In recent years, many reservoir dams have used geomembrane for seepage prevention, especially for plain reservoirs or hills with gentle slopes in the reservoir area. The reservoir uses geomembrane to prevent seepage of the reservoir, and many scholars have studied it. Starting from the concept of contact scouring at the interface between the pavement and the permeable foundation, the composite geomembrane is used as the anti-seepage of the inclined wall pavement, and the limit pavement length and seepage flow are derived. According to the calculation formula, the permeability characteristics of the geomembrane were determined by experiments, the seepage flow of the anti-seepage layer of the geomembrane was calculated, and the influence of various related factors on the seepage flow was analyzed. The calculation formula of the seepage volume of the membrane composite anti-seepage layer is obtained, and the interpretation of the seepage volume of the geomembrane composite anti-seepage layer is obtained. For the analysis of the permeability characteristics of the large-area geomembrane impermeable body, the traditional method of analyzing the formation permeability cannot be used, and the traditional experimental method cannot simulate the permeability characteristics of the large-area geomembrane impermeable body. There are few studies. Geomembrane has strong anti-seepage performance and small permeability coefficient, so it is a good anti-seepage material; but geomembrane is flexible, and it is prone to damage and holes during construction and operation. It has become the main channel of seepage in the reservoir area. Therefore, when analyzing the permeability characteristics of the geomembrane impermeable body in the reservoir, it is particularly important to consider the impact of damaged holes caused by construction and operation on its permeability. The West's Law studies the permeability characteristics of the large-area geomembrane impermeable body in the warehouse, analyzes the influence of the damaged holes generated by the large-area geomembrane impermeable body in the warehouse on the permeability characteristics of the geomembrane, and calculates the permeability of the large-area geomembrane impermeable body. The method of analyzing the permeability characteristics of large-area geomembrane impermeable body is obtained as a whole.

1 Penetration analysis of geomembrane impermeable body

The geomembrane anti-seepage structure of the reservoir plate is usually composed of 5 parts: protective layer, upper cushion layer, geomembrane, lower cushion layer and support layer, among which the geomembrane is the main body of the anti-seepage structure. Therefore, the integrity relationship of the geomembrane To the anti-seepage effect of the entire reservoir, the geomembrane is a non-voided medium. At present, the infiltration mechanism of the geomembrane under the action of the hydraulic gradient has not been fully understood. The permeability law of the lower liquid through the geomembrane is studied, and the permeability characteristics of the geomembrane impermeable body are studied from the perspective of water balance analysis.

2 Case Analysis

2.1 Project Overview

Xinjiang Shengli Reservoir is an injection-type hilly reservoir with dam body, dam foundation and geomembrane anti-seepage. The maximum dam height is 18.1m, the main dam is 1875m long, the dam crest width is 5m, the maximum water depth in the reservoir area is 21m, and the storage capacity is 2.0 ×107m, , the project is located in Chaha Township, Cele County, Hotan, Xinjiang, in the hilly area between Cele River and the middle reaches of Chaha River. The precipitation is only 35mm, and the annual evaporation is more than 2400mm.

Because the low depression in the middle of the reservoir is covered with about 2m of alluvial-proluvial silty loam, the permeability coefficient is 2.17×10-5cm/s; the rest is composed of gravel-bearing coarse sand and gravel, and the permeability coefficient is 5.41×10- 3~1.16×10-2cm/s. The lower part is a semi-cemented sand and gravel layer, but poor cementation and discontinuous seepage in the reservoir area are the main geological problems of the reservoir. Since the anti-seepage, the leakage problem in the reservoir area has been basically solved.

