CN111122619A - Water content measuring system based on parallel spiral type telescopic time domain reflection probe - Google Patents
Water content measuring system based on parallel spiral type telescopic time domain reflection probe Download PDFInfo
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Abstract
The invention discloses a water content measuring system based on a parallel spiral type telescopic time domain reflection probe, which comprises a time domain reflection probe, an insulation converter, a flexible cylinder and a time domain reflection signal generator, wherein the time domain reflection probe comprises an anode probe and at least one cathode probe, the anode probe and the cathode probe are spirally arranged on the inner side wall of the flexible cylinder in parallel, the arranged time domain reflection probe at least occupies 90% of the height of the flexible cylinder, and the anode probe and the cathode probe penetrate through the insulation converter and are connected with the time domain reflection signal generator through coaxial cables. The scheme can be used for nondestructively measuring the moisture content of samples such as frozen soil, soil containing natural gas hydrate, sandy soil, silt, clay, loess, special soil and rock in a triaxial shear test in real time, so that the measurement effect of the moisture content of the samples is ensured, the influence on triaxial shear test data of the samples is avoided, and the method has a wide application prospect in the aspect of the unsaturated soil and soil sample containing natural gas hydrate triaxial shear test.
Description
Technical Field
The invention belongs to the field of moisture content testing of geotechnical samples in the geotechnical engineering industry, agriculture and unconventional oil and gas industry, and particularly relates to a moisture content measuring system based on a parallel spiral type telescopic time domain reflection probe.
Background
The water content of the rock soil sample is a key parameter influencing the mechanical property and the seepage property of the rock soil sample, is also an important index for evaluating the water demand condition of soil plants, and has important significance for accurately and rapidly measuring the water demand condition of the soil plants. The time domain reflection technology is a method for measuring the internal water content of various soil bodies by using the propagation velocity difference of electromagnetic waves in different media, and has wide application in the geotechnical engineering industry, agriculture and unconventional oil and gas industry in the last four decades. At present, rigid time domain reflection probes such as parallel two-pin type probes, parallel three-pin type probes, parallel multi-pin type probes, parallel flat plate type probes and the like are common in the market, and the rigid time domain reflection probes have a good application effect when being applied to field soil body water content measurement.
The triaxial shear test is an effective means for determining the shear strength index of a sample in a laboratory, and is widely applied to the geotechnical engineering industry and the unconventional oil and gas industry. However, when triaxial shear tests are performed on unsaturated soil samples and soil samples containing natural gas hydrate, the water content of the samples is usually determined by a method of presetting before the test or retesting after the test, or the water content of the samples is determined according to displacement, so that the real-time performance is poor and nondestructive measurement is difficult. In recent years, the Canada university of Cargary applies a parallel rigid time domain reflection probe to the measurement of the water content of an unsaturated soil triaxial shear test sample, but the length of the probe is only about one third of the height of the sample, and although the water content of the sample can be measured in real time without damage, the water content in the remaining two thirds of the height range of the sample cannot be reflected.
In order to overcome the difficulties, the invention patents with the publication number of CN 104215499B and the publication number of CN 103196490B provide flexible probe design schemes, the flexible parallel two-pin time domain reflection probes are adopted to replace the traditional rigid time domain reflection probes, the length of the probes reaches more than 90% of the height of the sample, the water content of the whole sample can be well reflected by measuring the water content, and the influence on the stress-strain curve of the sample in the triaxial shear test is avoided. However, the above-mentioned flexible parallel two-pin time domain reflectometer probe puts high demands on sample preparation, and it is necessary to ensure that the spatial distribution position of the probe used in the triaxial shear test is consistent with the spatial distribution position of the probe used in the dielectric constant-water content correlation empirical calibration test, for example, it is necessary to ensure that the two probes are vertically consistent and parallel and the two probes are located in the same plane, which is usually difficult to achieve in recent practical application processes. Because the flexible probes (soft copper wires) are vertically arranged inside the sample, the flexible probes are straightened while sand is filled, the fact that it is difficult to guarantee that the distances between the probes inside the flexible probes are equal after the sand is filled is caused, the effect on the time domain reflection method measurement effect is greatly influenced is caused because an empirical relation is required to be calibrated in advance when the time domain reflection probes are used for measurement, the empirical relation is related to the probes, if the space distribution of the probes used in the experiment is inconsistent with the calibration, the effect of the empirical relation is also greatly influenced, and further obvious uncertainty deviation exists in the moisture content measurement result. In addition, the flexible parallel two-needle time domain reflection probe can be continuously curled in the shearing process, and the measuring effect of the water content of the sample is also influenced.
