Rock Excavation and Underpinning below a 19-Story Building-Paper

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The project created an entrance to Manhattan’s Second Avenue Subway 72nd Street Station through the basement of an existing 19-story residential building. Three escalators descend approximately 80 feet through an inclined rock cavern beneath that building and an adjacent five-story structure. Excavation threatened the foundations of both buildings, so columns, a stone masonry party wall, and supporting columns were underpinned before work began.<br /><br />Temporary steel frames spanning the cavern carried the building loads while permanent concrete transfer girders and the cavern lining were constructed. Loads were transferred in two stages using hydraulic jacks: first to the temporary frames, then to the permanent supports. Because the buildings had brittle finishes and very strict movement limits, the work used incremental jacking and continuous monitoring of vertical movement, rotation, and structural response. Limits included 0.07 inches of vertical movement and 0.003 inches per foot of column rotation.<br /><br />Excavation took place in confined conditions beneath occupied residences, requiring controls for noise, dust, vibration, and equipment access. Rock was initially drilled and mechanically split, then broken and removed with compact equipment. Adding Da-Mite expansive splitting mortar to the drilling process increased excavation productivity from about 25 to 60 cubic yards per week and reduced removal costs from roughly $2,000 to $800 per cubic yard.<br /><br />The cavern was waterproofed, reinforced, and lined with cast-in-place concrete using a custom form system. Careful engineering, staged load transfers, monitoring, and controlled excavation allowed construction to proceed without adverse impacts to either building or its residents.

Keywords

Second Avenue Subway

72nd Street Station

Manhattan

subway entrance

underground excavation

hydraulic jacking

temporary steel frames

load transfer

rock cavern

construction monitoring