CN113916744B - Method and system for testing permeability coefficient of foam soil under total stress - Google Patents

Abstract

The invention discloses a method and a system for testing the permeability coefficient of foam soil under total stress, for this reason, the method for testing the permeability coefficient of foam soil under total stress provided by the embodiment of the invention comprises the steps of filling a foam soil sample in a loading cavity of a permeameter; n +1 pore water pressure gauges are uniformly distributed on the loading cavity along the height direction of the loading cavity; wherein, a pore water pressure gauge is arranged at the top and the bottom of the loading cavity; connecting the pore water pressure gauge with a data acquisition instrument; applying a constant pressure water head and vertical total stress on a foam soil sample in the permeameter, opening an automatic drain valve on a drain pipe, measuring the water pressure in a loading cavity by using a pore water pressure gauge, and acquiring data by using a data acquisition instrument; and processing the data acquired by the data acquisition instrument, and calculating to obtain the permeability coefficient. The testing method can realize the measurement of the permeability coefficient of the foam soil under the conditions of total stress and high water pressure, and the measured permeability coefficient is more in line with the actual situation on site.

Description

A method and system for testing the permeability coefficient of foamed soil under total stress

technical field

The invention belongs to the technical field of geotechnical test measurement, in particular to a method and a system for measuring the permeability coefficient of foamed soil under total stress.

Background technique

Earth pressure balance shield is widely used in urban underground tunnel construction because of its high efficiency and safety. With the continuous increase of the number of earth pressure balance shield tunnels under construction, it is inevitable to encounter water-rich sandy strata. When the earth pressure balance shield passes through the water-rich sandy strata, it is very prone to spewing accidents, which may delay the construction period, economic losses, and cause casualties in severe cases. In the field, modifiers are usually added to the face to improve the impermeability of the muck. At present, the most used modifier in sandy formations is foam, which is filled between the pores of soil particles to block its seepage channels, but foam is a kind of foam. It is a metastable system. As it bursts or is carried away by water flow, the impermeability of foamed soil will gradually decrease with time. Especially under high water pressure conditions, foam is more likely to burst and migrate, so the measurement The change of its permeability coefficient with time has important engineering significance for guiding shield construction.

Some scholars have proposed some permeability coefficient measuring devices or methods. For example, the Chinese invention patent (application number: 201911182461.4, patent name: a soil penetration test device and method under variable stress conditions) provides a soil penetration test device under variable stress conditions, which can simulate the real sample in the test process. The stress state, the stress state of one or more surfaces of the sample can be changed according to the needs, and the change law of the permeability coefficient of the soil in the process of stress increase and decrease can be accurately obtained. Chinese invention patent (application number: 201810683637.3, patent name: a test method and test device for the permeability coefficient of foam-modified sandy slag under high water pressure) provides a permeability coefficient test device, which can measure high water by providing a high-position water head Depressed foam to improve the permeability coefficient of sandy slag. Chinese utility model patent (application number: 201510108411.7, patent name: compound penetration test device with constant and variable water heads) relates to a kind of exhaust valve respectively provided on the sample bucket and the water injection device. By adjusting the air pressure and exhaust in the water injection device The valve realizes adjustable water pressure and is compatible with devices for constant head and variable head penetration tests. The above patents do not consider the influence of total stress on the permeability of foamed soil, and cannot truly reflect the permeability of foamed soil in the soil bin.

SUMMARY OF THE INVENTION

Aiming at the failure to fully consider the influence of total stress and high water pressure in the existing foam soil permeability coefficient measurement test, the invention provides a foam soil permeability coefficient test method and system under total stress, which can realize total stress, high water pressure The measurement of the permeability coefficient of foamed soil under pressure, the measured permeability coefficient is more in line with the actual situation on site.

To this end, the method for testing the permeability coefficient of foamed soil under total stress provided in one aspect of the embodiments of the present invention includes:

Fill the foamed soil sample in the loading chamber of the permeameter;

On the loading chamber, there are n+1 pore water pressure gauges evenly distributed along its height direction; wherein, one pore water pressure gauge is arranged at the top and bottom positions of the loading chamber;

Connect the pore water pressure gauge to the data acquisition instrument;

Apply constant pressure water head and vertical total stress to the foam soil sample in the permeameter, open the automatic drain valve on the drain pipe, use the pore water pressure gauge to measure the water pressure in the loading chamber, and collect data through the data acquisition instrument;

The data collected by the data acquisition instrument are processed to calculate the permeability coefficient.

