CN114411616B - 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 PDFInfo
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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 gas in the exhaust process 1 from the soil body unit to the blind ditch is the same as that of the gas 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 invention not only ensures the smoothness of gas discharge under the geomembrane, reduces the air resistance under the geomembrane and 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 air discharge caused by overlong air discharge path in the horizontal direction of the plain reservoir at present.
Description
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, which indicates 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 the design of blind ditches, so that the risk of engineering operation and maintenance is reduced, and the technical problem to be solved urgently is formed.
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 an exhaust process 1 from the soil body unit to the blind ditch in the blind ditch exhaust system is the same as that of the air in an 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 saturation 1 ；
V 1 ＝[(1-S r )*H*S 2 *e]/2 (1)
Wherein: e is the average pore ratio of the soil under the geomembrane; s r Is 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 p t And p is t Pressure 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 p 1 (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 ：
γ p1 ＝ρ p1 g (4)
Q 1 ＝V 1 *γ p1 (5)
Wherein, Δ p t For a set sub-geomembrane air pressure threshold, Δ p 1 The air pressure difference between the soil body unit at the center of the reservoir and the blind ditch is obtained; rho p1 At a pressure p 1 Density of the gas under conditions; p is a radical of 1 The 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 ray p1 At a pressure p 1 Gas severity under conditions; g is a gravity conversion coefficient, and 9.832N/kg is taken; q 1 The 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 ：
Wherein k is a1 Is the air permeability coefficient of the soil body unit; l is 1 The length of the seepage path from the soil body unit to the blind ditch; a. The 1 The contact area between the soil body unit and the blind ditch is defined; v. of 1 The gas flow velocity when the soil body unit exhausts to the blind ditch; t is t 1 The 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 ：
Δp 2 ＝p t -Δp 1 (8)
Wherein p is t To set sub-geomembrane pressure threshold, Δ p 1 Is the soil body at the center of the reservoirAir pressure difference between the unit and the blind drain;
(5) Weight Q of gas discharged from the center of reservoir to atmosphere by blind ditch 2
Q 2 ＝Q 1
(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 ：
Wherein k is a2 The air permeability coefficient of the blind ditch; l is a radical of an alcohol 2 The 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. The 2 The cross section area of the blind ditch; v. of 2 The gas flow rate when the blind ditch exhausts gas from the center of the reservoir to the atmosphere; t is t 2 The 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 apron dam; q 2 The 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 ditch 1 And (4) repeating the calculation from the step (2) to the step (6) until t is met 1 ＝t 2 ；
(8) Obtaining the total exhaust time t under the soil unit geomembrane at the center of the reservoir 12 Is 2t 1 。
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 step (2), by increasingIncreasing critical value p of air pressure under geomembrane by pressure load on geomembrane t And further increase Δ p of exhaust process 1 and exhaust process 2 1 、Δp 2 、v 1 、v 2 And the exhaust time is shortened.
In the step (6), the cross section area of the blind ditch is increased to further increase A 1 、A 2 Increasing v 1 、v 2 And 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 unit flow rate and the air resistance of the unit length are equal to the power formed by the air pressure difference, the flow rate and the pressure difference are in 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
The gas volume flow through the seepage path under a certain gas pressure difference is calculated and determined by the formula (11):
in the formula:
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 seepage 3 ) (ii) a Δ t is the duration of gas infiltration(s); k is a radical of a Is 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; rho p Being gaseous under pressure pDensity (10) 3 kg/m 3 ) (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 seepage air resistance is the air permeability coefficient k a And 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 ditches a Thereby 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:
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 the underground water risesThe air pressure under the geomembrane is increased due to the compression of the pore water, and the air pressure under the geomembrane is set to be delta p t When the sub-geomembrane pressure is Δ p t Greater 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 p 1 And 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 p 2 And satisfy Δ p 2 ＝Δp t -Δp 1 。
Considering the seepage process 2 of blind drain to atmosphere exhaust: 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 p 2 。
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 same t The 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 ditches 1 And the air exhaust of the seepage process 2 of exhausting air to the atmosphere with the blind ditches at the periphery of the soil body unitTime t 2 So that the total air pressure deltap under the geomembrane t Load on the geomembrane is smaller 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 drain below 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 reservoir area site.
(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 phenomenon of air inflation 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, 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 level 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 then the condition that the geomembrane bulges can occur.
