CN114411616A - Blind ditch exhaust optimization method for seepage prevention of reservoir geomembrane - Google Patents

Blind ditch exhaust optimization method for seepage prevention of reservoir geomembrane Download PDF

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CN114411616A
CN114411616A CN202210241588.4A CN202210241588A CN114411616A CN 114411616 A CN114411616 A CN 114411616A CN 202210241588 A CN202210241588 A CN 202210241588A CN 114411616 A CN114411616 A CN 114411616A
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blind ditch
reservoir
geomembrane
exhaust
gas
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CN114411616B (en
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曹雪山
袁俊平
丁国权
王骏
吴琦
沈正茂
廖志彬
白鹭
党仕全
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Yinchuan China Railway Water Group Co ltd
Hohai University HHU
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Yinchuan China Railway Water Group Co ltd
Hohai University HHU
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    • E—FIXED CONSTRUCTIONS
    • E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
    • E02B—HYDRAULIC ENGINEERING
    • E02B3/00—Engineering works in connection with control or use of streams, rivers, coasts, or other marine sites; Sealings or joints for engineering works in general
    • E—FIXED CONSTRUCTIONS
    • E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
    • E02B—HYDRAULIC ENGINEERING
    • E02B3/00—Engineering works in connection with control or use of streams, rivers, coasts, or other marine sites; Sealings or joints for engineering works in general
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Abstract

The invention discloses a blind ditch exhaust optimization method for seepage prevention of reservoir geomembranes. When the invention exhausts through the blind ditch exhaust system, the air resistance balance is considered, even if the weight of the air in the exhaust process 1 of the air from the soil body unit to the blind ditch is the same as that in the exhaust process 2 of the air from the blind ditch to the atmosphere, the exhaust time is the same, and the air pressure under the geomembrane is less than or equal to the critical value of the air pressure. The invention not only ensures the smoothness of gas discharge under the geomembrane, reduces the air resistance under the geomembrane, further reduces the gas pressure under the geomembrane, but also ensures that the air pressure under the geomembrane does not exceed a critical value, and solves the problems of air inflation under the geomembrane caused by the rising of the underground water level of the reservoir and difficult gas discharge caused by overlong horizontal direction gas discharge path of a plain reservoir at present.

Description

Blind ditch exhaust optimization method for seepage prevention of reservoir geomembrane
Technical Field
The invention relates to a blind ditch exhaust system and an exhaust method, in particular to a blind ditch exhaust optimization method for seepage prevention of reservoir geomembranes.
Background
The blind ditch is a fluid drainage channel arranged below the ground surface of a field and is a blind ditch for filling crushed stones, gravels and other coarse-grained materials, paving a reverse filter layer and arranging water-permeable pipes and intercepting underground water gas fluid.
When the reservoir site distribution stratum of the reservoir is a silt, sandy soil or gravel soil stratum with large thickness and strong water permeability and is lack of an effective water barrier, the seepage problem of the site stratum is generally solved by adopting reservoir bottom geomembrane for seepage prevention at present, but the following problems occur: for example, a thick polyethylene PE geomembrane seepage-proofing scheme of 0.3mm is adopted in a new city reservoir in Zibo city in Shandong province, the reservoir water level reduction speed in the operation process is 1.0-1.5 cm/day, and the water level in the seepage-stopping trench is changed under the influence of the reservoir water level, so that the reservoir still leaks.
When the underground water level is relatively deep, a large amount of pore gas exists under the reservoir bottom geomembrane; if the underground water level rises, the gas under the geomembrane can be caused to gather; however, the plain reservoir has a large plane size, and the length of the horizontal exhaust path is too large, so that the exhaust is difficult. Therefore, the reservoir seepage reason of the current geomembrane seepage-proofing scheme is mostly caused by the inflation under the geomembrane. The factors influencing the flatulence under the membrane are mainly the water level of the reservoir and the underground water level. When the blind ditch is arranged under the geomembrane, the air permeability and the exhaust effect of the blind ditch structure are considered, and the air resistance of the blind ditch is closely related to the exhaust effect of the blind ditch.
Therefore, how to optimize the blind ditch exhaust method for geomembrane seepage prevention and further avoid the problems in blind ditch design, thereby reducing the risk of engineering operation and maintenance, and becoming a technical problem to be solved urgently.
Disclosure of Invention
The purpose of the invention is as follows: aiming at the defects that gas under the geomembrane is accumulated due to rising of underground water level, gas expansion under the geomembrane is caused, and exhaust is difficult due to overlong exhaust path in the horizontal direction of a plain reservoir in the prior art, the invention provides the blind ditch exhaust optimization method for seepage prevention of the reservoir geomembrane.
