Device and method for preparing original soft clay sample by low-position vacuum loading combined with preloading

Abstract

The present invention relates to a device and method for preparing an original soft clay sample by low-position vacuum loading combined with pre-pressing, the device comprises a model box system and a vacuum pump system; the model box system comprises a model box and a stack of carrier plates, a sealed top cover is arranged on the top of the model box for sealing the model box, and an air extraction hole is arranged on the side wall of the model box at a certain distance from the bottom of the model box; a cross-shaped vacuum filter tube is arranged at the bottom of the model box; a plurality of plastic drainage belts are arranged closely to the inner wall of the model box; any end of the vacuum filter tube is connected to the air extraction hole, and the other end is closely to the bottom of the plastic drainage belt; the vacuum pump system comprises a water ring vacuum pump with a water vapor separation box; the water ring vacuum pump is connected to the air extraction hole on the model box through an air extraction pipe. The method is to inject mud into the model box, first drain and consolidate under the action of dead weight, and then vacuum consolidate to obtain the original soft clay sample. The method of the present invention can ensure the uniformity of sample moisture content, density and strength.

Description

Technical Field

[0001] The invention belongs to the field of geotechnical technology and relates to a device and a method for preparing an original soft clay sample by low-position vacuum loading combined with pre-pressing. Background Art

[0002] In geotechnical engineering, a large number of undisturbed soil samples are often required for indoor testing to study the physical and mechanical properties of the soil. However, the undisturbed samples taken from the engineering site usually need to be wrapped, transported, and stored, which is a cumbersome process and causes great disturbance to the samples, which can easily cause stress release and uneven sampling. In view of the many shortcomings of taking undisturbed samples from engineering sites, it is of great significance to study a device and technical method for preparing undisturbed soft clay samples indoors.

[0003] At present, the existing methods for preparing original soil samples indoors are mainly vacuum preloading, that is, the soil is preloaded into shape by vacuuming the upper part. It has the advantages of easy operation and simple equipment, but it still has many shortcomings. For example, the upper vacuuming method has poor sealing and it is difficult to reach the target vacuum pressure; and the drainage effect is poor, the vacuuming takes a long time, the sample moisture content is high, and the density is uneven, resulting in an uneven surface of the soil after molding, uneven force on the soil, and uneven strength of the sample. In view of the above problems, it is urgent to improve the existing vacuum preloading sample device and method. Summary of the invention

[0004] In order to solve the problems existing in the prior art, the present invention provides a device and method for preparing original soft clay samples by low-position vacuum loading combined with pre-pressing; that is, a method of low-position vacuuming and loading combined with pre-pressing is adopted to pre-press high-water content mud into shape, so as to quickly and efficiently prepare multiple original soft clay samples at one time indoors.

[0005] To achieve the above object, the scheme adopted by the present invention is as follows:

[0006] A device for preparing original soft clay samples by low-position vacuum loading combined with pre-pressing, comprising a model box system and a vacuum pump system;

[0007] The model box system includes a model box and a pile of carrier plates;

[0008] A sealed top cover is arranged on the top of the model box to seal the model box;

[0009] An air extraction hole is provided on the side wall of the model box at a certain distance (preferably 2 cm) from the bottom of the model box;

[0010] A 1 cm thick sand cushion layer and a water-permeable and breathable geotextile are laid on the bottom of the model box, and then a cross-shaped vacuum filter tube (which can be an integral part or spliced) is arranged on the water-permeable and breathable geotextile; the four ends of the vacuum filter tube are all open structures (i.e., non-closed ends); a number of fine holes are evenly arranged on the circumference of the vacuum filter tube (the function is to ensure that the vacuum pressure at the bottom of the box is evenly transmitted to the upper soil and to improve the drainage efficiency);

[0011] Several plastic drainage belts are arranged closely on the inner wall of the model box; the plastic drainage belts are composed of a plastic core plate and a filter layer (generally a non-woven geotextile) covering both sides thereof. The cross section of the plastic core plate is a parallel cross, which plays a supporting role and discharges the water infiltrated into the filter layer (manufacturer: Fujian Bangjie Plastic Products Co., Ltd.);

