RU2416081C1 - Method to automatically measure pore and side pressure under conditions of soil compression - Google Patents

Abstract

FIELD: construction.

SUBSTANCE: method to measure pore and side pressure under conditions of soil compression includes the following stages: preparation of an odometer for tests with its subsequent connection to a back pressure unit and an automatic control unit. Underpressure of 15…30 kPa is pulled in the odometer chamber. With computer control, a motion sensor is used to monitor and measure value of a soil sample displacement, when its value reaches the value of the soil sample elevation, a step motor is switched off, fixing the soil sample in a working position inside a rubber shell. Air suction is closed, the working chamber is filled with degassed water from a reservoir. Upper and lower valves are opened in the back pressure unit, the back pressure value is pulled with an air compressor and monitored by the sensor in the reservoir, water is pushed at the top and bottom of the soil sample, the sensor is used to monitor pore pressure at the lower border of the sample. Steps of 15…30 kPa are used to increase the back pressure until full water saturation of the soil sample (S_r=92…95%), and when the measured pore pressure and the back pressure are equal, water saturation is stopped. Using the developed program, the soil is tested for compression with constant speed of axial deformation to the specified efficient strain. In process of tests full and efficient vertical and side strains are measured, as well as pore pressure and axial deformation.

EFFECT: increased accuracy of measurement of the soil pore and side pressure, expansion of research range, automation and increased efficiency of tests.

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Description

Technical field

The invention relates to the field of construction and is intended to determine the mechanical properties of soils in laboratory conditions.

State of the art

An analogue of this proposed invention is a COMPRESSION-FILTRATION DEVICE (certificate for utility model RU 54597 U1, application 2005138873 dated 12.13.2005, patent holder and author Solomin SF, IPC E02D 1/02, publ. 07/10/2006).

1. A compression-filtration device containing a base, a working ring, a separator liner, a stamp with a section for applying a load and connected with measuring devices, characterized in that the inner surface of the working ring is equipped with a spring loaded and axially movable sleeve.

2. The compression-filtration device according to claim 1, characterized in that the height of the sleeve is commensurate with the thickness of the test soil sample.

3. The compression-filtration device according to claim 1, characterized in that the inner diameter of the sleeve corresponds to the diameter of the peripheral annular groove of the separator insert, and the lower inner end of the sleeve interacts with the side surface of the peripheral annular groove of the separator insert.

4. The compression-filtration device according to claim 1, characterized in that the upper inner end of the sleeve interacts with the lateral annular surface of the stamp.

5. The compression-filtration device according to claim 1, characterized in that the compressive force of the spring, spring-loaded movable sleeve, is greater than the total weight of the sleeve with a soil sample.

6. The compression-filtration device according to claim 1 or 4, characterized in that between the sleeve and the spring is placed a flat support washer.

7. The compression-filtration device according to claim 1, characterized in that the connection of the base and the working ring is made by means of a union nut and a sealing ring between the contacting ends of the base and ring.

The disadvantage of the analogue is the inability to conduct soil tests with the measurement of pore and side pressures.

The next analogue of the claimed proposed invention is a DEVICE FOR COMPRESSION TESTS OF SOILS (patent for invention RU 2045756 C1, application 93014282/33 of 03.23.1993, patentee and author Vorobyov EA, IPC G01N 3/08, E02D 1/00, publ. 10.10.1995), including the base, working rings of different diameters for soil samples, mounted one below the other in increasing diameter from top to bottom, upper and lower perforated dies, means for creating vertical unloading and strain gauges, characterized in that it is made in in and e three detachable sections mounted coaxially under each other, each of which is formed by the upper and lower dies and the working ring placed between them and the pressure ring connected to the dies, while the upper dies are made in the form of tanks for pouring water with flat perforated bottoms and inclined solid side walls having shoulders for installing strain gauges, annular protrusions with horizontal through holes are formed on the outer surface of the lower dies of the upper and middle sections And on the upper sections of the lower die surface facing thereto perforated bottoms corresponding protrusions annular grooves, the inner wall of the clamping rings has a tapered surface for self-centering of the working ring.

The disadvantage of the analogue is the inability to conduct soil tests with the measurement of both pore and side pressures.