The technical scheme is as follows: when the blind ditch exhaust optimization method for seepage prevention of the reservoir geomembrane is implemented, firstly, a blind ditch exhaust system is laid; the blind ditch exhaust system comprises a chessboard-type blind ditch positioned below the geomembrane at the bottom of the reservoir and a plurality of soil body units divided by the blind ditch. When the air is exhausted through the blind ditch exhaust system, the air resistance balance is considered, namely the weight of the air in the exhaust process 1 from the soil body unit to the blind ditch of the air in the blind ditch exhaust system is the same as that of the air in the exhaust process 2 from the blind ditch to the atmosphere, the exhaust time is the same, and the air pressure under the geomembrane is less than or equal to the critical value of the air pressure.
The exhaust process 1 includes steps (1) to (3); the exhaust process 2 includes steps (4) to (6); the method comprises the following specific steps:
(1) obtaining the volume V of the exhaust gas in the exhaust process 1 according to the blind ditch space at the center of the reservoir, the underground water level design burial depth, the soil body pore ratio and the gas saturation1；
V1＝[(1-Sr)*H*S2*e]/2 (1)
Wherein: e is the average pore ratio of the soil under the geomembrane; srIs the gas saturation; s is the blind ditch spacing; h is the designed buried depth of the underground water level;
(2) setting the critical value of the air pressure under the geomembrane as ptAnd p istPressure load on geomembrane ≦ geomembrane; simultaneously setting the air pressure difference between the soil body unit at the center of the reservoir and the blind ditch as delta p1(ii) a The average air pressure p between the soil body unit and the blind ditch is obtained by the formula (2)1(ii) a The gas density rho in the soil body unit is obtained by the formula (3)p1(ii) a Gas gravity γ from equation (4)p1(ii) a The weight Q of the gas discharged from the soil body unit at the center of the reservoir to the blind ditch is obtained by the formula (5)1：
Figure BDA0003542308570000021
Figure BDA0003542308570000022
γp1＝ρp1g (4)
Q1＝V1*γp1 (5)
Wherein, Δ ptTo set sub-geomembrane pressure threshold, Δ p1The air pressure difference between the soil body unit at the center of the reservoir and the blind ditch is obtained; rhop1At a pressure p1Density of the gas under conditions; p is a radical of1The average air pressure from the soil body unit at the center of the reservoir to the blind ditch; t is the temperature of gas under the geomembrane, and the unit is centigrade; mu is the molar mass of gas molecules under the geomembrane, and the value of mu for air is 29; gamma rayp1At a pressure p1Gas severity under conditions; g is a gravity conversion coefficient, and 9.832N/kg is taken; q1The weight of the gas discharged from the soil body unit at the center of the reservoir to the blind ditch;
(3) obtaining the exhaust time t from the soil body unit at the center of the reservoir to the blind ditch in the exhaust process 1 according to the formula (6) and the formula (7)1：
Figure BDA0003542308570000023
Figure BDA0003542308570000024
Wherein k isa1Is the air permeability coefficient of the soil body unit; l is1The length of the seepage path from the soil body unit to the blind ditch; a. the1The contact area between the soil body unit and the blind ditch is defined; v. of1The gas flow rate when the soil body unit exhausts to the blind ditch; t is t1The exhaust time from the soil body unit at the center of the reservoir to the blind ditch is determined;
(4) the air pressure delta p of the blind ditch exhausting from the center of the reservoir to the atmosphere is obtained by the formula (8)2：
Δp2＝pt-Δp1 (8)
Wherein p istTo set sub-geomembrane pressure threshold, Δ p1The air pressure difference between the soil body unit at the center of the reservoir and the blind ditch is obtained;
(5) weight Q of gas discharged from the center of reservoir to atmosphere by blind ditch2
Q2＝Q1
(6) The time t for exhausting the blind ditch from the center of the reservoir to the atmosphere is obtained by the formula (10)2：
Figure BDA0003542308570000031
Figure BDA0003542308570000032
Wherein k isa2The air permeability coefficient of the blind ditch; l is2The length of a seepage path for exhausting air from the center of the reservoir to the atmosphere is the blind ditch in the short side direction of the reservoir; a. the2The cross section area of the blind ditch; v. of2The gas flow rate when the blind ditch exhausts gas from the center of the reservoir to the atmosphere; t is t2The duration of the air exhaust from the center of the short side direction of the reservoir to the blind ditch at the side of the dyke is determined; q2The amount of gas discharged from the center of the reservoir to the atmosphere for the culverts;
(7) by adjustingAir pressure difference delta p between soil body unit at center of whole reservoir and blind ditch1And (4) repeating the calculation from the step (2) to the step (6) until t is met1＝t2；
(8) Obtaining the total exhaust time t under the soil unit geomembrane at the center of the reservoir12Is 2t1。
The blind ditch material is usually medium coarse sand, the step (6) shows that the medium coarse sand of the blind ditch material is changed into pebble so as to increase the permeability coefficient of the blind ditch, and then the exhaust time t of the exhaust process 2 is shortened by the calculation of the formulas (9) to (10)2。
Filling pebbles in the blind ditch, and adding the geotechnical blind pipe to increase the permeability coefficient of the blind ditch, so that the permeability coefficient of the blind ditch is further increased, and the exhaust time t of the exhaust process 2 is shortened by further calculating the formula (9) to (10)2。
In the step (2), the critical value p of the air pressure under the geomembrane is increased by increasing the pressure load on the geomembranetAnd further increase Δ p of exhaust process 1 and exhaust process 21、Δp2、v1、v2And the exhaust time is shortened.
