As part of the 7-Line Extension Project in New York City, the 42nd Street Station of the A, C, E subway needed to be underpinned while the existing rock grade was lowered and then new structural steel installed. This work was completed with no impact on the operating subway or traveling public. Extensive instrumentation was required to monitor deflection in structure during the entire excavation and installation process. This underpinning supported 4 subway tracks, a mezzanine level, 10 feet of earth and utilities, and 6 lanes of traffic. Rock was excavated using drill and split methods working alongside existing footings and girders supporting the subway above. Specialized equipment to work in the tight confines were brought in using work trains to help with the excavation. Removal of the existing rivets and installation of new steel and bolts had to be done while existing steel was under load. A detailed erection sequence was required for the installation. The load transfers sequence was planned in detail and required chaining a series of hydraulic jacks to reach the high loads that were unique to each column.
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