[0012] One end of the vacuum filter tube is connected to the air extraction hole in the model box, and the other end is close to the bottom of the plastic drainage belt; then a layer of sand cushion with a thickness of 2 cm and a layer of water-permeable and breathable geotextile are laid on the vacuum filter tube in sequence; the 2 cm thick sand cushion completely covers the vacuum filter tube and the connection between the vacuum filter tube and the plastic drainage pipe;

[0013] The outer edge of the stacking plate is kept at a certain distance from the inner wall of the model box (preferably 1-2 mm); the purpose of leaving a certain distance is to prevent the stacking plate from being rubbed by the inner wall of the model box and hindering its downward movement;

[0014] The vacuum pump system includes a water ring vacuum pump with a water vapor separation box;

[0015] The water ring vacuum pump is detachably connected to the exhaust hole on the model box through an exhaust pipe, and is used to realize vacuuming in the model box.

[0016] As the preferred technical solution:

[0017] As described above, in a device for preparing original soft clay samples by low-position vacuum loading combined with preloading, the exhaust hole is connected to an exhaust hole valve (the exhaust hole can be a threaded hole, and the two are threadedly connected), the exhaust pipe is connected to the exhaust hole valve (threaded connection is adopted for easy disassembly), and the connection is located outside the model box; a stop valve is arranged at the connection between the exhaust pipe and the water ring vacuum pump.

[0018] The device for preparing undisturbed soft clay samples by low-position vacuum loading combined with preloading as described above, the device also includes a data acquisition system;

[0019] The data acquisition system includes a rebound displacement sensor, a pore water pressure sensor and a fully automatic data acquisition instrument;

[0020] The rebound displacement sensor is fixedly mounted on the inner wall of the model box, and the probe of the rebound displacement sensor is in contact with the stacking plate;

[0021] The pore water pressure sensor is installed in the middle of the sample preparation height (generally estimated, and the estimation method is well known to those skilled in the art) in the model box;

[0022] The rebound displacement sensor and pore water pressure sensor are connected to the fully automatic data acquisition instrument through a data transmission line to monitor the vacuum pressure changes and settlement inside the soil in real time, so as to calculate the consolidation degree of the soil after forming and evaluate the effect of vacuum loading pre-compression sampling.

[0023] In the device for preparing original soft clay samples by low-position vacuum loading combined with pre-pressing, the model box is cylindrical; the material of the model box is organic glass; and scale lines are attached to the outer wall of the model box.

[0024] In the device for preparing undisturbed soft clay samples by low-position vacuum loading combined with preloading as described above, the vacuum filter tube is wrapped with a water-permeable and breathable geotextile; the wrapping is to prevent sediment from entering and blocking the drainage channel.

[0025] In the device for preparing original soft clay samples by low-position vacuum loading combined with preloading as described above, a lead hole with a diameter of 1.5 cm is opened on the loading plate for passing the data transmission line of the pore water pressure sensor.

[0026] In the device for preparing original soft clay samples by low-position vacuum loading combined with preloading as described above, a plurality of weights are evenly stacked on the loading plate; the device is used for preloading in combination with vacuum pressure to ensure the flatness of the soil surface in the box, the uniformity of force and the uniformity of sample strength.

[0027] As described above, a device for preparing undisturbed soft clay samples by low-position vacuum loading combined with preloading has two lead bolt holes with a diameter of 1 cm set on the sealed top cover; the lead bolt holes are used to lead out the data transmission lines of the rebound displacement sensor and the pore water pressure sensor, and the device can be sealed to maintain the vacuum degree in the box.

