Failure of a Dredged Slope in a Sensitive Clay

During construction of a new wharf facility in Portland, Maine, an underwater slope failed during dredging and subsequent driving of piles through the slope. The construction and failure of the slope are described. The major factors which contributed to the failure were: 1) high sensitivity of the silty clay, 2) placement of riprap on the crest of the slope to 4 to 6 ft above the design elevation, 3) method of dredging which caused high shear stresses and probable disturbed zones near the toe of the slope, 4) dredging slope steeper than design slope, 5) pile driving causing localized disturbed zones with low strength around the piles, and 6) sequence of dredging and pile driving