Upgrading and modernizing New York City Transit’s track ventilation system is a key part of the Metropolitan Transportation Authority ’s Capital Program. For two vent plants in lower Manhattan, general contractor Nab Construction Corporation of New York awarded Moretrench the contract to install a total of 65, 80-ton micropiles at each location to support new fan chambers.
The challenge of the project lay in that the micropile work, including drilling, casing installation and full scale load testing, was to be accomplished in tight conditions some 40 feet below street level and adjacent to the active subway tracks. This required that all equipment and materials be lowered from street level to working level by crane, making attention to safety the highest priority.
Soils through which the micropiles were to be installed consisted of approximately 60 feet of silty sands, the bottom 10 to 15 feet of which contained large boulders and cobbles, overlying rock. To expedite the work, Moretrench mobilized two, track-mounted, electric hydraulic drill rigs fitted with rotary duplex, internal-flush drilling systems and down-the-hole hammers. At each micropile location, 9.625-inch diameter permanent casing was advanced through the overburden and boulders and seated three feet into rock. The drill string was than advanced further to cut the 14-foot deep rock socket. Number 14 threadbar, with centralizers, was inserted the full depth of the hole and 4,500 psi concrete tremied in place to complete the pile.
Full-scale static axial load tests were conducted on two test piles at each fan chamber location to verify design parameters. At 200% of design capacity, settlements of 0.42 and 0.45 inch were recorded, well within acceptable limits.