Failure of a column-supported embankment over soft ground

As part of improvements to a roadway in coastal South Carolina, a portion of the road crossing reclaimed marshland was widened. The new lanes were constructed on a column-supported embankment consisting of sand fill reinforced by geogrid and supported by vibro-concrete columns that penetrate through the underlying very soft clays and into the local basement stratum. Shortly after construction, the roadway on the column-supported embankment began to experience distress characterized by an irregular surface with humps at the column locations and depressions in the areas between column locations. The differential vertical deformation between the high and low points was sufficiently significant that the owner closed the roadway almost immediately after completion. Forensic study illustrated that the design applied state-of-practice design techniques; however, certain design assumptions were not consistent with the fundamentals controlling the column-supported embankment behavior. This paper describes the original design, construction, and the authors' forensic study for this failure.