Specifically, the specific calculation process of the permeability coefficient is as follows:

Step 1: Calculate the permeability coefficient of soil samples between two adjacent pore water pressure gauges. The calculation formula is as follows:

where: ξ is the reduction factor, n _x is the porosity of the foamed soil under the vertical total stress, v is the kinematic viscosity of water, g is the acceleration of gravity, n _fi is the porosity of the foam under the water pressure p _i , d _{10 , fi} is the effective particle size of the foam under the water pressure _pi , and the water pressure _pi is the average value of the two adjacent pore water pressure gauges;

Step 2: Calculate the overall equivalent permeability coefficient, the calculation formula is as follows:

where l is the height of the foamed soil sample, _hi is the seepage path of the soil sample between two adjacent pore water pressure gauges;

The process of solving the reduction factor ξ is as follows:

where d _10,s is the effective particle size of the soil particles in the foamed soil sample.

Specifically, the porosity n _fi of the foam under the water pressure p _i is calculated by the following formula:

where e _fi is the porosity of the foam under the water pressure p _i , FER is the foam expansion ratio, and p is the atmospheric pressure.

Specifically, the effective particle size d _10,fi under the water pressure p _i is calculated by the following formula:

where: d _10,f is the effective particle size of the foam at atmospheric pressure p.

Another aspect of the embodiment of the present invention provides a system for testing the permeability coefficient of foamed soil under total stress, including a permeameter and a data acquisition device, wherein the permeameter includes:

infiltration cylinder;

a bottom plate, fixedly arranged at the bottom of the infiltration cylinder;

a piston, which is slidably arranged inside the infiltration cylinder;

The constant pressure water head is communicated with the water inlet hole on the piston through the water inlet pipe;

a drain pipe, communicated with the drain hole on the bottom plate;

also includes an axial loading system acting on the piston, the axial loading system is used to apply a vertical total stress to the foamed soil sample;

A loading cavity for accommodating foamed soil samples is formed between the infiltration cylinder, the piston and the bottom plate, a plurality of pore water pressure gauges are arranged on the loading cavity along its height direction, and displacement is arranged on the piston. a sensor, the displacement sensor and the pore water pressure gauge are connected to the data acquisition instrument;

Wherein, the pore water pressure gauge is used to measure the water pressure inside the loading chamber, the pore water pressure gauge at the bottom end is arranged at the bottom plate, and the pore water pressure gauge at the top end is arranged on the piston .

Specifically, the axial loading system includes an air pump, a first air delivery pipe, and a first servo air pressure valve, a top plate is fixed on the top of the permeation cylinder, and the permeation cylinder, the piston and the top plate are between An air pressure chamber is formed, two ends of the first air delivery pipe are respectively connected with the air pump and the air pressure chamber, and the first servo air pressure valve is arranged on the first air delivery pipe.

Specifically, the constant-pressure water head is provided by a high-pressure water injection system, and the high-pressure water injection system includes a constant-pressure water tank. The constant-pressure water tank is a closed box, the bottom of which is connected to the water inlet pipe, and the top is connected to the second air pipe. In communication with the air pump, a second servo air pressure valve is arranged on the second air delivery pipe.

Specifically, the high-pressure water injection system further includes a water storage tank and an intelligent water injector, the intelligent water injector is connected between the constant pressure water tank and the water storage tank through a water replenishment pipe, and the water replenishment pipe is inserted into the constant pressure water tank The end of one end is provided with a water level sensor, and the water level sensor is electrically connected with the intelligent water injector.

Specifically, a flow meter is provided on the drain pipe, the flow meter is connected to the data acquisition instrument, and a water stop valve is provided on both the water inlet pipe and the drain pipe.

