CN114411616B - Blind ditch exhaust optimization method for seepage prevention of reservoir geomembrane - Google Patents

Abstract

The invention discloses a blind ditch exhaust optimization method for reservoir geomembrane anti-seepage. First, a blind ditch exhaust system is laid. The blind ditch exhaust system includes a checkerboard-type blind ditch located under the geomembrane at the bottom of the reservoir. And multiple soil units divided by blind ditch. When the invention exhausts through the blind ditch exhaust system, considering the balance of air resistance, even if the gas is exhausted from the soil unit to the blind ditch in the process 1 and the gas through the blind ditch to the atmosphere in the exhaust process 2, the weight of the gas The same, the exhaust time is the same, and the air pressure under the geomembrane ≦ the air pressure critical value. The invention not only ensures the smoothness of the 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 problem of the current reservoir groundwater level. The rise leads to inflation under the geomembrane, and the exhaust path in the horizontal direction of the plain reservoir is too long, which makes it difficult to exhaust.

Description

A Blind Ditch Exhaust Optimization Method for Geomembrane Seepage Prevention in Reservoirs

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 geomembrane seepage prevention in reservoirs.

Background technique

The blind ditch is a fluid drainage channel set up below the surface of the site. It is an underground ditch filled with crushed, gravel and other coarse-grained materials and paved with reverse filter layers, permeable pipes, and intercepted groundwater and gas fluids.

When the stratum distributed at the reservoir site is silt, sandy soil or gravel stratum with large thickness and strong water permeability, and lacks an effective water-resisting layer, the seepage of the strata at the site is generally solved by using geomembrane anti-seepage at the bottom of the reservoir. However, the following problems occurred: For example, the Xincheng Reservoir in Zibo City, Shandong Province adopted a 0.3mm thick polyethylene PE geomembrane anti-seepage solution. Changes affected by the water level of the reservoir indicate that the reservoir is still leaking.

When the groundwater level is relatively deep, there is a large amount of pore 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 horizontal exhaust path is too large, resulting in difficulty in exhausting . Therefore, the reason for the reservoir leakage of the current geomembrane anti-seepage scheme is mostly caused by the inflation under the geomembrane. The main factors affecting the inflation under the membrane are the drop of reservoir water level and the rise of groundwater level. When the blind ditch is set under the geomembrane, the air permeability and exhaust effect of the blind ditch structure should be 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 anti-seepage, so as to avoid the problems 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.

Contents of the invention

Purpose of the invention: In view of the defects in the prior art that the rise of the groundwater level leads to the accumulation of gas under the geomembrane, causing inflation under the geomembrane, and the fact that the exhaust path in the horizontal direction of the plain reservoir is too long and the exhaust is difficult, the present invention proposes a method for The blind ditch exhaust optimization method for geomembrane anti-seepage in reservoirs, by adjusting the gas pressure under the geomembrane, not only ensures the smoothness of gas discharge in the entire system, but also ensures that the air pressure under the geomembrane does not exceed the critical value, which solves the problem of reservoir site stratum leakage problem.

Technical solution: when the blind ditch exhaust optimization method for reservoir geomembrane anti-seepage is implemented, the blind ditch exhaust system is firstly laid; ditch, and multiple soil units divided by blind ditch. When the present invention exhausts air through the blind ditch exhaust system, the air resistance balance is considered, that is, the exhaust process 1 of the gas in the blind ditch exhaust system from the soil unit to the blind ditch and the discharge of gas to the atmosphere through the blind ditch The gas weight in gas process 2 is the same, the exhaust time is the same, and the pressure under the geomembrane is less than or equal to the critical value of pressure.

