On the shores of Rockland County, at the western terminus of the New NY Bridge, Tappan Zee Constructors, LLC (TZC) recently used an innovative construction technique to help build rock-solid landings with an eye – and ear – towards minimizing noise for nearby residents.
As with every part of the new bridge, the abutments – structures that connect the bridge to the highway on land – are immense and require a firm, unyielding base. The existing soft soil found near the river’s edge in Rockland was unsuitable, and required reinforcement.
The traditional approach to this problem would be to drive steel piles into the soil to give the abutment the needed stability. Instead, TZC installed a series of subterranean piers comprised of compacted crushed stone, called aggregate. The large, tubular masses of stone strengthen the adjacent soil with their collective compressive strength and provide support for enormously heavy structures such as abutments.
The process began with a thorough analysis of soil conditions to determine the size and number of aggregate piers needed, which were then plotted in a grid. Using a high-energy ram attached to a hollow casing, called a mandrel, tubular holes were pressed into the soil at designated locations.
Workers then poured aggregate down the mandrel and used the ram to tightly pack the material in the ground. The compaction made the aggregate and surrounding soil much denser. With the holes ranging in depth from seven to 30 feet, the process was repeated in layers until each hole was entirely filled.
Installation of these stiff, high-density aggregate piers moved at a swift pace, saving time and helping to keep construction costs down. Now that the soil-reinforcing piers are finished, the abutments will be built, moving the project closer toward completion in 2018.