[0028] The present invention also provides a method for preparing an original soft clay sample by using the device for preparing an original soft clay sample by low-position vacuum loading combined with pre-pressing as described above, comprising the following steps:

[0029] (1) Device preparation: First, a sand cushion layer of a certain thickness is laid on the bottom of the model box, and then several layers of water-permeable and breathable geotextiles are laid; then, the vacuum filter tube is placed on the water-permeable and breathable geotextile, and one end of the vacuum filter tube is kept connected to the air extraction hole at the bottom of the model box; then, the plastic drainage belt is vertically arranged close to the inner wall of the model box, so that the bottom of the plastic drainage belt is in close contact with the other three ends of the vacuum filter tube; finally, a 2-cm-thick sand cushion layer and a layer of water-permeable and breathable geotextile are laid on the vacuum filter tube in sequence; the 2-cm-thick sand cushion layer completely covers the vacuum filter tube and the connection between the vacuum filter tube and the plastic drainage pipe;

[0030] (2) Inject mud with a water content of 100% to 120% into the model box. When the mud liquid level is a certain distance (generally 10 cm) from the top of the model box and lower than the plastic drainage belt, stop grouting and close the model box with a sealed top cover;

[0031] (3) Open the exhaust hole valve at the bottom of the model box and disconnect the exhaust hole valve from the exhaust pipe to allow the mud with a moisture content of 100% to 120% to drain and consolidate under its own weight for 7 to 10 days;

[0032] (4) opening the sealed top cover and placing a loading plate on the slurry that has been drained and consolidated under the action of its own weight;

[0033] (5) Connect the air extraction hole valve to the air extraction pipe and open the air stop valve and the air extraction hole valve, start the water ring vacuum pump, and control the vacuum pressure of the water ring vacuum pump to be no less than 85 kPa. When the target water content is reached, stop vacuuming;

[0034] (6) Sampling to obtain the original soft clay sample.

[0035] As the preferred technical solution:

[0036] In the method as described above, in the step, a plurality of weights are evenly stacked on the loading plate.

[0037] The principle of the present invention is:

[0038] The present invention can control the moisture content of the soil in the model box by measuring the drainage volume in the water vapor separation box; by arranging the plastic drainage belt, the vacuum pressure is evenly transmitted and drained inside the soil, and at the same time, combined with the loading effect of the loading plate, the soil surface is subjected to uniform stress and settlement deformation, thereby ensuring the uniformity of the moisture content and strength of the sample.

[0039] Beneficial Effects

[0040] (1) Compared with the upper vacuuming method, the method of the present invention can not only effectively solve the sealing problem and ensure that the vacuum pressure meets the requirements; it can also significantly reduce the time consumption of sample preparation by reasonably arranging the plastic drainage belt and applying the heap load, ensure the uniformity of the moisture content and density of the sample, and the uniformity of the strength, thereby improving the sample preparation efficiency;

[0041] (2) The method of the present invention can control the moisture content of the sample after molding, and monitor the pore water pressure and settlement in real time during the sample molding process, so as to calculate the consolidation degree of the soil after molding and reasonably evaluate the effect of vacuum loading pre-compression sampling. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1It is an overall cross-sectional view of a device for preparing original soft clay samples by low-position vacuum loading combined with pre-pressing according to the present invention;

[0043] Figure 2 is a top view of the vacuum filter tube of the present invention;

[0044] Figure 3 It is a cross-sectional view of the bottom structure of the model box of the present invention;

[0045] In the figure: 1-base; 2-permeable and breathable geotextile; 3-sand cushion; 4-plastic drainage belt; 5-loading plate; 6-weight; 7-sealed top cover; 8-vacuum filter tube; 9-exhaust hole valve; 10-lead bolt hole; 11-lead hole; 12-rebound displacement sensor; 13-pore water pressure sensor; 14-data transmission line; 15-automatic data acquisition instrument; 16-exhaust pipe; 17-stop valve; 18-water ring vacuum pump; 19-pressure gauge. DETAILED DESCRIPTION

[0046] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0047] The overall cross-sectional view of the device for preparing original soft clay samples by low-position vacuum loading combined with pre-pressing of the present invention is as follows: Figure 1 shown.

[0048] Example 1

[0049] A device for preparing original soft clay samples by low-position vacuum loading combined with pre-pressing, comprising a model box system and a vacuum pump system;

[0050] The model box system includes a model box (for stability during placement, the model box can be provided with a base 1) and a stack of carrier plates 5; the model box is cylindrical and made of plexiglass, and scale lines are attached to the outer wall of the model box for monitoring the settlement of the mud during self-weight consolidation.