Specifically, a permeable plate is provided between the foamed soil sample and the piston, and between the foamed soil sample and the bottom plate

Specifically, the drain pipe is communicated with the pressure chamber, the bottom of the pressure chamber is provided with an automatic drain valve, the air pump is communicated with the pressure chamber through a third air pipe, and the third air pipe is provided with an automatic drain valve. There is a third servo air valve.

Compared with the prior art, at least one embodiment of the present invention has the following beneficial effects: the present invention can realize the measurement of the total stress and the permeability coefficient of foamed soil under high water pressure, and the measured permeability coefficient is more in line with the actual situation on site. It has the advantages of automatic water pressure control, easy operation and high test accuracy.

Description of drawings

In order to illustrate the technical solutions in the embodiments of the present invention more clearly, the following briefly introduces the accompanying drawings used in the description of the embodiments. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can also be obtained from these drawings without creative effort.

1 is a schematic structural diagram of a test system provided by an embodiment of the present invention;

2 is a schematic structural diagram of a permeameter involved in an embodiment of the present invention;

Among them: 1. Infiltration instrument; 101. Infiltration cylinder; 102, Bottom plate; 103, Piston; 104, Drain pipe; 105, Top plate; 2. Data acquisition instrument; 3. Axial loading system; 301, Air pump; 302, Section 1. Air pipeline; 303. First servo air valve; 4. Foam soil sample; 5. Pore water pressure gauge; 6. Display; 7. Displacement sensor; 8. Air pressure chamber; 9. High pressure water injection system; 901, Constant pressure water tank 902, the second gas pipeline; 903, the second servo air valve; 904, the water storage tank; 905, the intelligent water injector; 906, the water supply pipe; 907, the water level sensor; 10, the water stop valve; 12. The third gas pipeline; 13. The third servo air valve; 14. The automatic drain valve; 15. The automatic switching valve; 16. The flow meter.

Detailed ways

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. Obviously, the described embodiments are only a part of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", " Rear, Left, Right, Vertical, Horizontal, Top, Bottom, Inner, Outer, Clockwise, Counterclockwise, Axial, The orientations or positional relationships indicated by "radial direction", "circumferential direction", etc. are based on the orientations or positional relationships shown in the accompanying drawings, which are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying the indicated devices or elements. It must have a specific orientation, be constructed and operate in a specific orientation, and therefore should not be construed as a limitation of the present invention.

In addition, the terms "first" and "second" are only used for descriptive purposes, and should not be construed as indicating or implying relative importance or implying the number of indicated technical features. Thus, a feature defined as "first" or "second" may expressly or implicitly include one or more of that feature. In the description of the present invention, "plurality" means two or more, unless otherwise expressly and specifically defined.

Referring to Figures 1 and 2, a system for testing the permeability coefficient of foamed soil under total stress includes a permeameter 1, a data acquisition device 2 and an axial loading system 3. The permeameter 1 includes a permeation cylinder 101, a bottom plate 102 and a piston 103, A constant pressure water head and a drain pipe 104, the bottom plate 102 is fixedly arranged at the bottom of the permeation cylinder 101, the piston 103 is slidably arranged inside the permeation cylinder 101, and a foam soil sample 4 is formed between the permeation cylinder 101, the piston 103 and the bottom plate 102 In the loading chamber, the axial loading system 3 acts on the piston 103 to apply vertical total stress to the foam soil sample 4. The constant pressure water head communicates with the water inlet hole on the piston 103 through the water inlet pipe, and the drain pipe 104 is connected to the bottom plate 102. The pore water pressure gauge 5 is used to measure the water pressure in the loading cavity, and the bottom pore water pressure gauge 5 is provided with a bottom plate 102 The topmost pore water pressure gauge 5 is arranged on the piston 103, linked with the piston 103, the data acquisition instrument 2 is connected to the external display 6, and the piston 103 is also provided with a displacement sensor 7, through which the displacement sensor 7 can realize the movement distance of the piston 103. For measurement, the displacement sensor 7 and the pore water pressure gauge 5 are connected to the data acquisition instrument 2 .

