Testing device and method for initial freezing temperature of frozen soil under negative pressure environment

Abstract

The present application relates to a kind of frozen soil starting freezing temperature testing device and method under negative pressure environment.The determination of frozen soil starting freezing temperature does not consider the influence of atmospheric pressure, and the starting freezing temperature of frozen soil under negative pressure environment cannot be measured.The device includes compressed gas tank, electromagnetic control valve, jet vacuum, pressure sensor, temperature sensor and sample copper cup;Compressed gas tank is connected to jet vacuum through airflow pipeline, jet vacuum is connected to sample copper cup through airflow pipeline, and temperature sensor is placed in the soil sample in sample copper cup;Pressure sensor is arranged on the airflow pipeline between jet vacuum and sample copper cup, electromagnetic control valve is arranged on the airflow pipeline between compressed gas tank and jet vacuum;Pressure sensor and electromagnetic control valve are electrically connected.The present application can be integrated into freezing temperature test, provide different vacuum degree for the container of soil sample, vacuum generation speed is fast, can be automatically adjusted and controlled, and is safe and reliable.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of geotechnical testing, in particular to a device and method for testing the initial freezing temperature of frozen soil in a negative pressure environment. BACKGROUND

[0002] The freezing temperature is the temperature at which the pore water in the soil is stably frozen, and is an important indicator for determining whether the soil is frozen. Under standard conditions, water begins to freeze at 0 O C, however, in actual engineering, the initial freezing temperature of frozen soil is often lower than 0 O C, affected by various factors such as soil properties, soil moisture content, liquid limit moisture content, and salt content in the soil. The initial freezing temperature of soil plays a key role in studying soil frost heaving and thawing, determining the freezing depth and thawing depth of soil, etc. For example, in actual engineering, the freezing temperature of soil affects the depth of frozen soil wall and the use of coolant, and the freezing temperature is the basis for determining the freezing depth of subgrade and the thickness of artificial frozen soil freezing wall, and is also a physical quantity that must be measured around the soil during mine shaft and urban subway construction by the freezing method.

[0003] After long-term research and development, four methods for testing the freezing temperature have been formed. The first method is the no external load method, however, this measurement method has many shortcomings, first of all, many instruments and materials need to be prepared before the test, and the on-site assembly is very troublesome and inefficient; secondly, the low-temperature solution required during the test needs to be prepared in advance; moreover, the data of the potentiometric cell needs to be read every minute during the test, and the recording task is heavy, finally, the data obtained by the test also needs to be calculated to obtain the final temperature. The second method is the freezing potential method based on porous media. However, this method also has a sudden change in potential in the non-freezing temperature range, and there is still a deviation in accurately determining the change of freezing temperature. The third method is the differential calorimetry method, which has high sensitivity to temperature and high measurement accuracy, and can obtain the necessary thermal data in real time. The fourth method is a research method based on the electrical resistance characteristics of soil, which can reflect the temperature of the soil during the freezing process through the change of the electrical resistance of the soil layer.

[0004] Currently, the research on the influencing factors of soil freezing mainly focuses on soil properties, dry density, salt content, and water content. However, the change of pressure can also change the freezing point of water in the soil and the pore structure of the soil, resulting in different freezing temperatures compared to the freezing temperature under normal pressure. The above methods do not consider the influence of atmospheric pressure on the freezing temperature of frozen soil, and there is currently a lack of a method for measuring the initial freezing temperature of frozen soil in a negative pressure environment. Therefore, a method for measuring the initial freezing temperature of frozen soil in a negative pressure environment is needed to provide a reference for the construction of frozen soil regions. SUMMARY

[0005] The application aims to provide a frozen soil initial freezing temperature testing device and method under negative pressure environment to at least solve the problem that the prior art does not consider the influence of atmospheric pressure on the freezing temperature of frozen soil and cannot measure the initial freezing temperature of frozen soil under negative pressure environment.

[0006] In order to achieve the above-mentioned purpose, the application adopts the following technical solution:

[0007] The frozen soil initial freezing temperature testing device under negative pressure environment comprises a compressed gas tank, an electromagnetic control valve, a jet vacuum device, a pressure sensor, a temperature sensor and a sample copper cup.

[0008] The compressed gas tank is connected to the jet vacuum device through an airflow pipeline, the jet vacuum device is connected to the sample copper cup through an airflow pipeline, and the temperature sensor is arranged in the soil sample in the sample copper cup.

[0009] A pressure sensor is arranged on the airflow pipeline between the jet vacuum device and the sample copper cup, an electromagnetic control valve is arranged on the airflow pipeline between the compressed gas tank and the jet vacuum device, and the pressure sensor and the electromagnetic control valve are electrically connected.

[0010] Further, the jet vacuum device comprises a vacuum device air inlet, a vacuum device air outlet, a first Laval tube, a second Laval tube, a first vacuum port, a second vacuum port, a vacuum cavity and an expansion tube.

[0011] The first Laval tube and the second Laval tube are arranged in series between the vacuum device air inlet and the vacuum device air outlet, the first Laval tube and the second Laval tube are connected to the vacuum cavity through the first vacuum port and the second vacuum port respectively, and the expansion tube is arranged between the second Laval tube and the vacuum device air outlet.

