settlement (m);

[0024] (2) Through the constant temperature circulating water supply system and the head difference seepage system, the seepage of the seepage freezing model device is formed, and the seepage velocity of different sand layers is obtained by the following formula:

[0025]

[0026] Where: v ai is the water flow velocity of each sand layer (m / d); v i is the seepage velocity of the i-th sand layer (m / d); K i is the permeability coefficient of the i-th sand layer (m / d); Δh i / Δl i is the hydraulic gradient of the i-th sand layer; n i is the porosity of the i-th sand layer;

[0027] Based on the permeability coefficient of each sand layer K1, K2, ..., K i There is a difference between their relative hydraulic gradients Δh / Δl, so the total seepage velocity is:

[0028]

[0029] The water flow velocity of each sand layer and its permeability coefficient K1, K2, ..., K i Related:

[0030] v a1 :v a2 :…:v ai =K1:K2:…:K i ;

[0031] Then, the total seepage velocity can be used to directly measure the seepage flow rate Q and the seepage cross-sectional area A, and the following can be inferred:

[0032]

[0033] (3) Through the liquid nitrogen cooling system and monitoring system, the data on the temperature field change and displacement change during the liquid nitrogen seepage freezing process are obtained, and then the coupling relationship between the liquid nitrogen freezing temperature field and the seepage field under multiple groups of different sand layers and different seepage velocities is obtained.

[0034] Beneficial effects: Compared with the prior art, the significant advantages of the present invention are: the simulated freezing system can maintain a constant seepage velocity by changing the density of different sand layers, and can study the coupling relationship between the liquid nitrogen freezing temperature field and the seepage field under different densities; and can also change the seepage velocity of each layer of sand by keeping the density of different sand layers constant, and study the coupling relationship between the liquid nitrogen freezing temperature field and the seepage field under different seepage velocities. That is, the coupling relationship between the liquid nitrogen freezing temperature field and the seepage field under different seepage velocities can be obtained by one experiment, and multiple groups of experiments can also be carried out to obtain the coupling relationship between the liquid nitrogen freezing temperature field and the seepage field under different seepage velocities or different densities of the same sand mold. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a three-dimensional structural diagram of the test system of the present invention;

[0036] Figure 2 An exploded view of the seepage freezing model device of the present invention;

[0037] Figure 3 This is a three-dimensional structural diagram of the sand storage box of the present invention;

[0038] Figure 4 It is a side plan view of the sand storage box of the present invention and a partial enlarged view of the sand outlet;

[0039] Figure 5 It is a planar layout diagram of measuring points of the monitoring system of the present invention;

[0040] Figure 6 This is a structural diagram of the sensor module of the present invention;

[0041] Figure 7 This is a cross-sectional view of the multiple soil layers inside the box of the seepage freezing model device. DETAILED DESCRIPTION

[0042] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings.

[0043] like Figure 1 As shown, the sand layer liquid nitrogen freezing model test system of the present invention can simulate the coupling relationship between the liquid nitrogen freezing temperature field and the seepage field of different seepage environments under different sand layer density conditions. Specifically, the test system includes a seepage freezing model device for storing sand for model tests and performing seepage freezing tests, a head difference seepage system that provides seepage conditions for the seepage freezing model device, a sand layer free implementation system for filling sand layers in the seepage freezing model device, a liquid nitrogen cooling system that provides frozen low-temperature cold for the seepage sand layer in the seepage freezing model device, a constant temperature circulation water supply system that provides constant temperature boundary conditions and insulation effect for the seepage environment in the seepage freezing model device, and a monitoring system arranged in the sand layer in the seepage freezing model device to obtain temperature data and frost heave displacement. The detection system is connected to the data acquisition system, and then the temperature and displacement data received by the data acquisition system are combined with the seepage velocity generated by the head difference to obtain the relationship between the sand layer temperature field, seepage field and displacement field during the liquid nitrogen freezing method construction process.