The process of using the test system of the above structure to test the permeability coefficient of foamed soil under total stress is as follows:

Fill the foamed soil sample in the loading chamber of the permeameter;

Install n+1 pore water pressure gauges on the pore body on the loading cavity, where n is a positive integer and greater than or equal to 2;

Connect the pore water pressure gauge to the data acquisition instrument;

Apply constant pressure water head and vertical total stress to the foam soil sample in the permeameter, open the automatic drain valve on the drain pipe, use the pore water pressure gauge to measure the water pressure in the loading chamber, and collect data through the data acquisition instrument;

The data collected by the data acquisition instrument are processed to calculate the permeability coefficient.

Specifically, the specific calculation process of the permeability coefficient is as follows:

Step 1: Calculate the permeability coefficient of soil samples between two adjacent pore water pressure gauges. The calculation formula is as follows:

Step 2: Calculate the overall equivalent permeability coefficient, the calculation formula is as follows:

Among them, the porosity n _fi of the foam under the water pressure p _i is calculated by the following formula:

The effective particle size d _10,fi under the water pressure p _i is calculated by the following formula:

The process of solving the reduction factor ξ is as follows:

where: ξ is the reduction factor, n _x is the porosity of the foamed soil under the applied vertical total stress, v is the kinematic viscosity of water, g is the acceleration of gravity, n _fi is the porosity of the foam under the water pressure p _i , d _10,fi is the effective particle size of the foam under the water pressure _pi , the water pressure _pi is the average value of the two adjacent pore water pressure gauges, l is the height of the foam soil sample, and _hi is the adjacent two pore water The seepage path of the soil sample between the pressure gauges, e _fi is the porosity of the foam under the water pressure p _i , FER is the foaming ratio of the foam, d _10,f is the effective particle size of the foam under the atmospheric pressure p, d _10,s is Effective particle size of soil particles in foamed soil samples.

The invention can realize the measurement of the total stress and the permeability coefficient of the foamed soil under high water pressure, and provides a specific formula for solving the permeability coefficient. Simple, higher measurement accuracy and other advantages.

Referring to FIG. 1 , in some embodiments, the axial loading system 3 includes an air pump 301 , a first air delivery pipe 302 and a first servo air pressure valve 303 , a top plate 105 is fixed on the top of the permeation cylinder 101 , the permeation cylinder 101 , the piston An air pressure cavity 8 is formed between 103 and the top plate 105, the two ends of the first air delivery pipe 302 are respectively communicated with the air pump 301 and the air pressure cavity 8, the first servo air pressure valve 303 is arranged on the first air delivery pipe 302, and the air pump 301 outputs The high-pressure gas from the first servo air pressure valve 303 enters the air pressure chamber 8 with a set constant air pressure, thereby pushing the piston 103 to apply an axial load to the foamed soil sample 4. The servo control technology is used to realize the loading of the axial load, and the degree of intelligence is high. , the control is also simpler.

1, in other embodiments, the constant pressure water head is provided by the high pressure water injection system 9, the high pressure water injection system 9 includes a constant pressure water tank 901, the constant pressure water tank 901 is a closed box, and the bottom is communicated with the water inlet pipe, and the top passes through The second air delivery pipe 902 is communicated with the air pump 301, the second air delivery pipe 902 is provided with a second servo air pressure valve 903, and the high-pressure gas output by the air pump 301 enters the constant pressure water tank 901 with a set constant air pressure through the second servo air pressure valve 903, The water in the water tank is forced to enter the loading chamber at a constant pressure through the water inlet pipe to achieve the provision of constant pressure water head. In this embodiment, the servo air pressure control technology is used to realize the water pressure control, which has the advantages of automatic water pressure control, simple operation and high test accuracy.

1 , specifically, the high-pressure water injection system 9 further includes a water storage tank 904 and an intelligent water injector 905. The intelligent water injector 905 is connected between the constant pressure water tank 901 and the water storage tank 904 through a water supply pipe 906, and the water supply pipe 906 is inserted into the constant pressure water tank 906. The bottom of the water tank 901 is connected to the water level sensor 907, and the water level sensor 907 is electrically connected to the intelligent water injector 905. When the water level sensor 907 senses that the water level is lower than 1/5 of the height of the constant pressure water tank 901, the intelligent water injector 905 starts to realize automatic water replenishment. When the replenishment reaches the set time, for example, when the replenishment reaches 4/5 of the height of the constant pressure water tank 901 , the intelligent water injector 905 stops the replenishment. In addition, a faucet may also be installed above the water storage tank 904, and the water storage tank 904 can be replenished through the faucet.

