DESIGN ISSUES FOR MINI-CAISSONS IN THE NYC AREA-Paper

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Mini-caissons are widely used in the New York City area because they can pass through difficult soils, reach bedrock at variable depths, and be installed with relatively little vibration or disruption to nearby structures. Typically, a permanent steel casing supports drilling through overburden, while an uncased rock socket provides most of the foundation’s axial capacity. Reinforcement and grout or concrete fill the completed element.<br /><br />Despite their widespread use, important design questions remain unresolved. The paper compares allowable structural stresses under the 1968, 2008, and 2014 New York City Building Codes. Although construction quality has improved, the 2014 code generally yields lower allowable capacities than earlier versions, including a pronounced reduction in allowable tensile capacity for larger-diameter mini-caissons. The authors note that the reasons for these changes are not always clear.<br /><br />Geotechnical design is also inconsistent. Designers generally rely on side resistance along the grout-rock interface and ignore end bearing, but disagree about whether—and how much—compressive load in the steel casing transfers directly to bedrock. Some assume the full casing load is transferred; others allow only a limited share or none, citing a lack of supporting research and uncertainty about the casing-rock interface.<br /><br />The casing’s structural reliability is similarly uncertain. Threaded joints between casing sections may not reliably transmit the assumed axial load; welding can improve continuity but slows construction. Some designers therefore reduce the effective casing area in calculations.<br /><br />The authors call for clarification of the 2014 code provisions and further research on load transfer and casing behavior. Instrumented field tests and load testing of rock-socketed mini-caissons could help establish more reliable design parameters.

Keywords

mini-caissons

New York City Building Code

rock socket

allowable structural stress

allowable tensile capacity

side resistance

grout-rock interface

steel casing load transfer

casing joints

load testing