Installation of Secant Pile Wall in Rock Adjacent to Fragile Building-Paper

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The paper describes construction of a secant pile wall for a 65-foot-deep excavation at Ancillary Building 1 of Manhattan’s Second Avenue Subway, only two feet from a fragile masonry building. Highly variable Manhattan Schist elevations—spanning about 30 feet—complicated pile installation, with rock locally rising above the excavation’s invert slab.<br /><br />The wall replaced an initially proposed slurry wall because chiseling or milling through shallow rock could have disturbed the neighboring building. Piles were fully cased through overburden, and rock sockets were drilled with a core barrel using non-vibratory methods; down-the-hole hammering was prohibited. The adjacent building had been underpinned with micropiles before excavation.<br /><br />Twelve supplementary probe holes, combined with existing borings and observations from micropile installation, helped map rock levels and guide construction. Contingency toe anchors were designed for piles that might not reach their intended depth, and potential gaps between piles at deep rock locations were assessed. The anchors were ultimately unnecessary. Primary piles in areas of shallow rock were permitted to terminate one foot below the invert slab, provided they achieved at least two feet of rock embedment; secondary piles retained the original depth requirements.<br /><br />Installation took seven months. Overburden drilling lengths ranged from about 51 to 83 feet, while rock drilling varied substantially, reaching 13.6 feet for some secondary piles. Monitoring showed vibrations remained below the specified limit, and building settlements were small, with no significant horizontal movement. The authors conclude that probing, fully cased drilling, careful contingency planning, and continuous monitoring enabled safe installation without compromising the fragile building.

Keywords

secant pile wall

Manhattan Second Avenue Subway

deep excavation

Manhattan Schist

micropiles

fully cased drilling

probe holes

toe anchors

vibration monitoring

building settlement