Exhaust process 1 includes steps (1) to (3); exhaust process 2 includes steps (4) to (6); the specific steps are as follows:

(1) Obtain the exhaust volume V in the exhaust process ₁ according to the blind ditch spacing at the center of the reservoir, the design buried depth of the groundwater level, the soil void ratio, and the gas saturation;

V ₁ =[(1-S _r )*H*S ² *e]/2 (1)

Among them: e is the average void ratio of the soil under the geomembrane; S _r is the gas saturation; S is the blind ditch spacing; H is the design buried depth of the groundwater level;

(2) Set the critical value of air pressure under the geomembrane to p _t , and p _t ≦ pressure load on the geomembrane; at the same time, set the air pressure difference between the soil unit at the center of the reservoir and the blind ditch to be Δp ₁ ; from formula (2 ) to get the average air pressure p ₁ between the soil unit and the blind ditch; get the gas density ρ _p1 in the soil unit from formula (3); get gas density γ _p1 from formula (4); get Calculate the gas weight Q ₁ discharged from the soil unit at the center of the reservoir to the blind ditch:

γ _p1 = ρ _p1 g (4)

Q ₁ =V ₁ *γ _p1 (5)

Among them, Δp _t is the critical value of air pressure under the geomembrane, Δp ₁ is the air pressure difference between the soil unit at the center of the reservoir and the blind ditch; ρ _p1 is the gas density under the condition of pressure p ₁ ; p ₁ is the reservoir The average air pressure from the soil unit at the center to the blind ditch; T is the gas temperature under the geomembrane, in degrees Celsius; μ is the molar mass of the gas molecule under the geomembrane, and the value for air is 29; γ _p1 is the pressure at p ₁ The gas weight under the condition; g is the gravity conversion factor, which is 9.832N/kg; Q ₁ is the weight of the gas discharged from the soil unit at the center of the reservoir to the blind ditch;

(3) According to formula (6) and formula (7), in exhaust process 1, the exhaust time t ₁ of the soil unit at the center of the reservoir to the blind ditch is obtained:

Among them, k _a1 is the air permeability coefficient of the soil unit; L ₁ is the seepage path length of the soil unit to the blind ditch; A ₁ is the contact area between the soil unit and the blind ditch; v ₁ is the drainage of the soil unit to the blind ditch The gas flow rate when the gas is gas; _t1 is the exhaust time of the soil unit at the center of the reservoir to the blind ditch;

(4) From the formula (8), the air pressure Δp ₂ of the blind ditch exhausting from the center of the reservoir to the atmosphere is obtained:

Δp ₂ = _pt -Δp ₁ (8)

Among them, p _t is the critical value of air pressure under the geomembrane, and Δp ₁ is the air pressure difference between the soil unit at the center of the reservoir and the blind ditch;

(5) Set the gas weight Q ₂ discharged from the center of the reservoir to the atmosphere by the blind ditch

Q ₂ =Q ₁

(6) The time t ₂ for the blind ditch to exhaust from the center of the reservoir to the atmosphere is obtained from formula (10):

Among them, k _a2 is the air permeability coefficient of the blind ditch; L ₂ is the seepage path length of the blind ditch in the direction of the short side of the reservoir from the center of the reservoir to the atmosphere; A ₂ is the cross-sectional area of the blind ditch _; The gas flow rate when the center of the reservoir is exhausted to the atmosphere; _t2 is the duration of exhausting from the center of the short side of the reservoir to the blind ditch beside the embankment; _Q2 is the weight of gas discharged from the center of the reservoir to the atmosphere by the blind ditch;

(7) By adjusting the air pressure difference Δp ₁ between the soil unit at the center of the reservoir and the blind ditch, repeat steps (2) to (6) until t ₁ =t ₂ is satisfied;

(8) It is obtained that the total exhaust time t ₁₂ under the geomembrane of the soil unit at the center of the reservoir is 2t ₁ .

The material of the blind ditch is usually medium-coarse sand. According to step (6), changing the medium-coarse sand of the blind ditch material into pebbles can increase the permeability coefficient of the blind ditch, and then calculated by formulas (9)-(10), shortening Degassing time t _{2 for degassing process 2} .

Fill the blind ditch with pebbles, and add geotechnical blind pipes to increase the permeability coefficient of the blind ditch, thereby further increasing the permeability coefficient of the blind ditch, and further calculating from formulas (9) to (10), shortening the discharge process 2 Gas time t ₂ .