[0044] Among them, the seepage freezing model device is as follows Figure 2 As shown, it includes a box body 1 with supporting feet, and the box body 1 is a seamless welded steel structure. The outer side of the box body 1 is a heat-insulating structure. For example, it can be wrapped with a composite heat-insulating material composed of aerogel, ultra-fine glass fiber and PE material on the outer side, or it can be wrapped with some heat-insulating materials known in the art. The box body 1 is provided with a cavity for accommodating test sand, and the circumference of the cavity is nested with a constant temperature water area structure 2. The constant temperature water area structure 2 can be a frame structure with a cavity inside for filling constant temperature water, that is, a square water tank structure. The box body 1 is provided with a water inlet hole 6 and a water outlet hole, and then the constant temperature water is infiltrated into the sand layer filled therein through the water inlet hole 6, and a seepage cycle is formed through the water outlet hole. The top of the box body 1 is also provided with a cover plate 10, and the cover plate 10 is provided with an exhaust hole 11.

[0045] The constant temperature water area structure 2 is provided with a liquid inlet 8 and a liquid outlet 9. A uniform seepage structure is provided between the constant temperature water area structure 2 and the box body 1. The uniform seepage structure is an upstream permeable plate 4 and a downstream permeable plate 5, and a polymer permeable film and a highly permeable engineering mesh fabric combination structure are laid on the inner side of the permeable plate, so that the water can flow into the sand layer at a uniform speed and evenly. A number of permeable holes arranged horizontally and vertically are provided on the upstream permeable plate 4 and the downstream permeable plate 5, and a number of through holes are provided on the side of the constant temperature water area structure 2 located at the upstream permeable plate 4, so that the constant temperature water can flow smoothly from the water inlet 6 of the box body 1 into the box body 1.

[0046] The constant temperature circulating water supply system includes a constant temperature circulating water tank 19, a pressure pump 26, a control panel 27, and a water inlet pipe 20 and a water outlet pipe 21 provided on the constant temperature circulating water tank 19. The water inlet pipe 20 is connected to the liquid inlet 8 on the constant temperature water area structure 2, and the water outlet pipe 21 is connected to the liquid outlet 9 on the constant temperature water area structure 2. A heating pump is also provided in the constant temperature circulating water tank 19, and the water temperature of the constant temperature circulating water tank 19 is regulated by the control panel 27.

[0047] The head difference seepage system includes a hanger 28, an upstream water supply tank 22, a downstream water supply tank 23, a lifting mechanism 30 and a control panel. The lifting mechanism 30 is controlled by the control panel 27 to adjust the head difference between the upstream water supply tank 22 and the downstream water supply tank 23 to generate a pressure difference, thereby providing the seepage freezing model device with the high flow rate seepage conditions required for the test. The lifting mechanism can adopt a lifting device known in the art, such as a hydraulic lifting structure, etc., which will not be described in detail in the present invention. An inlet pipe 20 and an outlet pipe 21 are provided on the upstream water supply tank 22. The outlet pipe 21 is connected to several water inlet holes 6 on the seepage freezing model device box 1, and the inlet pipe 10 is connected to the constant temperature circulating water tank 19. An inlet pipe 20 and an outlet pipe 21 are provided on the downstream water supply tank 23. The outlet pipe 21 is connected to the constant temperature circulating water tank 19, and the inlet pipe 20 is connected to the outlet hole on the seepage freezing model device box 1 to form a circulation. By injecting 80% of the volume of 20°C domestic water into the constant temperature circulating water tank 19, opening the control valve, and controlling the pressure pump 26 through the control panel 27 to pump the water in the constant temperature circulating water tank 19 to the upstream water supply tank 22 and fill it up, and at the same time pumping the constant temperature water to the liquid inlet 8 of the constant temperature water area structure 2.

