NYCDOT Builds a Bridge Under a Bridge Under a Bridge in Brooklyn

A roadway bridge in a residential Brooklyn neighborhood is an unlikely poster child for the complexity of New York City’s dense web of infrastructure. However, the NYCDOT's reconstruction of the Fifth Avenue Bridge in Bay Ridge, Brooklyn certainly fits the bill: micropiles, H piles, soldier piles, subways, railroads, a petroleum pipeline, an interstate highway viaduct… and about twenty feet of working headroom. This case study addresses the load testing and construction of deep foundations that were employed to support the south integral abutment of the new roadway bridge, which will span 170 feet across active railroad and subway tracks below and replace the existing 106-year-old bridge in its exact location. To maintain two-way traffic at all times, the foundations were constructed from underneath the existing bridge prior to its staged demolition. Working under 12 to 23 feet of headroom, eighteen (18) HP16x183 H-piles, twenty-four (24) 24-inch diameter pipe soldier piles, and thirty-eight (38) hollow-bar micropiles were installed up to 65 feet deep in dense sands. Solutions employed to overcome the challenges of the confined environment without adversely impacting nearby sensitive infrastructure are presented, with particular focus on the H piles and the micropiles. The H piles were driven using a forklift-mounted hydraulic impact hammer and were dynamically load tested. For the micropiles, an alternative hollow-bar bond zone installation method was proposed by the pile subcontractor and subjected to a static load test program.