CN113447413A - Unsaturated loess osmotic coefficient curve testing arrangement of steerable stress condition - Google Patents
Unsaturated loess osmotic coefficient curve testing arrangement of steerable stress condition Download PDFInfo
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- 238000012360 testing method Methods 0.000 title claims abstract description 138
- 230000003204 osmotic effect Effects 0.000 title claims description 3
- 239000002689 soil Substances 0.000 claims abstract description 114
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims abstract description 95
- 239000000523 sample Substances 0.000 claims abstract description 69
- 239000002184 metal Substances 0.000 claims abstract description 64
- NIXOWILDQLNWCW-UHFFFAOYSA-N acrylic acid group Chemical group C(C=C)(=O)O NIXOWILDQLNWCW-UHFFFAOYSA-N 0.000 claims abstract description 42
- 230000035699 permeability Effects 0.000 claims abstract description 38
- 239000004033 plastic Substances 0.000 claims abstract description 18
- 238000006073 displacement reaction Methods 0.000 claims abstract description 17
- 239000004575 stone Substances 0.000 claims abstract description 9
- 229920001971 elastomer Polymers 0.000 claims description 8
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- 230000008595 infiltration Effects 0.000 abstract description 22
- 238000001764 infiltration Methods 0.000 abstract description 22
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N15/00—Investigating characteristics of particles; Investigating permeability, pore-volume or surface-area of porous materials
- G01N15/08—Investigating permeability, pore-volume, or surface area of porous materials
- G01N15/082—Investigating permeability by forcing a fluid through a sample
- G01N15/0826—Investigating permeability by forcing a fluid through a sample and measuring fluid flow rate, i.e. permeation rate or pressure change
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- G—PHYSICS
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Abstract
The invention relates to a unsaturated loess permeability coefficient curve testing device capable of controlling a stress state, which comprises a computer, a loading frame, a loading rod, an electronic displacement meter, a sleeve, an instrument panel bracket, a water valve, a porous metal loading plate, an acrylic hollow barrel, a testing earth column, a three-pin type metal probe, a tensiometer, a base, a voltage signal collector, a Ma bottle, a stabilized voltage power supply, a plastic barrel and an electronic scale, wherein the loading rod is arranged on the loading frame; the testing soil column is arranged in the acrylic hollow barrel, permeable stones are arranged at the upper end and the lower end of the testing soil column, a porous metal loading plate is arranged above the permeable stones at the upper end of the testing soil column, and a cavity is reserved between the porous metal loading plate and the top of the acrylic hollow barrel; the invention has complete functions, high test precision, convenient operation and quick response, and has important value for evaluating the infiltration and the collapsibility of unsaturated loess and solving the engineering construction problem caused by rainwater infiltration in loess areas.
Description
Technical Field
The invention belongs to the field of unsaturated soil hydraulics characteristic tests in the field of geotechnical engineering, and particularly relates to a unsaturated loess permeability coefficient curve testing device capable of controlling a stress state.
Background
Loess is typically unsaturated soil, and compacted or rammed loess may be used as an engineering filler for damming, embankments, soil-retaining structures, and slopes in actual engineering construction. However, the collapsibility of loess is a troublesome problem for project builders, and a large number of engineering practices indicate that rainwater infiltration is one of important factors causing the occurrence of project disasters in loess areas after rainy seasons or heavy rains. In order to solve the problem of water-force coupling seepage of the geotechnical structures and unsaturated slopes built on loess, accurate test and evaluation of the hydraulic characteristics of the geotechnical structures and unsaturated slopes are of great importance. The permeability coefficient of unsaturated loess as a core parameter of unsaturated water-force coupling analysis has important theoretical significance for engineering landslide research caused by loess collapsibility and rainwater infiltration.
The stress state directly affects the pore structure of the unsaturated soil body, and further the permeability characteristic of the soil body is changed. In particular, in the case of special soils having collapsibility such as loess, the pore structure is more significantly affected by the stress state. The existing methods for testing the permeability coefficient curve of the unsaturated soil body mainly comprise steady-state and transient methods. The technology for testing the permeability coefficient curve of unsaturated soil in a steady state has the defect of time consumption, for example, a constant head test based on Darcy's law of unsaturated soil is more suitable for unsaturated soil with larger permeability coefficient. For soil with low permeability coefficient, the amount of water flowing out is small, so that the test period is long, and the requirement on the measurement accuracy of the volume of the inflow water and the outflow water is high. In view of the above-mentioned drawbacks of the steady-state method, some researchers have proposed a transient test method and related devices for the permeability coefficient curve of unsaturated soil. Although the existing transient method greatly saves the testing time, the control of the stress state of the soil body in the infiltration process and the measurement of deformation are not considered, and the method is not suitable for the special soil with collapsibility, such as loess. Therefore, how to test the hydraulic characteristics of unsaturated loess quickly and flexibly in a stress state is still a problem.
