The municipal solid waste disposal facility of Cherry Island in Wilmington Delaware, operating since 1985, started reaching its full operational capacity by the early 2000s, making the Delaware Solid Waste Authority (DSWA) to consider solutions for extending the life of the landfill. The geographical location of the facility, meeting the Delaware River on the east, the Christina River on the south, as well as Interstate 495 to the west, and dredge lagoons to the north, made the vertical expansion of the landfill to be the only viable solution. However, the subsurface conditions of the area required the application of special geotechnical measures for the improvement and reinforcement of the ground.
Subsurface conditions at the site consisted of medium to dense residual sand, overlain by a 60- to 100-ft thick layer of weak and low-permeability alluvial deposits and dredge materials, with significantly low undrained shear strengths, in the order of 200lbs per square foot (psf). A vertical expansion of the landfill would therefore lead to significant settlements and excess pore pressures. To deal with this challenge, the consulting engineers adopted a composite foundation scheme, consisting of PVDs and a mechanically stabilized earth (MSE) berm. The MSE berm allowed the compression and consolidation of the soft foundation soils, providing extra waste disposal space, while PVDs provided dissipation of the generated excess pore pressures from the foundation layer to an underlying sand deposit.
Construction sequence started with the installation of over 81,000 PVDs, in the foundation area of the berm, measuring over 6.8million ft in total length. Two layers of a high-strength reinforcement geotextile with drainage sand were placed on top of the PVDs, providing quality backfill at the base of the bern as well as a drainage reservoir for the water produced during sub-soil consolidation. Additional reinforcement at the mid-height of the berm was guaranteed by two high strength geotextile layers. Additionally, the berm construction included the use of polyester geogrid with embedment lengths ranging from 20ft to 80ft. The final inclination of the berm was 1H:3V (one horizontal : three vertical), significantly steeper compared to the 8H:1V initial inclination estimated without the application of the above composite foundation solution.
Watch more information on this awarded an Award of Merit by the American Society of Civil Engineers (ASCE) in the following video!
Sources: Geosyntheticsmagazine, DSWA
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