420 Carroll Street Abstract: Soil Mix Excavation Adjacent to the Gowanus Canal-PPT Presentation

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The 420 Carroll Street project in Brooklyn involved constructing two residential towers connected by a below-grade tunnel, with deep elevator pits and excavations extending below the groundwater table beside the contaminated Gowanus Canal. Highly permeable soils and nearby contaminated groundwater created risks of water inflow and contaminant migration.<br /><br />The design team compared several excavation-support options and selected a soil-mix cutoff system with a grouted bottom. During design-build value engineering, the contractor and engineer replaced soil-mix secant piles with a reinforced soil-mix mass, incorporating core beams for bracing and foundation piles to help control hydrostatic uplift. This reduced the cutoff thickness and optimized material use.<br /><br />Construction proceeded on an accelerated schedule, with soil mixing coordinated alongside displacement-pile installation. Mixing adjacent to previously installed piles proved more difficult because pile installation compacted the soil. Quality-control results were uncertain: wet-grab samples were below the specified unconfined compressive strength, while core tests varied widely and had poor recovery. Despite these results, excavation proceeded with precautions for unmixed areas and potential leakage.<br /><br />The soil mix generally stood up well during excavation, though some areas required mechanical chipping and small leaks needed patching. The excavation was ultimately reported as completely dry, demonstrating that the system effectively limited groundwater infiltration.<br /><br />Key lessons included the value of soil mixing to reduce groundwater inflow in permeable soils; the need for reliable wet-sampling equipment; the variability of coring results in low-strength intermediate geomaterials; and the increased difficulty of mixing near displacement piles. Omitting a test section led to conservative cement content and a harder-to-excavate mass. Extending core beams above the soil mix also provided a practical way to support kickboards and safety fencing.

Keywords

Brooklyn construction

soil-mix cutoff system

groundwater infiltration

Gowanus Canal

deep excavation

elevator pits

contaminated groundwater

hydrostatic uplift

excavation support

quality control