In summary, the time domain reflection probe for measuring the moisture content applicable to the existing triaxial shear test has the following defects: (1) the rigid parallel short probe can not measure the integral water content of the sample; (2) the spatial distribution of the flexible parallel probes is difficult to accurately control during sample preparation; and because the flexible probe is in the sample, after the sample is compressed, the flexible probe can also be compressed, and the flexible parallel probe can be continuously curled due to the three-axis shearing. Due to the defects, the probe is difficult to be suitable for a triaxial shear test in advance due to the fact that the empirical relation is calibrated in advance, the water content of a sample is difficult to accurately measure in the shearing process, the water content measuring result is not representative, and the like, and the application effect of the time domain reflection technology in the triaxial shear test is seriously influenced.
Therefore, a set of time domain reflection probes capable of overcoming the defects are developed, the method has important significance for developing a real-time accurate measurement technology for the moisture content of unsaturated soil and soil triaxial shear test samples containing natural gas hydrates, and meanwhile, reliable moisture content change data can be provided for determination of the synthetic amount and the decomposition amount of the natural gas hydrates in the samples.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a water content measuring system based on a parallel spiral type telescopic time domain reflection probe, and the real-time nondestructive measurement of the water content of a sample in a triaxial shear test of unsaturated soil and soil containing natural gas hydrate is realized by combining the special design of the parallel spiral type telescopic time domain reflection probe.
The invention is realized by adopting the following technical scheme: a water content measuring system based on a parallel spiral type telescopic time domain reflection probe comprises a time domain reflection probe, an insulation converter, a flexible cylinder and a time domain reflection signal generation collector, wherein the insulation converter is connected with the flexible cylinder in a sealing manner, and the flexible cylinder is used for coating a sample to be measured;
the time domain reflection probe comprises an anode probe and at least one cathode probe, the anode probe and the cathode probe are made of conductive and soft metal, the anode probe and the cathode probe are spirally arranged on the inner side wall of the flexible cylinder at equal intervals in parallel, and the anode probe and the cathode probe penetrate through the insulation converter and are connected with the time domain reflection signal generator through coaxial cables.
Furthermore, the time domain reflection probe comprises a positive probe and two negative probes, the three probes are spirally arranged on the inner wall of the flexible cylinder in parallel, the probe positioned in the middle is marked as a central probe, and the probes positioned on two sides of the central probe are marked as side probes.
Furthermore, in order to ensure the measurement effect of the water content of the sample, the corresponding central angles of the side probe and the central probe on the circumference are equal, and the central angle theta is not more than 20 times of the ratio of the diameter of the probe to the diameter of the sample, wherein the diameter of the probe refers to the diameter of a single probe in the time domain reflection probe, and the diameters of the positive probe and the negative probe are the same.
Further, the central angle θ is determined by:
wherein D is0And H0Respectively the diameter and height of the sample, NcIs the effective spiral turn number of the time domain reflection probe, and G is the rigidity modulus of the probe wire.
Further, from the determined central angle θ, the diameter d of the individual positive or negative probe can be determined:
further, the material used for the time domain reflection probe includes, but is not limited to, copper, aluminum and their derivative alloys.
Further, the number of spiral turns of the time domain reflection probe on the inner side wall of the flexible cylinder is more than 0 turn.
Furthermore, the time domain reflection probe is directly adhered and fixed on the inner side wall of the flexible cylinder, or a spiral groove is formed in the inner side wall of the flexible cylinder, and the time domain reflection probe is fixed in the groove.
Further, the arranged time domain reflection probe at least occupies 90% of the height of the flexible cylinder.
Further, the flexible tube is made of heat-shrinkable tubes or rubber films.
