Micropiles in an Urban Setting - TBTA Verrazano Narrows Bridge HOV/Bus Ramps in Brooklyn, NY and the NYSDOT Kosciuszko Bridge Replacement Brooklyn NY-Paper

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The paper examines micropile use on two Brooklyn-area infrastructure projects: the Verrazano Narrows Bridge HOV/bus ramp improvements and Phase I of the Kosciuszko Bridge replacement. Both sites had dense existing infrastructure and challenging ground conditions, including glacial till with cobbles and boulders.<br /><br />At Verrazano, foundation design was constrained by existing foundations and utilities, limited working space and overhead clearance, active traffic, and the need to protect nearby structures. Shallow foundations would have required extensive excavation and dewatering; driven piles raised drivability and vibration concerns; and drilled shafts were difficult to install in the available space. Micropiles offered a small footprint, low vibration, and the ability to pass through existing foundations.<br /><br />At the Kosciuszko abutments, low clearance beneath the operating expressway, nearby shallow foundations and retaining-wall elements, corrosion, settlement-related downdrag, and limits on foundation depth ruled out conventional alternatives. Micropiles were used to carry axial loads, while a reinforced-concrete deadman and tie rods resisted lateral loads. Polyethylene wrapping isolated micropiles from proposed fill to reduce downdrag.<br /><br />Both projects used Type B pressure-grouted micropiles bonded in glacial till and followed AASHTO LRFD design procedures. Verification and proof load tests were conducted. Four verification tests reached verified nominal grout-to-ground bond resistances of 4.2–7.0 ksf; none was taken to failure. Results suggest resistance in Brooklyn till may approach or exceed 7 ksf, but variability in the soil and installation methods makes project-specific testing important.<br /><br />The case studies demonstrate micropiles’ value in constrained urban construction, particularly where clearance, access, vibration, and protection of existing facilities govern foundation selection. The authors also emphasize monitoring nearby structures and utilities throughout construction to manage risk.

Keywords

micropiles

Brooklyn infrastructure

Verrazano Narrows Bridge

Kosciuszko Bridge

glacial till

Type B pressure-grouted micropiles

AASHTO LRFD

verification load tests

downdrag mitigation

urban foundation construction