In the step (6), the cross section area of the blind ditch is increased to further increase A1、A2Increase v1、v2And the exhaust time is shortened.
The working principle is as follows: when the blind ditch exhaust system exhausts, because the pore diameter of the blind ditch material is small and the pore channel is changed in a zigzag manner, the contact area between gas in pores and solid particles is large, and the mutual friction force between gas molecules and the surface of the material consumes the motion energy of the gas, thereby generating gas resistance and gas resistance effect.
When the air resistance of unit flow and unit length is equal to the power formed by the air pressure difference, the flow and the pressure difference have a linear relation and satisfy Darcy's law, so the pressure difference is regarded as the air resistance. Seepage air resistance of gas in blind ditch
Figure BDA0003542308570000036
The gas volume flow through the seepage path under a certain gas pressure difference is calculated and determined by the formula (11):
Figure BDA0003542308570000033
Figure BDA0003542308570000034
in the formula:
Figure BDA0003542308570000035
the air resistance of seepage in the seepage process, and the delta p is the pressure difference (kPa) in the seepage process; Δ V is the volume flow (m) of gas seepage3) (ii) a Δ t is the duration of gas infiltration(s); k is a radical ofaIs the air permeability coefficient (m/s) of the material; l is the length of the percolation path; a is the cross-sectional area of seepage; rhopIs the density of the gas at a pressure p (10)3kg/m3) (ii) a p is the gas pressure (kPa) in the percolation path; g is a gravity conversion coefficient, and 9.832N/kg is taken; v is the gas flow rate.
Wherein, formula (11) is a calculation formula of seepage air resistance, and is obtained from (11): the factor negatively correlated with the air-permeability resistance is the air permeability coefficient kaAnd a percolation cross-sectional area a; the factors positively correlated with the permeate gas lock are the gas flow rate v and the gas permeate path length L. From (12), the gas flow rate V is proportional to the gas volume Δ V, and Δ t is inversely proportional to time.
In engineering practice, when blind ditches are arranged, pebbles with large particle size are selected, or a geotechnical blind pipe is additionally arranged in the middle of the blind ditches to increase the permeability coefficient k of the blind ditchesaThereby further reducing the seepage air resistance.
For the cul-de-sac, the air-lock effect is length dependent. In the formula (11), the seepage air resistance increases with the increase of the path length; meanwhile, as for the discharged gas volume, the time for which the gas correspondingly flows increases as the path is lengthened, so if the gas passing through the path length is kept unchanged and the gas quantity is kept unchanged, the cumulative effect of the gas resistance is increased, and the cumulative effect of the gas resistance is the blind ditch gas resistance effect Z.
The formula (13) between the blind ditch length L and the blind ditch air resistance effect Z accumulated along the blind ditch length L is:
Figure BDA0003542308570000041
the blind air-lock effect Z in equation (13) is proportional to the square of the blind length L. Because the blind ditch length L is obviously influenced by the plane size of the plain reservoir, particularly the size of the short side, when the plane size of the reservoir is large, the air resistance of the blind ditch is correspondingly large.