Disclosure of Invention
In order to solve the problem that the existing permeable soil column device cannot control the stress state, the invention provides the unsaturated loess permeability coefficient curve testing device capable of controlling the stress state.
The invention adopts the following technical scheme:
a unsaturated loess permeability coefficient curve testing device capable of controlling a stress state comprises a computer, a loading frame, a loading rod, an electronic displacement meter, a sleeve, an instrument panel bracket, a water valve, a porous metal loading plate, an acrylic hollow barrel, a testing earth column, a three-pin metal probe, a tensiometer, a base, a voltage signal collector, a Ma's bottle, a stabilized voltage power supply, a plastic barrel and an electronic scale;
the testing soil column is arranged in the acrylic hollow barrel, permeable stones are arranged at the upper end and the lower end of the testing soil column, a porous metal loading plate is arranged above the permeable stones at the upper end of the testing soil column, and a cavity is reserved between the porous metal loading plate and the top of the acrylic hollow barrel;
a water valve is arranged on the side wall of the upper part of the acrylic hollow barrel and is connected with the March bottle;
the electronic scale is positioned on one side of the base, the plastic barrel is placed on the electronic scale, and the lower end of the acrylic hollow barrel is communicated with an inlet at the top of the plastic barrel;
the base is symmetrically provided with studs, and an instrument panel bracket and a loading frame are sequentially arranged above the acrylic hollow barrel; the loading frame and the instrument panel bracket can move up and down along the stud, and are fixed on the stud through bolts; the loading rod is positioned between the loading frame and the instrument panel bracket; the sleeve is arranged on the instrument panel support, the loading frame is provided with a pressure cylinder, a piston of the pressure cylinder downwards penetrates through the loading frame and vertically acts on the loading rod, and the lower end of the loading rod penetrates through the sleeve and vertically acts on the porous metal loading plate; the electronic displacement meter records the vertical displacement of the loading rod when the loading rod is stressed to move; a plurality of three-needle metal probes are arranged on one side of the test soil column, a plurality of tensiometers are arranged in the test soil column and correspond to the three-needle metal probes one by one; the three-pin type metal probes are electrically connected with a voltage-stabilized power supply; the tension meter is connected with a computer electric signal through a voltage signal collector.
The water valve is connected with the Mariotte bottle through a rubber tube.
The lower end of the acrylic hollow barrel is communicated with the plastic barrel through an L-shaped water pipe.
The loading rod is T-shaped, the diameter of the upper end of the loading rod is larger than that of the lower end of the loading rod, and the diameter of the lower end of the loading rod is matched with the sleeve.
The three-pin type metal probes are electrically connected with a stabilized voltage power supply through a time domain reflectometer and a coaxial cable multiplexer, the three-pin type metal probes are connected with the coaxial cable multiplexer through corresponding coaxial cables, the coaxial cable multiplexer is connected with the time domain reflectometer, the time domain reflectometer is electrically connected with the stabilized voltage power supply, and the time domain reflectometer is connected with a computer through an electric wire.
And air valves are arranged on the side and the upper part of the pressure cylinder.
And lubricating oil is coated on the contact surface of the sleeve and the loading rod.
The porous metal loading plate is a metal cylinder which is fully distributed with round holes, and the diameter of the porous metal loading plate is the same as the inner diameter of the acrylic hollow barrel.
The inner diameter of the March's bottle is 20cm, the height of the March's bottle is 50cm, the March's bottle is positioned on one side of the test soil column, and the bottom of the March's bottle is higher than the top of the test soil column.
The test soil column is a cylindrical soil sample with the diameter of 14-15 cm and the height of 80-90 cm.
The invention has the beneficial effects that:
the invention has simple structure and can quickly test the unsaturated loess permeability coefficient curve related to the stress state. The invention can accurately test the water content of unsaturated loess and the suction change of the matrix in the process of water infiltration; the method can be used for quickly testing the permeability coefficient curve related to the unsaturated loess stress state based on a transient section method.
The following will be further described with reference to the accompanying drawings.
Drawings
FIG. 1 is a schematic view of the structure of the present invention.
Fig. 2 is a schematic sectional view taken along line a-a in fig. 1.