[0012] Further, the vacuum device air inlet is connected to the compressed gas tank through an airflow pipeline, the vacuum cavity is connected to the sample copper cup through an airflow pipeline, and the vacuum device air outlet is connected to a muffler through an airflow pipeline.

[0013] Further, the electromagnetic control valve comprises an electromagnetic valve body, S-pole magnets and N-pole magnets arranged in the electromagnetic valve body in transverse opposition, and a coil arranged between the S-pole magnets and the N-pole magnets.

[0014] One end of the coil is connected to one end of an electromagnetic valve piston, the other end of the electromagnetic valve piston is connected to one end of an electromagnetic valve compression spring, the other end of the electromagnetic valve compression spring is connected to the inner wall of the electromagnetic valve body, and the coil, the electromagnetic valve piston and the electromagnetic valve compression spring are arranged longitudinally in the electromagnetic valve body.

[0015] The electromagnetic valve piston is provided with an electromagnetic valve air inlet and an electromagnetic valve air outlet on the corresponding electromagnetic valve body on the two sides of the electromagnetic valve piston.

[0016] Further, the electromagnetic valve air inlet is connected to the compressed gas tank through an air flow pipeline, and the electromagnetic valve air outlet is connected to the jet vacuum through an air flow pipeline.

[0017] Further, a one-way valve is arranged on the air flow pipeline between the jet vacuum and the sample copper cup; the one-way valve comprises a one-way valve body, two ends of the one-way valve body are respectively a one-way valve air inlet and a one-way valve air outlet, a cavity is arranged between the one-way valve air inlet and the one-way valve air outlet, the inner diameter of the cavity is greater than the inner diameter of the one-way valve air inlet and the one-way valve air outlet, and a one-way valve piston is arranged in the cavity.

[0018] The one-way valve piston is cylindrical, the outer diameter of the one-way valve piston is the same as the inner diameter of the cavity, a through hole is arranged on the cylinder wall of the one-way valve piston, the inner diameter of the cavity outside the through hole is greater than the outer diameter of the one-way valve piston, the longitudinal length of the one-way valve piston is less than the longitudinal length of the cavity, and a piston cone is arranged at the air inlet end of the one-way valve piston.

[0019] Further, a pressure relief safety valve is arranged on the air flow pipeline between the compressed gas tank and the electromagnetic control valve; the pressure relief safety valve comprises a valve body, a valve cap is arranged at one end of the valve body, and an air inlet hole is arranged on the valve cap.

[0020] A safety valve piston is arranged in the valve body; the outer diameter of one end of the safety valve piston is the same as the inner diameter of the inner cavity of the valve body and is close to the air inlet hole, the outer diameter of the other end of the safety valve piston is less than the inner diameter of the inner cavity of the valve body and is connected to one end of a safety valve compression spring, the other end of the safety valve compression spring is connected to the inner wall of the valve body, and air outlet holes are arranged on the valve body on the two sides of the other end of the safety valve compression spring.

[0021] Further, the sample copper cup comprises a cup body, a copper cup cover is screwed on the top of the cup body, the copper cup cover is provided with an air pipe, an air suction pipe is inserted into the air pipe, the air pipe is provided with external threads and is connected to an air flow pipeline through an aviation plug.

[0022] On the other hand, a frozen soil initial freezing temperature test method under a negative pressure environment is provided by using the device, and the method comprises the following steps.

[0023] The soil sample is prepared, dried, crushed, and sieved.

[0024] The soil sample is weighed and placed in a copper sample cup, a temperature sensor is placed in the soil sample in the copper sample cup, the temperature sensor wire is connected with the air suction pipe through the air pipe of the copper cup cover, the air suction pipe inlet is placed on the surface of the soil sample, the copper cup cover is covered, the aviation plug is installed on the air pipe, the aviation plug is tightened, and the temperature sensor wire and the air suction pipe are pulled out from the aviation plug;

[0025] The temperature sensor is a resistance temperature sensor, and data is collected through an A / D converter connected to a data acquisition device;

[0026] The copper sample cup is placed in the organic glass tube filled with dry sand, and the copper sample cup is covered with a rubber plug, and the temperature sensor wire and the air suction pipe are pulled out from the rubber plug;

[0027] A target vacuum degree is set, a compressed gas tank is opened to make the jet type vacuumizer work, the vacuum degree reaches the set value, and the copper sample cup is kept at the set vacuum degree;

[0028] The organic glass tube is placed in a low-temperature constant-temperature tank set to a negative temperature in advance, and the temperature sensor data is read through the data acquisition device; when the temperature of the temperature sensor reaches-15℃ or below, it can be determined that the soil is completely frozen, and at this time, the test is ended and the collected temperature data is read, which is the initial freezing temperature of frozen soil in a negative pressure environment.

