In response to Subtitle D regulations and the need to address specific site issues, the industry is developing alternative liner systems that use new materials and other innovative technologies.
New landfill designs have included provisions for liner systems intended specifically to protect groundwater quality. The composite liner requirement establishes that all liners systems must conform to a minimum national standard including a combination of two feet of clay covered with a synthetic geomembrane (see chart).
Engineers are now approaching landfill design as an overall integrated system; for example, liner design engineers must incorporate the final capping system, stormwater management and operation features into consideration. Specific site conditions and constructability issues also should be addressed in liner design.
Finally, in some areas of the country, alternative materials are necessary because materials commonly used to construct the liner and leachate collection system are unavailable or expensive. Clay, for example, can be costly in some areas and may perform erratically under certain site conditions such as a high groundwater table.
More commonly used materials include geotextiles and geonets, made from a net-like material used as the drainage layer for the leachate collection system. At some sites geotextiles can be used under the clay liner to improve constructability or improve the stability of the sub-base upon which the composite liner is constructed.
Geonets may be substituted for the sand drainage layer and in some areas they can cost less than the sand. Geonets also can improve the hydraulic capabilities of the drainage system, which gives design engineers more flexibility in laying out the leachate collection system.
Using an advanced or alternative liner system from the start can be more environmentally sound, reduce future costs, increase operational flexibility and provide environmental reliability. Alternative liner technologies are a good option to consider for landfill sites that require modification to the standard composite liner to address specific site conditions and constructability issues.
Other advanced technologies include coupling various polyethylene materials, which forms a more crack and tear resistant material with greater stretch to meet specific site conditions. These materials also have been used with a relatively thin clay layer and are factory manufactured. They tend to be easier to install and often are used as the upper liner in multiple liner systems.
In a collaborative effort between Camp Dresser & McKee, Cambridge, Mass., Gundle Lining Systems, Houston, and the University of South Florida, Tampa, Fla., engineers developed a model to evaluate the performance of alternative liner systems and pilot systems to measure their performance. The project relates mathematical predictions to real-world performance of alternative liner systems. The results of the pilot tests will provide guidelines for engineers and designers of liner systems to predict liner performance and its overall effectiveness.