The Throgs Neck Bridge is an important regional connector for New York City and Long Island, traveled by over 100,000 vehicles per day. Carrying I-295 over the East River between the Bronx and Queens, the suspension bridge is one of only three direct roadway connections between Long Island and the mainland. To enhance the safety of this crucial crossing and extend its service life, MTA C&D is undertaking a design-build project to replace the existing steel truss and timber fenders protecting the bridge’s two towers with a new concrete fender system that meets current standards.

EnTech provided geotechnical instrumentation and monitoring as a member of the design-build team. The new fender system will be supported on both existing caissons and new H-piles, and EnTech’s geotechnical team developed an instrumentation and monitoring plan to measure vibrations and settlement of the tower pedestals and steel structures during pile driving and other vibration-causing construction activities. The plan also encompassed monitoring of temporary support structures and ancillary structures within the project’s zone of influence. EnTech installed vibration monitoring units near each tower, despite challenges posed by the location of the structures in the middle of the river, allowing instruments to be installed and the monitoring program to proceed throughout construction.