In step (2), the critical value p _t of air pressure under the geomembrane is increased by increasing the pressure load on the geomembrane, and then the Δp ₁ , Δp ₂ , v ₁ , v ₂ of the exhaust process 1 and exhaust process 2 are increased , to shorten the exhaust time.

In step (6), by increasing the cross-sectional area of the blind ditch, A ₁ and A ₂ are increased, v ₁ and v ₂ are increased, and the exhaust time is shortened.

Working principle: When the blind ditch exhaust system of the present invention is exhausting, due to the small pore diameter of the blind ditch material and the tortuous change of the pore channel, the contact area between the gas in the pores and the solid particles is large, and the interaction between the gas molecules and the material surface Friction consumes gas movement energy, resulting in air resistance and air resistance effect.

由一定气压差下通过渗流路径气体体积流量的公式(11)计算确定：When the air resistance per unit flow and unit length is equal to the power formed by the air pressure difference, the flow and pressure difference have a linear relationship, which satisfies Darcy's law, so the pressure difference is regarded as air resistance. Seepage air resistance of gas in blind ditch

Calculated and determined by the formula (11) of the gas volume flow through the seepage path under a certain pressure difference:

为渗流过程中渗流气阻，Δp为渗流过程中的压力差(kPa)；ΔV为气体渗流的体积流量(m³)；Δt为气体渗流所持续时间(s)；k_a为材料的渗气系数(m/s)；L为渗流路径长度；A为渗流横截面面积；ρ_p为在压力p条件下气体的密度(10³kg/m³)；p为渗流路径内气体压力(kPa)；g为重力换算系数，取9.832N/kg；v为气体流速。In the formula:

Δp is the pressure difference during the seepage process (kPa); ΔV is the volume flow rate of the gas seepage (m ³ ); Δt is the duration of the gas seepage (s); k _a is the gas percolation of the material coefficient (m/s); L is the length of the seepage path; A is the cross-sectional area of the seepage; ρ _p is the density of the gas under the condition of pressure p (10 ³ kg/m ³ ); p is the gas pressure in the seepage path (kPa) ; g is the gravity conversion factor, take 9.832N/kg; v is the gas flow rate.

Among them, formula (11) is the calculation formula of seepage air resistance. From (11), it can be concluded that the factors negatively correlated with seepage air resistance are the gas permeability coefficient k _a and the seepage cross-sectional area A; positively correlated with seepage air resistance The factors of are the gas flow rate v and the gas permeation path length L. It can be seen from (12) that the gas flow rate v is proportional to the gas volume ΔV, and Δt is inversely proportional to the time.

In engineering practice, when setting blind ditch, large-sized pebbles are selected, or a geotechnical blind pipe is added in the middle of the blind ditch to increase the permeability coefficient _ka of the blind ditch, thereby reducing seepage air resistance.

For blind trenches, the air resistance effect is related to the length. In formula (11), the percolation air resistance increases with the increase of the path length; at the same time, for the discharged gas volume, the length of the path increases the time for the gas to flow through, so if the time for the gas to pass through the path length, the gas If the values are all constant, the cumulative effect of air resistance increases, and the cumulative effect of air resistance is the blind ditch air 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 ditch air resistance effect Z in formula (13) is proportional to the square of the blind ditch length L. Since the length L of the blind ditch is significantly affected by the plane size of the plain reservoir, especially 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.

Implementation principle of the present invention is as follows:

First of all, for the reservoir with geomembrane anti-seepage at the bottom of the entire reservoir, the groundwater of the reservoir is lower than the surface of the geomembrane, there is unsaturated soil under the membrane, and the pores of the unsaturated soil are filled with gas. Because the geomembrane at the bottom of the reservoir isolates the connection between the unsaturated soil at the bottom of the reservoir and the atmosphere, when the groundwater rises, the pore gas is compressed by the pore water, which causes the air pressure under the geomembrane to increase. Let the air pressure under the geomembrane be Δp _t , when When the air pressure Δp _t under the geomembrane is greater than the load on the membrane, it will cause the geomembrane body to bulge.

