Structural Details

Double-liner systems are more prone to defects in the structural details (anchorage, access ramps, collection standpipes, and penetrations) than single-liner systems.

26.3.3.31 Anchorage

Anchor trenches can cause FMLs to fail in one of the two ways: by ripping or by pulling out. The pullout mode is easier to correct. It is possible to calculate pullout capacity for FMLs placed in various anchorage configurations (Figure 26.18). In the "V" anchor configuration, resistance can be

Cover soil

Cover soil cs q = >'cs dcs q = >'cs dcs isinß

Horizontal anchor d cs b

"V"anchor

T Concrete anchor

FIGURE 26.18 Forces and variables—anchor analysis. (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

T Concrete anchor

FIGURE 26.18 Forces and variables—anchor analysis. (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

increased by increasing the "V" angle. A drawback to using the "V" design for getting an accurate estimate of pullout capacity is that it uses more space.

Most facilities have access ramps (Figure 26.19), which are used by trucks during construction and by trucks bringing waste into the facility. The figure also depicts a cross section of a typical access ramp. The double FML integrity must be maintained over the entire surface of the ramp. Because ramps can fail due to traffic-induced sliding, roadway considerations, and drainage, these three factors must be considered during the design and construction of access ramps.

The weight of the roadway, the weight of a vehicle on the roadway, and the vehicle braking force— all these must be considered when evaluating the potential for slippage due to traffic. The vehicle braking force should be much larger than the dead weight of the vehicles that will use it. Wheelloads also have an impact on the double FML system and the two leachate collection systems below the roadway. Trucks with maximum axle loads (some much higher than the legal highway loads) and 90psi tires should be able to use the ramps. Swells or small drains may be constructed along the inboard side of a roadway to ensure that the ramp will adequately drain water from the roadway. The liner system, which must be protected from tires, should be armored in the area of the drainage swells. A sand subgrade contained by a geotextile beneath the roadway can prevent local sloughing and local slope failures along the side of the roadway where the drains are located. The sand subgrade tied together with geotextile layers forms, basically, 800-ft long sandbags stacked on top of one another.

26.3.3.3.3 Vertical Standpipes

Landfills have two LCRSs: a primary LCRS and a secondary LCRS. Any leachate that penetrates the primary system and enters the secondary system must be removed. Vertical standpipes are used

18' Typical
FIGURE 26.19 Geometry and cross section of a typical access ramp. (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

to access the primary leachate collection sumps. As waste settles over time, downdrag forces can have an impact on standpipes. Those downdrag forces can lead to puncture of the primary FML beneath the standpipe.

To reduce the amount of downdrag force on the waste pile, standpipes can be coated with viscous or low friction coating. Standpipes can be encapsulated with multiple layers of HDPE. This material has a very low coefficient of friction that helps reduce the amount of downdrag force on the waste piles.

Downdrag forces also affect the foundation or subgrade beneath the standpipe. If the foundation is rigid, poured concrete, there is a potential for significant strain gradients. A flexible foundation will provide a more gradual transition and spread the distribution of contact pressures over a larger portion of the FML than will a rigid foundation. To soften rigid foundations, encapsulated steel plates may be installed beneath the foundation.

26.3.3.3.4 Standpipe Penetrations

The secondary leachate collection system is accessed by collection standpipes that must penetrate the primary liner. There are two methods of making these penetrations: rigid or flexible. In the rigid penetrations, concrete anchor blocks are set behind the pipe with the membranes anchored to the concrete. Flexible penetrations are preferred since these allow the pipe to move without damaging the liner. In either case, standpipes should not be welded to the liners. If a vehicle hits a pipe, there is a high potential for creating major tears in the liner at depth.

26.3.3.3.5 Wind Damage

During the installation of FMLs, care must be taken to avoid damage from wind. Designers should determine whether wind will affect an installation and, if so, how many sandbags will be needed to anchor the FML panels as they are being placed in the field.

26.3.3.3.6 Surface Impoundments versus Landfills

There are significant differences in structural considerations between landfills and surface impoundments. First, liners used in surface impoundments have a long-term exposure to the waste and to sunlight. In addition, surface impoundments have a potential for gas in the LCRS because there will always be the potential for organic material beneath the system.

Long-term exposure can be stopped using either soil or a nonwoven fabric to cover the membrane in a surface impoundment. Another option is to drape a heavy, nonwoven fabric with base anchors in it over the membrane. This nonwoven material is cheaper, safer, and more readily repaired than a soil cover.

Gas- or liquid-generated "whales" can be a serious problem in surface impoundments. Water-induced "whaling" can be a problem in facilities that are located where there is a high water table. Stormwater can also enter a collection system through gas vents. In gas vent designs, care should be taken to ensure that the vent is placed higher than the maximum overflow level. If excess water in the leachate collectors is causing whaling, the perimeter should be checked to determine where water is entering. To repair a water-generated whale, the excess water should be pumped out of the sump and its source stopped. If there is gas in the whale (the liner is inflated and visible above the water surface), the facility must be rebuilt from scratch.

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