Rehabilitation of Drydock No. 2, Portsmouth Navy Shipyard: Innovative Solutions for a 108 Year Old Drydock

This paper discusses the design of a $15,000,000 to $30,000,000 rehabilitation to Drydock No. 2 at the Portsmouth Naval Shipyard in Kittery Maine. Drydock No. 2 was constructed following the Spanish American War under President Theodore Roosevelt. The current project addresses the replacement of the entrance structures and caisson gate. The project has a critical delivery timeline with design completion in June 2009 and construction completed within a one-year duration. The drydock structure is founded on bedrock and generally consists of granite blocks and concrete. Past studies determined that the entrance structures were deteriorating and temporary bracing was installed to mitigate movement of the walls during dewatered conditions. The replacement of the entrance structures requires the construction of a cofferdam outboard of the drydock to facilitate dewatering and construction. The entrance structures were designed to be stabilized by the balancing pressure of the sea and therefore require specialized tie-down and tie-back anchoring to stabilize the 35-foot walls in a dewatered condition. The caisson gate needs to resist a 35-foot hydrostatic head, contain the systems required for ballasting during seating operations and have through gate flooding tubes capable of super flooding the dock. Super flooding operations increase the water elevation above the tidal height to allow docking of deeper draft vessels. This project schedule requires the identification of critical path tasks and development of a comprehensive overall project schedule. Measured progress and close communication between the Government and design team is essential in realizing success of this challenging engineering project.