MICROPILE DESIGN AND CONSTRUCTION IN LIMITED ACCESS WETLAND HABITAT-Paper

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The Susquehanna-to-Roseland 500 kV transmission project replaced seven 230 kV towers with double-circuit monopoles across Troy Meadows, a protected New Jersey wetland. Conventional foundations would have required extensive temporary access roads, so the project team selected helicopter-supported construction and micropile foundations to reduce environmental disturbance.<br /><br />A design-build approach enabled collaboration between the overhead-line designers and the foundation contractor. Value engineering replaced the original bolted steel pile caps with concrete caps, improving pile-to-cap fixity and allowing smaller casing and more efficient pile arrangements. Site-specific tower loading further reduced cap size and permanent disturbance. Because subsurface conditions were uncertain, foundation schedules provided pre-designed options that crews could adapt using observations and soil samples collected while drilling the first pile at each site.<br /><br />Lightweight, modular drilling equipment was flown to the work areas. Closed-cell steel cofferdams provided stable platforms in soft, waterlogged soils, contained drilling fluids and cuttings, and formed the concrete caps. Depending on field conditions, crews installed gravity-grouted Type A or pressure-grouted Type B micropiles, typically 90–150 feet deep. High-early-strength concrete allowed poles to be erected less than a week after cap placement.<br /><br />Construction was constrained by the electrical outage schedule and a nearby bald eagle nest, which limited wetland work to a 106-day window, including only 60 days for foundations. Despite these challenges, foundation work finished ahead of schedule, all seven monopoles were erected in three days, and the 500 kV segment entered service on April 1, 2014. The approach met schedule requirements while avoiding extensive matted access roads and minimizing impacts to the protected habitat.

Keywords

Susquehanna-to-Roseland

500 kV transmission

Troy Meadows

wetland construction

micropile foundations

helicopter-supported construction

double-circuit monopoles

design-build

environmental mitigation

value engineering