The closest analogue (prototype) of the claimed invention is a DEVICE FOR MEASURING LATER PRESSURE IN SOIL (USSR author's certificate for the invention No. 939638, application 2868832 / 29-33 dated 01.10.80, applicant Leningrad Order of the Red Banner of the Red Banner of Civil Engineering, authors A .V. Golly and L.K. Tikhomirova, M.C. ³ E02D 1/00, publ. 30.06.82), including a housing with a porous bottom, a strain ring with load cells, load and measuring devices, characterized in that in order to increase precision and expansion dia In the research area, it is equipped with a manometer with flexible needles, and the housing with a rigid ring connected to the tensor ring with a sealed, elastic material, the rigid ring and the tensor ring made with lateral coaxial holes in which the flexible needles are passed, and the tensor ring is mounted above the porous bottom with a gap equal to 0.005-1 mm.

The disadvantage of the prototype is the low accuracy of the measurement of pore pressure, the influence of the design of the pore pressure sensor and the load device on the deformation of the soil sample, the inability to conduct tests with water saturation of the soil sample with reverse pressure, and the complexity of preparing the device for testing.

Explanation of the disadvantages of the prototype.

1. In the prototype, a pore pressure needle is inserted inside the soil sample, which prevents the movement of soil particles, increases the stiffness of the soil, introduces heterogeneity into the sample, thereby reducing the accuracy of the axial strain measurement. It is also known that when a needle is pressed into a sample, it is easily clogged with soil, which can completely exclude the possibility of measuring pore pressure. From the prototype it is not clear how to remove air from the needle cavity and the manometer chamber and fill this volume with water, otherwise the presence of air bubbles will not allow to accurately measure pore pressure.

2. In the prototype, the rod of the loading device passes through a soil sample. There is a need to create a through hole in the soil sample, which cannot be done in sandy and sandy loamy soils. Secondly, friction forces arise between the rod of the loading device and the soil, which affects the measured deformation of soil compression.

3. In the prototype, when measuring the pore pressure of a soil sample, it is impossible to fully saturate a soil sample beforehand, for example, using the back pressure method. The design of the device does not allow you to perform this operation due to leaks in the guide glass.

Explanations According to GOST 12248-96, compression compression tests are performed with a soil sample with a diameter of 71 mm and a height of 25 mm, which is located in a steel ring, which makes it impossible to expand in the radial direction. The tests are carried out under conditions of one-dimensional deformation. The load on the soil sample is applied through the stamp in steps with stabilization of compression deformations at each loading stage. The test duration for clay soils is up to 2-3 days, for peat or silt 6-10 days. Only axial deformation and axial force are measured. To determine the compressibility index of the soil (compression modulus of deformation), K. Tertzagi's theoretical solutions are used. This solution was obtained by filtering pore water in two directions: to the upper and lower boundaries of the soil sample.

Also known is a theoretical solution obtained by Wissa, AEZ, Christian, J. T., Davis, EH, and Heiberg, S., "Consolidation at Constant Rate of Strain", Journal of the Soil Mechanics and Foundations Division, ASCE, Vol 97, No. SM10, 1971, pp. 1393-1413, for the case of one-sided filtration when compressing the soil sample, not with a step load (kN), but with a load with a given strain rate (mm / min). In this case, the test duration is reduced to several hours. This type of loading, one-way filtration and measurement of pore and lateral pressure are implemented in the present invention.

The essence of the technical solution

A known method of measuring pore and lateral pressure under compression of the soil using a set of the following technical means: an odometer installed in the press, a unit for generating back pressure, an automatic control unit consisting of electronic converting equipment and a personal computer.

The technical task of the invention is to increase the accuracy of the measurement of pore and lateral pressure of the soil, expanding the range of studies, automation and improving the performance of the tests.

This problem is solved by installing a ring with soil in the odometer, installing the cover and fixing it with screws to the base, installing the odometer on the base of the press, and the movement indicator in the working position, adjusting the screw to eliminate the gap between the force sensor and the hollow piston, connecting the odometer to the automatic block for creating back pressure and the automatic control block, connect the sensors of force, displacement, lateral and pore pressure; they connect a stepper motor, an automatic control unit, a press and an odometer via an interface to a computer and complete the test preparation, then automatically shut off the taps of the back pressure unit from the control panel, connect a vacuum ejector to the fitting, open the tap and create a vacuum in the odometer chamber of 15 ... 30 kPa, and this pulls the outer rubber shell of the odometer; controlling from the panel, they turn on the stepper motor of the press, create an axial compressive force and transfer it through the hollow die and ceramic filter to the soil sample, overcome the friction forces between the steel ring and the side surface of the soil sample, vertically move the soil sample along with the perforated platform, controlling from the panel a displacement sensor, they control and measure the amount of displacement of the soil sample, and when the amount of displacement of the height of the soil sample is reached, the stepper motor is automatically turned off from the computer and fix the soil sample in the working position inside the rubber shell, block the air suction, disconnect the vacuum ejector from the fitting, controlling from the remote control, fill the working chamber with degassed water from the tank, remove the air from the working chamber with a drain cock and close the water supply tap, saturate the soil sample with water from the reservoir, the back pressure is monitored and the set value is created by the compressor, the air is dissolved in the pores of the soil sample in water, the water is forced through the bottom and top of the soil sample, regulated the pressure difference between the back and pore pressure is not more than 10 ... 15 kPa, the pore pressure is controlled, steps 15 ... 30 kPa increase the back pressure until the soil sample is completely saturated (S _r = 92 ... 95%) and if the measured pore pressure is back pressure - water saturation is stopped using the developed program, the soil is tested for compression with a constant axial deformation rate (mm / min) to a given effective stress, and during the tests, full and effective vertical and lateral stresses are measured eniya, pore pressure and axial strain.

