CN115266526B - A starting pressure gradient simulation test device and its use method and application - Google Patents

Abstract

The invention discloses a starting pressure gradient simulation test device and a using method and application thereof, and provides a reliable technical scheme for determining a starting pressure gradient by determining a starting pressure gradient simulation test device and a using method of a blind ditch material on one hand; and secondly, providing a blind ditch material selection method, testing the gas permeability coefficient of the blind ditch material through a starting pressure gradient simulation test device, substituting the material gas permeability coefficient into a functional formula of the starting pressure gradient, solving the starting pressure gradient of the blind ditch material, then solving the blind ditch exhaust starting pressure difference according to the product of the seepage path length of blind ditch exhaust and the starting pressure gradient, and finally comparing the blind ditch exhaust starting pressure difference with the earth covering weight on the membrane to determine whether the blind ditch material type is feasible or not, thereby avoiding the problems of exhaust smoothness and exhaust displacement in blind ditch design, and further reducing the risk of engineering operation and maintenance.

Description

A starting pressure gradient simulation test device and its use method and application

Technical Field

The invention belongs to the technical field of blind ditch engineering, and in particular relates to a starting pressure gradient simulation test device and a use method and application thereof.

Background Art

A blind ditch is a fluid drainage channel set up below the surface of the site. It is a hidden ditch filled with crushed stone, gravel and other coarse-grained materials and paved with a filter layer and permeable pipes to drain and intercept groundwater fluid.

When the reservoir site is located in a thick, highly permeable silt, sandy soil or gravel layer, and lacks an effective waterproof layer, the leakage problem of the site stratum is currently generally solved by using geomembrane at the bottom of the reservoir. However, the following problems arise: For example, the Xincheng Reservoir in Zibo City, Shandong Province uses a 0.3mm thick polyethylene PE geomembrane for anti-seepage. The reservoir water level drops at a rate of 1.0 to 1.5 cm/day during operation, and the water level in the interception ditch changes due to the reservoir water level, indicating that the reservoir is still leaking.

When the groundwater level is relatively deep, there is a large amount of porous gas under the geomembrane at the bottom of the reservoir; if the groundwater level rises, it can cause gas accumulation under the geomembrane; but the plane size of the plain reservoir is large, and the length of the seepage path for horizontal exhaust is too long, which makes exhaust difficult. Therefore, the cause of reservoir leakage in the current geomembrane anti-seepage scheme is mostly caused by gas expansion under the geomembrane. The factors affecting the gas expansion under the membrane are mainly the drop in reservoir water level and the rise in groundwater level. When setting a blind ditch under the geomembrane, the exhaust property and exhaust effect of the blind ditch structure should be considered. Although the blind ditch material is coarse-grained and has high permeability, the huge seepage path length not only greatly reduces the seepage pressure gradient and reduces the exhaust volume, but also increases the impact of the initial pressure gradient on the smoothness of blind ditch exhaust.

Therefore, optimizing the ventilation effect of the blind ditch used for geomembrane waterproofing, and avoiding the problems of ventilation smoothness and exhaust volume in the blind ditch design, thereby reducing the risk of project operation and maintenance, has become a technical problem that needs to be solved urgently.

Summary of the invention

Technical problem solved: In response to the above technical problem, the present invention provides a starting pressure gradient simulation test device and its use method and application, which can effectively solve the shortcomings of improper selection of blind ditch materials, resulting in poor exhaust effect and further leakage.

Technical solution: In a first aspect, the present invention provides a starting pressure gradient simulation test device, comprising a first pressure stabilizing chamber, a sample tube, a second pressure stabilizing chamber, a first cover, a first negative pressure gauge, a first vacuum pump, an air inlet pipe, a first pipe, a second cover, a second negative pressure gauge, a second vacuum pump, an air outlet pipe, and a second pipe;

The first sealing cover is arranged at the top of the first pressure stabilizing chamber, and is provided with four through holes, which are respectively connected to the first negative pressure gauge, the first vacuum pump, the first pipeline and one end of the air intake pipeline through connecting pipes, the other end of the air intake pipeline is connected to the air intake end of the sample tube, and the first pipeline is provided with a first negative pressure regulating valve;

The second cover is arranged on the top of the second pressure stabilizing chamber, and is provided with four through holes, which are respectively connected to the second negative pressure gauge, the second vacuum pump, the second pipeline and one end of the gas outlet pipeline through connecting pipes, the other end of the gas outlet pipeline is connected to the gas outlet end of the sample tube, and the second pipeline is provided with a second negative pressure regulating valve;

A gas flow meter is provided on the gas outlet pipeline.

