Underwater Retrogressive Slope Failure: Observations and Analyses

This paper presents an analysis of an underwater retrogressive slope failure caused by the concurrent construction of a wharf access causeway and dredging near the end of the causeway for the wharf structure cofferdams. A cross section was developed along the causeway and analyzed to simulate the retrogressive slope failure using limit equilibrium analyses. The compound failure surface for the inverse stability analysis of each of the five retrogressive failure masses agrees with observations before, during, and after the failure. Soil stratigraphy is discussed and the mobilized undrained strength of the seabed clay underlying the causeway fill was estimated using an inverse analysis of each of the five failure masses. The inverse analysis shows that the concurrent dredging and causeway construction reduced the factor of safety (FoS) of the causeway underwater slope, which eventually initiated the retrogressive failure. The stability analyses show that the causeway construction contributed to the reduction of the FoS but dredging triggered the first slope failure, which started the retrogressive failure. Nevertheless, had dredging not occurred, a slope failure would still have occurred if the causeway construction had extended another 5 m from where the first slope failure occurred.

There are several practical lessons from the slope failure discussed in this paper. First, in any slope modification, the existing soils in the area of construction should be sampled and tested. Engineers and owners should be cautious of construction methods that make implicit assumptions about soil strength and/or other engineering characteristics. Even though gathering underwater geotechnical data is difficult, it is recommended to perform field tests to evaluate the shear strength of soil layers to perform slope stability analyses. Further, planned modifications to a slope that could destabilize it should be thoroughly analyzed, and such analyses should not be restricted to a circular failure surface. If possible, the construction sequence should be modified to reduce the extent to which the slope is destabilized. In this case, that would mean constructing the wharf before the causeway. Finally, construction should be monitored to verify that the proposed methods are working as intended. In this case, it was assumed that the rockfill would displace the marine clay when constructing the causeway. This should have been verified as filling progressed. Photographs of the dredged marine indicated a medium to medium stiff clay, which indicated that the clay was not as soft as anticipated.