Replacing Subaqueous Water Mains to City Island Using Secant Shaft Cofferdams: Fid-Bau Portal

Replacing Subaqueous Water Mains to City Island Using Secant Shaft Cofferdams

Beer, Ira A. / McDermott, Martin / Palmer, Richard

As part of a $26 million infrastructure project, twin 508 mm (20-in.) water mains were installed under Eastchester Bay, Bronx, NY. These mains provide increased capacity and redundancy for water supply to the residents and businesses on City Island. A traditional surface-to-surface HDD operation was planned; however, there was insufficient space available at the exit side of the drill path. In response, the vertical alignment was lowered, a cofferdam was constructed, and all drilling operations were conducted from the entry side. The cofferdam was sized to accommodate the installation and removal of drill steel segments and tooling more than 12.2 m (40-ft) below mean sea level. The project team required a cofferdam that would minimize water inflow to reduce around-the-clock manning of pumps as well as to mitigate the negative impact of salt water on the drilling operation. With early collaboration of all parties, the contractor was able to install a successful cofferdam with the best technology for the challenging site conditions.