The slope ratio of the upstream design of the reservoir using geomembrane seepage prevention should be slower than the critical slope between the materials, and because the friction coefficient between the geotextile and the geomembrane is smaller than the friction coefficient between the geotextile and the earth and stone, the sliding surface It will be produced between the geotextile and the geomembrane, and a more gentle pool slope is needed to maintain stability. At the same time, considering the construction factors and operational safety requirements, the upstream pool slope should be set to be slow and not steep. The slope of the upstream pool in the cold area should also consider the influence of freezing. It is not advisable to set up the horse track on the water surface. In order to ensure the stability of the slope protection and reduce the length of the single-stage water-facing slope, it is good to provide concrete anti-skid beams in the middle of the slope.
Geomembrane anti-seepage layer structure type
The geomembrane is the main body of the anti-seepage layer of the reservoir. However, the protective (protective) layer, the upper and lower cushions and the supporting transition layer should be installed on both sides of the composite geomembrane to form a complete anti-seepage system.
Protective layer material and type
Commonly used protective surface types are precast concrete slabs, cast-in-place concrete slabs, reinforced mesh or wire mesh concrete slabs, dry block stones and slab masonry stones . In the early construction of reservoirs and reservoirs, the protective layer materials used more dry stone and masonry, but in the cold areas, this type of protective layer was damaged by freezing, and the reason was analyzed. The stone masonry surface is rough, and the stone has a certain angular angle, the rough surface area is relatively large, and the contact area between the ice and the stone in the winter operation and the adhesion between the two, the ice in the temperature rise The pushing effect of the ice expansion force on the masonry protective layer, and the damage and instability of the protective layer caused by the thrust and pull-down force of the ice on the protective layer when the water level rises and falls.
Geomembrane anti-seepage material and type
Membrane material and thickness
Geomembrane is the main body of the anti-seepage layer of the reservoir. The choice of membrane material and film thickness is very important. The PE project should be used in the project of drinking water project and water contact . The thickness of the membrane is controlled by the amount of leakage and the thickness of the soil particles on the contact surface, and the voids under the membrane cause the membrane to undergo uneven deformation under the action of the water head, so that the membrane is locally pulled and the thickness is calculated. .
(1) Control the thickness of the membrane by the amount of geomembrane leakage
On the one hand, it is the amount of leakage caused by welding and puncture during construction, which is unpredictable and does not control the thickness of the membrane; on the other hand, the permeability of the membrane itself is produced by the unevenness of the membrane manufacturing. Relevant information, the factory geomembrane has about one hole per 1.0m2. When the permeability of the 0.3mm thick film is measured, the permeability coefficient is 1×10-9~1×10-10cm/s , the permeability coefficient For hydraulic engineering, the requirements and permissible requirements of the specification can be met. Therefore, the leakage amount does not control the thickness of the membrane.
(2) The film thickness is determined by the degree of the thickness of the soil particles of the film and the contact surface or the gap between the particles being broken.
The calculation uses the curve intersection method of the thin film theory. Since the assumptions of the calculation basis often do not conform to the actual conditions of the project, the calculated thickness is often very small, and it is difficult to ensure the safety according to the thickness of the film. The film of 0.25 mm was tested for no damage under the action of 67.1% gravel, 32.9% sand, and 200 m water head. Therefore, as long as the underlying film layer is firm, uniform and reliable, a small film thickness is hard to be broken by the water pressure and can meet the requirements of the anti-seepage, and the thickness is not controlled to control the thickness of the film. In engineering practice, the film thickness is often determined by the same type or similar foundation conditions, the successful experience of the same type of film upper and lower cushions, and the smaller range allowed by the specification. The specification stipulates that the thickness of geomembrane for anti-seepage engineering should not be less than 0.5mm. According to the level of buildings in water conservancy projects, the thickness of geomembrane of 1~2 building should not be less than 0.5mm, and the thickness below 3 should not be less than 0.3mm. . Considering the construction factors, safety and standard requirements, combined with the statistics of the geomembrane anti-seepage tanks that have been built, the thickness of the membrane should not be less than 0.4mm, and the important projects can be appropriately increased.
Impervious layer selection
The anti-seepage layer is divided into a single-layer geomembrane anti-seepage layer and a geomembrane composite anti-seepage layer. The geomembrane composite anti-seepage layer refers to a composite layer composed of “geomembrane + cement soil or clay”. Regardless of the impact of construction man-made damage, the leakage of the membrane is mainly caused by the amount of leakage. According to GirordJ.P, there is a defect of about 0.4 million m2. The size of these defects is set to a pool with an equivalent aperture of about 2mm. The water depth is 10.0m. It is made of a single layer of 0.5mm PE anti-seepage film and 0.5mm PE thickening. The combined anti-seepage layer of 0.5m clay has a laying area of 40,000 m2, and the leakage amount is estimated separately. According to the calculation results, the leakage of the single-layer membrane anti-seepage layer under the same conditions is 24.70m3/d, which is much larger than the leakage of the geomembrane composite anti-seepage layer by 1.06m3/d, which is 23 times different. Therefore, the material of the bottom of the pool and the material of the pool are homogenous pools of sand and gravel. The material permeability of the material under the membrane is large and the water permeability is strong. Even if there is leakage of geomembrane, it is not easy to aggregate, and the membrane is drained smoothly. A single layer of anti-seepage layer (as shown in Figure 4). For the bottom of the pool and the material of the pool, the permeability coefficient of the material under the membrane such as loam is small, and the water leakage after the membrane is easy to aggregate, so that the reservoir of the soil layer is deformed and deformed, and the composite membrane composed of “geomembrane + cement soil” should be adopted. Infiltration structure type to ensure project safety.
By analyzing and summarizing the failed and successful engineering examples of the established reservoir, the following conclusions can be drawn.
(1) For the storage tank on the collapsible site, the foundation treatment is based on the original soil turning, and it is necessary to combine the “10% cement soil + geomembrane” composite anti-seepage layer type.
(2) For the water storage tank with composite geomembrane seepage prevention, it is not enough to calculate the stability of the slope only considering the influencing factors of the pool slope, and the stability calculation of the slope protection under buoyancy is not enough. The slope ratio should be selected not less than the protection The critical slope ratio between the materials, the graded horse track is not set on the slope.
(3) The protective layer of slope protection in cold areas should adopt the surface material with large surface smoothness to avoid the instability of freezing damage during winter operation. At the same time, the anti-slip beam is strengthened and the stability of the slope block is maintained.