Referring to FIG. 1 and FIG. 2 , it can be understood that, in the actual design, the permeation cylinder 101 is a transparent hollow cylinder, the height of which is greater than 2.5 times the diameter, and the top plate 105 and the bottom plate 102 pass through the flange and the hollow cylinder respectively. Connection, the top plate 105 is provided with an air inlet, a piston 103 hole and a vent valve, the first air delivery pipe 302 is docked with the air inlet, the hollow piston 103 rod seal of the piston 103 passes through the piston 103 hole and is connected to the water inlet pipe, and the bottom plate is connected to the water inlet pipe. A water outlet is provided below the 102, and the water outlet is docked with the drain pipe 104. A water stop valve 10 is provided on both the water inlet pipe and the drain pipe 104. The piston 103 slides up and down along the permeable cylinder 101, and the diameter of the piston 103 is the same as that of the permeable cylinder 101. The inner diameters are the same, the effective stroke of the piston 103 is less than the length of the rod of the hollow piston 103 , and the high-pressure water flows into the loading chamber through the rod of the hollow piston 103 .

1 , in some embodiments, the drain pipe 104 communicates with the pressure chamber 11, the bottom of the pressure chamber 11 is provided with an automatic drain valve 14, and the air pump 301 communicates with the pressure chamber 11 through the third air delivery pipe 12. The third air supply pipe 12 is provided with a third servo air pressure valve 13, and the water flowing out of the permeable cylinder 101 flows into the pressure chamber 11. The stop height is 1/5 of the height of the pressure chamber 11 . This embodiment can simultaneously provide high-pressure water heads for the upper and lower sides of the foamed soil, and the hydraulic gradient is more in line with the actual situation on site, so the measured permeability coefficient is more in line with the actual engineering.

Referring to FIG. 1 , specifically, the number of pressure chambers 11 is two, and the water flow from the permeation cylinder 101 flows into the two pressure chambers 11 by selecting one of the automatic switching valves 15 . Specifically, the air pressure in the air pressure chamber 8 is greater than the air pressure in the constant pressure water tank 901 , the air pressure in the constant pressure water tank 901 is greater than the air pressure in the lower pressure chamber 11 , and the volume of the water storage tank 904 is twice the volume of the constant pressure water tank 901 . The drain pipe 104 is provided with a flow meter 16, the flow meter 16 is connected to the data acquisition instrument 2, and a permeable plate is provided between the foamed soil sample 4 and the piston 103 and between the foamed soil sample 4 and the bottom plate 102. Of course, by adjusting the air pressure in the constant pressure water tank 901 and the pressure chamber 11, the permeation path can be exchanged.

The working process of the test system of the above structure is as follows:

S1: According to the connection relationship of the test system, assemble the test system, close the water stop valves on the water inlet pipe and the drain pipe, and open the faucet to inject water into the water storage tank;

S2: Measure the maximum porosity n _max of the dry sand, add water and foam to the dry sand and stir to form foam soil, measure the porosity n of the foam soil, the porosity n of the foam soil is greater than the maximum porosity n _max of the dry sand;

S3: Put a permeable board on the bottom plate of the permeameter, then pour the stirred foamed soil into the permeable cylinder and flatten it. Place a permeable board on the flattened foamy soil, and the filling height is the permeameter. more than twice the diameter;

S4: Align the piston into the permeable cylinder, and quickly tighten the bolts between the top plate and the permeable cylinder, further pre-press the piston to contact the permeable plate above, and connect the water inlet pipe at the bottom of the constant pressure water tank to the top of the piston rod, Connect the first air line to the top plate of the permeameter;

S5: Close the air relief valve, set and open the pressure values of the first servo air valve, the second servo air valve and the third servo air valve in turn, and turn on the air pump;

S6: Open the water stop valves on the water inlet pipe and the drain pipe at the same time, and use the data acquisition instrument to collect data;

S7: Process the data collected by the data acquisition instrument, and calculate the permeability coefficient.

