Aerial Photo of Coos Bay Wetlands
Libby Dike protects approximately 100 acres of residential and farm properties near Coos Bay, Oregon. The dike was originally constructed before 1920 from dredge spoils removed from the slough and placed on the banks.
Improvements were being considered to raise and widen the 5,000-foot long dike and replace three tidebox structures. Cornforth Consultants conducted a field exploration program consisting of eight 40-foot deep vertical borings and two 6-foot deep angle borings (into the embankment side). Three borrow sources were also evaluated. Laboratory testing consisted of Atterberg limits, unit weight tests, gradation analyses, unconfined compression tests, compaction tests, and consolidation tests.
The silts and clays in the upper 20 feet of the foundation were extremely soft, averaging 200 psf shear strength. Recommendations were made to raise the dike by staged construction to prevent slope instability and lateral spreading. Recommendations were also provided on estimated settlements and tidebox construction.