Compared with the prior art, the invention has the advantages and positive effects that:
1. the time domain reflection probe provided by the scheme is fixed on the inner wall of the flexible cylinder, so that the difficult problem of probe space distribution control during sample preparation is effectively avoided, the probe space distribution used in a pre-calibration test and a triaxial shear test has good consistency, and the use effect of a dielectric constant and water content calibration relational expression is ensured;
2. the time domain reflection probe adopts a spiral parallel structure, is similar to a spring to compress and deform under the influence of triaxial shearing during test, is not curled as a whole, only has the height reduced, and has the influence on time domain reflection measurement obviously reduced, thereby effectively avoiding continuous curling of the probe caused by triaxial shearing sample compression, ensuring that more than half of the side surface of the probe is contacted with the sample, and ensuring the sensitivity of water content measurement;
3. the height of the spiral probe reaches more than 90% of the height of the sample, the water content measurement range has good representativeness, the spiral probe is easy to axially compress, and the influence of the traditional rigid probe on a triaxial shear stress-strain curve of the sample is effectively avoided.
By carrying out targeted design aiming at the defects of the common time domain reflection probe, the time domain reflection probe is easy to calibrate and has small influence on the stress-strain result of the triaxial shear test, thereby ensuring the measurement effect of the water content of the sample, avoiding the influence on the triaxial shear test data of the sample, and having wide application prospect in the aspect of the triaxial shear test of unsaturated soil and soil samples containing natural gas hydrate; the method can be used for nondestructively measuring the water content of samples such as frozen soil, natural gas hydrate-containing soil, sandy soil, silt soil, clay, loess, special soil, rock and the like in a triaxial shear test in real time.
Drawings
Fig. 1 is a schematic structural diagram of a parallel spiral-type scalable time domain reflectometry probe according to an embodiment of the present invention, in which fig. 1(1) is a schematic overall diagram, and fig. 1(2) is a schematic top view of each point of a cross section at different heights in fig. 1(1) to illustrate a spatial spiral structure thereof;
FIG. 2 is a schematic diagram of a parallel spiral-type telescopic time domain reflectometry probe according to an embodiment of the present invention applied to a triaxial shear test;
wherein, a negative electrode probe; b. a positive electrode probe; c. a negative probe; d. a rubber flexible tube; e. a coaxial cable; f. an insulating adapter; g. a sample; h. a time domain reflection signal generation collector; i. an inner cavity of the reaction kettle for the triaxial shear test; k. sample cap and shear piston.
Detailed Description
In order that the above objects and advantages of the present invention may be more clearly understood, particular embodiments, structures, features and functions of the present invention are described in detail below with reference to the accompanying drawings:
the embodiment provides a moisture content measurement system based on parallel spiral telescopic time domain reflection probe to realize the real-time nondestructive measurement of sample moisture content in unsaturated soil and the triaxial shear test of soil containing natural gas hydrate, moisture content measurement data can be further used for confirming the hydrate saturation of the soil sample containing natural gas hydrate.
Specifically, the water content measuring system based on the parallel spiral telescopic time domain reflection probe comprises a time domain reflection probe, an insulation converter, a flexible cylinder and a time domain reflection signal generation collector, wherein the insulation converter is hermetically connected with the flexible cylinder, the flexible cylinder is used for coating a sample to be measured (namely, an unsaturated soil sample or a soil sample containing natural gas hydrate is arranged in the flexible cylinder), the flexible cylinder is made of materials such as a rubber film or a heat-shrinkable tube, the time domain reflection probe comprises an anode probe and at least one cathode probe, the anode probe and the cathode probe are spirally arranged on the inner side wall of the flexible cylinder in parallel (namely, the time domain reflection probe is positioned on an interface between the sample and the flexible cylinder), and the arranged time domain reflection probe at least accounts for 90% of the height of the flexible cylinder; the positive probe and the negative probe penetrate through the insulation converter, are connected with the time domain reflection signal generator through the coaxial cable, and are matched with the flexible cylinder and the time domain reflection signal generation collector to complete the water content measurement.
In this embodiment, a three-pin spiral probe is taken as an example for description, the flexible tube is preferably made of a rubber film, as shown in fig. 1-2, the three-pin spiral probe includes a positive probe b and two negative probes a and c, the three probes are spirally fixed on the inner wall of the flexible rubber film at equal intervals in parallel, the probes a, b and c are directly adhered and fixed on the inner wall of the rubber film d, or a spiral groove is formed on the inner wall of the rubber film, the probes are fixed in the groove, the center probe and the side probe (the center and the side are only relative to the positions of the three probes, and are not specifically limited, for example, the center probe in this embodiment is the center probe, and a and c are the side probes) are respectively corresponding to and woven and welded with the copper core conductor and the copper wire of the coaxial cable e through an insulating converter f, so as to ensure good insulation between the two, the other end of the coaxial cable e is connected with a time domain reflection signal generation collector h, during a specific test, the flexible cylinder and the insulation converter are both arranged in an inner cavity i of the triaxial shear test reaction kettle, and the top of the flexible cylinder is provided with a sample top cover and a shear piston k.