Preferably, support frames are provided below the air inlet pipe and the air outlet pipe.

Preferably, a first valve is provided on the air inlet pipe, and a second valve is provided on the air outlet pipe.

Preferably, the sample tube is a polyfluoro tube, and is provided with connecting covers at both ends. The connecting covers are provided with connecting holes, and the two ends of the sample tube are respectively connected to the air inlet pipe and the air outlet pipe through the connecting holes.

In a second aspect, the present invention provides a method for using the device according to the first aspect, comprising the following steps:

S1, close the first valve and the second valve, open the first vacuum pump, adjust the first negative pressure regulating valve, and when the indication of the first negative pressure gauge reaches the target value, close the first vacuum pump and the first negative pressure regulating valve; open the second vacuum pump, adjust the second negative pressure regulating valve, and when the indication of the second negative pressure gauge reaches the target value, close the second vacuum pump and the second negative pressure regulating valve;

S2. Open the first valve and the first vacuum pump to allow the gas in the first pressure-stabilizing chamber to enter the sample tube. Apply soapy water to the connection between the air inlet pipe and the sample tube. If there are no obvious bubbles or air leakage, it indicates that the connection between the air inlet pipe and the sample tube is well sealed. Close the first valve and the first vacuum pump, and perform the same operation to detect the sealing of the connection between the air outlet pipe and the sample tube to ensure that the overall sealing of the device meets the test requirements.

S3, installing a sample tube containing a sample material;

S4, opening the first valve and the second valve, adjusting the first negative pressure regulating valve and the second negative pressure regulating valve, so that the readings of the first negative pressure gauge and the second negative pressure gauge are target values, and a stable seepage state is formed in the sample tube containing the sample material;

S5. Record the indication V of the gas flow meter, the indication P1 of the first negative pressure gauge, and the indication P2 of the second negative pressure gauge.

In a third aspect, the present invention provides an application of the device described in the first aspect in blind ditch material selection, comprising the following steps:

1) Determine the gas flow rate v and the gas pressure gradient i by using a simulation test device; wherein,

Where: V-flow rate, m ³ /s; A-sample tube cross-sectional area, m ² ; v-flow rate, kPa/s; g is the gravity conversion factor, which is 9.832N/kg; ρ _p is the density of the gas under pressure p, kg/m ³ , determined by the following formula (2):

Where p is the average air pressure in the sample tube, kPa, T is the sample temperature during the test, °C; μ is the molar mass of the gas molecule, which is 29 for air;

Where: i-pressure gradient, kPa/m; P1-pressure in the first stabilizing chamber, kPa; P2-pressure in the second stabilizing chamber, kPa; l-length of the sample tube, m;

2) The gas flow rate v and the pressure gradient i in step 1) are fitted using a linear relationship function, as shown in the following formula (4):

v＝k(i-ξ) (4)

Where: v-gas flow rate, kPa/s; i-gas pressure gradient, kPa/m; k-gas permeability coefficient, m/s; ξ-initial pressure gradient, kPa/m;

3) Establish the functional relationship between the initial pressure gradient ξ and the permeability coefficient k, as shown in the following formula (5):

ξ(k)＝A ₁ k ^B (5)

Where: A ₁ and B are fitting constants;

4) Determine the gas permeability coefficient k of the blind ditch material and substitute it into formula (5) to obtain the initial pressure gradient ξ of the blind ditch;

5) The seepage path length L of the blind ditch is multiplied by the initial pressure gradient ξ to obtain the seepage starting pressure difference Δp _m , that is:

Δp _m = L·ξ (6)

The seepage path length L is the equivalent circle radius of the reservoir geomembrane anti-seepage area Area, that is:

Where, Area is the anti-seepage area of the reservoir geomembrane, ^m2 ;

The seepage start pressure difference Δp _m is the minimum pressure for blind ditch exhaust start;

6) Compare the blind ditch exhaust starting pressure difference with the soil weight on the geomembrane. If the blind ditch exhaust starting pressure difference is less than the soil weight on the geomembrane, it means that the selected blind ditch material is feasible.