[0048] The sand layer free implementation system includes a sand box frame 12, a pulley 31, and a sand storage box 13. Figure 3 and Figure 4 The bottom of the sand storage box 13 is provided with a sand outlet, a screening plate 14 is provided at the sand outlet, and a screening net 15 is provided on the screening plate. The height and swing amplitude of the sand storage box 13 are adjusted by the lifting mechanism, and the opening spacing of the screening plate 14 at the sand outlet 14 is controlled by the control lever, so that the sand is screened out from the screening net 15.

[0049] The liquid nitrogen cooling system includes a liquid nitrogen tank 16, a liquid nitrogen flowmeter 18, a control valve and a plurality of liquid nitrogen freezing tubes 17. The liquid nitrogen tank 16 is provided with a liquid nitrogen outlet 33 to connect the liquid nitrogen freezing tubes 17, the liquid nitrogen freezing tubes 17 are inserted into the saturated sand layer in the seepage freezing model device box 1, the control valve is opened to pass liquid nitrogen, the liquid nitrogen consumption is monitored by the liquid nitrogen flowmeter 18, and the temperature of the liquid nitrogen freezing tubes 17 is controlled.

[0050] The monitoring system includes a displacement sensor 24, a temperature sensing needle 34, a temperature sensor 25 and a data acquisition system 32. The displacement sensor 24 is placed on the sand layer, and the temperature sensor 25 is quickly and conveniently inserted into the sand layer of the seepage freezing model device box 1 using a quick-insert interface at a specified point, and silicone is applied to the interface to prevent water seepage. The displacement sensor 24 and the temperature sensor 25 respectively measure the frost heave amount and temperature field change law of the sand layer during the freezing process, and transmit the collected data to the data acquisition system 32 through data transmission.

[0051] The method for simulating the freezing of sand layers in different seepage environments by the test system of the present invention comprises the following steps:

[0052] (1) According to the actual application occasions, the density of different sand layers is determined in combination with the following formula, and the filling is carried out through the sand layer free filling system;

[0053]

[0054] Where: ρ(h) is the density of the sand layer; γ d is the dry density of sand (kg / m 3 ); γ s is the particle density of sand (kg / m 3 );v s is the falling speed of sand particles (m / s); h is the height of the sand outlet from the sand filling box; E k is the kinetic energy of the sand; m is the mass of the sand; ρ min is the minimum density, corresponding to zero height (m); ρ max is the maximum density, corresponding to infinite height (m); κ is the kinetic energy influence coefficient, among which medium-coarse sand: 0.67, fine sand: 1.15, silt sand: 0.79; β is the compaction coefficient, among which medium-coarse sand: 0.03, fine sand: 0.02, silt sand: 0.01; Δh is the settlement (m);

[0055] (2) Through the constant temperature circulating water supply system and the head difference seepage system, the seepage of the seepage freezing model device is formed, and the seepage velocity of different sand layers is obtained by the following formula:

[0056]

[0057] Where: v aiis the water flow velocity of each sand layer (m / d); v i is the seepage velocity of the i-th sand layer (m / d); K i is the permeability coefficient of the i-th sand layer (m / d); Δh i / Δl i is the hydraulic gradient of the i-th sand layer; n i is the porosity of the i-th sand layer;

[0058] Based on the permeability coefficient of each sand layer K1, K2, ..., K i There is a difference between their relative hydraulic gradients Δh / Δl, so the total seepage velocity is:

[0059]

[0060] The water flow velocity of each sand layer is related to its permeability coefficient:

[0061] v a1 :v a2 :…:v ai =K1:K2:…:K i ;

[0062] Then, the total seepage velocity can be used to directly measure the seepage flow rate Q and the seepage cross-sectional area A, and the following can be inferred:

[0063]

[0064] (3) Through the liquid nitrogen cooling system and monitoring system, the data on the temperature field change and displacement change during the liquid nitrogen seepage freezing process are obtained, and then the coupling relationship between the liquid nitrogen freezing temperature field and the seepage field under multiple groups of different sand layers and different seepage velocities is obtained.