[0029] Further, the target vacuum degree is set, the compressed gas tank is opened to make the jet type vacuumizer work, the vacuum degree reaches the set value, and the copper sample cup is kept at the set vacuum degree, which includes:

[0030] A target vacuum degree is set, a compressed gas tank valve is opened, compressed gas flows through the opened electromagnetic control valve at high speed, flows through the jet type vacuumizer composed of two Laval tubes in series, forms a negative pressure in the vacuum cavity of the jet type vacuumizer, and absorbs the air in the copper sample cup to create a negative pressure environment; the compressed gas flowing through the Laval tube is discharged into the atmosphere after noise reduction by the silencer;

[0031] When the vacuum degree in the copper sample cup reaches the set value, the pressure sensor outputs a control signal to the electromagnetic control valve, the coil current is disconnected, the magnetic force disappears, the electromagnetic valve compression spring pushes the electromagnetic valve piston to move left to close the air path, at this time the jet type vacuumizer no longer passes through the airflow, the jet type vacuumizer restores to normal pressure state, at this time the pressure in the copper sample cup is the set negative pressure value, the internal gas pressure is less than the external atmospheric pressure, but the external normal pressure gas cannot flow back into the copper sample cup due to the action of the one-way valve, and the copper sample cup still maintains the set vacuum degree;

[0032] The freezing test continues, when the pressure sensor senses that the pressure in the copper cup of the sample is higher than the set value, the pressure sensor input electrical signal connects the circuit, and the electromagnetic control valve is started again to open, the coil is energized to generate a magnetic force, which pushes the electromagnetic valve piston to move to the right to open the gas path, and the jet vacuum pump works again to reduce the vacuum degree in the copper cup of the sample to the set value, so as to achieve the purpose of keeping the copper cup of the sample at the set vacuum degree, so as to carry out the soil freezing temperature test under different negative pressure environments.

[0033] Compared with the prior art, the beneficial effects of the present application are as follows:

[0034] The negative pressure environment under the frozen soil initial freezing temperature testing device provided by the present application is a negative pressure generating device that can be integrated into a freezing temperature test, which can provide different vacuum degrees for the container of the test soil sample, can provide a stable negative pressure environment to simulate different atmospheric pressures, and can test and measure the soil freezing temperature under different vacuum degree environments. The vacuum generation speed is fast, and the automation adjustment and control can reduce the manual operation steps. Compared with the traditional jet vacuum circuit, the maximum vacuum degree is higher, different vacuum degrees can be maintained, and the flow valve, filter valve and other devices are reduced, the pipeline is short, safe and reliable, and can meet the testing needs of most earth negative pressure environment frozen soil. BRIEF DESCRIPTION OF DRAWINGS

[0035] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and those skilled in the art can obtain other drawings of embodiments according to these drawings without creative labor.

[0036] Figure 1 It is a structure diagram of the frozen soil initial freezing temperature testing device under negative pressure environment.

[0037] Figure 2 It is a structure diagram of the sample copper cup.

[0038] Figure 3 It is a structure diagram of the pressure relief safety valve

[0039] Figure 4 It is a structure diagram of the jet vacuum pump.

[0040] Figure 5 It is a structure diagram of the one-way valve.

[0041] Figure 6 It is a structure diagram of the electromagnetic control valve.

[0042] In the drawings, the following are marked:

[0043] 1 - compressed gas tank, 2 - electromagnetic control valve, 3 - jet vacuum, 4 - muffler, 5 - check valve, 6 - pressure sensor, 7 - pressure relief valve, 8 - A / D converter, 9 - data acquisition device, 10 - aviation plug, 11 - temperature sensor, 12 - sample copper cup, 13 - low pressure airflow duct, 14 - high pressure airflow duct;

[0044] 201 - filter screen, 202 - electromagnetic valve air inlet, 203 - electromagnetic valve air outlet, 204 - electromagnetic valve piston, 205 - electromagnetic valve compression spring, 206 - coil, 207 - S pole magnet, 208 - N pole magnet;

[0045] 301 - vacuum inlet, 302 - vacuum outlet, 303 - first Laval tube, 304 - second Laval tube, 305 - first vacuum port, 306 - second vacuum port, 307 - vacuum chamber, 308 - expansion tube;

[0046] 501 - check valve air inlet, 502 - through hole, 503 - check valve piston, 504 - check valve air outlet, 505 - cavity;

[0047] 701 - air inlet hole, 702 - safety valve piston, 703 - air outlet hole, 704 - safety valve compression spring, 705 - valve body, 706 - valve cap;

[0048] 1201 - cup body, 1202 - copper cup inner cavity, 1203 - internal thread, 1204 - external thread, 1205 - air pipe, 1206 - copper cup cover. DETAILED DESCRIPTION

[0049] In order to facilitate the understanding of the present application, the present application will be described in more detail below with reference to the relevant drawings. The preferred embodiments of the present application are shown in the drawings. However, the present application can be implemented in many different forms, and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present application more thorough and comprehensive.

[0050] In the description of the present patent, it should be understood that the terms "upper", "lower", "left", "right", "top", "bottom", "inner", "outer" and the like indicate the orientation or positional relationship shown in the drawings, and are only for the purpose of facilitating the description of the present patent and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present patent. The terms "first", "second" and the like are described only for the purpose of more clearly describing the structural features of the structure, and should not be understood as a limitation on the relationship, order, importance, etc.

