Abstract:
The proposed development at 4650 Broadway is a 20 story mixed-use commercial building on Manhattan’s far north side. The site is on the margins of the Inwood Hill thrust fault and contacts between the Inwood Marble and the Manhattan Schist geological units. This paper focusses on the geological variability within the project site and its unique impacts on foundation design and construction and the innovative solutions to address these challenges.
An extensive subsurface investigation, including implementation of geophysical methods, was conducted to better understand the geological features and profiles of the site. The general soil profile consisted of manmade fill underlain by glacial soil deposits, and highly variable bedrock. The bedrock was generally encountered from about 30 to 60 feet below grade and consisted of marble, gneiss, and schist. A significant challenge was to estimate the extent and degree of weathering of the bedrock for deep foundation elements across the site. Continuous Flight Auger (CFA) piles were ultimately selected after evaluating several other options; index piles and a detailed load testing program were implemented to optimize the pile design and develop an acceptance criteria for establishing final pile depths in light of the variable bedrock conditions present within the site.