Failure Investigation and Restoration of Two Cellular Sheetpile Structures

Chicago, Illinois

The Trainer - Delco Tap - Mickleton 220-38 kilo-volt transmission lines are carried across the Delaware River by two 332-foot high steel latticed towers each founded on a man-made foundation island structure. Each island structure is comprised of four interconnected cellular sheetpile structures. One island, suffered a severe partial failure due to long-term scour in the Delaware River, near Chester, Pennsylvania. The other island exhibited early symptoms of potential failure, also due to scour. The client was the Philadelphia Electric Company, now known as PECO, acting on behalf of the owner, the Atlantic Electric Company (AECO). The author served as project manager and principal investigator for AECOM (formerly Earth Tech, formerly TAMS). The paper describes the failure investigation, including the structures before and after failure, the original installation (1959-1960), the condition survey of each island, condition of the failed sheetpiles, divers’ findings of an underwater survey, hydrographic studies, scour and loss of sheetpile embedment. Also described are the subsurface investigation, soils laboratory testing, the soil/rock profile, the probable cause(s) of failure, the sequence comprising the failure mechanism, metallurgical findings, circumferential stress analysis and brittle failure of the sheetpile panels outside the interlocks. Remedial measures are described and the design and construction of the selected restoration/stabilization solution via a crushed stone buttress is presented. The author established the construction sequence and provided technical liaison to PECO during the underwater staged construction, which included geo-instrumentation and hydrographic monitoring of an 80,000 cubic yard crushed stone and riprap protected circumferential stabilizing buttress, over 50 ft high, placed around the failed island in the Delaware River. The failure investigation, the design and the restorative construction occurred during 1991-1994, yet the lessons learned from this case history are as aptly important today.