Sidescan sonar examination of deteriorated revetments and bulkheads along Chicago's lake front

Lakefill for parks along Chicago's lake front is primarily defended with aged, deteriorated revetments. Survey by 100- and 500-kHz sidescan sonar documents that structural failure of the revetments is not related to lake-bottom undermining. The size and weight of the rock fill in the cribs is identified as a critical factor. Long-term effects of wave surge combined with ice action and gravity can remove rock fill either through breaks in the timber crib wall, or where revetment design offers lakeward exposure of the rock fill. Capstones are displaced when they lose underlying support. Revetments with a lakeward face of steel sheetpile, or toe protection mounded high against the structure, have no significant capstone displacement and are models for improved design. This study supports a rebuilding project that will bury the old revetments with new structures having a lakeward face of either steel sheetpile or a rubble mound.