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 between the blind ditches is S =75m. 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
And filling pebbles around the geotechnical blind pipe. The reservoir disc field is divided into square soil body units by the checkerboard blind ditches, wherein the side length S =75m of the square soil body units, and the shortest distance from the center of the soil body unit at the center of the reservoir to the center of the dyke of the reservoir area is 516m.
As shown in fig. 4 to 6, the square soil body unit at the center of the reservoir area is taken as an object to be examined, and the side length S =75m. Underground water is below ground H =1m, soil is breccia soil, porosity e =0.2, and saturation S r Is 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 unit is surrounded by blind ditches at the front, the back, the left and the right, namely 4 blind ditches are surrounded to form a soil body unit, each blind ditch only has 1/2 of the exhaust function of the soil body unit, the volume of the gas to be exhausted by the corresponding single blind ditch is 1/2 of the total exhaust volume from the soil body unit to the blind ditch, and then V is the volume of the gas to be exhausted by the corresponding single blind ditch, namely the volume of the gas to be exhausted by the soil body unit is 1/2 of the total exhaust volume from the soil body unit to the blind ditch, and the whole volume of the gas to be exhausted by the soil body unit is V 1 (m 3 )＝75*75*1*0.2/2＝562.5m 3 . 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 this embodiment, the geomembrane is filled with the fine sand protection layer SC and the excavation material protection layer EC, and the total load on the geomembrane is 20kPa, so that the air pressure Δ p under the geomembrane is set to ensure that the geomembrane does not swell t ≦ 20kPa. 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 p t And (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 =75 × 0.3 × 1.5=33.75m 2 . 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).
The specific calculation process for the exhaust process 1 is:
setting the critical value of the air pressure under the geomembrane as p t And p is t Pressure load 20kPa on geomembrane ≦; the permeability coefficient k of the soil body unit a1 Length L of seepage path from soil body unit to blind ditch 1 Contact area A of soil body unit and blind ditch 1 The seepage velocity v of the soil body unit discharged to the blind ditch is obtained by the formula (6) 1 ：
TABLE 1 seepage velocity of exhaust air of soil body units
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) p1 Then the gas gravity gamma is obtained by the formula (4) p1 (ii) a The weight conversion factor g =9.832N/kg, and further the weight 1.41 × 10 was obtained -2 kN·m -3 。
TABLE 2 calculation of gas severity mean
Total exhaust volume V of seepage process 1 for exhausting soil body unit to blind ditch 1 The 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 from equation (7).
TABLE 3 calculation of gas exhaustion time in soil cells
| Exhaust gas volume Q 1 /kN | Volume of exhaust V 1 /m 3 | Time t 1 /s | Time/day |
| 7.94 | 562.50 | 3.264E+04 | 0.378 |
Calculation of seepage process 2 of blind drain to atmosphere exhaust:
wherein, Δ Q 2 Weight of gas discharged to atmosphere for the blind ditch at the center end of reservoir, v 2 The 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, A 2 ＝0.3*0.3＝0.09m 2 . 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 dike 2 =513-37.5=479.5m. The air pressure in the blind ditch was determined by the formula (8), i.e., 20 to 13.52=6.48kpa. 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 4 2 Equal to soilDischarge time t of body unit to blind drain exhaust process 1 1 (ii) a Finally, the total exhaust time t under the soil body unit geomembrane at the center of the reservoir is obtained 12 ＝2t 1 6.528E +4s.
TABLE 4 calculation of exhaust Capacity of Blind and gas Exit time of Process 2
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 V 1 (m 3 )＝75*1*1*0.2＝15m 3 (ii) a Exhaust cross area A 1 Is 1m 2 . The length of the percolation path is equal to the length L of the process 2 in this example 1 ＝479.5m。
Working condition 1: the air pressure difference is the total pressure difference p t ＝Δp 1 If the flow rate of the soil mass exhaust in table 5 is significantly lower than that in table 1, as determined by equation (6), 20kPa is obtained.
TABLE 5 soil mass air discharge Capacity calculation
From the formula (2), p can be obtained 1 =10kPa; calculating the gas density rho in the soil body by the formula (3) p1 Then, the gas gravity gamma is obtained from the formula (4) p1 Is 1.50X 10 -2 kN·m -3 . Further, the gas weight Q can be obtained from the formula (5) 1 Is 0.224kN. The discharge time t is determined from equation (7) 1 ＝2.69×10 5 s, total discharge time t relative to blind drain setting time 12 The increase is 4.11 times. This indicates that the provision of the trench under the geomembrane shortens the drainThe air inflation time can prevent the occurrence of the air inflation phenomenon under the geomembrane.