Secondly, considering the air resistance balance, the air resistance balance means that the gas is discharged from the soil unit to the blind ditch (process 1) and the gas is discharged to the atmosphere through the blind ditch (process 2). The same, but the air pressure under the geomembrane still meets the requirement of no inflation, which not only achieves the smoothness of gas discharge in the whole system, but also ensures that the air pressure under the geomembrane does not exceed the critical value of air pressure, meeting the anti-seepage requirements of the reservoir.

Consider the seepage process of the soil unit exhausting to the blind ditch 1: Lay the blind ditch at the bottom of the whole reservoir. When the pore pressure of the soil under the geomembrane increases, the permeability of the blind ditch is greater than that of the soil and it is connected to the atmosphere. , so the air pressure in the blind ditch is low, and then the soil unit surrounded by the blind ditch is formed to vent to the blind ditch, which is the seepage process of soil unit exhaust 1. The air pressure difference is Δp ₁ , and the air pressure difference between the air pressure in the blind ditch around the soil unit and the atmospheric pressure is Δp ₂ , and satisfies Δp ₂ = _Δpt -Δp ₁ .

Consider the seepage process 2 of the blind ditch exhausting to the atmosphere: in this process, one end of the blind ditch is connected to the atmosphere, the air pressure is zero, and the air pressure in the blind ditch gradually seeps to the atmosphere. The pressure difference of the blind ditch exhaust process 2 is Δp ₂ .

Then the exhaust process of the blind ditch exhaust system under the geomembrane is optimized. Considering the smoothness of the exhaust, the weight of the gas discharged in the seepage process 1 from the soil unit to the blind ditch and the seepage process 2 from the blind ditch to the atmosphere are the same, and the time taken is the same. At the same time, the geomembrane The lower total air pressure Δp _t still satisfies the requirement of being less than or equal to the load on the geomembrane. Therefore, the exhaust time t ₁ of the seepage process 1 of the soil unit venting to the blind ditch is balanced by adjusting the distance between the blind ditch, and the soil mass The exhaust time t ₂ of the seepage process 2 in which the blind ditch around the unit exhausts to the atmosphere makes the total air pressure under the geomembrane Δp _t ≦ the load on the geomembrane, which not only achieves the fluency of gas discharge in the entire system, but also ensures that the membrane The lower air pressure does not exceed the critical value of inflation to ensure the anti-seepage requirements of the reservoir.

Beneficial effect: compared with the prior art, the present invention has the following advantages:

(1) When the present invention optimizes the exhaust of the blind ditch under the geomembrane, the unobstructed exhaust under the geomembrane is considered, the air resistance under the geomembrane is reduced, and the gas pressure under the geomembrane is reduced, and the effect is remarkable , which not only ensures the fluency of gas discharge in the whole system, but also ensures that the air pressure under the geomembrane does not exceed the critical value, which solves the leakage problem of the formation in the reservoir area.

(2) The present invention provides two working conditions when no exhaust blind ditch is provided under the geomembrane at the same time in the embodiment, and then compares the present invention by optimizing the exhaust method, shortening the exhaust time, and It prevents the occurrence of inflation under the geomembrane.

Description of drawings

Fig. 1 is reservoir general plan of the present invention;

Figure 2 is a geological profile of the reservoir site;

Figure 3 is a schematic diagram of the exhaust gas under the geomembrane;

Fig. 4 is a schematic diagram of a square soil unit in an embodiment of the present invention;

Fig. 5 is a schematic diagram of the layout of blind ditches and soil units in an embodiment of the present invention;

FIG. 6 is a schematic diagram of a blind trench structure in an embodiment of the present invention.

Detailed ways

Example:

As shown in Figures 1 to 6, the present invention adopts geomembrane laying at the bottom of the reservoir for anti-seepage, the groundwater level is lower than the geomembrane, and a blind ditch 3 is arranged under the geomembrane at the bottom of the reservoir. 1 in Figure 1 is the soil unit at the center of the reservoir, and 2 is the embankment.

Among them, Figure 4 and Figure 5 show the arrangement of criss-cross blind ditches under the geomembrane. The soil at the bottom of the reservoir is covered with an airtight geomembrane, so the gas in the pores of the soil under the geomembrane cannot be discharged directly, but can only be discharged to the surrounding blind ditch, and then discharged to the atmosphere through the blind ditch. Process 1 in Fig. 3 is the exhaust of soil units to the blind ditch, and process 2 is the exhaust of blind ditch to the atmosphere.

