Underwater Load Transfer Platform in Compressible Marine Clay-Paper

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The Long Slip Canal Rail Enhancement Project in Hoboken, New Jersey, aims to reduce flood risk and prepare the site for future rail facilities. Filling the canal posed a geotechnical challenge: highly compressible marine clay could settle and spread laterally, potentially damaging nearby king-pile and sheet-pile structures.<br /><br />The original design called for deep soil mixing to strengthen the clay. However, a Phase 1 cofferdam restricted barge access and made it impractical to bring the required soil-mixing equipment into the work area. The contractor proposed a value-engineered alternative: a 9.5 m-wide, 1.2 m-thick underwater concrete load transfer platform (LTP), supported by driven steel piles bearing in glacial till. The platform would transfer backfill loads to the piles and limit settlement and lateral movement. This approach also used equipment and materials already available on site.<br /><br />Design analyses considered construction stages, consolidation, future rail loading, and effects on adjacent structures. A conservative 2D finite-element analysis predicted 19.8 cm of king-pile wall displacement; a 3D analysis, which captured support from bordering cofferdams, predicted 2.1 cm. The 3D model also assessed sheet-pile interlock rotation, estimated at 2 degrees—within the project’s 5-degree limit. Although initially conceived as a compression-only platform, the LTP was ultimately reinforced to meet the owner’s structural design requirements.<br /><br />Construction took place entirely underwater. Crews drove 96 piles through the soft clay into glacial till, installed a mud mat to separate the concrete from the clay, and placed the LTP using tremie concrete and divers. Soft clay movement, excavation infill, and displaced mud complicated the work. The completed platform enabled backfilling and surcharging to proceed, providing the intended ground-support function with improved constructability.

Keywords

Long Slip Canal

Hoboken New Jersey

flood risk reduction

geotechnical engineering

marine clay settlement

load transfer platform

driven steel piles

glacial till foundation

finite element analysis

underwater concrete construction