Fig. 3 is a three-pin type metal probe test waveform diagram of typical electromagnetic wave reflection of unsaturated loess.
Fig. 4 is a schematic diagram of calculating the permeability coefficient of unsaturated loess by an instantaneous profile method.
Fig. 5 is a schematic cross-sectional view of B-B in fig. 1.
In the figures, the reference numbers are: 1. a computer; 2. an electric wire; 3. an air valve; 4. a pressure cylinder; 5. An O-ring; 6. a loading frame; 7. a bolt; 8. a loading rod; 9. an electronic displacement meter; 10. a sleeve; 11. an instrument panel support; 12. a water valve; 13. a rubber tube; 14. a porous metal load plate; 15. an acrylic hollow barrel; 16. testing the soil column; 17. a three-pin metal probe; 18. a tensiometer; 19. a permeable stone; 20. a base; 21. a voltage signal collector; 22. a March bottle; 23. A regulated power supply; 24. a time domain reflectometer; 25. a coaxial cable multiplexer; 26. a coaxial cable; 27. a plastic barrel; 28. an electronic scale.
Detailed Description
In order to make the implementation objects, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all embodiments of the present invention. The components of the embodiments of the present invention generally described and illustrated in the figures herein may be arranged and designed in a wide variety of different configurations. Thus, the following detailed description of the embodiments of the present invention, presented in the figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of selected embodiments of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Example 1:
in order to overcome the problem that the existing permeable soil column device cannot control the stress state, the invention provides the unsaturated loess permeability coefficient curve testing device capable of controlling the stress state as shown in the figures 1-5.
A unsaturated loess permeability coefficient curve testing device capable of controlling a stress state comprises a computer 1, a loading frame 6, a loading rod 8, an electronic displacement meter 9, a sleeve 10, an instrument panel bracket 11, a water valve 12, a porous metal loading plate 14, an acrylic hollow barrel 15, a testing earth column 16, a three-pin metal probe 17, a tensiometer 18, a base 20, a voltage signal collector 21, a Ma's bottle 22, a voltage-stabilized power supply 23, a plastic barrel 27 and an electronic scale 28;
the device comprises a base 20, a test soil column 16, porous metal loading plates 14, a base 20, a base, a test soil column 16, a testing device and a testing device, wherein the acrylic hollow barrel 15 is arranged on the base 20, the test soil column 16 is arranged in the acrylic hollow barrel 15, the upper end and the lower end of the test soil column 16 are respectively provided with the porous stones 19, the porous metal loading plates 14 are arranged above the porous stones 19 at the upper end of the test soil column 16, and a cavity is reserved between the porous metal loading plates 14 and the top of the acrylic hollow barrel 15;
a water valve 12 is arranged on the side wall of the upper part of the acrylic hollow barrel 15, and the water valve 12 is connected with a March bottle 22;
the electronic scale 28 is positioned at one side of the base 20, the plastic barrel 27 is placed on the electronic scale 28, and the lower end of the acrylic hollow barrel 15 is communicated with an inlet at the top of the plastic barrel 27;
the base 20 is symmetrically provided with studs, and an instrument panel bracket 11 and a loading frame 6 are sequentially arranged above the acrylic hollow barrel 15; the loading frame 6 and the instrument panel bracket 11 can move up and down along the stud, and the loading frame 6 and the instrument panel bracket 11 are fixed on the stud through bolts 7; the loading rod 8 is positioned between the loading frame 6 and the instrument panel bracket 11; the sleeve 10 is arranged on an instrument panel bracket 11, the loading frame 6 is provided with a pressure cylinder 4, a piston of the pressure cylinder 4 downwards passes through the loading frame 6 and vertically acts on the loading rod 8, and the lower end of the loading rod 8 passes through the sleeve 10 and vertically acts on a porous metal loading plate 14; the electronic displacement meter 9 records the vertical displacement of the loading rod 8 when the loading rod is stressed to move; a plurality of three-pin metal probes 17 are arranged on one side of the test soil column 16, a plurality of tensiometers 18 are arranged in the test soil column 16 and correspond to the three-pin metal probes 17 one by one; the three-pin metal probes 17 are all electrically connected with a voltage-stabilized power supply 23; the tension meter 18 is electrically connected with the computer 1 through a voltage signal collector 21.