As shown in Figure 2, FC in the figure represents the intersection of vertical and horizontal blind ditches, FD represents the blind ditch, S represents the distance between the centers of two parallel blind ditches; RG represents the ground at the bottom of the reservoir, FD represents the blind ditch, and FG represents the blind ditch The crushed stone material for internal filling, FT represents the blind pipe in the blind ditch, GW represents the groundwater level line, H represents the height difference between the geomembrane surface and the groundwater level, that is, the buried depth of the groundwater level, and EC represents the excavated soil protection on the membrane layer, SC represents the fine sand protective layer on the membrane, GM represents the geomembrane; FGT is the filament geotextile between the soil and the blind ditch and wrapping the geotechnical blind pipe.

Fig. 5 is a sectional view of line 1-1 in Fig. 4 . GW in FIG. 5 is the groundwater level, and H is the buried depth of the groundwater level. When the groundwater level rises and H decreases, the pore water pressure in the soil under the geomembrane increases, and the pore water squeezes the pore air, causing the pore air pressure to increase. When the air pressure under the membrane is greater than the load on the membrane, the pressure on the geomembrane The bulge deforms, so the gas under the geomembrane is discharged to the surrounding blind ditch. If the blind ditch cannot exhaust effectively, the air pressure under the geomembrane will gradually increase, and the air pressure under the geomembrane will be greater than the load on the membrane, and then the geomembrane will bulge.

As shown in Fig. 6, the blind ditch in the blind ditch exhaust system of the present invention is composed of gravel FG, dead pipe FT, between soil and gravel, and geotextile FGT wrapped in the blind pipe. GD represents the bottom or ground of the reservoir; a represents the width of the blind ditch. The seepage flow is from high pressure to low pressure. The gas passes through the geotextile FGT from the soil to the gravel FG of the blind ditch, and then passes through the geotextile FGT from the gravel FG to the dead pipe FT. One end of the blind ditch is in the soil, and the other end is connected to the atmosphere. The higher the permeability of the material in the blind ditch, the lower the corresponding air pressure.

土工盲管，土工盲管周边回填卵石。库盘场地被棋盘式盲沟分割成方形土体单元，其中方形土体单元边长S＝75m，水库中心的土体单元的中心距离水库库区围堤中心最短距离为516m。As shown in Figures 1 to 6, in this embodiment, checkerboard blind ditches are arranged under the geomembrane at the bottom of the reservoir, and the distance between the blind ditches is S=75m. The cross section of the blind ditch is a square of 0.3m×0.3m. Lay out a blind ditch

Geotechnical dead pipe, backfill pebbles around the geotechnical dead pipe. The reservoir site is divided into square soil units by checkerboard blind ditches, where the side length of the square soil unit is S=75m, and the shortest distance between the center of the soil unit at the center of the reservoir and the embankment center of the reservoir area is 516m.

As shown in Figure 4 to Figure 6, the square soil unit in the center of the reservoir area is taken as the investigation object, and the side length S=75m. The groundwater is located H=1m below the surface, the soil is breccia, the porosity e=0.2, and the saturation S _r is 0.

When the reservoir is rectangular, the length of the air seepage path of the blind ditch on the long side of the reservoir is large, and the corresponding air resistance is large, so only the exhaust condition of the short side of the reservoir is considered. Considering that the soil unit is surrounded by blind ditches, that is, four blind ditches form a soil unit, and only 1/2 of each blind ditch has an effect on the exhaust of the soil unit, and the corresponding single blind ditches need The exhaust gas volume is 1/2 of the total exhaust volume of the soil unit to the blind ditch, so 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 depth of the groundwater table, and 75 is the blind ditch distance.

