Abstract:
The Potomac Formation is a Cretaceous aged deposit of soil that generally overlies basement rock east of the Fall Line throughout the Mid-Atlantic Coastal Plains that extend from New Jersey to Virginia. This over 66-million-year-old deposit includes very dense sands and highly plastic clays susceptible to significant weakening to very low residual strengths along planes of movement such as relict slip surfaces.
Much of the metropolitan area surrounding Washington. D.C. consists of natural slopes underlain by Potomac formation soils. Ambitious development plans throughout the area have introduced new loads on many of these slopes. These new loads have either resulted in slope instabilities (particularly where pre-existing failure surfaces are present) or required design of supplemental stabilization measures to maintain an adequate factor of safety.  Supplemental stabilization can include the use of drilled shafts or piles to provide additional capacity to reduce or prevent slope movement.  Analysis and design of these deep foundations often includes the use of limit-equilibrium slope stability analysis to define loads and a soil-structure interaction analysis of the on the stabilizing elements.
This presentation will provide an overview of the challenging Potomac soils including their shear strength properties (peak, fully softened, and residual) and an overview of the authors' experience with the use of deep foundations to improve stability of these slopes.