As shown in fig. 1, in the present invention, the height of the loading frame 6 is adjustable to a suitable height as needed, so that the loading frame 6 is integrally connected with the base 20. An O-shaped ring 5 is arranged at the position of a piston in the pressure cylinder 4, so that the internal tightness is ensured, and the air pressure is not dissipated. After the piston moves, air pressure is formed and is vertically transmitted downwards to be sequentially pressurized to the loading rod 8 and the porous metal loading plate 14, and finally the air pressure is transmitted to the test soil column 16 in the acrylic hollow barrel 15; similarly, the height of the instrument panel support 11 can be adjusted, an electronic displacement meter 9 and a sleeve 10 are fixed on the instrument panel support 11, the electronic displacement meter 9 records the vertical displacement when the loading rod 8 is stressed to move, and the sleeve 10 limits the horizontal displacement when the loading rod 8 is stressed to move, so that the loading is ensured to be vertical loading.
In the invention, the three-needle type metal probe 17 and the tensiometer 18 are respectively used for testing the volume water content and the matrix suction of the test soil column 16, and the soil-water characteristic curve of the test soil sample can be obtained by correlating the test values of the volume water content and the matrix suction.
The invention can flexibly control the stress state of the soil column and accurately test the deformation of the soil column under the action of water-force coupling, and is particularly suitable for the special soil with collapsibility, such as loess; the water-force boundary conditions of the unsaturated loess during infiltration can be flexibly controlled, such as: constant head and free drainage.
The invention can accurately test the water content of unsaturated loess and the suction change of the matrix in the process of water infiltration; the method can be used for quickly testing the permeability coefficient curve related to the unsaturated loess stress state based on a transient section method.
In conclusion, the device disclosed by the invention has the advantages of clear testing principle, complete functions, simple structure, stronger innovativeness and practical value, and capability of quickly testing the unsaturated loess permeability coefficient curve related to the stress state.
Example 2:
based on embodiment 1, in this embodiment, it is preferable that the water valve 12 is connected to the mahalanobis bottle 22 through a rubber tube 13.
Preferably, the lower end of the acrylic hollow barrel 15 is communicated with the plastic barrel 27 through an L-shaped water pipe.
In the present invention, the L-shaped water pipes are used to promote the infiltration and drainage of moisture in the test soil column 16.
Preferably, the loading rod 8 is in a T shape, the diameter of the upper end of the loading rod 8 is larger than that of the lower end of the loading rod, and the diameter of the lower end of the loading rod 8 is matched with that of the sleeve 10.
Preferably, the three-pin metal probes 17 are electrically connected with a regulated power supply 23 through a time domain reflectometer 24 and a coaxial cable multiplexer 25, the three-pin metal probes 17 are connected with the coaxial cable multiplexer 25 through corresponding coaxial cables 26, the coaxial cable multiplexer 25 is connected with the time domain reflectometer 24, the time domain reflectometer 24 is electrically connected with the regulated power supply 23, and the time domain reflectometer 24 is connected with the computer 1 through an electric wire 2.
In the invention, a three-pin type metal probe 17 and a tensiometer 18 are respectively arranged on each horizontal section of the acrylic hollow barrel 15 at different heights. In the invention, the volume water content data of the soil body measured by the time domain reflectometer 24 is transmitted to the control system, and the control system acts on the computer terminal and finally displays the data on the display of the computer 1; the soil matrix suction data measured by the tensiometer 18 is finally displayed on the display of the computer 1. The coaxial cable 26 of the present invention is a 50 Ω coaxial cable.
Preferably, air valves 3 are arranged on the side and the upper part of the pressure cylinder 4.
According to the invention, air enters the air valve 3 during pressurization, the air in the pressure cylinder 4 is compressed to a certain volume to form pressure intensity, the air valve 3 is used for exhausting air during pressure relief, and the air pressure in the pressure cylinder 4 is reduced.
Preferably, the contact surface of the sleeve 10 and the loading rod 8 is coated with lubricating oil.
In the invention, lubricating oil is coated on the contact surface of the sleeve 10 and the loading rod 8, so that the friction between components is reduced to the maximum extent possible while the absolute vertical transmission of air pressure is ensured, and the energy dissipation of the loading rod 8 is reduced.
Preferably, the porous metal loading plate 14 is a metal cylinder with round holes, and the diameter of the porous metal loading plate 14 is the same as the inner diameter of the acrylic hollow barrel 15.
In the invention, as shown in fig. 5, round holes are uniformly distributed on the porous metal loading plate 14, and water flows into the test soil column 16 from top to bottom along the round holes uniformly distributed on the porous metal loading plate 14 during infiltration test, so that not only the soil stress application process is not influenced, but also the water infiltration rate is not influenced.
