Case Study: Selection of an Appropriate Design Residual Strength for a Heavily Overconsolidated Clay

Potomac Clays are heavily overconsolidated cretaceous clays of the Potomac Formation within the Coastal Plain Physiographic Province. The Potomac Formation outcrops east of the Fall Line in a narrow region that stretches from Virginia to New Jersey. Due to their low residual strength, Potomac Clays presents problems for slope stability, particularly with regards to progressive failure, and such failure has been documented on several occasions leading some localities to specify a maximum friction angle of 10° for design within Potomac Clays. A major transportation improvement project is underway in southeastern Washington, DC. As Potomac Clays are present at this project site, selection of an appropriate residual friction angle for design was critical for both safety and cost. The correlation of residual strength to the Plasticity Index was examined and a statistical framework was developed for evaluating the residual friction angle across the project site. Additionally, the spatial variation the residual friction angle led to splitting the site into several areas, which decreased the variability in the data and allowed for the confident selection of appropriate design values within each area.