Performance of Rock Anchors for Underground Construction in Manhattan Schist

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The paper evaluates rock-anchor performance in Manhattan Schist using proof-test results from excavation support at New York City’s 63rd Street and 86th Street subway stations. Tests covered 23 tied-back anchors, four rock dowels, and two prestressed toe anchors.<br /><br />Measured grout–rock bond resistance varied with location and anchor type. At 86th Street, maximum mobilized resistance reached 98.4 psi for tied-back anchors, 56 psi for dowels, and 90.7 psi for a replacement toe anchor. Anchor response also varied across excavation faces, reflecting differences in rock quality. One toe anchor performed poorly and failed to sustain its test load; inadequate drill-hole cleaning in weak rock was identified as the cause. Cleaning the replacement hole improved performance. The deficient installation reduced bond resistance by an estimated 42.5%.<br /><br />The study finds that intact rock strength and rock quality designation (RQD) alone do not adequately characterize the complex geological conditions of Manhattan Schist. Estimating rock-mass strength using Rock Mass Rating (RMR)—which incorporates joint conditions and groundwater as well as rock quality—produced more consistent results than using RQD alone.<br /><br />For design, the paper recommends applying the Horvath and Kenny grout–rock bond relationship with rock-mass strength derived from RMR, using a bond-resistance coefficient of 4. For temporary anchors installed with good workmanship, a minimum factor of safety of 1.3 is suggested; permanent anchors may require a higher factor. The findings emphasize that geological conditions and installation quality must both be considered in anchor design.

Keywords

rock anchors

Manhattan Schist

proof-test results

grout-rock bond resistance

tied-back anchors

rock dowels

prestressed toe anchors

Rock Mass Rating

anchor design

New York City subway stations