[0051] In the description of the patent, it is necessary to explain that, unless otherwise explicitly specified and limited, the terms "access", "connection", "arrangement" should be understood broadly, for example, it can be fixedly connected, arranged, or detachably connected, arranged, or integrally connected, arranged. For those skilled in the art, the specific meaning of the above terms in the patent can be understood according to the specific circumstances.

[0052] At present, the research on the influencing factors of soil freezing mainly focuses on the soil quality, dry density, salt content and water content of soil. However, the change of pressure will also change the freezing point of water in soil and the pore structure of soil, resulting in different freezing temperature from that under normal pressure. The existing determination of the initial freezing temperature of frozen soil does not consider the influence of atmospheric pressure. The present application provides a frozen soil initial freezing temperature testing device under negative pressure environment to overcome the problem that the existing determination method cannot be implemented under different vacuum conditions.

[0053] As Figure 1 , the device comprises a compressed gas tank 1, an electromagnetic control valve 2, a jet vacuum 3, a pressure sensor 6, a temperature sensor 11 and a sample copper cup 12. The compressed gas tank 1 is connected to the jet vacuum 3 through an airflow pipeline, the jet vacuum 3 is connected to the sample copper cup 12 through an airflow pipeline, and the temperature sensor 11 is arranged in the soil sample in the sample copper cup 12. The airflow pipeline between the jet vacuum 3 and the sample copper cup 12 is provided with the pressure sensor 6, and the airflow pipeline between the compressed gas tank 1 and the jet vacuum 3 is provided with the electromagnetic control valve 2; the pressure sensor 6 and the electromagnetic control valve 2 are electrically connected. The airflow pipeline between the jet vacuum 3 and the sample copper cup 12 is further provided with a one-way valve 5. The airflow pipeline between the compressed gas tank 1 and the electromagnetic control valve 2 is further provided with a pressure relief safety valve 7. Among them, the compressed gas tank 1, the electromagnetic control valve 2, the jet vacuum 3, the one-way valve 5 and the pressure sensor 6 form a vacuum generating circuit, the sample copper cup 12 and the temperature sensor 11 form a freezing temperature testing assembly, and the vacuum generating circuit can create negative pressure conditions of different vacuum degrees for the freezing temperature testing assembly.

[0054] In the description of the specific embodiment, the structure of each device constituting the device is described, and when each device is described, the axial direction of the device is defined as the longitudinal direction, and the direction perpendicular to the axial direction is defined as the transverse direction.

[0055] As Figure 4The jet flow vacuum device 3 comprises a vacuum device air inlet 301, a vacuum device air outlet 302, a first Laval tube 303, a second Laval tube 304, a first vacuum port 305, a second vacuum port 306, a vacuum cavity 307 and an expansion tube 308. The first Laval tube 303 and the second Laval tube 304 are arranged in series between the vacuum device air inlet 301 and the vacuum device air outlet 302, and the first Laval tube 303 and the second Laval tube 304 are connected to the vacuum cavity 307 through the first vacuum port 305 and the second vacuum port 306 respectively, and the expansion tube 308 is arranged between the second Laval tube 304 and the vacuum device air outlet 302. The vacuum device air inlet 301 is connected to the compressed gas tank 1 through an airflow pipeline, the vacuum cavity 307 is connected to the sample copper cup 12 through an airflow pipeline, and the vacuum device air outlet 302 is connected to the muffler 4 through an airflow pipeline. The muffler 4 can adopt the expansion interference absorption type muffler in CN201510396194.6.

[0056] The present application adopts a 1mm nozzle diameter first-expansion-then-constriction supersonic Laval tube nozzle design scheme, the wall surface adopts a Vitoshinsky curve to ensure uniform and stable airflow at the throat outlet, a two-Laval-tube series connection mode is adopted to form a double-stage jet flow vacuum device to improve the maximum vacuum degree and response time, so as to balance the manufacturing cost and manufacturing difficulty. Heat-resistant and impact-resistant engineering plastics can be selected as the material of the Laval tube to ensure the strength of the device. Compressed gas flows into the air inlet, generates negative pressure through the pipeline formed by the two Laval tubes in series, and then flows into the next device after mixing with the injection gas in the negative pressure cavity.

[0057] As Figure 6 The electromagnetic control valve 2 comprises an electromagnetic valve body, an S-pole magnet 207 and an N-pole magnet 208 arranged transversely in the electromagnetic valve body, and a coil 206 arranged between the S-pole magnet 207 and the N-pole magnet 208. One end of the coil 206 is connected to one end of an electromagnetic valve piston 204, the other end of the electromagnetic valve piston 204 is connected to one end of an electromagnetic valve compression spring 205, the other end of the electromagnetic valve compression spring 205 is connected to the inner wall of the electromagnetic valve body, and the coil 206, the electromagnetic valve piston 204 and the electromagnetic valve compression spring 205 are arranged longitudinally in the electromagnetic valve body. An electromagnetic valve air inlet 202 and an electromagnetic valve air outlet 203 are arranged on the electromagnetic valve body corresponding to the transverse two sides of the electromagnetic valve piston 204. The electromagnetic valve air inlet 202 is connected to the compressed gas tank 1 through an airflow pipeline, and the electromagnetic valve air outlet 203 is connected to the jet flow vacuum device 3 through an airflow pipeline.