2.2 Analysis of the permeability characteristics of geomembrane

The Shengli Reservoir uses 0.5mm polyethylene smooth geomembrane and 200g/0.5mm/200g composite geomembrane to prevent seepage in the whole reservoir. The laying area is 2.64×105 square meters. The laying area of ​​the geomembrane in the whole reservoir of Shengli Reservoir ranks first in the same type of projects at home and abroad. After a detailed investigation, the operation of the geomembrane in Shengli Reservoir since the first water storage is: polyethylene smooth surface There are 1 155 holes in the geomembrane, and 84 holes in the composite geomembrane. Some of these holes are generated during construction, and they were not found and repaired in time, thus forming a leakage channel; another part is due to the upper cushion layer. There are soils with sharp edges and corners, and under the action of water pressure, the geomembrane is pierced to form holes, thus forming a leakage channel. The permeability of the membrane impermeable body and the permeability coefficient, and then compare the permeability law of the geomembrane impermeable body on the whole. The calculation method of the monthly average water surface area is: use the measured water width and cross-section spacing of each section of the same water flow, use the trapezoidal area formula to obtain the water surface area of ​​each section, and accumulate the measured water surface area of ​​the reservoir area for this water level, and The relationship curve between water volume and water surface area is established, and then the monthly and annual water surface area is obtained by interpolation based on the monthly average flow over the years.

Shengli Reservoir is a hilly reservoir with dam body, dam foundation and geomembrane anti-seepage. The infiltration amount calculated here is the infiltration amount of the geomembrane anti-seepage body; Therefore, the calculated infiltration amount is the total infiltration amount of the two geomembranes. In this paper, according to the laying area of ​​the geomembrane and the percentage of broken holes, the total infiltration amount is multiplied by the percentage to obtain the smooth geomembrane respectively. Permeability of membranes and composite geomembranes.

Before laying the geomembrane, the permeability coefficient of the reservoir area was 2.17×10-5~1.16×10-2cm/s. In the first year of water storage (2003), the permeability coefficient of the geomembrane impermeable body was 1×10 -9~1×10-8cm/s, indicating that the geomembrane impermeable body has good permeability under the action of water pressure gradient; after 1 year of operation, in 2004, its infiltration volume and permeability coefficient have changed. This is because the geomembrane anti-seepage body produces leaking holes under the action of water pressure, which increases the penetration of the geomembrane anti-seepage body; after 2004, after the inspection and repair of the geomembrane anti-seepage body , the infiltration volume and permeability coefficient of the geomembrane impermeable body have been reduced, so the geomembrane impermeable body has not been laid, and the leakage problem in the reservoir area has been significantly improved.

It should be noted that the infiltration capacity and permeability coefficient of geomembrane are the calculation results obtained by macroscopically considering the impact of damaged holes during construction and reservoir operation, and the annual average water depth is used. As a whole, the permeability of the geomembrane impermeable body was evaluated.

3 Conclusion

1. The analysis of the permeability characteristics of the large-area geomembrane impermeable body in the warehouse cannot be solved by the traditional method of analyzing the permeability characteristics of the soil layer. This paper analyzes the permeability characteristics of the large-area geomembrane impermeable body from the perspective of water balance.

2. Through the analysis of the permeability characteristics of the geomembrane impermeable body in the reservoir plate of Shengli Reservoir, it is concluded that the permeability coefficient of the geomembrane impermeable body in the initial stage of water storage (2003) is relatively small, generally in the order of 10-9~1~ 10-8cm/s, after a period of operation, the permeability coefficient of the geomembrane impermeable body has increased, and then gradually stabilized. Compared with the non-laid geomembrane impermeable body, the permeability characteristics of the reservoir area have been significantly improved.

3. The permeability coefficient of the intact and non-destructive geomembrane is in the order of 10-13~10-12cm/s. After considering the influence of the damaged pores of the geomembrane on its permeability, the number of permeability coefficients is as large as 10-8~10-7cm/s. s, this is because the damaged holes generated by the geomembrane during construction and use increase the amount of infiltration of the geomembrane. Therefore, attention should be paid to the protection of the geomembrane during construction. During the maintenance of the reservoir, the geomembrane should be protected against seepage. body for overhaul.