Challenges of Designing and Constructing CFA Piles in Highly Variable Bedrock in Manhattan, New York-PPT Presentation

PPT Presentation

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The 19-story mixed-use building at 4650 Broadway in northern Manhattan required deep foundations because of highly variable subsurface conditions and nearby sensitive buildings. Investigations included geotechnical borings, seismic cone penetration tests, geophysical surveys, and laboratory testing. The site contained overburden soils, decomposed bedrock, and competent rock; geophysical methods were more effective at locating the soil–decomposed-rock interface than the boundary with competent rock.<br /><br />Continuous flight auger (CFA) piles were selected as a flexible alternative to drilled or driven piles and minicaissons. An index-pile program used 15 piles and 12 load tests—four each for compression, tension, and lateral loading—to refine the design and verify performance. Testing showed that pile behavior varied with length and bearing conditions, underscoring the need to assess each pile type and rock condition. CFA piles also replaced minicaissons, improving construction efficiency.<br /><br />Quality assurance included automated monitoring of grout volume, pile profile, drilling rate, and crowd pressure, as well as thermal integrity profiling to evaluate pile integrity, concrete quality, and shape. Drilling acceptance criteria specified minimum pile lengths or rock embedment, together with thresholds for rig crowd pressure and drilling rate, to confirm that piles reached the required bearing conditions.<br /><br />The project demonstrated that CFA piles can accommodate complex and variable bedrock conditions when supported by careful investigation, full-scale load testing, and clear construction acceptance criteria. Its success depended on an experienced foundation contractor, extensive quality control, and continuous communication among project participants.

Keywords

mixed-use building

deep foundations

geotechnical investigation

subsurface conditions

seismic cone penetration tests

continuous flight auger piles

index-pile program

load testing

thermal integrity profiling

bedrock embedment