Horry County Solid Waste Authority
Subtitle D Landfill Closure Project: Alternate Final Cover
Conway, South Carolina
The mission statement of the Horry County Solid Waste Authority, Inc. reads:
“To be an independent, innovative, responsive organization that aggressively provides comprehensive, cost-effective solid waste management in an environmentally sound manner, incorporating state-of-the-art methods and technology, and educating the public on responsible waste management.”
The recent Subtitle D landfill closure project is another success story demonstrating how the SWA continues to meet this mission statement.
The SWA began operating in Phase 1 of the Subtitle D landfill in 1997 and the permitting engineer at the time specified a typical industry standard final cover system. In 2006, the SWA had filled most of the Phase 1 area and was operating in Phase 2. Therefore most of the Phase 1 area had reached final grade and was ready for closure.
Garrett & Moore, Inc. was retained in 2006 to prepare construction drawings and specifications for the Phase 1 closure. In planning and collaborating for the project with SWA staff, it was determined that the industry standard final cover system was not ideal and could be improved.
For discussion purposes, Figure 1 depicts an industry standard final cover system that meets the Subtitle D regulatory requirements and therefore could be used in the closure of a lined municipal solid waste landfill.
The solid waste industry as a whole has gained experience in recent years about the performance, constructability, liner integrity, slope stability, and long-term maintenance of final cover systems for lined municipal solid waste landfills. Primarily because only recently have some of the landfills, which were initially permitted upon the promulgation of the Subtitle D rules, begun the facility’s closure and post-closure care periods.
Based on lessons learned by others and as allowed in the regulations, the SWA and Garrett & Moore permitted an alternate final cover system for the Subtitle D Landfill, which was the first true alternate final cover system approved in South Carolina and reduced overall costs. Figure 2 illustrates the alternate final cover system.
Advantages of the alternate final cover system include:
- Superior Flexible Membrane Liner (FML) integrity
- Improved slope stability
- Improved infiltration reduction
- Improved long-term vegetative growth
- Less weather-dependent construction
Each of these advantages is described in detail below.
Superior FML Integrity
The alternate final cover system nearly doubles the thickness of the soil cover over the FML compared to the industry standard final cover system. This increases the soil bridge thickness and thereby reduces the likelihood of damage to the FML during cover soil layer placement.
Improved slope stability
The alternate final cover system improves slope stability and the ability of the final cover system to resist sliding below and above the liner, and therefore exceeds the protection standards specified in the regulations.
Improved infiltration reduction
Based on the HELP model runs, the annual average rate of infiltration for the alternate final cover system is 2.3 cubic feet per year, per acre verses 72.58 cubic feet per year,per acre for the regulatory cap system. This means with respect to infiltration reduction, the alternate final cover system is about 30 times more restrictive than the alternate final cover system.
Improved long-term vegetative growth
The alternate final cover system offers the advantage of eliminating the 18”-thick infiltration layer altogether, which greatly reduces the impact of weather on the project and allows the FML to be installed over larger areas at one time minimizing the need for multiple liner crew mobilizations.
The SWA competitively bid the industry standard final closure system and the alternate final closure system to understand the cost differences between the two options. It is important to note that the alternate final closure system was not intended to be a cost-savings measure, however, there was a concern as to how much of a premium would be paid to construct the alternate final closure system if it was more expensive. The bid results showed that the alternate final closure system was about 10% less than the industry standard final closure system. In the end the SWA was able to provide an improved final closure system and save cost.