Beneficial effects: On the one hand, the present invention provides a simulation test device and a method for determining the initial pressure gradient of a blind ditch material, which provides a reliable technical solution for determining the initial pressure gradient;

On the other hand, the present invention proposes a method for selecting blind ditch materials, in which the gas permeability coefficient of the blind ditch material is tested by an initial pressure gradient simulation test device, and the material gas permeability coefficient is substituted into the function of the initial pressure gradient to calculate the initial pressure gradient of the blind ditch material, and then the blind ditch exhaust starting pressure difference is calculated according to the product of the seepage path length of the blind ditch exhaust and the initial pressure gradient, and finally the blind ditch exhaust starting pressure difference is compared with the soil cover weight on the membrane to determine whether the blind ditch material type is feasible, thereby avoiding the exhaust smoothness and exhaust volume problems in the blind ditch design, thereby reducing the risk of engineering operation and maintenance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic structural diagram of a starting pressure gradient simulation test device according to the present invention;

FIG2 is a schematic diagram of a sample tube of the present invention after being loaded with different blind channel materials, wherein (A) is a sample tube loaded with medium-coarse sand material, (B) is a sample tube loaded with gravel, and (C) is a test tube loaded with gravel and empty tube material;

FIG3 is a graph showing the relationship between the gas flow rate v and the gas pressure gradient i in Example 2;

FIG4 is a functional relationship diagram of the initial pressure gradient ξ and the permeability coefficient k in Example 2;

FIG5 is a schematic diagram of two blind groove materials in Example 3;

Serial numbers in the figure: 1. first pressure-stabilizing chamber, 2. sample tube, 3. second pressure-stabilizing chamber, 4. first cover, 5. second cover, 6. first negative pressure gauge, 7. second negative pressure gauge, 8. first vacuum pump, 9. second vacuum pump, 10. air inlet pipe, 11. air outlet pipe, 12. support frame, 13. gas flow meter, 14. first pipe, 15. second pipe, 16. first valve, 17. second valve.

DETAILED DESCRIPTION

The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments:

Example 1

As shown in Fig. 1-Fig. 2, a starting pressure gradient simulation test device comprises a first pressure stabilizing chamber 1, a sample tube 2, a second pressure stabilizing chamber 3, a first cover 4, a first negative pressure gauge 6, a first vacuum pump 8, an air inlet pipe 10, a first pipe 14, a second cover 5, a second negative pressure gauge 7, a second vacuum pump 9, an air outlet pipe 11 and a second pipe 15, wherein the first cover 4 is arranged at the top of the first pressure stabilizing chamber 1, and 4 through holes are opened thereon, and the 4 through holes are respectively connected to the first negative pressure gauge 6, the first vacuum pump 8, the first pipe 14 and the air inlet pipe 10 through connecting pipes. The first pipe 14 is provided with a first negative pressure regulating valve; the second sealing cover 5 is provided on the top of the second pressure stabilizing chamber 3, and 4 through holes are opened thereon, and the 4 through holes are respectively connected to the second negative pressure gauge 7, the second vacuum pump 9, the second pipe 15 and one end of the gas outlet pipe 11 through connecting pipes, the other end of the gas outlet pipe 11 is connected to the gas outlet end of the sample tube 2, and the second pipe 15 is provided with a second negative pressure regulating valve; the gas outlet pipe 11 is provided with a gas flow meter 13.

A support frame 12 is provided below the air inlet pipe 10 and the air outlet pipe 11 .

The air inlet pipe 10 is provided with a first valve 16 , and the air outlet pipe 11 is provided with a second valve 17 .

The sample tube 2 is a polytetrafluoroethylene tube, and is provided with connection covers at both ends. The connection covers are provided with connection holes, and the two ends of the sample tube 2 are connected to the air inlet pipe 10 and the air outlet pipe 11 respectively through the connection holes.

Example 2

The inner diameters of the PTFE tubes were 0.5 mm, 0.6 mm, 0.8 mm, 1.0 mm, 1.2 mm, and 1.5 mm, and the length l was 0.5 m, which was used to determine the correlation between the pressure gradient i and the gas permeability coefficient k. The test data when the sample tube length l = 0.5 m and the inner diameter of the PTFE tube = 0.5 mm are shown in Table 1.

Table 1 Test data of temperature T = 10°C, sample tube length l = 0.5m, PTFE tube inner diameter = 0.5mm

Note: The cross-sectional area of the PTFE tube with an inner diameter of 0.5 mm is A = 0.1963 mm ² ; Q = Av, that is, the gas flow rate is the product of the cross-sectional area A of the PTFE tube and the flow velocity v.