[0058] The electromagnetic control valve 2 of the application adopts an electromagnetic structure with external power supply. The pressure sensor 6 adopts a piezoresistive pressure sensor with good dynamic response, small size and high precision. In order to avoid the influence of temperature on the precision of the piezoresistive pressure sensor, the piezoresistive pressure sensor is installed in the negative pressure circuit between the one-way valve 5 and the electromagnetic control valve 2, and is connected with the electromagnetic control valve 2 and the A / D converter 8 through wiring. The electromagnetic control valve 2 controls the left and right movement of the piston through the electromagnetic force between the energized coil and the permanent magnet and the spring force, thereby controlling the on-off of the air path. The electromagnetic control valve 2 and the pressure sensor 6 jointly keep the air pressure inside the device always near the set value. In order to prevent impurities in the air source from damaging the device, a filter screen 201 with a filtering precision of 100 to 200 meshes is installed at the air inlet of the electromagnetic control valve 2.

[0059] As Figure 5 , the one-way valve 5 includes a one-way valve body, the two ends of the one-way valve body are a one-way valve air inlet 501 and a one-way valve air outlet 504, and the space between the one-way valve air inlet 501 and the one-way valve air outlet 504 is a cavity 505. The inner diameter of the cavity 505 is greater than the inner diameter of the one-way valve air inlet 501 and the one-way valve air outlet 504, respectively. A one-way valve piston 503 is arranged in the cavity 505. The one-way valve piston 503 is cylindrical, and the outer diameter of the one-way valve piston 503 is the same as the inner diameter of the cavity 505. A through hole 502 is arranged on the cylinder wall of the one-way valve piston 503, and the inner diameter of the cavity 505 outside the through hole 502 is greater than the outer diameter of the one-way valve piston 503. The longitudinal length of the one-way valve piston 503 is less than the longitudinal length of the cavity 505, and the air inlet end of the one-way valve piston 503 is provided with a piston cone.

[0060] The application adopts a piston type one-way valve design. The traditional spring one-way valve design inevitably reduces the maximum vacuum degree that the device can reach due to the existence of spring force. The one-way valve solves this problem through the design of the piston and the cavity, thereby achieving the purpose of saving gas and keeping the sealed device maintained near the set vacuum degree. In order to avoid the influence of the gravity of the piston on the maximum vacuum degree of the device, the one-way valve needs to be placed horizontally in use.

[0061] As Figure 3 , the pressure relief safety valve 7 includes a valve body 705, one end of the valve body 705 is provided with a valve cap 706, and the valve cap 706 is provided with an air inlet hole 701. A safety valve piston 702 is arranged in the valve body 705. The outer diameter of one end of the safety valve piston 702 is the same as the inner diameter of the inner cavity of the valve body 705 and is close to the air inlet hole 701. The outer diameter of the other end of the safety valve piston 702 is smaller than the inner diameter of the inner cavity of the valve body 705 and is connected to one end of a safety valve compression spring 704. The other end of the safety valve compression spring 704 is connected to the inner wall of the valve body 705. The valve body 705 on the two sides of the other end of the safety valve compression spring 704 is provided with an air outlet hole 703.

[0062] The valve body 705 of the pressure relief safety valve 7 is mainly composed of a safety valve piston 702 and a safety valve compression spring 704, when the pressure in the gas circuit exceeds the safety value, the piston moves to the exhaust port to discharge the excess gas in the gas circuit, the pressure relief safety valve 7 is installed between the compressed gas tank 1 and the electromagnetic control valve 2, so that when any part or multiple parts of the device is blocked, the high-pressure gas flow is directly discharged through the pressure relief safety valve 7, and explosion is prevented.

[0063] As Figure 2 , the sample copper cup 12 includes a cup body 1201, the top of the cup body 1201 is capped by a threaded copper cup cover 1206, the copper cup cover 1206 is provided with a breather pipe 1205, an air suction pipe is inserted into the breather pipe 1205, the breather pipe 1205 has an external thread 1204 and is connected with the airflow pipe through the aviation plug 10.

[0064] In addition to the sample copper cup 12, the other parts of the device are permanently sealed by using sealing rings and sealing strips made of butyronitrile rubber with good wear resistance. The sample copper cup 12 is connected and disconnected every time the test is conducted, so an air-electric hybrid aviation plug is used as the connector, which is simple and fast to install, has good air tightness, strong pressure resistance and anti-interference performance, and can effectively improve the sealing performance of the device. The copper cup cover 1206 is redesigned to have a threaded shape, the small opening originally connected only with the temperature sensor is enlarged, and the breather pipe 1205 is modified to have an external thread, which matches the aviation plug 10 with the same size thread, so that the requirements of convenient disassembly and reassembly and air tightness of the device are met.

[0065] The pressure sensor 6 and the temperature sensor 11 can be connected to the data acquisition device 9, such as an Agilent data acquisition instrument, through an A / D converter 8 to read the data of the pressure sensor 6 and the temperature sensor 11.

