One of the most challenging locations for any construction project is New York City. The densely populated area of Manhattan presents many obstacles for any project. Critical to success is ensuring a strong foundation for structures. When a new construction project began a few blocks away from the Empire State Building and the Midtown Tunnel at an extremely visible location, the design and construction team wanted to ensure that they had a competent foundation so that the building would not settle. The construction manager for the project contacted Structural Preservation Systems (SPS) because of its previous experience with the firm and reputation for tackling challenging geotechnical projects.

The site is surrounded by other occupied buildings, and the construction team wanted to minimize any disruption to the surrounding community. To ensure the integrity of the structure, the design team opted to install mini-caissons. The owner and construction manager originally considered driving piles, but that was not a viable option for several reasons. First, an Amtrak rail line is located underneath the facility. Second, driving the piles could cause settling in the surrounding buildings. Therefore, the team opted to drill mini-caissons, which was a safer option for this project. There were three different compression loads (150, 200 and 250 tons) for the 175 mini-caissons that were socketed into the bedrock. The use of mini-caissons is a relatively new process and was recently written into the building code of NYC. Advantages of mini-caissons over caissons include the ability to install them quicker, drill through obstructions and achieve high capacities through the use of reinforcing steel.

Several challenges occurred on this project. The most critical challenge was the fact that an Amtrak tunnel runs directly underneath the site, and precautionary measures were taken to ensure that the drill rigs would not drill through the top of the tunnel. The site also contained a large amount of clay and silt with a high water table, which made it hard to drive equipment in certain areas and presented a safety hazard. To combat this obstacle, SPS had a meeting with the client to improve the dewatering system and then added a gravel and stone sub-base to the site. These efforts made the site much more manageable for the heavy equipment. The schedule was critical on this project, and the construction manager needed SPS to complete its portion of the work as quickly as possible. As such, SPS had three crews on the site at the same time, which allowed the project to move quickly. SPS drilled simultaneously with two rigs and installed reinforcing steel with the third crew using an all-terrain variable reach forklift.

Completed in two months, the owner was very pleased with the outcome.