One way for landfill owners and operators to reduce costs is by considering the latest options for post-construction landfill monitoring. There have been significant variations in solid waste landfill design in the northeastern United States within the past year, which have reduced costs and met health and environmental standards.
Three specific variances, approved by state or federal regulators as recently as September 1998, include:
* Reducing the minimum final cover cap thickness to less than 24 inches;
* Extending the liner system over the common face for horizontal expansions; and
* Incorporating provisions for future vertical expansions into the landfill's support systems.
Each landfill expansion and cover design approach has its advantages and disadvantages, as well as financial costs and benefits.
Cover Approaches. To reduce the potential for leachate generation, a final cover system is placed on top of a closed landfill cell to minimize rainwater infiltration and prevent contact with the waste. A typical cover system includes a low-permeability geomembrane. On top of this layer is 18 inches of drainage material - sand or a geocomposite - that is topped with natural soils and 6 inches of topsoil. Where low permeability soils are available locally, these materials replace the geomembrane.
Facilities that closed before 1988 may not have this cover system. However, they still must comply with U.S. Environmental Protection Agency (EPA), Washington, D.C., regulations or have a system equivalent to the EPA's recommended system. Consequently, landfill owners are hiring engineering consultants to help demonstrate that their existing or proposed alternate system is effective and to help reduce the final cover cap thickness.
To demonstrate that an equivalent cover system exists, consultants typically will conduct a field investigation to map the existing soil's thickness and geotechnical properties (grain size and permeability). Using this information, models, such as the EPA's Hydrologic Evaluation of Landfill Performance (HELP), are used to estimate infiltration through soil profiles and compare the existing conditions with the EPA's required cover system.
This type of evaluation spared the 60-acre Berks Landfill, Sinking Spring, Pa., the costs of constructing a new cover system. Its landfill operators hired Golder Associates, Mt. Laurel, N.J., who found that its existing low-permeability cover soils were a minimum thickness of 12 inches, which demonstrated through technical analyses that this system is sufficient and equal to the regulatory cover system.
Horizontal Expansions. Ten years ago, the solid waste market in the Northeast was booming, but by 1992, the rapid development ended. Three years later, the landfill design focus had shifted toward horizontal expansions to existing, unlined landfills.
To help secure regulatory approval, the expansion's liner system is extended up the face of the existing landfill to the expansion's maximum filling grades. This extension increases liner system construction costs but reduces long-term leachate management costs of the existing unlined landfill because the final low-permeability cover soils on the existing landfill eliminates infiltration and allows its leachate management system to be shut down quicker after the final cover is placed. This shutdown could occur before the horizontal expansion is filled, depending on the site's waste composition.
Placing the impermeable liner atop the existing facility along the expansion interface may reduce the minimum storage requirement for managing, containing, storing, shipping and treating leachate.
Vertical Expansions. An additional landfill trend, which occurs during horizontal expansion design, includes evaluating future vertical expansion potential. To allow for a vertical expansion, existing soils may need to be reinforced with geosynthetics or soil admixtures. Slope stability analyses (static and pseudo-static) will show whether the interim proposed horizontal expansion grades need to be altered to ac- commodate future vertical expansion grades.
For example, consultants may discover that the maximum slope incline that provides stability for an increased vertical elevation is flatter than the maximum slope incline for a lower elevation. This should be taken into account when developing the initial horizontal expansion grades.
Depending on the materials selected for the liner system, additional reinforcements may be necessary as well. For example, if angular stone is proposed for the leachate collection layer and a geomembrane is proposed for the landfill's underlying barrier layer, a thicker cushion layer between the stone and geomembrane may be required. As the overlying waste's mass increases, there should be enough cushion to prevent puncturing the geomembrane.
Analyzing the cover system also may dictate the maximum inclination of the final cover. The waste may maintain a stable slope at an inclination of 2-horizontal-to-1 vertical, while the cover system remains stable at an inclination of a 3-1 ratio. Incorporating the additional provisions required for a vertical expansion can mean an insignificant cost increase during horizontal expansion.
All three approaches can be viable options. In fact, several Northeast owners and operators have found these options useful when negotiating with regulators to further reduce their costs. But before deciding on the best option for your landfill, or making a recommendation to a client, landfill owners and operators, and consulting engineers should assess all the technical variables and evaluate related costs.