[0066] Based on the above device, a stable negative pressure environment with different vacuum degrees can be continuously provided for the soil sample in the sample copper cup 12, and the determination of the initial freezing temperature of frozen soil can be carried out, and the specific process is as follows:

[0067] Step 1: prepare the soil sample according to the method in the "Standard for Soil Test Methods", dry, crush and sieve;

[0068] Step 2: weigh the soil sample and put it into the sample copper cup 12, place the temperature sensor 11 in the soil sample in the sample copper cup 12, pass the wire of the temperature sensor 11 and the air suction pipe through the breather pipe 1205 of the copper cup cover 1206, place the air suction pipe inlet end on the surface of the soil sample, cover the copper cup cover 1206, install the aviation plug 10 on the breather pipe 1205, tighten the aviation plug 10, and pass the wire of the temperature sensor 11 and the air suction pipe out of the aviation plug 10;

[0069] The temperature sensor 11 is a resistance temperature sensor, and data is collected through the A / D converter 8 and the data collection device 9;

[0070] Step 3: Put the sample copper cup 12 into the organic glass tube filled with dry sand about 2 cm thick, continue to add dry sand to cover the top of the sample copper cup 12 by 2 to 3 cm, ensure that the copper cup is not in contact with the inner wall of the organic glass tube, cover the rubber plug, and the wiring of the temperature sensor 11 and the air suction pipe are pulled out from the rubber plug;

[0071] Step 4: Set the target vacuum degree, open the compressed gas tank 1 to make the jet vacuum device 3 work, and after about a few seconds, the vacuum degree reaches the set value, the sample copper cup 12 is kept at the set vacuum degree, at this time the loop is automatically disconnected, and the copper cup is kept at the set vacuum value;

[0072] Step 5: Put the organic glass tube into the low-temperature constant-temperature tank set in advance to negative temperature, connect the instrument according to the steps of the freezing temperature test and set the data collection time, and start the test. Read the data of the temperature sensor 11 through the data collection device 9, and when the temperature of the temperature sensor 11 reaches-15℃ or below, it can be determined that the soil is completely frozen, at which time the test is ended and the collected temperature data is read, which is the initial freezing temperature of frozen soil under negative pressure environment.

[0073] Step 5 can use resistance temperature measurement method to measure soil freezing temperature, and the device can refer to the structure of "Zhou Jiazu, Tan Long, Wei Changfu, et al. Experimental study on freezing temperature and supercooling temperature of soil [J]. Rock and Soil Mechanics, 2015, 36(03): 777-785. DOI: 10.16285 / j.rsm.2015.03.023".

[0074] Repeat the above steps to perform the next group of tests until the test is completed. After the test is completed, first close the compressed gas tank 1, then take out the organic glass tube, open the rubber plug, pour out the dry sand, unscrew the aviation plug 10 to take out the copper cup, remove the air suction pipe and the temperature sensor, and arrange the instrument to complete the test.

[0075] Specifically, step 4 includes:

[0076] Step 401: Set the target vacuum degree, open the valve of the compressed gas tank 1, and the compressed gas passes through the open electromagnetic control valve 2 at high speed, flows through the jet vacuum device 3 composed of two Laval tubes in series, forms negative pressure in the vacuum cavity 307 of the jet vacuum device 3, sucks the air in the sample copper cup 12 to create a negative pressure environment, and the compressed gas flowing through the Laval tube is discharged into the atmosphere after noise reduction by the silencer 4;

[0077] Step 402: When the vacuum degree in the sample copper cup 12 reaches the set value, the pressure sensor 6 outputs a control signal to the electromagnetic control valve 2, the current of the coil 206 is cut off, the magnetic force disappears, the electromagnetic valve compression spring 205 pushes the electromagnetic valve piston 204 to move left to close the gas path, at this time the gas flow in the jet type vacuum device 3 stops, the normal pressure state in the jet type vacuum device 3 is restored, at this time the pressure in the sample copper cup 12 is the set negative pressure value, the internal gas pressure is less than the external atmospheric pressure, but the external normal pressure gas cannot flow back into the sample copper cup 12 due to the action of the one-way valve 5, and the sample copper cup 12 still maintains the set vacuum degree;

[0078] Step 403: Freeze test continues, when the pressure sensor 6 senses that the pressure in the sample copper cup 12 is higher than the set value, the pressure sensor 6 inputs an electric signal to close the circuit, and the electromagnetic control valve 2 is started again to open, the coil 206 is energized to generate a magnetic force, the electromagnetic valve piston 204 is pushed to move right to open the gas path, and the jet type vacuum device 3 works again to reduce the vacuum degree in the sample copper cup 12 until the set value, so as to achieve the purpose that the sample copper cup 12 is always near the set vacuum degree, so as to perform the soil freezing temperature test under different negative pressure environments.

[0079] The air suction pipe of the application is provided with an expanded polytetrafluoroethylene film, which can effectively prevent the moisture and soil particles in the sample soil from being sucked out by the vacuum generating device and affecting the test results or damaging the device, the end of the suction pipe is folded and the outlet is aligned with the side wall of the sealed container or the copper cup cover, the possibility of sucking out soil particles by air flow during air suction is minimized, and the normal flow of gas is ensured.

