Stemmers Run Steel Sheet Pile Cofferdam and Trestle across the Back River

Project documents and permits for construction of the Stemmers Run Relief Wastewater Force Main across the Back River in Baltimore County, Maryland specified a minimal width, sheet pile cofferdam and pile supported, work trestle approximately 518 m long. The contractor combined the impractically narrow cofferdam and trestle structures to minimize wetland disturbance and maximize access. The resulting structure was a braced, sheet pile cofferdam with timber deck mats as the work trestle for excavation, pipe installation, and backfill. Construction of this cofferdam/trestle progressed from the north shoreline, toward the south shoreline, to the two-thirds point of the structure. Then, working northward, the contractor excavated, installed the 1372 mm diameter pipe and support bents and backfilled the trench in increments of approximately 18.3 m. As the pipe installation progressed toward the north, the sheet piling and timber deck were removed. As sufficient materials were removed, the contractor began the process anew from the south shoreline using the reclaimed materials. Deep, very soft silts and organic clays and an unbalanced, hydrostatic head complicated the project. Concurrent pile driving, excavating, and backfilling in successive cofferdam sections required large backhoes and crawler cranes which controlled the design of the trestle and supporting sheet piles.