The NYU College of Dentistry was a perfect example of a difficult foundation construction in a tight urban environment. The tight site conditions, soil conditions, and the depth of the excavation made this project a particular challenge. The project started off with the construction of a full secant wall socketed into bedrock, along with the installation of caissons to support building loads. The secant wall and caisson work was installed by specialty subcontractor Underpinning and Foundation Skanska. After completion of the specialty work, the excavation could begin. The secant wall was braced internally via a system of steel walers and corner braces, three levels in all, designed by The Laquila Group’s in-house engineering staff. Careful sequencing of the bracing installation allowed the use of larger machinery to reach the subgrade depth and to remove the large quantity of hard rock below the surface. With most of the excavation out of the way, the larger equipment could be removed from the excavation to make way for smaller equipment, which completed the excavation work and was then removed from the excavation via a large crane. A thick concrete foundation mat was poured to seal the bottom of the excavation and provide the necessary building support.
The secant wall was faced with shotcrete to allow for the application of waterproofing membrane and later a concrete liner wall. After the intermediate cellar slab and first floor slabs were poured, the difficult operation of removing the interior bracing and pouring the foundation liner walls could commence.This required the use of specialized equipment, chain falls, a concrete pump, and a boom truck in order to successfully clear and remove the steel bracing from the completed cellar of the building, pour the liner wall, and complete the project.