The implementation principle of the invention is as follows:
firstly, aiming at the reservoir bottom of the whole reservoir, a reservoir with geomembrane for seepage control is adopted, the underground water of the reservoir is lower than the geomembrane surface, unsaturated soil is arranged under the reservoir, and the pores of the unsaturated soil are filled with gas. Because the reservoir bottom geomembrane isolates the connectivity of reservoir bottom unsaturated soil and atmosphere, when underground water rises, pore gas is compressed by the pore water to cause the air pressure under the geomembrane to increase, and the air pressure under the geomembrane is set to be delta ptWhen the sub-geomembrane pressure is Δ ptGreater than the load on the membrane, causes the geomembrane body to bulge.
Secondly, considering the air resistance balance, the air resistance balance refers to the same weight and the same time of the gas in the two processes of discharging the gas from the soil body unit to the blind ditch (process 1) and discharging the gas to the atmosphere through the blind ditch (process 2), and the air pressure under the geomembrane still meets the requirement of no air expansion, thereby not only achieving the smoothness of gas discharge in the whole system, but also ensuring that the air pressure under the geomembrane does not exceed the critical value of the air pressure and achieving the requirement of reservoir seepage prevention.
Considering the seepage process 1 of exhausting the soil body unit to the blind ditch: setting blind ditches on the foundation of the whole reservoir, when the pore pressure of soil under the geomembrane is increased, because the permeability of the blind ditches is larger than that of the soil and is communicated with the atmosphere, the air pressure in the blind ditches is low, so that a process that soil units enclosed by the blind ditches exhaust to the blind ditches is formed, namely a seepage process 1 for exhausting the soil units, wherein the air pressure difference between the centers of the corresponding soil units and the blind ditches is delta p1And the air pressure difference between the air pressure at the blind ditches at the periphery of the soil body unit and the atmospheric pressure is delta p2And satisfy Δ p2＝Δpt-Δp1。
Consider the seepage process 2 of blind trench exhaust to atmosphere: in the process, one end of the blind ditch is communicated with the atmosphere, the air pressure is zero, the air pressure in the blind ditch is gradually leaked and exhausted to the atmosphere, and the air pressure difference in the exhaust process 2 of the blind ditch is delta p2。
And then optimizing the exhaust process of the blind ditch exhaust system under the geomembrane. Considering the air exhaust smoothness, the weight and the time of the air exhaust in the two processes of the seepage process 1 of exhausting the air from the soil body unit to the blind ditch and the seepage process 2 of exhausting the air from the blind ditch to the atmosphere are the same, and meanwhile, the total air pressure delta p under the geomembrane is the sametThe requirement of the load on the geomembrane is still met and is less than or equal to that on the geomembrane, therefore, the air exhaust time t of the seepage process 1 of exhausting the soil body unit to the blind ditches is balanced by adjusting the space between the blind ditches1And the exhaust time t of the seepage process 2 for exhausting the blind ditches around the soil body units to the atmosphere2So that the total pressure deltap under the geomembranetLoad on the geomembrane is less than or equal to the load on the geomembrane, so that the smoothness of gas discharge in the whole system is achieved, the air pressure below the geomembrane is ensured not to exceed an air inflation critical value, and the seepage prevention requirement of a reservoir is ensured.
Has the advantages that: compared with the prior art, the invention has the following advantages:
(1) when optimized exhaust is carried out on the blind ditch under the geomembrane, the invention takes the smoothness of exhaust under the geomembrane into consideration, reduces the air resistance under the geomembrane, further reduces the gas pressure under the geomembrane, has obvious effect, not only ensures the smoothness of gas exhaust in the whole system, but also ensures that the gas pressure under the geomembrane does not exceed the critical value, and solves the problem of leakage of the stratum of the site in the reservoir area.
(2) In the embodiment of the invention, two working conditions are provided simultaneously when the exhaust blind ditch is not arranged under the geomembrane, so that the exhaust method is optimized, the exhaust time is shortened, and the occurrence of ballooning phenomenon under the geomembrane is prevented.
Drawings
FIG. 1 is a general plan view of a reservoir of the present invention;
FIG. 2 is a geological profile of a reservoir site;
FIG. 3 is a schematic diagram of the sub-geomembrane venting operation;
figure 4 is a schematic view of a square earth element in an embodiment of the invention;
FIG. 5 is a schematic layout of a blind ditch and soil mass units in an embodiment of the present invention;
fig. 6 is a schematic view of a blind trench structure in an embodiment of the invention.
Detailed Description
Example (b):
as shown in fig. 1 to 6, the present invention adopts geomembrane laid at the reservoir bottom of the reservoir for seepage prevention, the underground water level is lower than the geomembrane, and a blind ditch 3 is arranged under the geomembrane at the reservoir bottom of the reservoir. In fig. 1, 1 is a soil body unit in the center of a reservoir, and 2 is a dike.