Then, the relationship between the pressure gradient i and the gas flow rate v is plotted with the pressure gradient i as the ordinate and the gas flow rate v as the abscissa, as shown in Figure 3. The linear function equation is i=16.084v+1.9231, the correlation R ² =0.9963, and the intercept 1.9231 is the starting pressure gradient value, indicating that the gas flow can only be started when the pressure gradient i is greater than 1.9231 kPa/m. Further calculations show that the gas permeability coefficient k=1/16.084=6.217 cm/s, and the starting pressure gradient ξ=1.9231 kPa/m.

The test data of PTFE tubes with different inner diameters were analyzed to obtain the corresponding gas permeability coefficient k and initial pressure gradient ξ. The results are shown in Table 2:

Table 2 Results of initial pressure gradient ξ of PTFE tube under different permeability coefficients k

According to the results in Table 2, the functional relationship between the initial pressure gradient ξ and the permeability coefficient k is shown in Figure 4, and it is obtained:

ξ＝1132k ^-3.41 (5)

That is, in formula (5), A ₁ =1132, B =-3.41.

Example 3

First, select typical blind ditch structural materials. As shown in Figure 5, there are two types of sample tubes 2, namely, structure A is a combination of gravel and geotextile blind tubes, and structure B is gravel; then, measure the gas permeability coefficient k of the blind ditch material, and then according to

ξ＝1132k ^-3.41 (5)

Determine the initial pressure gradient ξ; furthermore, according to the plane size or area of the reservoir, calculate the seepage path length of the blind ditch by formula (7), that is,

Where, Area is the anti-seepage area of the reservoir geomembrane, ^m2 ;

Finally, according to the product of the seepage path length L of the blind ditch exhaust and the initial pressure gradient ξ,

Δp _m = L·ξ (6)

Calculate the blind ditch exhaust starting pressure difference Δp _m , and compare the blind ditch exhaust starting pressure difference Δp _m with the soil weight on the membrane to determine whether the blind ditch type is feasible.

For example, the area of Xixia Reservoir Expansion Project is 2.09 km ² , i.e. 2.09×10 ⁶ m ² . Formula (7) is equivalent to a circle of equal area, and its radius is calculated as the length of the blind ditch from the center of the reservoir to the outside of the dike, which is about 815 m. According to the calculation results in Table 3, the blind ditch under the membrane of Xixia Reservoir Expansion Project can only ensure the smoothness of exhaust if the structure of gravel and blind pipe is adopted.

Table 3 Gas permeability coefficients of different materials in blind ditch

Blind ditch structure type Gravel and dead-end Gravel 1 Gravel 2 Gravel 3 Average particle size d ₅₀ (mm, by weight) / 35 14 10 Unequal particle size coefficient d ₆₀ /d ₁₀ / 2.7 2 6.3 Permeability coefficient (cm/s) 50 20 10 5 Starting pressure gradient (kPa/m) 0.00182 0.04145 0.44063 4.68370 Blind ditch seepage path length (m) 815 815 815 815 Starting pressure difference (kPa) 1.49 33.78 359.12 3817.21 Less than the soil weight on the membrane (20kPa) yes no no no Feasibility determination of blind ditch exhaust feasible Not feasible Not feasible Not feasible

The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims (7)

1. The utility model provides a starting pressure gradient simulation test device which characterized in that: the device comprises a first pressure stabilizing cavity (1), a sample tube (2), a second pressure stabilizing cavity (3), a first sealing cover (4), a first negative pressure meter (6), a first vacuum pump (8), an air inlet pipeline (10), a first pipeline (14), a second sealing cover (5), a second negative pressure meter (7), a second vacuum pump (9), an air outlet pipeline (11) and a second pipeline (15);

The first sealing cover (4) is arranged at the top of the first pressure stabilizing cavity (1), 4 through holes are formed in the first sealing cover, the 4 through holes are respectively connected with one end of a first negative pressure meter (6), a first vacuum pump (8), a first pipeline (14) and one end of an air inlet pipeline (10) through connecting pipes, the other end of the air inlet pipeline (10) is connected with the air inlet end of the sample tube (2), and a first negative pressure regulating valve is arranged on the first pipeline (14);

The second sealing cover (5) is arranged at the top of the second pressure stabilizing cavity (3), 4 through holes are formed in the second sealing cover, the 4 through holes are respectively connected with one end of a second negative pressure meter (7), a second vacuum pump (9), a second pipeline (15) and one end of an air outlet pipeline (11) through connecting pipes, the other end of the air outlet pipeline (11) is connected with the air outlet end of the sample tube (2), and a second negative pressure regulating valve is arranged on the second pipeline (15);

the air outlet pipeline (11) is provided with an air flowmeter (13).

