Effect of Ruptured Sheet Pile Interlocks on Resistance against Piping Failure

The described project involved replacement of an existing railroad bridge over a river estuary. Two of the three piers were located in open water, requiring the use of temporary internally braced sheet pile cofferdams to allow installation of open-ended pipe piles, pile cap, and pier substructure in the dry. During excavation of one of the cofferdams, water and soil were observed entering the cofferdam through ruptured sheet pile interlocks. This inflow increased markedly as excavation within the cofferdam progressed, resulting in the development of a number of large sinkholes just outside the cofferdam and close to the river bank. The cofferdam was immediately flooded to avoid failure. Following analysis, it was decided to drive the pier support piles from a higher elevation within the cofferdam to reduce the risk of piping through ruptured interlocks. Damaged interlocks above the higher working grade in the cofferdam were repaired by welding steel plates over the ruptured portions to exclude water and soil. Upon completion of pile installation, the cofferdam was flooded, and excavation was completed under water to the design elevation. A 4-ft thick tremie concrete seal was placed and allowed to gain sufficient strength before dewatering the cofferdam and completing the pile cap and a portion of the pier structure in the dry. Upon completion of the work below river level, the sheet piles were extracted. It was found that many interlocks had been unzipped above toe level. Calculations were made to demonstrate the relationship between average hydraulic gradient and factor of safety against piping. It was shown that if the sheet pile interlocks are ruptured or unzipped, the effective length of the shortest seepage path around the sheet piles is reduced by twice the length of interlock rupture. The length of sheet piles used in the cofferdams was 61.5 ft, corresponding to a factor of safety in excess of 1.5, which satisfied the contract requirement. However, piping failure would be initiated for rupture lengths of about 12 ft. Actual rupture lengths measured upon sheet pile extraction typically exceeded 20 ft. Subsequent investigations, along with pieces of structural steel recovered during partial drilling out of the open-ended piles, strongly indicate that the sheet pile damage occurred during installation upon encountering buried steel from an earlier railroad bridge, which collapsed in a major storm and flood in 1927.