Preferably, the inner diameter of the mahalanobis bottle 22 is 20cm, the height of the mahalanobis bottle 22 is 50cm, the mahalanobis bottle 22 is positioned on one side of the test earth pillar 16, and the bottom of the mahalanobis bottle 22 is higher than the top of the test earth pillar 16.
In the invention, the March bottle 22 provides a water source for a soil body infiltration test, and water flow enters the acrylic hollow barrel 15 through the water valve 12 and the rubber tube 13 and then enters the infiltration test soil column 16. The bottom of the acrylic hollow barrel 15 is provided with a moisture collecting device. The moisture collecting device consists of a water pipe, a water valve on the water pipe, a plastic barrel 27 and an electronic scale 28. A rubber water pipe is arranged in the base 20 for draining water, water flow enters the plastic barrel 27 after passing through the rubber water pipe, and the drainage of the test soil column 16 is weighed by the electronic scale 28.
During the infiltration test, the mah-jong bottle 22 provides water source, and the height of the bottom of the glass tube below the water surface inside the mah-jong bottle is always consistent with the height of the liquid level inside the acrylic hollow barrel 15. Utilize the mahalanobis bottle 22 to provide the flood peak border at test earth pillar 16 top, moisture infiltration from top to bottom in the experimentation soaks test earth pillar 16 gradually, and the data that the later stage of being convenient for measured are calculated to infiltration's rivers comparatively stable during the experiment. The daily drainage quality of the test soil column 16 is recorded, and the soil body permeability coefficient can be obtained according to the unsaturated soil seepage theory by combining the soil-water characteristic curve.
Preferably, the test soil column 16 is a cylindrical soil sample with the diameter of 14-15 cm and the height of 80-90 cm. The test soil column 16 of the present invention has an initial dry density and moisture content.
The inside diameter of the acrylic hollow barrel 15 is 15cm, the height is 100cm, holes are drilled on the barrel wall for inserting a tension meter 18 and a three-pin metal probe 17, and sealing and seepage-proofing treatment is carried out before a test by using sealant.
In the invention, the base 20 is provided with the acrylic hollow barrel 15 as a soil sample container, and the top of the acrylic hollow barrel 15 is provided with the loading device which at least comprises a pressure cylinder 4 and a loading rod 8. A volume water content testing system and a matrix suction testing system are arranged between the base 20 and the soil sample container, the volume water content testing system is connected with the electromagnetic wave emitter, and the matrix suction testing system is connected with the voltage signal collector 21; by applying load to the soil body, the volume water content testing system and the matrix suction testing system respectively measure the volume water content and the matrix suction of the tested soil sample, the soil-water characteristic curve of the soil sample can be tested quickly in a transient state under a stress state. The precise electronic balance is placed at the bottom of the plastic bucket 27 beside the base 20 and used for detecting the daily drainage quality of the soil sample, and the soil body permeability coefficient is obtained according to the unsaturated soil seepage theory by combining the soil-water characteristic curve. According to the invention, the electronic scale 28 selects the precision according to the requirement, and the high-precision electronic scale 28 can avoid measurement errors.
The three-pin metal probe 17, the stabilized voltage power supply 23, the time domain reflectometer 24, the coaxial cable multiplexer 25 and the 50 omega coaxial cable 26 form a water content testing system. The voltage is provided by the voltage-stabilized source 23 according to the requirement, preferably, the voltage is 12V by the voltage-stabilized source 23, and the volume water content is tested by the time domain reflectometer 24 through an incident signal and a reflected signal which are transmitted to the three-pin metal probe 17 finally by transmitting an electromagnetic signal. The tensiometer 18 and the voltage signal collector 21 form a matrix suction test system, the top of the tensiometer 18 horizontally extends into the acrylic hollow barrel 15 to be in close contact with the test soil column 16, the matrix suction value is read through the pressure sensor in the voltage signal collector 21, and meanwhile, the periphery and the bottom of the tensiometer 18 are sealed through sealant, so that the quick response and the sensitive test of the tensiometer during the test are ensured. And combining the matrix suction test system and the water content test system to obtain a suction water head section and a volume water content section of the soil column at different moments, and calculating a soil unsaturated permeability coefficient curve by using a transient section method.
In the invention, as shown in figure 2, the tensiometer 18 is cylindrical ceramic with the diameter of 4-6 mm and the length of 6-8 mm, and the length of the top extending end is 1-2 mm. The air inlet value is set to be 100kPa, which is close to the vaporization pressure of the non-air water in the acrylic hollow barrel 15. The device comprehensively considers the test response sensitivity and the test system failure caused by water vaporization, so that the test efficiency is maximized. On the other hand, the top extending length of the tension meter 18 meets the requirement of the minimum contact area with the soil sample, and meanwhile, the structural damage caused by excessive insertion of the soil sample is reduced.
