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