A Case Study in Cellular Cofferdam Life Extension Design

A large portion of the nation's dry docks were constructed using cellular cofferdam walls between 1960 and 1980, and their design service lives have been lengthened to meet project schedules and budgets. As a result, customized repair programs that provide for cellular cofferdam dry dock life extension have been developed and implemented at Electric Boat in Groton, CT for Graving Docks 1 and 2. The design strategies for this life extension program are complicated by the inherent interdependence of the cofferdams cells upon one another for stability. The resulting consequence being the partial or complete demolition of one cofferdam cell may require that the adjacent cells be stabilized prior to demolition. Strengthening the cofferdam cells often involves partial demolition, especially where there is no space for additional structure outside of the existing cofferdam arrangement; thus, requiring that all strengthening be performed inside the cell. In 2010 a long-term life extension study and design for strengthening the existing outboard sheet pile cofferdam cells was conducted for Electric Boat Graving Docks 1 & 2. Due to the close proximity of the outboard cells to the graving dock gate, all strengthening was required to be performed within the existing footprint of the cells. During two design phases, the life extension design concept progressed from a repair, to a complete removal of the outboard cells with deep soil mixing of the final inboard cell, to constructing partial height reinforced concrete walls within the sheet pile cell. In this paper, a case study will be presented which will address the progression of the Outboard Cell Life Extension design, including the impact of ship building activities and schedule, cell stabilization techniques, and benefits of early contractor involvement.