The design form of the probe effectively avoids the difficult problem of controlling the spatial distribution of the probes a, b and c during sample g preparation, so that the spatial distribution of the probes a, b and c used in the pre-calibration test and the triaxial shear test has good consistency, and the use effect of the dielectric constant and moisture content calibration relational expression is ensured. And the probes a, b and c adopt a spiral parallel structure, so that the continuous curling of the probes a, b and c caused by the compression of a triaxial shearing sample g is effectively avoided, the probes are positioned in the grooves on the interface of the sample g and the rubber film d, at least more than half of the side surfaces of the probes are contacted with the sample, and the representativeness of the water content measurement result is ensured. The height of the spiral probe reaches more than 90% of the height of the sample g, the water content measurement range has good representativeness, and the spiral probe is easy to compress, so that the influence of the spiral probe on a triaxial shear stress strain curve of the sample g is effectively avoided.
The time domain reflection probe is made of metal with good conductivity and soft texture, such as but not limited to copper, aluminum and derivative alloys thereof, in order to ensure the measurement effect of the water content of the sample, the central angles of the two side probes and the central probe on the circumference are equal, as shown in FIG. 1, the central angles ∠ aOb and ∠ bOc of the two side probes a and c and the central probe b on the circumference are consistent (the central angle is marked as theta hereinafter), and the central angle theta is not more than 20 times of the ratio of the diameter of the probe (which refers to a single probe herein) to the diameter of the sample.
The central angle θ is determined by:
wherein D is0And H0Respectively the diameter and height of the sample, NcThe effective spiral turns of the spiral time domain reflection probe, and G is the rigidity modulus of the probe wire; therefore, after the probe wire and the effective number of turns are determined, the center of a probe circle in the middle of the spiral probe can be determined according to the effective number of turnsThe diameter of the probe can be determined according to the following formula:
therefore, the diameter of the probe can be determined according to the factors such as the rigidity, the thread pitch and the effective turn number of the wire rod, the stiffness coefficient of the spiral probe is ensured to be small enough, namely the axial supporting force of the spiral probe is less than 0.01kPa when the sample is compressed by 15% strain, because the shearing strength of the unsaturated soil sample is usually in the magnitude of dozens and hundreds of kPa, the shearing strength of the soil sample containing hydrate can reach several MPa magnitudes, and the shearing strength error caused by the spiral probe is within 0.1% and can be ignored; the probe height is determined from the sample height, at least over 90% of the sample height.
The insulating converter f is made of high-strength materials with good insulating property, and can be made of metal or nonmetal, can bear the high voltage of 20MPa while ensuring the insulating property between probes, and has good working performance under the condition of low temperature close to zero. In addition, the connection part of the insulating converter and the flexible rubber film is hermetically connected through a high-pressure sealing structure, the diameter of the insulating converter is determined by the diameter of the sample, the diameter of the insulating converter is larger than the diameter of the sample and the overall size of the high-pressure sealing structure, and the thickness of the insulating converter is determined according to the material withstand voltage parameter and the withstand voltage upper limit.
It should be noted that, in this embodiment, the coaxial cable is a common cable in the market, and has a copper core conductor and a copper braided structure, the copper core conductor is located at the center of the cable, and the copper braided structure is surrounded by the copper conductor, so that good insulation and shielding effects are provided between the copper core conductor and the copper braided structure, the whole is wrapped by an insulating sheath, and the length of the insulating sheath is determined according to actual test requirements. The flexible rubber film and the time domain reflection signal generation collector are matched with the parallel spiral compressible time domain reflection probe for use, and are market conventional instruments and accessories.
The structure and the material of the probe in the scheme comprehensively consider the requirements of low-temperature and high-pressure test conditions, and the probe can be used for not only a triaxial shear test of an unsaturated soil sample, but also a triaxial shear test of soil containing natural gas hydrate and a frozen soil sample; the spiral structure probe is arranged around the sample, and the height of the spiral structure probe reaches more than 90% of the height of the sample, so that the overall optimization of the moisture content measurement of the sample and the acquisition of stress-strain data of a triaxial shear test is realized; the probe has the advantages of ingenious structural design, low production cost and easy practical application and popularization of engineering.