In the invention, a three-pin metal probe 17 is inserted into a test earth pillar 16 to collect incident signals and reflected signals of electromagnetic waves. As shown in fig. 2, each three-pin metal probe 17 is composed of three metal round bar probes arranged in parallel. The probe penetrates through the cross section of the soil column and is fixed at the two ends of the soil column, and the probe is arranged to monitor the average water content of the whole cross section area. The probes for testing the volume water content have the advantages of simple calibration process and accurate test, and the probes with different heights can be arranged to monitor the infiltration amount of soil bodies with different heights respectively. The three-needle type metal probe 17 is arranged in parallel, so that electromagnetic wave guide can be well simulated, and clear electromagnetic wave reflection signals can be obtained. The length and the interval of every two adjacent probes in each group of probes of each three-needle type metal probe 17 are equal, the diameters of all the probes are the same, the length is 12.5cm, the interval between every two adjacent probes is 1cm, the diameters of the probes are 0.5cm, the skin effect of the electromagnetic waveguide is avoided as much as possible, the propagation and diffusion volume of electromagnetic waves in the soil sample is enlarged as much as possible, the water content difference of different positions of the soil sample is evaluated, the water content uniformity of the soil sample is judged, and the test precision of the volume water content can be improved.
In the description of the present invention, it is to be understood that the terms "center", "upper", "lower", "vertical", "horizontal", "top", "bottom", "inner", "outer", and the like, indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, are used only for convenience in describing the present invention and for simplification of description, and do not indicate or imply that the device or element being referred to must have a particular orientation, be constructed and operated in a particular orientation, and thus, should not be construed as limiting the present invention. Furthermore, the terms "first", "second" and "first" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of the present invention, "a plurality" means two or more unless otherwise specified.
The invention has complete functions, high test precision, convenient operation and quick response, and has important value for evaluating the infiltration and the collapsibility of unsaturated loess and solving the engineering construction problem caused by rainwater infiltration in loess areas. The device can flexibly control the stress state of the sample soil column, the boundary flux/water head during infiltration, and accurately test the deformation, the water content and the matrix suction under the water-force coupling load. The device can measure the permeability coefficient curve of unsaturated soil under different stress states by using a transient section method.
The main working principle of the device of the invention is as follows:
1. principle for testing volume water content of soil sample
When the volume water content of the soil sample is tested, an electromagnetic pulse is transmitted through an electromagnetic wave transmitter, as shown in figure 3, the pulse is transmitted through a coaxial cable and a metal probe, reflection occurs at a place with discontinuous impedance, a reflection signal is recorded by an oscilloscope, finally, the dielectric constant of the soil body is obtained by analyzing the reflection signal, and then the volume water content theta is measuredw：
Wherein, thetawTo test the volumetric water content of the soil sample,. epsilona,soilTo test the dielectric constant of the soil sample.
2. Principle of soil sample matrix suction
When the substrate suction is tested, firstly, the tensiometer 18 is subjected to airless water saturation treatment, the periphery of the tensiometer is subjected to sealant treatment, so that the tensiometer 18 can only contact with a soil sample through the head and is subjected to suction balance with the soil sample, the suction is transmitted to a voltage signal collector 21 at one end through water in a rigid plastic pipe for reading, and the quality of the saturation effect is judged by monitoring the response time in the air.
3. Instantaneous profile method for measuring permeability coefficient of unsaturated soil under different stress states
Based on soil strain measurement and pore water pressure measurement, the water flow rate and the hydraulic gradient can be respectively determined, so that permeability parameters in the states of moisture absorption and moisture removal can be obtained according to Darcy's law; meanwhile, the soil-water characteristic curves related to the rapid moisture absorption and the moisture removal stress in different stress states can be obtained by establishing the volume water content and the negative pore water pressure, for example: the substrate suction and the relationship. Any bulk of the earth pillar can be measured by the displacement rod of the surface.
In the experimental process, continuous water flow is introduced into the cylindrical soil column and flows from the top to the bottom of the column, and according to Darcy's law, the permeability coefficient K of unsaturated soil at any depth can be calculated according to the following formula:
where v is the flow velocity of the water flow in the earth and i is the water-force gradient.