[0080] The device of the application is a negative pressure generating device that can be integrated into a freezing temperature test, so that the whole device can generate different vacuum degrees in the container containing the test soil sample, simulate different atmospheric pressures, and measure the freezing temperature of soil under different vacuum degrees.

[0081] The above application of specific examples is used to illustrate the application, and is used to help understand the application, and does not limit the application. According to the idea of the application, those skilled in the art can make some simple deductions, deformations or substitutions.

Claims

1. A device for testing the initial freezing temperature of frozen soil under negative pressure environment, characterized in that: the device comprises a compressed gas tank (1), an electromagnetic control valve (2), a jet vacuum device (3), a pressure sensor (6), a temperature sensor (11) and a sample copper cup (12); the compressed gas tank (1) is connected to the jet vacuum device (3) through an airflow pipeline, the jet vacuum device (3) is connected to the sample copper cup (12) through an airflow pipeline, and the temperature sensor (11) is arranged in the soil sample in the sample copper cup (12); a pressure sensor (6) is arranged on the airflow pipeline between the jet vacuum device (3) and the sample copper cup (12), an electromagnetic control valve (2) is arranged on the airflow pipeline between the compressed gas tank (1) and the jet vacuum device (3), and the pressure sensor (6) and the electromagnetic control valve (2) are electrically connected.

2. The device for testing the initial freezing temperature of frozen soil under negative pressure environment according to claim 1, characterized in that: the jet vacuum device (3) comprises a vacuum device air inlet (301), a vacuum device air outlet (302), a first Laval tube (303), a second Laval tube (304), a first vacuum port (305), a second vacuum port (306), a vacuum cavity (307) and an expansion tube (308); the first Laval tube (303) and the second Laval tube (304) are arranged in series between the vacuum device air inlet (301) and the vacuum device air outlet (302), the first Laval tube (303) and the second Laval tube (304) are connected to the vacuum cavity (307) through the first vacuum port (305) and the second vacuum port (306) respectively, and the expansion tube (308) is arranged between the second Laval tube (304) and the vacuum device air outlet (302).

3. The device for testing the initial freezing temperature of frozen soil under negative pressure environment according to claim 2, characterized in that: the vacuum device air inlet (301) is connected to the compressed gas tank (1) through an airflow pipeline, the vacuum cavity (307) is connected to the sample copper cup (12) through an airflow pipeline, and the vacuum device air outlet (302) is connected to a muffler (4) through an airflow pipeline.

4. The device for testing the initial freezing temperature of frozen soil under negative pressure environment according to claim 3, characterized in that: the electromagnetic control valve (2) comprises an electromagnetic valve body, an S-pole magnet (207) and an N-pole magnet (208) are arranged in the electromagnetic valve body in transverse opposition, and a coil (206) is arranged between the S-pole magnet (207) and the N-pole magnet (208); one end of the coil (206) is connected to one end of an electromagnetic valve piston (204), the other end of the electromagnetic valve piston (204) is connected to one end of an electromagnetic valve compression spring (205), the other end of the electromagnetic valve compression spring (205) is connected to the inner wall of the electromagnetic valve body, and the coil (206), the electromagnetic valve piston (204) and the electromagnetic valve compression spring (205) are arranged longitudinally in the electromagnetic valve body. ​ ​ ​ ​ ​ ​ ​ ​ The electromagnetic valve piston (204) is provided with an electromagnetic valve air inlet (202) and an electromagnetic valve air outlet (203) on the corresponding electromagnetic valve body on the lateral two sides.

5. The device for testing the initial freezing temperature of frozen soil in a negative pressure environment according to claim 4, characterized in that: The electromagnetic valve air inlet (202) is connected to the compressed gas tank (1) through an air flow pipeline, and the electromagnetic valve air outlet (203) is connected to the jet vacuum device (3) through an air flow pipeline.

6. The device for testing the initial freezing temperature of frozen soil in a negative pressure environment according to claim 5, characterized in that: A one-way valve (5) is further arranged on the air flow pipeline between the jet vacuum device (3) and the sample copper cup (12); the one-way valve (5) comprises a one-way valve body, the two ends of the one-way valve body are a one-way valve air inlet (501) and a one-way valve air outlet (504), a cavity (505) is arranged between the one-way valve air inlet (501) and the one-way valve air outlet (504), the inner diameter of the cavity (505) is greater than the inner diameter of the one-way valve air inlet (501) and the one-way valve air outlet (504), and a one-way valve piston (503) is arranged in the cavity (505). The one-way valve piston (503) is cylindrical, the outer diameter of the one-way valve piston (503) is the same as the inner diameter of the cavity (505), a through hole (502) is arranged on the cylinder wall of the one-way valve piston (503), the inner diameter of the cavity (505) outside the through hole (502) is greater than the outer diameter of the one-way valve piston (503), the longitudinal length of the one-way valve piston (503) is less than the longitudinal length of the cavity (505), and a piston cone is arranged at the air inlet end of the one-way valve piston (503).