[0051] A sealing top cover 7 is provided on the top of the model box to seal the model box; an air extraction hole is provided on the side wall of the model box 2 cm away from the bottom of the model box;

[0052] The bottom of the model box is arranged with a cross-shaped vacuum filter tube 8 (such as Figure 2); the four ends of the vacuum filter tube 8 are all open structures (i.e., non-closed ends); fine holes are evenly arranged on the circumference of the vacuum filter tube 8 and the vacuum filter tube 8 is wrapped with a water-permeable and breathable geotextile; a number of plastic drainage belts 4 are arranged closely to the inner wall of the model box; the plastic drainage belt 4 is composed of a plastic core plate and a filter layer covering both sides thereof, and the cross section of the plastic core plate is a parallel cross, which plays a supporting role and discharges the water infiltrated into the filter layer; any end of the vacuum filter tube 8 is connected to the air extraction hole, and the other end is closely attached to the bottom of the plastic drainage belt 4; the cross-sectional view of the bottom structure of the model box is as follows Figure 3 As shown;

[0053] The distance between the outer edge of the stacking plate 5 and the inner wall of the model box is 1 to 2 mm;

[0054] The vacuum pump system includes a water ring vacuum pump 18 with a water vapor separation box; the water ring vacuum pump 18 is connected to the air suction hole on the model box through an air suction pipe 16; the air suction hole is connected to the air suction hole valve 9 (the air suction hole is a threaded hole, and the two are threadedly connected), the air suction pipe 16 is connected to the air suction hole valve 9 (threaded connection is adopted), and the connection is located outside the model box; a stop valve 17 is also provided at the connection between the air suction pipe 16 and the water ring vacuum pump 18. The pressure gauge 19 is installed on the water vapor separation box to monitor the vacuum pressure value.

[0055] A plurality of weights 6 are evenly stacked on the loading plate 5 .

[0056] Example 2

[0057] A device for preparing undisturbed soft clay samples by low-position vacuum loading combined with preloading, based on the device of Example 1, further includes a data acquisition system; the data acquisition system includes a rebound displacement sensor 12, a pore water pressure sensor 13 and a fully automatic data acquisition instrument 15; the rebound displacement sensor 12 is fixedly mounted on the inner wall of the model box, and the probe of the rebound displacement sensor 12 is in contact with the loading plate; the pore water pressure sensor 13 is mounted in the middle of the sample preparation height in the model box; the rebound displacement sensor 12 and the pore water pressure sensor 13 are connected to the fully automatic data acquisition instrument 15 through a data transmission line 14. A lead hole 11 is provided on the loading plate 5; and two lead bolt holes 10 with a diameter of 1 cm are provided on the sealing top cover 7.

[0058] Example 3

[0059] A method for preparing an original soft clay sample by low-position vacuum loading combined with pre-pressing is carried out using the device in Example 2, and the steps are as follows:

[0060] (1) The soil obtained on site is dried, crushed, sieved, homogenized and sampled in sequence, and water is added to prepare mud with a designed moisture content of 100% to 120%, which is then sealed and stored;

[0061] (2) First, a 1 cm thick sand cushion layer is laid on the bottom of the model box, and then a layer of water-permeable and breathable geotextile is laid, and then the cross-shaped vacuum filter tube 8 wrapped with the water-permeable and breathable geotextile is placed, and one end of the vacuum filter tube 8 is connected to the air extraction hole at the bottom of the model box;

[0062] (3) Three plastic drainage belts 4 are arranged vertically close to the inner wall of the model box, and the bottom of the plastic drainage belts 4 is in close contact with the other three ends of the vacuum filter tube 8; finally, a 2 cm thick sand cushion layer 3 and a water-permeable and breathable geotextile 2 are laid on the vacuum filter tube 8 in sequence; the 2 cm thick sand cushion layer 3 completely covers the vacuum filter tube 8 and the connection between the vacuum filter tube 8 and the plastic drainage pipe 4;

[0063] (4) The high-water-content mud prepared in step (1) is injected into the model box gradually, and the pore water pressure sensor 13 is pre-buried. When the mud liquid level is a certain distance (10 cm) from the top of the model box and lower than the plastic drainage belt 4, the grouting is stopped, and the model box is closed with a sealed top cover 7.

