Investigations of Stability Improvements for Submerged Berm on Soft Clay in

Rotational deformation of the submerged rock berm of the hurricane protection Floodwall over the Mississippi River Gulf Outlet in South Louisiana was observed during its construction. The deep seated slip surface profile of soft soil beneath the rock berm was investigated based on stability back analyses and bathymetric surveys. Remolded strengths of soft clays under the deformed rock dike were determined by field CPTs and vane shear tests, and they were verified with the back analysis as mentioned above. In order to restore the stability requirement of the submerged rock berm for the Floodwall, several remediation approaches were studied using finite element and limit equilibrium stability analyses. These stability analyses indicated that both rock surcharge and soil cut-off structure should be used to stabilize submerged berm on the soft soil during its further construction. Such soil cut-off structures can be constructed by means of stone trench fill or sheet-pile. The later option was finally adopted based on the constructability and cost estimate.