2. The initial pressure gradient simulation test apparatus according to claim 1, wherein: the lower parts of the air inlet pipeline (10) and the air outlet pipeline (11) are respectively provided with a supporting frame (12).

3. The initial pressure gradient simulation test apparatus according to claim 1, wherein: the air inlet pipeline (10) is provided with a first valve (16), and the air outlet pipeline (11) is provided with a second valve (17).

4. The initial pressure gradient simulation test apparatus according to claim 1, wherein: the sample tube (2) is a tetrafluoro tube, connecting covers are arranged at two ends of the sample tube, connecting holes are formed in the connecting covers, and two ends of the sample tube (2) are respectively connected with the air inlet pipeline (10) and the air outlet pipeline (11) through the connecting holes.

5. A method of using the device of any one of claims 1-4, comprising the steps of:

S1, closing a first valve (16) and a second valve (17), opening a first vacuum pump (8), adjusting a first negative pressure regulating valve, and closing the first vacuum pump (8) and the first negative pressure regulating valve when the indication of a first negative pressure meter (6) reaches a target value; opening a second vacuum pump (9), adjusting a second negative pressure regulating valve, and closing the second vacuum pump (9) and the second negative pressure regulating valve when the indication of the second negative pressure gauge (7) reaches a target value;

S2, opening a first valve (16) and a first vacuum pump (8) to enable gas in the first pressure stabilizing cavity (1) to enter the sample tube (2), coating soapy water on the joint of the air inlet pipeline (10) and the sample tube (2), and ensuring that the joint of the air inlet pipeline (10) and the sample tube (2) is good in tightness when no obvious bubble or air leakage phenomenon exists, and closing the first valve (16) and the first vacuum pump (8), and detecting the tightness of the joint of the air outlet pipeline (11) and the sample tube (2) by the same operation to ensure that the integral tightness of the device meets test requirements;

S3, installing a sample tube (2) filled with sample materials;

S4, opening a first valve (16) and a second valve (17), and adjusting the first negative pressure regulating valve and the second negative pressure regulating valve to enable the indication numbers of the first negative pressure gauge (6) and the second negative pressure gauge (7) to be target values, so that a stable seepage state is formed in the sample tube (2) filled with the sample material;

S5, recording an indication V of the gas flowmeter (13), an indication P1 of the first negative pressure meter (6) and an indication P2 of the second negative pressure meter (7).

6. Use of the device of any one of claims 1-4 for blind drain material selection.

7. The use according to claim 6, characterized by the steps of:

1) Determining the gas flow velocity v and the gas pressure gradient i through a simulation test device; wherein,

Wherein: v-flow, m ³/s; a-cross-sectional area of sample tube, m ²; v-flow rate, kPa/s; g is a gravity conversion coefficient, and 9.832N/kg is taken; ρ _p is the density of the gas under pressure p, kg/m ³, determined by the following formula (2):

Wherein, p is the average air pressure in the sample tube, kPa, P1-the air pressure in the first pressure stabilizing cavity, kPa; p2-the air pressure in the second pressure stabilizing cavity, kPa; t is the temperature of the sample during the test, the temperature is lower than the temperature; the molar mass of the gas molecules is 29 for air;

wherein: i-pressure gradient, kPa/m; l-sample tube length, m;

2) Fitting the gas flow velocity v and the gas pressure gradient i in the step 1) by adopting a linear relation function, wherein the linear relation function is shown as the following formula (4):

Wherein, k-gas permeability coefficient, m/s, xi-initial pressure gradient, kPa/m;

3) Establishing a functional relation between the initial pressure gradient xi and the gas permeability coefficient k, wherein the functional relation is shown in the following formula (5):

Wherein: a ₁ and B are fitting constants;

4) Measuring the gas permeability coefficient k of the blind ditch material, and substituting the gas permeability coefficient k into the formula (5) to obtain the initial pressure gradient xi of the blind ditch;

5) The seepage path length L of the blind ditch and the initial pressure gradient xi are integrated to obtain a seepage starting pressure difference p _m, namely:

The seepage path length L is the equivalent circular radius of the geomembrane seepage-proof Area of the reservoir disc, namely:

Wherein, the seepage prevention Area of the geomembrane of the Area-reservoir plate, m ²;

the seepage start pressure difference, p _m, is the minimum pressure of blind drain exhaust start;

6) And comparing the blind drain exhaust starting pressure difference with the earth covering weight on the geomembrane, and if the blind drain exhaust starting pressure difference is smaller than the earth covering weight on the geomembrane, indicating that the selected blind drain material is feasible.