Figure 1 shows the design of the odometer.

Figure 2 - design of the odometer after moving the soil sample to the working position.

Figure 3 - odometer installed in the press.

Figure 4 - diagram of the control and measurement of pore and lateral pressure.

Figure 5 shows an axonometric image of the odometer (in section).

An example of the implementation of a technical solution

In figures 1 and 2, the odometer contains a base 1 with three non-through cylindrical holes 2. Three movable rods 4, on which the perforated platform 5 rests, are inserted into the holes 2 on the springs 3.

The odometer has a pore pressure sensor 6 in the center of the base and a ceramic filter 7. The odometer is equipped with channels 8 for creating back pressure and a fitting 9 for connecting to the back pressure unit.

The working chamber is made of a steel or transparent shell 10, in the side surface of which a fitting 11 is mounted for connection with a back pressure unit, as well as a side pressure sensor 12 that transmits the pressure value to the automatic control unit, and a trigger valve 13.

The cylindrical rubber shell 14 is mounted on the protrusions 15 of the shell 10.

The odometer cover 16 includes a ceramic filter 17, a hollow piston 18, a steel ring 19 with a soil sample embedded in it 20. A pipe 21 passes through the hollow piston 18.

The sealing of the hollow piston 18 is provided by the lateral sealing rings 22 through which it passes. The sealing of the odometer cover is provided by the support ring 23. The cover 16 is connected to the base 1 with three screws 24. The tightening of the screws ensures that the shell 14 is fixed and sealed.

Figure 3 shows the connection in a single unit of the odometer and press. The press has a base 25 with a stepper motor 26 inside the base 25. The press is equipped with axial displacement sensors 27 and forces 28, and is also equipped with an adjusting screw 29. The press is designed to impact the odometer 30 with vertical force from the bottom up, i.e. press plate 31 presses on odometer 30 from bottom to top.

Figure 4 shows a diagram of automatic control and measurement of pore and lateral pressure, which consists of three blocks:

“A” - a compression unit containing an odometer integrated in the press;

“B” - block for creating back pressure;

“C” - automatic control unit.

All three blocks are combined into a single automatic control system and form a compression device.

Block "B" creating a back pressure consists of a tank 32, equipped with fittings 33, 50 and a pressure sensor 34, has connecting piping 43, 44, 45 and valves 46-49.

The automatic control unit “C” receives commands from the computer 35, connected by an interface 36 to the automatic converting apparatus 37 (EPA).

Automatic control unit “C” is connected by cables 38 (force), 39 (displacement), 40 (lateral pressure), 41 (pore pressure), 42 (stepper motor drive) with compression unit “A” and cable 51 with unit “B” creating back pressure. Automatic converting equipment 37 amplifies and converts signals into digital form from sensors: pore pressure 6, lateral pressure 12 (Fig. 1, 2), back pressure 34 (Fig. 4), force 28 and displacement 27 (Fig. 3).

The compression unit "A" is connected by cables 38-42 to the automatic control unit "C", pipelines 43-45 with the back pressure unit "B".

Compression device operates as follows.

Stage 1. Preparation of the odometer for testing

1.1. Having unscrewed the fixing screws 24, remove the odometer cover 16 and install a ring 19 with a soil sample 20 into the support ring 23 of the cover. Then, cover 16 is put back and hermetically fixed with screws 24 to the base of the odometer.

1.2. Set the odometer base 1 (Fig.3) on the plate 31 of the press 25, then set the axial displacement transducer 27 in the working position and eliminate the gap between the force transducer 28 and the hollow piston 18 (Fig.1, 2) with the adjusting screw 29 (Fig.3) .

Stage 2. Connecting the odometer 1 to the back pressure unit "B" and the automatic control unit "C" (figure 4)

2.1. Pipelines 43-45 (FIG. 4) of the back pressure unit “B” are connected to the odometer fittings 9, 11, 21 (FIGS. 1, 2), respectively.

