Abstract:
Potomac Clay, named for its exposures along the slopes of the Potomac River around Washington, DC, consist of massive beds of heavily overconsolidated, highly plastic clay with zones of fractures and slickensides. While the Potomac Clay typically has high undrained strengths but exhibits very low residual shear strengths in the long-term drained condition. Potomac Clay has historically been the source of many slope failures in the region. Continuous flight auger (CFA) piles were used to stabilize Potomac Clay slopes supporting new MSE walls constructed for the DDOT I-295 Malcolm X Ave Interchange project. The constructed CFA piles varied in depth between 55 and 95 ft, and were 24 inch in diameter with full-length reinforcing steel cages. The use of CFA piles saved over $1 million in construction costs compared when with the alternative plan to stabilize the slopes with drilled shafts. The authors summarize the project and subsurface conditions to show why slope stabilization was required. They review the engineering methods and approach used to design the CFA pile slope stabilization system, including an evaluation of the strength and service limit states. They discuss why CFA piles were selected over alternatives stabilization methods, and describe the QA/QC program. The authors discuss the construction approach and methodology to install the CFA piles, and describe the unexpected challenges as well as lessons learned during construction.