In this embodiment, the geomembrane is filled with a fine sand protective layer SC and an excavated material protective layer EC, and the total load on the geomembrane is 20kPa, so in order to ensure that the geomembrane does not bulge, the air pressure under the geomembrane Δp _t ≤ 20kPa. The exhaust under the geomembrane is completed in two processes: Process 1 is that the square soil unit surrounded by the blind ditch at the center of the reservoir vents to the blind ditch; process 2 is that the blind ditch at the center of the reservoir connects to the atmosphere inside the embankment, and the The blind ditch vents to the embankment, that is, the atmosphere. In order to ensure unimpeded exhaust and no inflation, the following three conditions must be met:

(1) The total amount of exhaust gas in process 1 and process 2 is the same, which is the gas volume in the central soil unit of the reservoir area;

(2) The exhaust time of process 1 and process 2 is the same;

(3) The sum of the pressure difference between process 1 and process 2, that is, the pressure under the geomembrane Δp _t ≦ the inflation critical pressure value.

Calculation of exhaust process 1: The seepage path length is half the width of the soil unit, because each soil unit seeps from the center to the surroundings, so it is half the width. The seepage cross-sectional area A is the contact area between the blind ditch and the soil unit, that is, A=75*0.3*1.5=33.75m ² . Among them, 1.5 is the side length of half a blind ditch; 0.3 is the side length of a blind ditch.

According to Darcy's law formula (1), the gas percolation velocity v and the pressure difference Δp within the gas permeation path length L are calculated.

The specific calculation process for exhaust process 1 is:

Set the critical value of air pressure under the geomembrane to p _t , and p _t ≦ pressure load on the geomembrane 20kPa; from the air permeability coefficient k _a1 of the soil unit, the seepage path length L ₁ from the soil unit to the blind ditch, the soil unit The contact area A ₁ with the blind ditch, the seepage velocity v ₁ from the soil unit to the blind ditch can be obtained from the formula (6):

Table 1 Seepage velocity of exhaust gas in soil unit

The gas density is related to the pressure. The gas density ρ _p1 in the soil unit is obtained from the formulas (2) to (3), and then the gas weight γ _p1 is obtained from the formula (4); the weight conversion factor g=9.832N/kg, Furthermore, the weight was found to be 1.41×10 ^-2 kN·m ^-3 .

Table 2 Calculation of the average value of gas gravity

The total exhaust volume V ₁ of the seepage process 1 of the soil unit exhausting to the blind ditch is obtained by formula (1); then the gas weight Q ₁ discharged from the soil unit to the blind ditch is obtained by formula (5); The time is obtained by formula (7).

Table 3 Calculation of gas emission time in soil unit

Displacement Q₁/kN Exhaust volume V₁/m³ Time t₁/s time/day 7.94 562.50 3.264E+04 0.378

Calculation of seepage process 2 of blind ditch venting to atmosphere:

Among them, ΔQ ₂ is the weight of gas discharged from the blind ditch at the central end of the reservoir to the atmosphere, and v ₂ is the gas flow rate when the blind ditch at the central end of the reservoir is exhausted to the atmosphere.

The exhaust cross-sectional area of the blind ditch is determined by the cross-sectional size of the blind ditch, A ₂ =0.3*0.3=0.09m ² . The length of the blind ditch is from the center of the soil unit at the center of the reservoir area to the inner boundary of the embankment, that is, L ₂ =513-37.5=479.5m. The air pressure in the blind ditch is obtained by formula (8), i.e. 20-13.52=6.48kPa. Calculate the seepage velocity v according to the formula (1); then calculate the discharge time t ₂ according to the formula (6) based on the exhaust volume and flow rate. In Table 4, the discharge time t 2 of the blind ditch to the atmosphere exhaust process ₂ is equal to the discharge time t ₁ of the soil unit to the blind ditch exhaust process 1; The total gas time t ₁₂ =2t ₁ is 6.528E+4s.