[0065] Specifically, the method for simulating the freezing of sand layers under different seepage environments by the test system of the present invention comprises the following steps:

[0066] (1) The shape and size of the box are determined by dimensional analysis. The position of the liquid nitrogen freezing tube is arranged on the central axis of the box. Several liquid nitrogen freezing tubes are arranged horizontally in the vertical seepage direction. The temperature sensor of the monitoring system is arranged according to Figure 5 The displacement sensors are arranged at equal intervals on the surface of the sand layer around each freezing tube. The temperature sensor is set at the liquid inlet of the liquid nitrogen freezing tube, and then connected to the data acquisition system through a homemade constantan wire data cable. Figure 6 shown.

[0067] (2) According to the actual application occasions, the density of different sand layers is determined in combination with the following formula, and the filling is carried out through the sand layer free filling system;

[0068]

[0069] Where: ρ(h) is the density of the sand layer; γ d is the dry density of sand (kg / m 3 ); γ s is the particle density of sand (kg / m 3 );v s is the falling speed of sand particles (m / s); h is the height of the sand outlet from the sand filling box; E k is the kinetic energy of the sand; m is the mass of the sand; ρ min is the minimum density, corresponding to zero height (m); ρ max is the maximum density, corresponding to infinite height (m); κ is the kinetic energy influence coefficient, among which medium coarse sand: 0.67, fine sand: 1.15, silt sand: 0.79; β is the compaction coefficient, among which medium coarse sand: 0.03, fine sand: 0.02, silt sand: 0.01; Δh is the settlement (m).

[0070] The sand layers are separated by geotextile fabrics, and the sand layers are filled from bottom to top. The permeability coefficient of the sand layers decreases from top to bottom. Figure 7 As shown. According to the density of the sand layer required for different tests, after the implementation is completed, use a tamping hammer to gently tamp the surface of the sand layer to control the sinking amount of the sand layer surface within 5cm, then use a scraper to flatten the sand layer on the surface of the box, connect the top steel cover to the box through nuts and apply silicone on the joints.

[0071] (3) Inject 80% of the volume of 20℃ domestic water into the constant temperature circulating water tank, open the control valve on the water inlet pipe of the upstream water supply tank and introduce constant temperature water. After the water tank is filled, open the control valve on the water outlet pipe of the upstream water supply tank to completely immerse the sand layer in the tank body in water. At the same time, raise the downstream side of the tank body by 3cm (tilt the tank body about 3 / 100) to facilitate the discharge of air from a high place. Perform saturated exhaust and drainage work through the exhaust hole on the top of the tank body. This process continues until no bubbles are discharged. The process lasts for about 20-30 minutes. After the initial exhaust is completed, use a vacuum pump to extract 30kPa negative pressure from the exhaust hole to further absorb the air inside the tank body until the test sand layer inside the tank body is completely saturated with water.

[0072] (4) Through the constant temperature circulating water supply system and the head difference seepage system, the seepage of the seepage freezing model device is formed, and the seepage velocity of different sand layers is obtained by the following formula:

[0073]

[0074] Where: v ai is the water flow velocity of each sand layer (m / d); v i is the seepage velocity of the i-th sand layer (m / d); Ki is the permeability coefficient of the i-th sand layer (m / d); Δh i / Δl i is the hydraulic gradient of the i-th sand layer; n i is the porosity of the i-th sand layer;

[0075] Based on the permeability coefficient of each sand layer K1, K2, ..., K i There is a difference between their relative hydraulic gradients Δh / Δl, so the total seepage velocity is:

[0076]

[0077] The water flow velocity of each sand layer and its permeability coefficient K1, K2, ..., K i Related:

[0078] v a1 :v a2 :…:v ai =K1:K2:…:K i ;

[0079] Then, the total seepage velocity can be used to directly measure the seepage flow rate Q and the seepage cross-sectional area A, and the following can be inferred:

[0080]

[0081] The required height difference between the water tanks is calculated according to the size of the seepage velocity required for the test, and the height difference between the upstream water supply tank and the downstream water supply tank is controlled by the CNC panel to provide the seepage velocity for the model box.