Wherein fig. 4 and 5 show the arrangement of criss-cross blind ditches under the geomembrane. The soil body at the bottom of the reservoir is covered with the air-tight geomembrane, so that gas in pores of the soil body under the geomembrane cannot be directly discharged, and can only be discharged to the peripheral blind ditches, and then is discharged to the atmosphere through the blind ditches. In fig. 3, the process 1 is that the soil body unit exhausts to the blind ditch, and the process 2 is that the blind ditch exhausts to the atmosphere.
As shown in fig. 2, FC in the figure represents the intersection of longitudinal and transverse blind trenches, FD represents the blind trench, and S represents the distance between the centers of two parallel blind trenches; RG represents a reservoir bottom ground surface, FD represents a blind ditch, FG represents crushed stones filled in the blind ditch, FT represents a blind pipe in the blind ditch, GW represents an underground water line, H represents a high difference value from a geomembrane surface to an underground water level, namely an underground water level burial depth value, EC represents an excavated soil protection layer on the membrane, SC represents a fine sand protection layer on the membrane, and GM represents a geomembrane; FGT is filament geotextile between soil body and blind ditch and wrapping the geotechnical blind pipe.
Fig. 5 is a sectional view of fig. 4 taken along line 1-1. In fig. 5, GW denotes the ground water level, and H denotes the ground water level burial depth. When the groundwater level rises, H is reduced, the pore water pressure in the soil body under the geomembrane is increased, pore water extrudes pore gas, so that the pore gas pressure is increased, when the gas pressure under the geomembrane is greater than the load on the geomembrane, the geomembrane is bulged and deformed, and then the gas under the geomembrane is discharged to the blind ditches on the periphery. If the blind ditch can not effectively exhaust, the air pressure under the geomembrane is gradually increased, the condition that the air pressure under the geomembrane is greater than the load on the geomembrane can occur, and further the geomembrane is expanded.
As shown in fig. 6, the blind ditch in the blind ditch exhaust system of the invention consists of gravel FG, blind pipes FT, geotextile FGT between soil and gravel and wrapped around the blind pipes. GD stands for the basement floor or ground; a represents the width of the blind trench. The seepage is from high pressure to low pressure, and the gas enters the blind ditch gravel FG from the soil body through the geotextile FGT and then enters the blind pipe FT from the gravel FG through the geotextile FGT. One end of the blind ditch is arranged in the soil body, and the other end of the blind ditch is communicated with the atmosphere, so that the higher the permeability of the material in the blind ditch, the lower the corresponding air pressure.
As shown in fig. 1 to 6, in this embodiment, checkerboard-type blind ditches are arranged under the geomembrane at the bottom of the reservoir, and the space S between the blind ditches is 75 m. The cross section of the blind ditch is a square of 0.3m multiplied by 0.3 m. One strip is arranged in the blind ditch
Figure BDA0003542308570000061
And filling pebbles around the geotechnical blind pipe. The reservoir disc field is divided into square soil units by the checkerboard blind ditches, wherein the side length S of each square soil unit is 75m, and the shortest distance from the center of the soil unit at the center of the reservoir to the center of the dyke of the reservoir area is 516 m.
As shown in fig. 4 to 6, the square soil body unit at the center of the reservoir area is used as an object to be examined, and the side length S is 75 m. Underground water is 1m below ground, soil is gravel soil, porosity e is 0.2, and saturation SrIs 0.
When the reservoir is rectangular, the length of the blind ditch air seepage path on the long side of the reservoir is large, and the corresponding air resistance is large, so that the air exhaust working condition in the short side direction of the reservoir is only considered. Considering that the soil body units are surrounded by blind ditches at the front, the back, the left and the right, namely 4 blind ditches are surrounded to form one soil body unit, each blind ditch only has 1/2 to play a role in exhausting the soil body unit, the volume of the gas to be exhausted by the corresponding single blind ditch is 1/2 of the total volume of the exhausted gas from the soil body unit to the blind ditch, and then V is formed by V1(m3)＝75*75*1*0.2/2＝562.5m3. Here, the porosity is 0.2, the gas saturation is 0, 1 is the groundwater level depth, and 75 is the blind ditch spacing.
