DELAWARE — A recent issue of Geo-Strata, the geotechnical magazine published by the American Society of Civil Engineers' Geotechnical Institute, featured an article on the geotechnical engineering challenges associated with the expansion of the Cherry Island Landfill, Delaware, a project led by Geosyntec practitioners based in the company's Columbia, Md., office. The article, titled "Design Consideration for Expansion of an Existing Landfill over Extremely Compressible Soils," was authored by Geosyntec's David Espinoza, Ph.D., P.E., Carlos Lazarte, Ph.D., P.E, M.ASCE, and Michael Houlihan, P.E., BCEE, F.ASCE, and staff of the Delaware Solid Waste Authority (DSWA), the project owner.
The Cherry Island Landfill expansion project provides an example of the use of geosynthetics for site improvement as well as the importance of integrating site investigation, characterization, and design with construction observation. The subsurface, which consists of very soft and extremely compressible materials, made the vertical expansion of the existing landfill an engineering design and construction challenge.
By carefully characterizing the site before design and by confirming the design assumptions through construction-phase monitoring, Geosyntec is able to deliver to DSWA and its customers a safe, stable long-term pollution for municipal waste disposal. To stabilize the less-than-ideal subsurface, Geosyntec's design consisted of installing thousands of 70-foot to 120-foot-long prefabricated vertical drains (PVDs) in the soft sediments, around the perimeter of the landfill, to allow relatively rapid consolidation of the saturated soft deposits. Our design also called for the construction of an 8,000-foot-long, 60-foot-high mechanically stabilized earth (MSE) berm over these drains to accelerate the dewatering of the dredge. As of May 2008, approximately 6 million linear feet of PVDs have been installed and up to 30 feet of fill have been placed in the MSE berm. The geotechnical monitoring results confirm the magnitude and rates of settlements predicted by analyses conducted for the berm design. The geotechnical monitoring results also indicate that the level of stability achieved during construction is adequate for the site-specific conditions and the arte of pore-pressure dissipation that is taking place in the underlying soft sediments.