2.2. Sensor cables: forces 38, displacement 39, lateral pressure 40, pore pressure 41 are connected to the automatic control unit "C".

Stage 3. Connection of odometer 1 and press 25 to computer 35 of automatic control unit “C”

3.1. Connect the cable 42 (figure 4) of the drive of the stepper motor 26 (figure 3) to the block "C".

3.2. Connect the automatic converting equipment 37 through the interface 36 (figure 4) to the computer 35.

Stage 4. Implementation of the automatic test method and test sequence

4.1. Close the valve 47 of the block "B" back pressure (figure 4). A vacuum ejector is connected to the nozzle 50, the valve 49 is opened, and a pressure of 15 ... 30 kPa is created in the odometer chamber 1, sufficient to expand the rubber shell 14.

4.2. At the command of the computer, the signal is transmitted to the automatic converting apparatus 37 (Fig. 4) to the stepper motor 26 of the press 25 (Fig. 3) and an axial force is generated, which is transmitted through the hollow piston 18 and the ceramic filter 17 to the soil sample 20 in the odometer (Fig. 4). 12). The frictional forces between the steel ring 19 and the lateral surface of the soil sample 20 are overcome. A vertical movement of the soil sample 20 is created together with the perforated platform 5. The movement of the sample 20 (Figs. 1, 2) is measured by a displacement sensor 27 (Fig. 3).

In automatic mode, the computer controls the amount of movement of the hollow piston 18, and when the amount of displacement reaches the height of the soil sample 20 (Figs. 1, 2), the computer automatically turns off the stepper motor 26 (Fig. 3). A sample of soil 20 occupied a working position inside the rubber shell 14 (figure 2).

4.3. Close the valve 49 and disconnect the vacuum ejector from the nozzle 50 (figure 4).

4.4. Open the drain valve 13 (FIGS. 1, 2), then the valve 47 (FIG. 4) and fill the odometer working chamber with degassed water from the reservoir 32. After removing air from the working chamber, the drain cock 13 and the cock 47 are closed.

4.5. Automation of water saturation of soil sample. The backpressure taps 46, 48 are opened and through the fitting 33 (FIG. 4) in the tank 32, a predetermined backpressure value is created by the air compressor. The back pressure value is controlled by a pressure sensor 34. The pressure sensor 6 (Fig. 1) automatically measures the pore pressure in the soil sample according to the developed program. The difference between the back and pore pressure should not be more than 10 ... 15 kPa. Then, in steps of 15 ... 30 kPa, the back pressure is increased until the soil sample is completely saturated with water S _r = 92 ... 95%. Water saturation is completed if the measured pore pressure is equal to the back pressure.

4.6. After automated water saturation of the soil sample, the cranes 46, 48 are closed and, using the developed program, a compression test is carried out with a constant speed of axial deformation (mm / min) to a given effective stress. During the tests, the total and effective vertical and lateral stresses, pore pressure and axial deformation are measured.

Industrial applicability

The use of the invention improves the accuracy of measurement of pore and lateral pressure, the introduction of an automatic control system for the loading process and the collection of information from sensors increases the productivity of the tests. In addition, the use of one-way filtration and continuous loading allows reducing the test time by up to 8 times, which can be seen from the attached test results (see Appendix). Thus, the invention also improves operational characteristics and creates a significant technical and economic effect.

Bibliography

1. GOST 12248-96. Soils. Laboratory methods for characterizing strength and deformability. Publishing House of Standards, 01/01/1997.

2. Certificate for utility model RU 54597 U1, IPC E02D 1/02, publ. 07/10/2006.

3. Golly A.V., Tikhomirova L.K. Device for measuring lateral pressure in soils. USSR copyright certificate No. 939638, M.Kl. E02D 1/00, bull. No. 24, 06/30/1982.

4. Vorobyov EA The device for compression testing of soils. Patent of Russia RU No. 2045756, M. Cl. G01N 3/08, E02D 1/00, 10/10/1995. (Prototype).

application

Test results in compression devices

general information

The tests were carried out in two stages. At the first stage, tests were carried out with samples of clay soil using the standard design of a compression device according to GOST 12248-96 and step loading. At the second stage, the tests were carried out using the proposed design of a compression device with pore pressure control and continuous loading, with a given strain rate (mm / min).

In both cases, the tests were performed using twin samples from the soil of a disturbed structure with physical properties, which are shown in Table 1.

