Design, Permitting, and CQA
Garrett & Moore, Inc. was retained to prepare construction drawings and specifications for the initial closure phase of an active Subtitle D landfill. In planning and collaborating for the project with the owner, we raised several concerns regarding the permit-approved final cover system. Although the permitted design would be considered by many to be an industry standard final cover system, Garrett & Moore believed the approach was not ideal and could be improved.
For discussion purposes, the adjacent figure illustrates 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 owner 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 the state of South Carolina. 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
Construction required careful coordination with the existing landfill gas active collection system to ensure continuous operation of the landfill gas to energy plant onsite. The closure system needed to account for raising existing gas collection wells.
The owner 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 cover 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 owner was able to provide an improved final closure system and save cost.
