Bulkhead Failure Investigation and Redesign

Abstract

A 300‐ft‐long anchored bulkhead at Kake, Alaska, failed following placement of fill behind the bulkhead wall. The entire wall rotated outward at the toe and moved downslope 40 to 60 ft. No on‐site over‐water borings had been made, but local geologic investigations and pile driving resistances noted during construction suggested that a 5‐ to 20‐ft‐thick zone of soft marine clay overlying relatively dense offshore sloping soils and shallow bedrock lay beneath the project site. The failure is believed to have resulted in the displacement of the soft clay down to the surface of the competent soils. After the failure, the granular fill originally placed behind the bulkhead lay immediately over competent soils along the bulkhead alignment. Observations from onshore borings, dredging, and offshore test pile driving after the failure support the conclusion that the clay was removed from the bulkhead and till area by the failure. The bulkhead was redesigned with its alignment about 24 ft inland, and with a higher dredgeline elevation. It was successfully completed using materials salvaged from the original construction. Postconstruction alignment surveys have not disclosed any movement of the reconstructed wall.