Table 1 Physical characteristics of soil samples ω, ω _L , ω _p I _p , i.e. I _L ρ, g / cm ³ ρ _d , g / cm ³ ρ _s , g / cm ³ n, i.e. e S _r 0.3 0.41 0.21 0.20 0.45 1.93 1.49 2.70 0.45 0.819 0.989

1. Tests with stepwise action of an external load in a compression device in accordance with GOST 12248-96

table 2 The average of the four tests σ, MPa h mm ε E, MPa m ₀ , MPa ^-1 e 0 0 0 0.00 0,000 0.819 0.005 0.05 0.0020 1.45 0.766 0.815 0.008 0.09 0.0035 1.42 0.779 0.812 0.011 0.10 0.0040 3.05 0.364 0.811 0.013 0.11 0.0044 2.92 0.380 0.811 0.017 0.12 0.0048 6.61 0.168 0.810 0,050 0.48 0.0192 1.39 0,800 0.784 0,099 0.85 0,0339 2.03 0.546 0.757 0,200 1.45 0,0580 2,56 0.433 0.713 0,300 1.93 0,0773 3.15 0.352 0.678 0.400 2,32 0.0928 3.93 0.283 0.650

Table 3 Test duration Test Number Test duration, hour one 84.26 2 77.57 3 81.12 four 85.08 Average - 82 h 13 min

2. Tests for compression compression with a constant strain rate in the proposed design of the device

Table 4 The average values of the three tests σ, MPa h mm ε E, MPa m ₀ , MPa ^-1 e 0. 0 0 0 0 0.819 0.05 0.64 0,03 1.45 0.92 0.772 0.10 1.01 0.04 2.24 0.53 0.745 0.20 1,57 0.06 2.72 0.41 0.704 0.30 1.98 0.08 3.84 0.29 0.675 0.40 2,30 0.09 4.72 0.24 0.652 0.50 2.69 0.11 4.11 0.28 0.623. 0.60 2.88 0.12 8.65 0.14 0.609

Table 5 Test duration Test Number Test duration, hour one 8.16 2 7.05 3 16.35 Average - 10 h 40 min

3. Comparison of compression tests

Fig. 3. Comparison of test results in compression devices of various designs

findings

1. Tests in the proposed design of the compression device are carried out 8 times faster.

2. The results of both types of tests almost coincide (Fig. 3). The discrepancies in the determined compression modulus of deformation E are not more than 20% (Tables 2 and 4).

Claims (1)

A method of measuring pore and lateral pressure under compression of the soil using a set of the following technical equipment: an odometer installed in the press, a back pressure generating unit, an automatic control unit consisting of electronic conversion equipment and a personal computer, characterized in that a ring is installed in the odometer with soil, install the cover and fasten it with screws to the base, set the odometer on the base of the press, and the indicator of movement in the working position, adjustable nym screw eliminate the gap between the sensor and force the hollow piston, the odometer is connected to the automatic creation of back pressure block and the automatic control unit, connected force sensors, displacement, pore pressure, and a side; they connect a stepper motor, an automatic control unit, a press and an odometer via an interface to a computer and complete the test preparation, then automatically shut off the taps of the back pressure unit from the control panel, connect a vacuum ejector to the fitting, open the tap and create a vacuum in the odometer chamber of 15 ... 30 kPa and this pulls the outer rubber shell of the odometer; controlling from the control panel, they turn on the stepper motor of the press, create an axial compressive force and transfer it through the hollow piston and ceramic filter to the soil sample, overcome the friction forces between the steel ring and the side surface of the soil sample, vertically move the soil sample together with the perforated platform, controlling from the remote control a displacement sensor, they control and measure the amount of displacement of the soil sample, and when the amount of displacement of the height of the soil sample is reached from the computer, the stepper motor is automatically turned off They fix the soil sample in the working position inside the rubber shell, block the air suction, disconnect the vacuum ejector from the nozzle, controlling it from the remote control, fill the working chamber with degassed water from the tank, remove the air from the working chamber with a drain cock and close the water supply tap, saturate the soil sample water from the reservoir, control the back pressure and create a predetermined value by the compressor, dissolve the air in the pores of the soil sample in water, push water through the bottom and top of the soil sample, adjust the pressure difference between the back and pore pressures is not more than 10 ... 15 kPa, the pore pressure is controlled, steps 15 ... 30 kPa increase the back pressure until the soil sample is completely saturated (S _r = 92 ... 95%), and if the measured pore pressure equal to back pressure - water saturation is stopped using the developed program, the soil is tested for compression with a constant axial strain rate (mm / min) to a given effective stress, and during the tests, the full and effective vertical and lateral pressure are measured yazheniya, pore pressure and axial strain.

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