[0082] (5) The data acquisition system obtains data on the temperature field changes and displacement changes during the liquid nitrogen seepage freezing process through temperature sensing needles, temperature sensors and displacement sensors. The data is imported into professional drawing software to obtain a cloud map of the liquid nitrogen temperature field and frost heave changes over time under the action of seepage. Then, the change law of the liquid nitrogen freezing temperature field and the frost heave process of the sand layer during the construction process are analyzed, and the relationship between the liquid nitrogen freezing temperature field, seepage field and displacement field is obtained.

[0083] In addition, the density of different sand layers can be kept constant, and the seepage velocity of each sand layer can be changed to study the coupling relationship between the liquid nitrogen freezing temperature field and the seepage field under different seepage velocities. That is, the coupling relationship between the liquid nitrogen freezing temperature field and the seepage field under different seepage velocities can be obtained by one experiment. Similarly, multiple groups of experiments can be carried out to obtain the coupling relationship between the liquid nitrogen freezing temperature field and the seepage field under different seepage velocities or different densities of the same sand mold.

Claims

1. A sand layer liquid nitrogen freezing model test system based on simulating different seepage environments under different sand layer density conditions, characterized in that: The test system includes: A seepage freezing model device, used to store sand for model tests and conduct seepage freezing tests; The sand layer free implementation system is used to fill the seepage freezing model device with test sand of different densities; Liquid nitrogen cooling system provides low-temperature cooling for freezing the seepage sand layer in the seepage freezing model device; The constant temperature circulating water supply system provides constant temperature boundary conditions and thermal insulation effect for the seepage environment in the seepage freezing model device; The water head difference seepage system provides the seepage conditions required for the test of the seepage freezing model device; A monitoring system is installed in the sand layer within the seepage freezing model device to obtain temperature data and frost heave displacement; The seepage freezing model device comprises a box (1) for accommodating a test sand cavity, a constant temperature water area structure (2) circumferentially nested inside the box (1), and an upstream permeable plate (4) and a downstream permeable plate (5) located between the constant temperature water area structure (2) and the box (1), wherein the upstream permeable plate (4) and the downstream permeable plate (5) are provided with a plurality of permeable holes arranged horizontally and vertically, a through hole is provided on the surface of the constant temperature water area structure (2) connected to the upstream permeable plate (4), and the box (1) is provided with a plurality of water inlet holes (6) and water outlet holes (7).

2. According to claim 1, the sand layer liquid nitrogen freezing model test system based on simulating different seepage environments under different sand layer densities is characterized in that: The constant temperature water area structure (2) is a frame-type structure with a cavity structure, on which a liquid inlet (8) and a liquid outlet (9) are respectively provided; the seepage freezing model device also includes a cover plate (10) located on the top of the box body (1), and an exhaust hole (11) is provided on the cover plate (10).

3. According to claim 1, the sand layer liquid nitrogen freezing model test system based on simulating different seepage environments under different sand layer densities is characterized in that: The sand layer free implementation system comprises a sand box frame (12), a sand storage box (13) connected to the sand box frame (12) through a lifting mechanism, a sand outlet (14) is provided at the bottom end of the sand storage box (13), and a screening plate (15) is provided at the sand outlet (14).

4. According to claim 1, the sand layer liquid nitrogen freezing model test system based on simulating different seepage environments under different sand layer density conditions is characterized in that: The liquid nitrogen refrigeration system comprises a liquid nitrogen tank (16), a plurality of liquid nitrogen freezing tubes (17) connected to the liquid nitrogen tank (16), the plurality of liquid nitrogen freezing tubes (17) being inserted into a sand layer of a seepage freezing model device, and a liquid nitrogen flow meter (18) and a control valve being arranged on the liquid nitrogen freezing tubes (17).

