Drilled Pile Design and Installation Challenges in an Urban Environment-Paper

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The paper describes the design and installation of 148 rock-socketed drilled piles for a 60-story residential tower on a small downtown New York City site. The project was constrained by nearby shallow-founded buildings, a landmark building, and a subway tunnel located as close as 1 foot from the property line. Subsurface conditions included fill, loose to medium-dense sand and silt, very dense glacial till containing cobbles and boulders, and mica schist bedrock about 110–150 feet below sidewalk level.<br /><br />Because driven piles could cause damaging vibration and settlement, the selected foundation used 16-inch-diameter steel-cased piles with rock sockets extending at least 17 feet into qualifying bedrock. The piles were designed for high compression and uplift loads, while the foundation layout and structural checks addressed stringent transit-agency requirements for tunnel protection and base-shear resistance. Visual verification of each socket was required in lieu of full-scale load testing.<br /><br />Installation proved difficult. Restrictions on drill-bit position during casing advancement, combined with high drilling-fluid pressure or slow casing penetration, caused external fluid flow and risked ground loss. A proprietary ring-bit system and a down-the-hole hammer also produced external flush and, in some cases, surface depressions, so both methods were discontinued. The contractor ultimately advanced casings through the sand, silt, and glacial till using duplex rotary drilling with a tri-cone roller bit and internal fluid circulation, which avoided observed ground loss.<br /><br />Variable bedrock elevations and weathered rock made it essential to confirm that casing tips had reached bedrock rather than stopping in glacial till boulders. Where this was not achieved, holes were stabilized with grout and redrilled. Rock sockets were inspected using video cameras or, when necessary, acoustic televiewer logging.<br /><br />The project demonstrates that drilled-pile work near sensitive infrastructure requires careful method selection and monitoring. In particular, drilling-fluid pressure and casing advancement rate must be controlled, and acoustic televiewer logging can provide an effective alternative for socket verification.

Keywords

rock-socketed drilled piles

foundation design

New York City high-rise

subway tunnel protection

duplex rotary drilling

steel-cased piles

glacial till

bedrock sockets

drilling-fluid pressure

acoustic televiewer logging