High Capacity Hollow Core Bar Piles-Paper

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A 17-story, 95,000-square-foot office building at the Brooklyn Navy Yard was founded on hollow-core bar piles in place of conventional caissons. The heavily developed site, located between two dry docks, contained obstructions and highly variable soils: fill and debris, weak organic soils, liquefiable sand, varved clay, glacial till, and gneiss bedrock at about 100 feet.<br /><br />The hollow-core piles were drilled through the soil and socketed into bedrock. Most included a 30-foot-long, 16-inch steel casing; the bar was grouted during installation. The pile design followed New York City Building Code micropile provisions. The value-engineering solution replaced Type A–C caissons at a ratio of five piles for every three caissons, while Type D caissons were replaced one-for-one.<br /><br />An extensive testing program informed both design and construction. Initial compression tests did not meet code criteria; investigation indicated that a test pile may have encountered a boulder and that the installed pile’s stiffness was lower than assumed. The contractor lengthened rock sockets—generally to at least six feet—and slowed drilling to improve grout placement. A minimum grout factor of 140% of theoretical volume was adopted for quality control.<br /><br />Compression tests reached 1,200 kips, described by the authors as the largest successful load test then conducted on a hollow-core bar pile. Although the design service compression load was initially 600 kips, it was reduced to 520 kips because of settlement concerns. Two lateral load tests met the project’s deflection criteria and helped validate the soil parameters used in analysis. Strain-gage installation below the casing proved difficult, limiting assessment of residual stresses.<br /><br />The project demonstrated that hollow-core bar piles could provide a high-capacity foundation solution in complex urban ground conditions, while emphasizing the importance of load testing and adapting installation procedures to observed performance.

Keywords

Brooklyn Navy Yard

hollow-core bar piles

micropile foundation

load testing

rock socket

grouted steel casing

urban foundation engineering

soil variability

settlement concerns

building code compliance