In addition, it should be noted that the time domain reflection probe may adopt a two-pin time domain reflection probe in addition to a three-pin time domain reflection probe, that is, one positive probe is connected to a copper core conductor of a coaxial cable, and the other negative probe is connected to a copper braid of the coaxial cable, and the requirements of the type of the probe wire, the spiral pitch or the central angle, and the diameter and the height of the probe are the same as those of the three-pin time domain reflection probe. In addition, the total number of turns of the three-pin and two-pin spiral probes is not limited to 1 turn in fig. 1, and any spiral form or spiral probe having a total number of turns greater than 1 belongs to a variant form of the probe.
The above description is only a preferred embodiment of the present invention, and not intended to limit the present invention in other forms, and any person skilled in the art may apply the above modifications or changes to the equivalent embodiments with equivalent changes, without departing from the technical spirit of the present invention, and any simple modification, equivalent change and change made to the above embodiments according to the technical spirit of the present invention still belong to the protection scope of the technical spirit of the present invention.
Claims (10)
1. The water content measuring system based on the parallel spiral type telescopic time domain reflection probe is characterized by comprising a time domain reflection probe, an insulation converter, a flexible cylinder and a time domain reflection signal generation collector, wherein the insulation converter is hermetically connected with the flexible cylinder, and the flexible cylinder is used for coating a sample to be measured;
the time domain reflection probe comprises an anode probe and at least one cathode probe, the anode probe and the cathode probe are made of conductive and soft metal, the anode probe and the cathode probe are spirally arranged on the inner side wall of the flexible cylinder at equal intervals in parallel, and the anode probe and the cathode probe penetrate through the insulation converter and are connected with the time domain reflection signal generator through coaxial cables.
2. The water cut measuring system based on the parallel spiral type telescopic time domain reflection probe as claimed in claim 1, wherein: the time domain reflection probe comprises an anode probe and two cathode probes, the three probes are spirally arranged on the inner wall of the flexible cylinder in parallel, the probe positioned in the middle is marked as a central probe, and the probes positioned on two sides of the central probe are marked as side probes.
3. The water cut measuring system based on the parallel spiral type telescopic time domain reflection probe as claimed in claim 2, wherein: the corresponding central angles of the side probe and the central probe on the circumference are equal, the central angle theta is not more than 20 times of the ratio of the probe diameter to the sample diameter, and the probe diameter refers to the diameter of a single probe in the time domain reflection probe.
4. The water cut measuring system based on the parallel spiral type telescopic time domain reflection probe as claimed in claim 3, wherein: the central angle θ is determined by:
wherein D is0And H0Respectively the diameter and height of the sample, NcIs the effective spiral turn number of the time domain reflection probe, and G is the rigidity modulus of the probe wire.
5. The water cut measuring system based on the parallel spiral type telescopic time domain reflection probe as claimed in claim 4, wherein: from the determined central angle θ, the diameter d of an individual one of the time domain reflectometry probes can then be determined:
6. the water cut measuring system based on the parallel spiral type telescopic time domain reflection probe as claimed in claim 1, wherein: the materials used for the time domain reflection probe include, but are not limited to, copper, aluminum and their derived alloys.
7. The water cut measuring system based on the parallel spiral type telescopic time domain reflection probe as claimed in claim 1, wherein: the spiral turns of the time domain reflection probe on the inner side wall of the flexible cylinder are more than 0 turn.
8. The water cut measuring system based on the parallel spiral type telescopic time domain reflection probe as claimed in claim 1, wherein: the time domain reflection probe is directly adhered and fixed on the inner side wall of the flexible cylinder, or a spiral groove is formed in the inner side wall of the flexible cylinder, and the time domain reflection probe is fixed in the groove.
9. The water cut measuring system based on the parallel spiral type telescopic time domain reflection probe as claimed in claim 1, wherein: the distributed time domain reflection probes at least account for 90% of the height of the flexible cylinder.
10. The water cut measuring system based on the parallel spiral type telescopic time domain reflection probe as claimed in claim 1, wherein: the flexible tube is made of heat-shrinkable tubes or rubber films.
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