The composite geomembrane waterproof structure consists of four layers: the base surface, the composite geomembrane, the protective layer, and the rubble (or concrete) slope protection. The design of each layer is described as follows:
① Base Surface Design (i.e., requirements for dam shell surface treatment)
To prevent the soil layer above the composite geomembrane from sliding along its surface, and to enhance the stability of the protective layer (including the slope protection), the upstream membrane-laying dam shell surface is excavated into terraces. Each level has a height difference of 0.3 meters, with the horizontal width determined by the slope ratio. The designed slope ratio is 1:1.0. After excavation, all bricks, stones, roots, and other debris on the surface are removed, and the surface is compacted and leveled using a rammer.
② Composite Geomembrane Design
Based on the main characteristics of the product and considering the specific conditions of the Zhou Head Reservoir, the WCD-1 type two-fabric-one-membrane composite geomembrane is selected for the design.
③ Protective Layer Design
The waterproof effect of the composite geomembrane depends on the integrity of the plastic membrane during construction and operation. To prevent trampling by humans and animals, damage by plants and animals, and to reduce the effects of light and heat (since they are high molecular fiber polymer materials), direct sunlight exposure should be particularly avoided. Therefore, a protective layer should be laid over the composite geomembrane.
The particle size of the protective layer cannot be too large, otherwise, it may puncture the composite geomembrane. Generally, the particle size of the protective layer can be estimated approximately using the following formula.
④ Slope Protection Design
Since the dam slope is a water-retaining structure that must withstand wind wave pressure, some form of slope protection is necessary. In this project, dry-laid large stone slope protection is used.
The slope protection stones must be hard, dense, able to withstand long-term weathering and solar radiation, and have sufficient individual weight to resist the pressure of wind waves and water flow scouring.
To prevent wind waves and water flow from carrying away the clay particles of the dam shell protective layer, a 0.1-meter-thick layer of crushed stone (gravel) is laid under the stone slope protection as a cushion and filter layer.
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