The invention can simultaneously provide high-pressure water head and apply high stress to the foamed soil, so that the permeability coefficient of the foamed soil can be measured under the condition of total stress, so the measured permeability coefficient is more in line with engineering practice.

Unless otherwise stated in any of the technical solutions disclosed in the present invention, if it discloses a numerical range, then the disclosed numerical range is the preferred numerical range, and any person skilled in the art should understand that: the preferred numerical range is only Among the many implementable numerical values, the technical effect is relatively obvious or representative. Since the numerical values are too numerous to be exhaustive, only some numerical values are disclosed in the present invention to illustrate the technical solutions of the present invention, and the above-mentioned numerical values shall not constitute a limitation on the protection scope of the present invention.

At the same time, if the above-mentioned invention discloses or involves parts or structural parts that are fixedly connected to each other, then, unless otherwise stated, fixed connection can be understood as: detachable fixed connection (for example, using bolts or screws), or It is understood as: non-removable fixed connection (such as riveting, welding), of course, the mutual fixed connection can also be replaced by an integral structure (such as using a casting process to integrally form) (except that it is obviously impossible to use an integral forming process).

In addition, unless otherwise stated, the terms used in any of the technical solutions disclosed in the present disclosure used to represent positional relationships or shapes include states or shapes that are similar to, similar to, or close to. Any component provided by the present invention may be assembled from a plurality of individual components, or may be a single component manufactured by an integral molding process.

The above-mentioned embodiments are only examples to clearly illustrate the present invention, and are not intended to limit the embodiments. For those of ordinary skill in the art, changes or modifications in other different forms can also be made on the basis of the above description. Neither need nor can all embodiments be exhaustive here. And the obvious changes or changes derived from this are still within the protection scope of the present invention.

Claims (3)

1. a foamed soil permeability coefficient testing method under a total stress, is characterized in that, comprises: Fill the foamed soil sample (4) in the loading cavity of the permeameter (1); N+1 pore water pressure gauges (5) are evenly distributed along the height direction of the loading chamber, wherein one pore water pressure gauge (5) is arranged at the top and bottom positions of the loading chamber; Connect the pore water pressure gauge (5) with the data acquisition instrument (2); Apply a constant pressure water head and vertical total stress to the foamed soil sample (4) in the permeameter (1), open the automatic drain valve (14) on the drain pipe (104), and use the pore water pressure gauge (5) to clean the inside of the loading chamber. The water pressure is measured, and the data is collected by the data acquisition device (2); Process the data collected by the data acquisition device (2), and calculate the permeability coefficient; The specific calculation process of the described permeability coefficient is as follows: Step 1: Calculate the permeability coefficient of soil samples between two adjacent pore water pressure gauges. The calculation formula is as follows:

where: ξ is the reduction factor, n _x is the porosity of the foamed soil under the vertical total stress, v is the kinematic viscosity of water, g is the acceleration of gravity, n _fi is the porosity of the foam under the water pressure p _i , d _{10 , fi} is the effective particle size of the foam under the water pressure _pi , and the water pressure _pi is the average value of the two adjacent pore water pressure gauges; Step 2: Calculate the overall equivalent permeability coefficient, the calculation formula is as follows:

where l is the height of the foamed soil sample, _hi is the seepage path of the soil sample between two adjacent pore water pressure gauges; The process of solving the reduction factor ξ is as follows:

where d _10,s is the effective particle size of the soil particles in the foamed soil sample. 2. foamed soil permeability coefficient testing method under total stress according to claim 1, is characterized in that: the porosity _nfi of foam under water pressure _pi is calculated by following formula:

In the formula: e _fi is the void ratio of the foam under the water pressure p _i , FER is the foaming ratio of the foam, and p is the atmospheric pressure. 3. foamed soil permeability coefficient testing method under total stress according to claim 1, is characterized in that: the effective particle diameter d 10 under water pressure _{pi , fi} is calculated by following formula:

where: d _10,f is the effective particle size of the foam at atmospheric pressure p.

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