Drilled and Driven Foundation Behavior in a Calcareous Clay

Most major structures in the Charleston, SC area of the U.S. are supported on deep foundations bearing in the Cooper Marl. The fine grained Cooper Marl, which typically classifies as highly plastic clay, has a calcium carbonate content of 60 to 80 percent. Unlike some other calcareous soils, the Cooper Marl is generally an effective foundation material for both driven and drilled foundations. In general, the performance of driven and drilled foundations is comparable but the marl's cementation does cause some differences. The relatively open structure collapses upon shearing which causes large excess pore pressures. As a result, the capacity of a driven pile is highly time dependent and increases as the excess pore pressures dissipate. Vibratory installation, which generates a large number of shear reversals, is particularly damaging to the soil structure and negatively impacts the performance of foundations installed in this manner. Drilled foundations often use a vibratory installed steel casing as a construction aid and performance data consistently indicate that the cased zone of the shaft within marl, even when the casing was extracted during concrete placement, experiences a reduction in side resistance as compared to an uncased excavation.