[0064] (5) Open the exhaust hole valve 9 at the bottom of the model box and disconnect the exhaust hole valve 9 from the exhaust pipe 16 to allow the mud with a moisture content of 100% to 120% to drain and consolidate under the action of its own weight for 7 days. Measure the drainage volume in real time and monitor the sedimentation of the mud in real time based on the scale lines on the outer wall of the box.

[0065] (6) After the self-weight consolidation is completed, open the sealed top cover 7, lay a layer of water-permeable and breathable geotextile 2, place the loading plate 5, and lead the data transmission line 14 of the pore water pressure sensor 13 out from the lead hole 11 on the loading plate 5, then stack the weights 6 with a total weight of 20 kg, and fix the rebound displacement sensor 12 on the side wall of the box.

[0066] (7) Lead out the data transmission lines 14 of the pore water pressure sensor 13 and the rebound displacement sensor 12 from the lead bolt holes 10 on the sealing top cover 7, and connect them to the fully automatic data acquisition instrument 15, debug the equipment, and complete the preparations before data acquisition.

[0067] (8) Connect the exhaust pipe 16 of the water ring vacuum pump 18 to the exhaust hole at the bottom of the box, open the exhaust hole valve 9 and the air stop valve 17, turn on the power, start vacuum loading pre-compression sampling, and collect pore pressure and displacement data. Monitor the vacuum pressure through the pressure gauge 19 to ensure that it is not less than 85kPa. The moisture content of the prepared sample is controlled according to the water discharge volume in the water ring vacuum pump water vapor separation box. When the target moisture content is reached, stop vacuuming. Based on the real-time monitored pore pressure and displacement data, calculate the soil consolidation degree and evaluate the vacuum loading pre-compression sampling effect, in order to further improve the sample preparation device and method and improve the sample preparation efficiency.

[0068] (9) After the vacuum preloading is completed, stop data collection, and take out the rebound displacement sensor 12, weight 6, loading plate 5, etc. from the model box in turn, analyze and organize the collected data, and calculate the soil consolidation degree.

[0069] An in-situ soil sampler was used to take a group of undisturbed samples at intervals of 5 cm from top to bottom in the middle of the model box. Each group of undisturbed samples contained one standard ring knife sample and three triaxial samples with a diameter of 39.1 mm and a height of 80 mm, which were used to test physical and mechanical indicators such as moisture content, density, cohesion, and internal friction angle, so as to evaluate the uniformity of moisture content and density, and strength of the prepared samples.

[0070] The test results show that after the low-position vacuum loading combined preloading is completed, the consolidation degree of the soil in the model box reaches 97.5%; the moisture content of each group of samples tested is within 2%, and the density difference is within 0.1g / cm 3 The cohesion difference is within 0.3 kPa, and the internal friction angle difference is within 0.5°. In summary, the sample preparation device and technical method discussed in the present invention can ensure the uniformity of moisture content and density and strength of the sample after molding, and the sample preparation effect is good.