In the present embodiment, the first and second electrodes are,the geomembrane is filled with a fine sand protective layer SC and an excavating material protective layer EC, and the total load on the geomembrane is 20kPa, so that the air pressure delta p under the geomembrane is ensured not to bulget≦ 20 kPa. The exhaust under the geomembrane is completed by two processes: the process 1 is that the square soil body unit surrounded by the blind ditch in the center of the reservoir exhausts to the blind ditch; and 2, the blind ditch at the center end of the reservoir is communicated with the atmosphere at the inner side of the dike, and the blind ditch at the center end of the reservoir exhausts air to the dike, namely the atmosphere. In order to ensure the smoothness of exhaust and avoid ballooning, the following three conditions are met:
(1) the total exhaust amount in the process 1 is the same as that in the process 2, and the total exhaust amount is the amount of gas in the central soil body unit of the reservoir area;
(2) the exhaust time of the process 1 and the process 2 is the same;
(3) sum of pressure differences between Process 1 and Process 2, i.e. sub-geomembrane pressure Δ ptAnd (4) the critical pressure value of the ballooning is less than or equal to.
Calculation of exhaust Process 1: the length of the seepage path is half the width of the soil body unit, and each soil body unit seeps from the center to the periphery, so that the seepage path is half the width. The seepage cross-sectional area A is the contact area of the blind ditch and the soil body unit, namely A is 75, 0.3, 1.5, 33.75m2. Wherein 1.5 is the side length of half of the blind ditch; 0.3 is the side length of the blind ditch.
The gas permeation velocity v, and the pressure difference Δ p within the gas permeation path length L are calculated according to darcy's law equation (1).
Figure BDA0003542308570000071
The specific calculation process for the exhaust process 1 is:
setting the critical value of the air pressure under the geomembrane as ptAnd p istPressure load on geomembrane ≦ 20 kPa; the permeability coefficient k of the soil body unita1Length L of seepage path from soil body unit to blind ditch1Contact area A of soil body unit and blind ditch1Obtaining the seepage velocity v of the soil body unit discharged to the blind ditch according to the formula (6)1：
TABLE 1 seepage velocity of exhaust air of soil body units
Figure BDA0003542308570000072
The gas density is related to the pressure, and the gas density rho in the soil body unit is obtained by the formulas (2) to (3)p1Then the gas gravity gamma is obtained by the formula (4)p1(ii) a The weight conversion factor g was 9.832N/kg, and the weight was 1.41X 10-2kN·m-3。
TABLE 2 calculation of gas severity mean
Figure BDA0003542308570000073
Total exhaust volume V of seepage process 1 for exhausting soil body unit to blind ditch1The result is obtained by formula (1); then, the weight Q of the gas discharged from the soil body unit to the blind ditch is obtained by the formula (5)1(ii) a The exhaust time is obtained by equation (7).
TABLE 3 calculation of gas discharge time in soil cells
Exhaust gas volume Q1/kN Volume of exhaust V1/m3 Time t1/s Time/day
7.94 562.50 3.264E+04 0.378
Calculation of seepage process 2 of blind drain to atmosphere exhaust:
wherein, is Δ Q2Weight of gas discharged to atmosphere for the blind ditch at the center end of reservoir, v2The gas flow rate when the blind ditch at the center end of the reservoir exhausts to the atmosphere.
The exhaust cross section area of the blind ditch is determined by the sectional size of the blind ditch, A2＝0.3*0.3＝0.09m2. The length of the blind ditch is L from the center of the soil body unit at the center of the reservoir area to the inner side boundary of the dike2513-37.5-479.5 m. The air pressure in the blind groove was determined by equation (8), i.e., 20 to 13.52 — 6.48 kPa. Solving the seepage velocity v according to the formula (1); then, the exhaust amount and flow velocity are used to calculate the exhaust time t according to the formula (6)2. Exhaust time t of exhaust process 2 from blind drain to atmosphere in Table 42Is equal to the discharge time t of the soil body unit to the blind ditch exhaust process 11(ii) a Finally, the total exhaust time t under the soil body unit geomembrane at the center of the reservoir is obtained12＝2t16.528E +4 s.
TABLE 4 calculation of exhaust Capacity of Blind and gas Exit time of Process 2
Figure BDA0003542308570000081
When no exhaust blind ditch is arranged under the geomembrane:
taking the reservoir scale in this embodiment as an example, but no exhaust blind ditch is arranged under the geomembrane, the reservoir with unit length is taken as a research object, at this time, the full section of the soil under the geomembrane directly exhausts to the atmosphere, the width is 1m, the depth is 1m of the underground water level buried depth, the porosity is 0.2, the gas saturation is 0, the underground water level depth is 1m, the exhaust length of the central soil unit in the reservoir area is still 75m, and then the gas amount of the soil is V1(m3)＝75*1*1*0.2＝15m3(ii) a Exhaust cross area A1Is 1m2. The length of the percolation path is equal to the length L of the process 2 in this example1＝479.5m。
Working condition 1: differential air pressure being total pressureDifference pt＝Δp1When 20kPa is reached, the flow rate of the soil mass discharge in table 5 is significantly lower than that in table 1, as determined by equation (6).
