Micropiles in an Urban Setting – TBTA Verrazano Narrows Bridge HOV/BUS Ramps, Brooklyn, NY and Nysdot Kosciuszko Bridge Replacement, Brooklyn, NY

"Construction of foundation elements in an urban environment is often complicated by multiple constraints such as existing foundations, utility conflicts, maintenance and protection of traffic on high volume roadways, low overhead clearances, and vibration and displacement monitoring to reduce risks of impact to existing facilities. Micropiles are commonly a preferred foundation alternative in an urban setting to mitigate some of these risks. This paper presents examples in which micropiles helped reduce risk in an urban setting on the two subject infrastructure improvement projects. The case studies presented include the Verrazano Narrows Bridge HOV and Bus Ramp Improvements in Brooklyn, NY and the Kosciuszko Bridge Replacement in Queens and Brooklyn, NY. The paper details the micropile design, how risks were mitigated, and the complications to each of these projects associated with the urban setting.INTRODUCTIONThe paper begins with background information about the Verrazano Narrows Bridge HOV and Bus Ramp Improvements Project, followed by background information for the Kosciuszko Bridge Replacement Project. This introductory background information is followed by discussion of the urban site constraints, which controlled the foundation type selection for both projects. In closing, the paper discusses the micropile design process, load testing, and construction.VERRAZANO NARROWS BRIDGE HOV & BUS RAMP IMPROVEMENTS BACKGROUNDProject ScopeThe project consists of two new ramps; one designated for HOV / Bus traffic between Staten Island and the Gowanus Expressway, and one to be utilized for storing the barrier transfer vehicle. Also included is the complete reconstruction of Ramps B and F to accommodate horizontal and vertical alignment clearances from the new ramps, removal of the median of the upper level to add one lane, utilities improvements, stormwater improvements, and miscellaneous structural repairs. The new HOV / Bus Ramp was designed for reversible traffic flow to accommodate the demand of the peak traffic direction by movable barrier.Geotechnical / Foundation Engineering ScopeThe geotechnical work included a desk study review of existing plans, geotechnical reports, and geologic maps. This data was subsequently supplemented with a subsurface exploration program. Once completed, foundation design was performed for 25 proposed bridge substructures. Four existing battered pilesupported foundations and two shallow foundations were evaluated for increased loading. Upon completion of the foundation design, foundation plans, specifications, and an engineer’s estimate were prepared. Foundation installation was observed on a part-time as needed basis."