TABLE 6 calculation of mean gas weight in soil
Working condition 2: the exhaust time is not changed, i.e. the total time t of Process 1 and Process 2 12 Is 6.528 × 10 4 And s. The gas volume was still 0.224kN, and the flow rate was 2.53E-06kN/s. The discharge time t is determined from equation (7) 1 ＝2.69×10 5 s, 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
| k a1 /m·s -1 | Δp 1 /kPa | Length L 1 /m | Cross sectional area A 1 /m 2 | Flow velocity v 1 /kN·s -1 |
| 2E-05 | 82.36 | 479.50 | 1 | 3.44E-06 |
Claims (5)
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 exhaust process 1 of the gas in the blind ditch exhaust system from the soil body unit to the blind ditch is the same as the exhaust process 2 of the gas 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;
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 saturation 1 ；
V 1 ＝[(1-S r )*H*S 2 *e]/2 (1)
Wherein: e is the average pore ratio of the soil under the geomembrane; s r Is the gas saturation; s is the blind ditch spacing; h is the designed burial depth of the underground water level;
(2) Setting the critical value of the air pressure under the geomembrane as p t And p is t Pressure 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 p 1 (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 [ gamma ] is obtained 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 ：
γ p1 ＝ρ p1 g (4)
Q 1 ＝V 1 *γ p1 (5)
Wherein, Δ p t To set sub-geomembrane pressure threshold, Δ p 1 The air pressure difference between the soil body unit at the center of the reservoir and the blind ditch is obtained;
at a pressure p 1 Density of the gas under conditions; p is a radical of 1 The 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 for air is 29; gamma ray p1 At a pressure p 1 Gas severity under conditions; g is a gravity conversion coefficient, and 9.832N/kg is taken; q 1 The 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 ：
Wherein k is a1 Is the air permeability coefficient of the soil body unit; l is 1 The length of the seepage path from the soil body unit to the blind ditch; a. The 1 The contact area between the soil body unit and the blind ditch is defined; v. of 1 The gas flow rate when the soil body unit exhausts to the blind ditch; t is t 1 The exhaust time from the soil body unit at the center of the reservoir to the blind ditch is determined;
(4) The formula (8) shows that the blind ditch is from the center of the reservoir to the centerAtmospheric pressure Δ p of atmospheric exhaust 2 ：
Δp 2 ＝p t -Δp 1 (8)
Wherein p is t For a set sub-geomembrane air pressure threshold, Δ p 1 The 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 ditch 2
Q 2 ＝Q 1
(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 ：
Wherein k is a2 The air permeability coefficient of the blind ditch; l is 2 The 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. The 2 The cross section area of the blind ditch; v. of 2 The gas flow rate when the blind ditch exhausts gas from the center of the reservoir to the atmosphere; t is t 2 The 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; q 2 The 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 ditch 1 And (5) repeating the calculation from the step (2) to the step (6) until t is met 1 ＝t 2 ；
(8) Obtaining the total exhaust time t under the soil unit geomembrane at the center of the reservoir 12 Is 2t 1 。
2. The blind ditch air discharge optimization method for seepage control of the reservoir geomembrane as claimed in claim 1, wherein: obtaining the blind ditch material by the step (6)Changing the medium coarse sand into pebbles to increase the blind ditch permeability coefficient, and reducing the exhaust time t of the exhaust process 2 by calculating according to the formulas (9) to (10) 2 。
3. The blind ditch air discharge optimization method for seepage control of the reservoir geomembrane as claimed in claim 1, wherein: 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 。
4. The blind ditch air discharge optimization method for seepage control of reservoir geomembrane according to claim 1, 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 geomembrane t And further increase Δ p of exhaust process 1 and exhaust process 2 1 、Δp 2 、v 1 、v 2 The exhaust time of the exhaust process 1 and the exhaust process 2 is shortened.
5. The blind ditch air discharge optimization method for seepage control of reservoir geomembrane according to claim 1, characterized in that: in the step (6), the cross section area of the blind ditch is increased to further increase A 1 、A 2 Increase v 1 、v 2 The exhaust time of the exhaust process 1 and the exhaust process 2 is shortened.
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