TABLE 5 soil mass air discharge Capacity calculation
Figure BDA0003542308570000082
From the formula (2), p can be obtained110 kPa; calculating the gas density rho in the soil body by the formula (3)p1Then, the gas gravity gamma is obtained from the formula (4)p1Is 1.50X 10-2kN·m-3. Further, the gas weight Q can be obtained from the formula (5)1Is 0.224 kN. The discharge time t is obtained from equation (7)1＝2.69×105s, total discharge time t of blind drain12The increase is 4.11 times. This shows that the provision of the subdural blind drain shortens the exhaust time and prevents the occurrence of the ballooning phenomenon under the geomembrane.
TABLE 6 calculation of mean gas weight in soil
Figure BDA0003542308570000091
Working condition 2: exhaust time is not changed, i.e. total time t of Process 1 and Process 212Is 6.528X 104And s. The gas volume was still 0.224kN, and the flow rate was 2.53E-06 kN/s. The discharge time t is obtained from equation (7)1＝2.69×105s, as can be seen from Table 7, the air pressure difference reached 82.36kPa, which is much higher than the loading pressure on the membrane by 20kPa, and therefore the geomembrane bulging phenomenon occurred. This indicates that higher air pressure is beneficial for venting when the groundwater level rises at a high rate.
TABLE 7 calculation of gas seepage and exhaustion Capacity in the soil
ka1/m·s-1 Δp1/kPa Length L1/m Cross sectional area A1/m2 Flow velocity v1/kN·s-1
2E-05 82.36 479.50 1 3.44E-06

Claims (6)

1. A blind ditch exhaust optimization method for seepage prevention of reservoir geomembranes is characterized by comprising the following steps: firstly, paving a blind ditch exhaust system, wherein the blind ditch exhaust system comprises a chessboard-type blind ditch and a plurality of soil body units divided by the blind ditch; the weight of gas in the exhaust process 1 from the soil body unit to the blind ditch of the gas in the blind ditch exhaust system is the same as that of gas in the exhaust process 2 from the blind ditch to the atmosphere, the exhaust time is the same, and the air pressure below the geomembrane is less than or equal to the critical value of the air pressure.
2. The blind ditch air discharge optimization method for seepage control of reservoir geomembrane according to claim 1, characterized in that: the exhaust process 1 includes steps (1) to (3); the exhaust process 2 includes steps (4) to (6); the method comprises the following specific steps:
(1) obtaining the volume V of the exhaust gas in the exhaust process 1 according to the blind ditch space at the center of the reservoir, the underground water level design burial depth, the soil body pore ratio and the gas saturation1；
V1＝[(1-Sr)*H*S2*e]/2 (1)
Wherein: e is the average pore ratio of the soil under the geomembrane; srIs the gas saturation; s is the blind ditch spacing; h is the designed buried depth of the underground water level;
(2) setting the critical value of the air pressure under the geomembrane as ptAnd p istPressure load on geomembrane ≦ geomembrane; simultaneously setting the air pressure difference between the soil body unit at the center of the reservoir and the blind ditch as delta p1(ii) a The average air pressure p between the soil body unit and the blind ditch is obtained by the formula (2)1(ii) a The gas density rho in the soil body unit is obtained by the formula (3)p1(ii) a Gas gravity γ from equation (4)p1(ii) a The weight Q of the gas discharged from the soil body unit at the center of the reservoir to the blind ditch is obtained by the formula (5)1：
Figure FDA0003542308560000011
Figure FDA0003542308560000012
γp1＝ρp1g (4)
Q1＝V1*γp1 (5)
Wherein, Δ ptTo set sub-geomembrane pressure threshold, Δ p1The air pressure difference between the soil body unit at the center of the reservoir and the blind ditch is obtained;
Figure FDA0003542308560000013
at a pressure p1Density of the gas under conditions; p is a radical of1The average air pressure from the soil body unit at the center of the reservoir to the blind ditch; t is the temperature of gas under the geomembrane, and the unit is centigrade; mu is the molar mass of gas molecules under the geomembrane, and the value of mu for air is 29; gamma rayp1At a pressure p1Gas severity under conditions; g is a gravity conversion coefficient, and 9.832N/kg is taken; q1The weight of the gas discharged from the soil body unit at the center of the reservoir to the blind ditch;
(3) the exhaust gas is obtained from the formula (6) and the formula (7)In journey 1, the exhaust time t from the soil body unit at the center of the reservoir to the blind ditch1：
Figure FDA0003542308560000014
Figure FDA0003542308560000015