Claims

1. A device for preparing original soft clay samples by low-position vacuum loading combined with preloading, characterized in that: Including model box system and vacuum pump system; The model box system comprises a model box and a stack of carrier plates (5), on which a plurality of weights (6) are evenly stacked; A sealing top cover (7) is provided on the top of the model box for sealing the model box; An air extraction hole is arranged on the side wall of the model box at a certain distance from the bottom of the model box; A cross-shaped vacuum filter tube (8) is arranged at the bottom of the model box; the four ends of the vacuum filter tube (8) are all open structures; and holes are evenly arranged on the circumference of the vacuum filter tube (8); A plurality of plastic drainage belts (4) are arranged closely to the inner wall of the model box; the plastic drainage belt (4) is composed of a plastic core plate and filter layers covering both sides thereof, and the cross section of the plastic core plate is in the form of a parallel cross, which plays a supporting role and discharges water infiltrated into the filter layer; Any end of the vacuum filter tube (8) is connected to the air extraction hole, and the other end is in close contact with the bottom of the plastic drainage belt (4); The outer edge of the stacking plate (5) and the inner wall of the model box are kept at a certain distance to avoid friction between the two; The vacuum pump system includes a water ring vacuum pump (18) with a water vapor separation box; The water ring vacuum pump (18) is detachably connected to the exhaust hole on the model box through an exhaust pipe (16); The exhaust hole is connected to an exhaust hole valve (9), and the exhaust pipe (16) is threadedly connected to the exhaust hole valve (9), and the connection point is located outside the model box; a stop valve (17) is provided at the connection point between the exhaust pipe (16) and the water ring vacuum pump (18); A method for preparing an original soft clay sample by using a device for preparing an original soft clay sample by low-position vacuum loading combined with pre-pressing comprises the following steps: 1) Device preparation: first, a sand cushion layer of a certain thickness is laid on the bottom of the model box, and then several layers of water-permeable and breathable geotextiles are laid; then, the vacuum filter tube (8) is placed on the water-permeable and breathable geotextile, and one end of the vacuum filter tube (8) is kept in communication with the air extraction hole at the bottom of the model box; then, the plastic drainage belt (4) is vertically arranged close to the inner wall of the model box, so that the bottom of the plastic drainage belt (4) is in close contact with the other three ends of the vacuum filter tube (8); finally, a layer of sand cushion layer (3) and a layer of water-permeable and breathable geotextile (2) are laid on the vacuum filter tube (8); the sand cushion layer (3) completely covers the vacuum filter tube (8) and the connection between the vacuum filter tube (8) and the plastic drainage belt (4); 2) injecting mud with a water content of 100% to 120% into the model box, stopping the injection when the mud liquid level is a certain distance from the top of the model box and lower than the plastic drainage belt (4), and sealing the model box with a sealing top cover (7); 3) opening the air extraction hole valve (9) at the bottom of the model box and disconnecting the air extraction hole valve (9) from the air extraction pipe (16), so that the mud with a water content of 100% to 120% is drained and consolidated under the action of its own weight for 7 to 10 days; 4) opening the sealing top cover (7) and placing a loading plate (5) on the slurry that has been drained and consolidated under the action of the deadweight; 5) connecting the air extraction hole valve (9) and the air extraction pipe (16), opening the air stop valve (17) and the air extraction hole valve (9), starting the water ring vacuum pump (18), controlling the vacuum pressure of the water ring vacuum pump (18) to be not less than 85 kPa, and stopping the vacuum extraction when the target water content is reached; 6) Sampling to obtain the original soft clay sample.

2. The device for preparing original soft clay samples by low-position vacuum loading combined with preloading according to claim 1 is characterized in that: The device also includes a data acquisition system; The data acquisition system comprises a rebound displacement sensor (12), a pore water pressure sensor (13) and a fully automatic data acquisition instrument (15); The rebound type displacement sensor (12) is fixedly mounted on the inner wall of the model box, and a probe of the rebound type displacement sensor (12) is in contact with the stacking plate (5); The pore water pressure sensor (13) is installed in the middle of the sample preparation height in the model box; The rebound displacement sensor (12) and the pore water pressure sensor (13) are connected to a fully automatic data acquisition device (15) via a data transmission line (14).

3. The device for preparing original soft clay samples by low-position vacuum loading combined with preloading according to claim 1 is characterized in that: The model box is cylindrical; the material of the model box is organic glass; and scale lines are attached to the outer wall of the model box.

4. The device for preparing original soft clay samples by low-position vacuum loading combined with preloading according to claim 1 is characterized in that: The vacuum filter tube (8) is wrapped with a water-permeable and breathable geotextile.

5. The device for preparing original soft clay samples by low-position vacuum loading combined with preloading according to claim 1 is characterized in that: A lead hole (11) is provided on the stacking plate (5).

6. The device for preparing original soft clay samples by low-position vacuum loading combined with preloading according to claim 1 is characterized in that: Two lead bolt holes (10) are provided on the sealing top cover (7).

7. The device for preparing original soft clay samples by low-position vacuum loading combined with preloading according to claim 1 is characterized in that: In step 4), a plurality of weights (6) are evenly stacked on the loading plate (5).

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