Abstract:
Located near the western tip of Coney Island, the 3514 Surf Avenue development consists of two 22-story towers, a three- to five-story podium, and a garage over an approximately 70,000 square-foot site that occupies an entire Brooklyn block. Our presentation describes the numerous challenges that arose during design and construction of this fast-track project: complex soil conditions, strict settlement tolerances, and flood considerations for post-Hurricane Sandy Coney Island.
Shallow foundations (spread footings and mats) were initially considered; however excessive total and differential settlements would result at the towers due to the presence of a deep seated compressible clay layer (about 65 to 105 feet below grade). To address differential settlement concerns and meet strict project tolerances, the geotechnical and structural engineers proposed a hybrid foundation system of deep pile foundations at the towers and spread footings at the podium and garage. Since the development plan involved locally raising grades above the design flood elevation, the piles were designed to consider down-drag.
Multiple rounds of subsurface exploration, index-pile driving, and pile load testing were performed during the design phase to optimize the foundation to meet the project demands. The load test program (which exceeded the minimum New York City Building Code requirements) was performed in two phases. The first phase was performed during the design development to evaluate the drivability and geotechnical capacity of several pile options and to provide in situ data to finalize the pile design. The second phase was performed before driving production piles to evaluate the contractor’s proposed modifications to the pile thickness and spicing connection details.
The project team considered multiple pile types, sizes, and splicing connections during an extensive index-pile program, as recommended by the geotechnical engineer and foundation contractor.