Wherein k isa1Is the air permeability coefficient of the soil body unit; l is1The length of the seepage path from the soil body unit to the blind ditch; a. the1The contact area between the soil body unit and the blind ditch is defined; v. of1The gas flow rate when the soil body unit exhausts to the blind ditch; t is t1The exhaust time from the soil body unit at the center of the reservoir to the blind ditch is determined;
(4) the air pressure delta p of the blind ditch exhausting from the center of the reservoir to the atmosphere is obtained by the formula (8)2：
Δp2＝pt-Δp1 (8)
Wherein p istTo set sub-geomembrane pressure threshold, Δ p1The air pressure difference between the soil body unit at the center of the reservoir and the blind ditch is obtained;
(5) weight Q of gas discharged from the center of reservoir to atmosphere by blind ditch2
Q2＝Q1
(6) The time t for exhausting the blind ditch from the center of the reservoir to the atmosphere is obtained by the formula (10)2：
Figure FDA0003542308560000021
Figure FDA0003542308560000022
Wherein k isa2The air permeability coefficient of the blind ditch; l is2Is a blind ditch in the short side direction of the reservoirThe length of a seepage path for exhausting air to the atmosphere from the center of the reservoir; a. the2The cross section area of the blind ditch; v. of2The gas flow rate when the blind ditch exhausts gas from the center of the reservoir to the atmosphere; t is t2The duration of the air exhaust from the center of the short side direction of the reservoir to the blind ditch at the side of the dyke is determined; q2The amount of gas discharged from the center of the reservoir to the atmosphere for the culverts;
(7) by adjusting the air pressure difference delta p between the soil body unit at the center of the reservoir and the blind ditch1And (4) repeating the calculation from the step (2) to the step (6) until t is met1＝t2；
(8) Obtaining the total exhaust time t under the soil unit geomembrane at the center of the reservoir12Is 2t1。
3. The blind ditch air discharge optimization method for seepage control of reservoir geomembrane according to claim 2, characterized in that: the step (6) shows that medium coarse sand of the blind ditch material is changed into pebbles so as to increase the permeability coefficient of the blind ditch, and the exhaust time t of the exhaust process 2 is reduced by calculating the formulas (9) to (10)2。
4. The blind ditch air discharge optimization method for seepage control of reservoir geomembrane according to claim 2, characterized in that: filling pebbles in the blind ditch, and adding the geotechnical blind pipe to increase the permeability coefficient of the blind ditch, so that the permeability coefficient of the blind ditch is further increased, and the exhaust time t of the exhaust process 2 is shortened by further calculating the formula (9) to (10)2。
5. The blind ditch air discharge optimization method for seepage control of reservoir geomembrane according to claim 2, characterized in that: in the step (2), the critical value p of the air pressure under the geomembrane is increased by increasing the pressure load on the geomembranetAnd further increase Δ p of exhaust process 1 and exhaust process 21、Δp2、v1、v2The exhaust time of the exhaust process 1 and the exhaust process 2 is shortened.
6. The blind ditch air discharge optimization for seepage control of reservoir geomembrane according to claim 2The chemical conversion method is characterized by comprising the following steps: in the step (6), the cross section area of the blind ditch is increased to further increase A1、A2Increase v1、v2The exhaust time of the exhaust process 1 and the exhaust process 2 is shortened.
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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN115266526A (en) * 2022-07-21 2022-11-01 河海大学 Starting pressure gradient simulation test device and use method and application thereof

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CN101748704A (en) * 2010-01-06 2010-06-23 宋玉田 Impermeable membrane for reservoir with exhaust function
CN113914291A (en) * 2021-09-09 2022-01-11 河海大学 Under-membrane exhaust blind ditch and design method thereof

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Publication number Priority date Publication date Assignee Title
CN101748704A (en) * 2010-01-06 2010-06-23 宋玉田 Impermeable membrane for reservoir with exhaust function
CN113914291A (en) * 2021-09-09 2022-01-11 河海大学 Under-membrane exhaust blind ditch and design method thereof

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN115266526A (en) * 2022-07-21 2022-11-01 河海大学 Starting pressure gradient simulation test device and use method and application thereof
CN115266526B (en) * 2022-07-21 2024-09-24 河海大学 Initial pressure gradient simulation test device and use method and application thereof

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