Abstract:
Douglas S. Roy P.E (Principal) GZA GeoEnvironmental of NY, Manhattan, NY, USA, 646-533-5765, douglas.roy@gza.com
Muktar H. Khatari P.E. (Associate Principal) GZA GeoEnvironmental of NY, Manhattan, NY, USA, 646-294-7356, muktar.khatari@gza.com
ABSTRACT
Drilled micropiles have become a common urban foundation element in the United States over the last two decades, and the use of internal flush drilling techniques is sometimes required due to limited site constraints, adjacent structures and subsurface conditions. On a recent project in the West Village neighborhood of New York City, an innovative external flush technology was successfully implemented for micropiles used for temporary and permanent support, with significant compression and uplift loads.
The project included gut renovation of a two-story landmark building, located in the Greenwich Village Historic District. Project plans included excavating the 9,900 square-foot site to up to 30 feet below existing grade, requiring 400 micropiles for both foundations and support of excavation. Typical for the West Village area, the site is on the edge of the former Hudson River and underlain by fill, former marsh deposits and alternate layers of alluvial river deposits atop bedrock.
Due to the presences of loose fill materials and running sands in the groundwater table, the project team utilized micropiles for the permanent foundation and tangent piles for support of excavation. The permanent foundation and support of excavation piles were installed with external flush drilling techniques using a diluted grout mix. The purpose of this drilling technique was to prevent soil-washout and replace loose soils encountered during drilling, thus creating a reliable bond zone for drilled micropiles. Permanent foundations included 13.375-inch drilled micropiles, designed for allowable loads of up to 180 tons in compression and 115 tons in tension. The tangent piles generally included 11.875-inch micropiles with casing extended down the bottom of pile depth, and tremie grouting upon drilling completion.
This presentation will provide insight to the external flush technology utilized in micropile installation, including design and construction limitations and challenges, pile installation methods, results of load test program, and benefits.