5. According to claim 2, the sand layer liquid nitrogen freezing model test system based on simulating different seepage environments under different sand layer densities is characterized in that: The constant temperature circulating water supply system comprises a constant temperature circulating water tank (19), on which a water inlet pipe (20) and a water outlet pipe (21) are provided, wherein the water inlet pipe (20) and the water outlet pipe (21) are respectively connected to a liquid inlet (8) and a liquid outlet (9) on a constant temperature water area structure (2).

6. According to claim 2, the sand layer liquid nitrogen freezing model test system based on simulating different seepage environments under different sand layer densities is characterized in that: The water head difference seepage system comprises an upstream water supply box (22) and a downstream water supply box (23) which form a height difference. The upstream water supply box (22) is connected with a water inlet pipe (20) and a water outlet pipe (21), the water outlet pipe (21) is connected with a plurality of water inlet holes (6) on a seepage freezing model device box (1), and the water inlet pipe (20) is connected with a constant temperature circulating water tank (19); the downstream water supply box (23) is connected with a water inlet pipe (20) and a water outlet pipe (21), the water outlet pipe (21) is connected with a constant temperature circulating water tank (19), and the water inlet pipe (20) is connected with a water outlet hole (7) on the seepage freezing model device box (1).

7. According to claim 1, the sand layer liquid nitrogen freezing model test system based on simulating different seepage environments under different sand layer densities is characterized in that: The monitoring system comprises a displacement sensor (24) located above the sand layer in the seepage freezing model device and a temperature sensor (25) located in the sand layer.

8. A method for simulating the freezing of sand layers in different seepage environments based on the test system of claim 1, characterized in that: The steps include: (1) According to the actual application occasions, the density of different sand layers is determined in combination with the following formula, and the filling is carried out through the sand layer free filling system; Where: ρ(h) is the density of the sand layer; γ d is the dry density of sand (kg / m 3 ); γ s is the particle density of sand (kg / m 3 );v s is the falling speed of sand particles (m / s); h is the height of the sand outlet from the sand filling box; E k is the kinetic energy of the sand; m is the mass of the sand; ρ min is the minimum density, corresponding to zero height (m); ρ max is the maximum density, corresponding to infinite height (m); κ is the kinetic energy influence coefficient, among which medium-coarse sand: 0.67, fine sand: 1.15, silt sand: 0.79; β is the compaction coefficient, among which medium-coarse sand: 0.03, fine sand: 0.02, silt sand: 0.01; Δh is the settlement (m); (2) Through the constant temperature circulating water supply system and the head difference seepage system, the seepage of the seepage freezing model device is formed, and the seepage velocity of different sand layers is obtained by the following formula: Where: v ai is the water flow velocity of each sand layer (m / d); v i is the seepage velocity of the i-th sand layer (m / d); K i is the permeability coefficient of the i-th sand layer (m / d); Δh i / Δl i is the hydraulic gradient of the i-th sand layer; n i is the porosity of the i-th sand layer; Based on the permeability coefficient of each sand layer K1, K2, ..., K i There is a difference between their relative hydraulic gradients Δh / Δl, so the total seepage velocity is: The water flow velocity of each sand layer and its permeability coefficient K1, K2, ..., K i Related: v a1 :v a2 :…:v ai =K1:K2:…:K i ; Then, the total seepage velocity can be used to directly measure the seepage flow rate Q and the seepage cross-sectional area A, and the following can be inferred: (3) Through the liquid nitrogen cooling system and monitoring system, the data on the temperature field change and displacement change during the liquid nitrogen seepage freezing process are obtained, and then the coupling relationship between the liquid nitrogen freezing temperature field and the seepage field under multiple groups of different sand layers and different seepage velocities is obtained.

Images