Fig. 4 is a schematic diagram of calculating the permeability coefficient of unsaturated loess by an instantaneous profile method. At different depths Z of the specimenA、ZB、ZCAnd ZDThe volume water content and the substrate suction of the cross section are monitored at any time, and the testing time of two adjacent times of the device is t1And t2. According to the one-dimensional continuous theorem, arbitrary depth ZBArbitrary average test time tave＝(t1+t2) Water flow rate of/2:
wherein Δ V means when t ═ t1And t2Theta betweenw(z,t)The area of the inner shadow; vze,tavThe seepage velocity at the Ze depth.
On the other hand, head gradient and test time at any depth, izB,taveCan be obtained by estimating the gradient of the measured water head section. The mathematical calculations may be tabulated as:
wherein-hzi,tjIs a completion time of tjj is 1,2, and zii-a, B, C and D, head at depth.
The device of the invention tests the permeability coefficient K of unsaturated loessiCan be calculated as follows:
wherein v iszIs the flow velocity of the water flow in the soil mass at depth Z, izIs the water-force gradient at the depth Z.
The invention relates to a device for testing unsaturated loess permeability coefficient curve capable of controlling stress state, which comprises the following specific operation methods:
and in the equipment assembling stage, the test soil column 16 is placed into the acrylic hollow barrel 15, the three-pin metal probe 17 and the tensiometer 18 are inserted into the test soil column 16 through holes which are punched in advance on the side wall of the acrylic hollow barrel 15, and gaps of the holes are sealed by using sealant to prevent moisture leakage. Before the test, the whole soil column is filled with water to verify the tightness of the device, and the volume water content and the matrix suction can be measured by checking that no water leakage occurs.
Firstly, stress is applied to a soil body, the piston of the pressure cylinder 4 moves to generate air pressure, the air pressure acts on the loading rod 8 in the vertical direction, vertical load is sequentially transmitted to the porous metal loading plate 14 and the test soil column 16 through the loading rod 8, and the vertical deformation of the test soil column is displayed on the electronic displacement meter 9 in real time.
Secondly, humidifying the infiltration of the test soil column 16 through a Mariotte bottle 22, wherein in order to prevent overlarge boundary flow and the occurrence of a sharp wetting front, the moisture absorption boundary flow of the whole soil column is generally controlled to be 0.2-5.0 ml/day in the transient section method test process of loess; the rubber water pipe connected with the lower part of the acrylic hollow barrel 15 and the water valve on the water pipe are used for promoting the water to seep and drain in the test soil sample, and the water outlet is always kept smooth.
And thirdly, starting the water content testing system and the matrix suction testing system to test basic parameters in an initial state. In testing the substrate suction, tensiometer 18 is internally saturated with airless water; during testing, the top of the test soil column 16 is contacted with the test soil column to balance the substrate suction force, and the substrate suction force is fed back to the computer 1 for reading through the electronic sensor in the voltage signal collector 21; when the volume water content of the soil sample is tested, an electromagnetic pulse is emitted through a time domain reflectometer 24, the pulse is transmitted through a 50 omega coaxial cable 26, a coaxial cable multiplexer 25 and a three-pin metal probe 17, reflection occurs at a place with discontinuous impedance, a reflection signal is recorded by an oscilloscope, finally, the dielectric constant of the soil sample is obtained by analyzing the reflection signal, and then the volume water content is calculated by utilizing the dielectric constant of the soil sample.
When the electromagnetic wave reflection oscillogram measured by the water content test system and the matrix suction test system is stable and the daily water flow monitored by the Mariotte 22 is basically kept unchanged, the whole earth pillar is considered to reach a steady-state seepage stage, and the constant water head seepage test is finished.
The method has important significance for realizing the measurement of the permeability coefficient curve related to the unsaturated loess stress state, and has important engineering value for accurately evaluating the loess collapsibility and solving the problem of engineering slope instability caused by rainwater infiltration in loess areas.
The above-mentioned contents are only for illustrating the technical idea of the present invention, and the protection scope of the present invention is not limited thereby, and any modification made on the basis of the technical idea of the present invention falls within the protection scope of the claims of the present invention. The device structures and method steps not described in detail in the present invention are prior art and will not be further described in the present invention.