Table 4 Calculation of exhaust capacity of blind ditch and gas exhaust time in process 2

When there is no exhaust blind ditch under the geomembrane:

Still take the scale of the reservoir in this embodiment as an example, but there is no exhaust blind ditch under the geomembrane, and the reservoir per unit length is taken as the research object. At this time, the whole section of the soil under the geomembrane is directly vented to the atmosphere, with a width of 1m, The depth is 1m buried depth of the groundwater table, the porosity is 0.2, the gas saturation is 0, the depth of the groundwater table is 1m, and the exhaust length of the soil unit in the center of the reservoir area is still 75m, so the gas volume of the soil is V ₁ (m ³ )=75*1*1*0.2=15m ³ ; the exhaust cross-sectional area A ₁ is 1m ² . The length of the seepage path is equal to the length L ₁ =479.5m in process 2 in this embodiment.

Working condition 1: The air pressure difference is the total pressure difference p _t =Δp ₁ =20kPa, then the air exhaust flow rate of the soil in Table 5 is obviously lower than the flow rate in Table 1 obtained from formula (6).

Table 5 Calculation of soil exhaust capacity

From the formula (2), p ₁ = 10kPa can be obtained; then the gas density ρ _p1 in the soil can be calculated from the formula (3), and then the gas density γ _p1 can be obtained from the formula (4), which is 1.50×10 ^-2 kN·m ^{- 3} . Furthermore, the gas weight Q1 obtained from formula ( ₅ ) is 0.224kN. The discharge time t ₁ =2.69×10 ⁵ s is obtained from formula (7), and the discharge time increases by 4.11 times relative to the total time t ₁₂ of setting the blind ditch. This shows that setting the blind ditch under the geomembrane shortens the exhaust time and can prevent the occurrence of inflation under the geomembrane.

Table 6 Calculation of average value of gas gravity in soil

Working condition 2: The exhaust time remains unchanged, that is, the total time t ₁₂ of process 1 and process 2 is 6.528×10 ⁴ s. The gas volume is still 0.224kN, so the flow rate is 2.53E-06kN/s. The discharge time t ₁ =2.69×10 ⁵ s is obtained from formula (7). It can be seen from Table 7 that the air pressure difference reaches 82.36kPa at this time, which is much higher than the load pressure on the membrane of 20kPa, so the geomembrane bulges. This shows that when the groundwater level rises rapidly, higher air pressure is beneficial to exhaust.

Table 7 Calculation of Gas Seepage and Exhaust Capacity in Soil

k_a1/m s^-1 Δp₁/kPa Length L₁/m Cross-sectional area A₁/m² Velocity v₁/kN s^-1 2E-05 82.36 479.50 1 3.44E-06

Claims (5)

1. A blind ditch exhaust optimization method for geomembrane anti-seepage in reservoirs, characterized in that: first lay the blind ditch exhaust system, the blind ditch exhaust system includes checkerboard blind ditch and the blind ditch divided into Multiple soil units; the gas in the blind ditch exhaust system has the same gas weight in the exhaust process 1 from the soil unit to the blind ditch and the gas exhaust process 2 through the blind ditch to the atmosphere, and the exhaust time is the same, and The air pressure under the geomembrane ≦ air pressure critical value; The exhaust process 1 includes steps (1) to (3); the exhaust process 2 includes steps (4) to (6); the specific steps are as follows: (1) Obtain the exhaust volume V in the exhaust process ₁ according to the blind ditch spacing at the center of the reservoir, the design buried depth of the groundwater level, the soil void ratio, and the gas saturation; V ₁ =[(1-S _r )*H*S ² *e]/2 (1) Among them: e is the average void ratio of the soil under the geomembrane; S _r is the gas saturation; S is the blind ditch spacing; H is the design buried depth of the groundwater level; (2) Set the critical value of air pressure under the geomembrane to p _t , and p _t ≦ pressure load on the geomembrane; at the same time, set the air pressure difference between the soil unit at the center of the reservoir and the blind ditch to be Δp ₁ ; from formula (2 ) to get the average air pressure p ₁ between the soil unit and the blind ditch; get the gas density ρ _p1 in the soil unit from formula (3); get gas density γ _p1 from formula (4); get Calculate the gas weight Q ₁ discharged from the soil unit at the center of the reservoir to the blind ditch:

γ _p1 = ρ _p1 g (4) Q ₁ =V ₁ *γ _p1 (5) Among them, Δp _t is the critical value of air pressure under the geomembrane, and Δp ₁ is the air pressure difference between the soil unit at the center of the reservoir and the blind ditch;

is the density of gas under the condition of pressure p ₁ ; p ₁ is the average air pressure from the soil unit at the center of the reservoir to the blind ditch; T is the gas temperature under the geomembrane, in degrees Celsius; μ is the mole of gas molecules under the geomembrane The mass is 29 for air; γ _p1 is the gas gravity under the condition of pressure p ₁ ; g is the gravity conversion factor, which is 9.832N/kg; Q ₁ is the gas discharged from the soil unit at the center of the reservoir to the blind ditch gas weight; (3) According to formula (6) and formula (7), in exhaust process 1, the exhaust time t ₁ of the soil unit at the center of the reservoir to the blind ditch is obtained:

Among them, k _a1 is the air permeability coefficient of the soil unit; L ₁ is the seepage path length of the soil unit to the blind ditch; A ₁ is the contact area between the soil unit and the blind ditch; v ₁ is the drainage of the soil unit to the blind ditch The gas flow rate when the gas is gas; _t1 is the exhaust time of the soil unit at the center of the reservoir to the blind ditch; (4) From the formula (8), the air pressure Δp ₂ of the blind ditch exhausting from the center of the reservoir to the atmosphere is obtained: Δp ₂ = _pt -Δp ₁ (8) Among them, p _t is the critical value of air pressure under the geomembrane, and Δp ₁ is the air pressure difference between the soil unit at the center of the reservoir and the blind ditch; (5) Set the gas weight Q ₂ discharged from the center of the reservoir to the atmosphere by the blind ditch Q ₂ =Q ₁ (6) The time t ₂ for the blind ditch to exhaust from the center of the reservoir to the atmosphere is obtained from formula (10):

Among them, k _a2 is the air permeability coefficient of the blind ditch; L ₂ is the seepage path length of the blind ditch in the direction of the short side of the reservoir from the center of the reservoir to the atmosphere; A ₂ is the cross-sectional area of the blind ditch _; The gas flow rate when the center of the reservoir is exhausted to the atmosphere; _t2 is the duration of exhausting from the center of the short side of the reservoir to the blind ditch beside the embankment; _Q2 is the weight of gas discharged from the center of the reservoir to the atmosphere by the blind ditch; (7) By adjusting the air pressure difference Δp ₁ between the soil unit at the center of the reservoir and the blind ditch, repeat steps (2) to (6) until t ₁ =t ₂ is satisfied; (8) It is obtained that the total exhaust time t ₁₂ under the geomembrane of the soil unit at the center of the reservoir is 2t ₁ . 2. the blind ditch exhaust optimization method for reservoir geomembrane anti-seepage according to claim 1, is characterized in that: draw by step (6), change the medium coarse sand of blind ditch material into pebbles and then Increase the permeability coefficient of the blind ditch, and then calculate by formulas (9)~(10), reduce the exhaust time t _{2 of the exhaust process 2} . 3. the blind ditch exhaust optimization method for reservoir geomembrane anti-seepage according to claim 1, characterized in that: fill pebbles in the blind ditch, and then add a geotechnical blind pipe to increase the blind ditch permeability coefficient, Therefore, the permeability coefficient of the blind ditch is further increased, and the exhaust time t ₂ of the exhaust process 2 is shortened by further calculation according to formulas (9)-(10). 4. the blind ditch exhaust optimization method for reservoir geomembrane anti-seepage according to claim 1 is characterized in that: in step (2), by increasing the pressure load on the geomembrane, the air pressure criticality under the geomembrane is increased value p _t , and then increase the Δp ₁ , Δp ₂ , v ₁ , v ₂ of exhaust process 1 and exhaust process 2, and shorten the exhaust time of exhaust process 1 and exhaust process 2. 5. The blind ditch exhaust optimization method for reservoir geomembrane anti-seepage according to claim 1, characterized in that: in step (6), A ₁ and A ₂ are increased by increasing the cross-sectional area of the blind ditch , increase v ₁ , v ₂ , and shorten the exhaust time of exhaust process 1 and exhaust process 2.

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