Vibroreplacement of Sand within a Closed-Cell Cofferdam Wall

A large closed-cell cofferdam breakwater was constructed offshore to protect docks and boats moored at the new Marina Pez Vela from storm surges and wave loading. The marina is located in earthquake-prone Quepos, Costa Rica and is situated in an area underlain by loose sand over bedrock that dips downward from the shoreline. The cells were constructed with interlocking flat-plate steel sheets and filled with un-compacted hydraulic sand. The cells settled about one-half meter during construction, creating concern for additional settlement during a design level earthquake. Liquefaction analyses indicated that 1 m of settlement could occur with no ground improvement. Consequently, vibroreplacement was used to densify the sand inside and below the cells. Post-densification explorations indicated that densification was limited to a distance of about 1.5 m from the center of the probe with little densification outside this zone, primarily because of higher silt contents than originally thought. Finite element analyses using PLAXIS 3D indicate that gravel replacement columns and the surrounding densified soil columns are sufficient to limit the cofferdam settlement to about 0.3 m, even if the soil in between the columns liquefies. Differential settlement along the cofferdam surface was estimated to be about 0.1 m, which was acceptable to the owner.