Claims (10)
1. The utility model provides a but unsaturated loess osmotic coefficient curve testing arrangement of controlled stress state which characterized in that: the device comprises a computer (1), a loading frame (6), a loading rod (8), an electronic displacement meter (9), a sleeve (10), an instrument panel support (11), a water valve (12), a porous metal loading plate (14), an acrylic hollow barrel (15), a test earth column (16), a three-pin metal probe (17), a tensiometer (18), a base (20), a voltage signal collector (21), a Ma's bottle (22), a stabilized voltage power supply (23), a plastic barrel (27) and an electronic scale (28);
the device comprises a base (20), a testing soil column (16), porous metal loading plates (14) and a hollow cavity, wherein the acrylic hollow barrel (15) is arranged on the base (20), the testing soil column (16) is arranged in the acrylic hollow barrel (15), the upper end and the lower end of the testing soil column (16) are respectively provided with a permeable stone (19), and the upper part of the permeable stone (19) at the upper end of the testing soil column (16) is provided with the porous metal loading plates (14);
a water valve (12) is arranged on the side wall of the upper part of the acrylic hollow barrel (15), and the water valve (12) is connected with a March bottle (22);
the electronic scale (28) is positioned on one side of the base (20), the plastic barrel (27) is placed on the electronic scale (28), and the lower end of the acrylic hollow barrel (15) is communicated with an inlet at the top of the plastic barrel (27);
the base (20) is symmetrically provided with studs, and an instrument panel bracket (11) and a loading frame (6) are sequentially arranged above the acrylic hollow barrel (15); the loading frame (6) and the instrument panel bracket (11) can move up and down along the stud, and the loading frame (6) and the instrument panel bracket (11) are fixed on the stud through bolts (7); the loading rod (8) is positioned between the loading frame (6) and the instrument panel bracket (11); the sleeve (10) is arranged on an instrument panel support (11), the loading frame (6) is provided with a pressure cylinder (4), a piston of the pressure cylinder (4) downwards penetrates through the loading frame (6) and vertically acts on the loading rod (8), and the lower end of the loading rod (8) penetrates through the sleeve (10) and vertically acts on the porous metal loading plate (14); the electronic displacement meter (9) records the vertical displacement of the loading rod (8) when the loading rod is stressed to move; a plurality of three-needle metal probes (17) are arranged on one side of the test soil column (16), a plurality of tensiometers (18) are arranged in the test soil column (16) and correspond to the three-needle metal probes (17) one by one; the three-pin metal probes (17) are electrically connected with a voltage-stabilized power supply (23); the tension meter (18) is in electrical signal connection with the computer (1) through the voltage signal collector (21).
2. The unsaturated loess permeability coefficient curve testing device of claim 1, wherein: the water valve (12) is connected with the Mariotte bottle (22) through a rubber tube (13).
3. The unsaturated loess permeability coefficient curve testing device of claim 1, wherein: the lower end of the acrylic hollow barrel (15) is communicated with the plastic barrel (27) through an L-shaped water pipe.
4. The unsaturated loess permeability coefficient curve testing device of claim 1, wherein: the loading rod (8) is T-shaped, the diameter of the upper end of the loading rod (8) is larger than that of the lower end of the loading rod, and the diameter of the lower end of the loading rod (8) is matched with the sleeve (10).
5. The unsaturated loess permeability coefficient curve testing device of claim 1, wherein: the three-pin metal probes (17) are electrically connected with a stabilized voltage power supply (23) through a time domain reflectometer (24) and a coaxial cable multiplexer (25), the three-pin metal probes (17) are connected with the coaxial cable multiplexer (25) through corresponding coaxial cables (26), the coaxial cable multiplexer (25) is connected with the time domain reflectometer (24), the time domain reflectometer (24) is electrically connected with the stabilized voltage power supply (23), and the time domain reflectometer (24) is connected with a computer (1) through an electric wire (2).
6. The unsaturated loess permeability coefficient curve testing device of claim 1, wherein: and air valves (3) are arranged on the side and the upper part of the pressure cylinder (4).
7. The unsaturated loess permeability coefficient curve testing device of claim 1, wherein: and lubricating oil is coated on the contact surface of the sleeve (10) and the loading rod (8).
8. The unsaturated loess permeability coefficient curve testing device of claim 1, wherein: the porous metal loading plate (14) is a metal cylinder which is fully distributed with round holes, and the diameter of the porous metal loading plate (14) is the same as the inner diameter of the acrylic hollow barrel (15).
9. The unsaturated loess permeability coefficient curve testing device of claim 1, wherein: the inner diameter of the March's bottle (22) is 20cm, the height of the March's bottle (22) is 50cm, the March's bottle (22) is located on one side of the test soil column (16), and the bottom of the March's bottle (22) is higher than the top of the test soil column (16).
10. The unsaturated loess permeability coefficient curve testing device of claim 1, wherein: the test soil column (16) is a cylindrical soil sample with the diameter of 14-15 cm and the height of 80-90 cm.
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| CN114563288A (en) * | 2022-01-14 | 2022-05-31 | 西安理工大学 | Roadbed settlement test device suitable for vibration condition |
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