520 Park Avenue was a particularly challenging project due to many factors. The site, a mid-block site of only 6,000 SF in a difficult to access part of Manhattan, close proximity to existing high rise buildings and a NYCT Subway tunnel, a very deep excavation which involved the installation of a secant pile wall, extensive bracing, staged excavation utilizing a crane for soil removal, and finally excavation of hard rock in confined space. Only then could the foundation concrete work begin.
The project started off with a general site clearing and the installation of a guidewall for the secant pile wall. Subcontractor Skanska installed a full secant pile wall through the overburden and deep into solid bedrock. At the same time, Skanska installed the 39” diameter caissons. Upon completion of Skanska’s work, Laquila installed a massive concrete cap beam atop the Secant wall and then commenced the excavation process. Excavation involved installing four layers of bracing to support the secant wall while excavating underneath the bracing with smaller equipment. Laquila worked closely with the owner’s geotechnical consultant to ensure safe and efficient prosecution of the work. A large, long reach excavator sat at street level to bring the excavated soil up to the surface, and then one of Laquila’s 100 ton crawler crane was used with a debris box once the excavator could no longer reach. At the bottom of the nearly 50’ cut was hard rock, which was removed by a combination of small excavators with hydraulic hammers, and hydraulic rock splitters. Once subgrade was reached, caisson caps were poured and a concrete mat slab was poured, allowing superstructure work to commence. Laquila worked hand in hand with the superstructure contractor to remove each level of bracing as each subbasement level slab was poured. Laquila came back and applied a waterproof shotcrete treatment to the secant pile wall to provide a smooth finish.