7. The device for testing the initial freezing temperature of frozen soil in a negative pressure environment according to claim 6, characterized in that: A pressure relief safety valve (7) is further arranged on the air flow pipeline between the compressed gas tank (1) and the electromagnetic control valve (2); the pressure relief safety valve (7) comprises a valve body (705), one end of the valve body (705) is provided with a valve cap (706), and an air inlet hole (701) is arranged on the valve cap (706). A safety valve piston (702) is arranged in the valve body (705); the outer diameter of one end of the safety valve piston (702) is the same as the inner diameter of the inner cavity of the valve body (705) and is close to the air inlet hole (701), the outer diameter of the other end of the safety valve piston (702) is less than the inner diameter of the inner cavity of the valve body (705) and is connected to one end of a safety valve compression spring (704), the other end of the safety valve compression spring (704) is connected to the inner wall of the valve body (705), and exhaust holes (703) are arranged on the lateral two sides of the valve body (705) at the other end of the safety valve compression spring (704).

8. The device for testing the initial freezing temperature of frozen soil in a negative pressure environment according to claim 7, characterized in that: The sample copper cup (12) comprises a cup body (1201), the top of the cup body (1201) is capped with a copper cup cover (1206) in a threaded manner, the copper cup cover (1206) is provided with a breather pipe (1205), an air suction pipe is inserted into the breather pipe (1205), the breather pipe (1205) has an external thread (1204) and is connected with an air flow pipeline through an aviation plug (10).

9. The method for testing the initial freezing temperature of frozen soil under negative pressure environment by using the device of claim 8, comprising the following steps: The method comprises: Preparation of soil sample, drying, crushing and sieving; The soil sample is weighed and placed in the sample copper cup (12), the temperature sensor (11) is placed in the soil sample in the sample copper cup (12), the wire of the temperature sensor (11) is inserted into the breather pipe (1205) of the copper cup cover (1206) together with the air suction pipe, the air suction pipe is placed on the surface of the soil sample, the copper cup cover (1206) is capped, the aviation plug (10) is installed on the breather pipe (1205), the aviation plug (10) is tightened, and the wire of the temperature sensor (11) and the air suction pipe are pulled out from the aviation plug (10); The temperature sensor (11) is a resistance temperature sensor, and data is collected through an A / D converter (8) connected to a data acquisition device (9); The sample copper cup (12) is placed in a plexiglass tube filled with dry sand, and the top of the sample copper cup (12) is covered with dry sand, a rubber plug is placed on the top, and the wire of the temperature sensor (11) and the air suction pipe are pulled out from the rubber plug; A target vacuum degree is set, the compressed gas tank (1) is opened to make the jet-type vacuum device (3) work, the vacuum degree reaches the set value, and the sample copper cup (12) is kept at the set vacuum degree; The plexiglass tube is placed in a low-temperature constant-temperature bath set to a negative temperature in advance, the data of the temperature sensor (11) is read through the data acquisition device (9), when the temperature of the temperature sensor (11) reaches-15℃ or below, it is determined that the soil is completely frozen, at this time the test is ended and the collected temperature data is read, which is the initial freezing temperature of frozen soil under negative pressure environment.

10. The method for testing the initial freezing temperature of frozen soil under negative pressure environment according to claim 9, comprising the following steps: Setting a target vacuum degree, opening the compressed gas tank (1) to make the jet-type vacuum device (3) work, the vacuum degree reaches the set value, and the sample copper cup (12) is kept at the set vacuum degree, including: Setting a target vacuum degree, opening the valve of the compressed gas tank (1), the compressed gas passes through the open electromagnetic control valve (2) at high speed, flows through the jet-type vacuum device (3) composed of two Laval tubes in series, forms negative pressure in the vacuum cavity (307) of the jet-type vacuum device (3), sucks the air in the sample copper cup (12) to create a negative pressure environment, and the compressed gas flowing through the Laval tube is discharged into the atmosphere after noise reduction by the silencer (4); When the vacuum degree in the sample copper cup (12) reaches the set value, the pressure sensor (6) outputs a control signal to the electromagnetic control valve (2), the current of the coil (206) is cut off, the magnetic force disappears, the electromagnetic valve compression spring (205) pushes the electromagnetic valve piston (204) to move left to close the gas path, at this time the gas flow in the jet vacuum device (3) stops, the jet vacuum device (3) returns to normal pressure, at this time the pressure in the sample copper cup (12) is the set negative pressure value, the internal gas pressure is less than the external atmospheric pressure, but the external normal pressure gas cannot flow back into the sample copper cup (12) due to the action of the one-way valve (5), the sample copper cup (12) still maintains the set vacuum degree; The freezing test continues, when the pressure sensor (6) senses that the pressure in the sample copper cup (12) is higher than the set value, the pressure sensor (6) inputs an electric signal to close the circuit, the electromagnetic control valve (2) is started again, the coil (206) is energized to generate a magnetic force, which pushes the electromagnetic valve piston (204) to move right to open the gas path, the jet vacuum device (3) works again to reduce the vacuum degree in the sample copper cup (12) until the set value, and the cycle is repeated to keep the sample copper cup (12) around the set vacuum degree, so as to conduct the soil freezing temperature test under different negative pressure environments.

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