Site Characterization and Subsurface Conditions for the Cooper River Bridge

At the expected time of its completion (2005), the new Cooper River Bridge in Charleston, South Carolina will be the longest cable-stayed bridge in the US. The $600+ million structure is supported on drilled shafts, which bear within the soils of the Cooper Group (colloquially known as the Cooper Marl). Geotechnical data necessary for foundation design were primarily obtained during three major site characterization programs performed for the preliminary design phases. The programs included soil test borings, cone penetration testing, shear wave velocity measurements, laboratory index testing, consolidation testing, and triaxial shear testing. The extensive data from these exploration efforts are summarized and evaluated. The Cooper Marl typically classifies as an overconsolidated (OCR of 3 to 6), highly plastic clay or silt with liquid limits often in excess of 100 and plasticity indices of more than 50. The fines content is generally in the range of 75% to 90% but the clay mineral content is small (<10%) and the primary mineral constituent is calcium carbonate (60% to 80%). The calcium carbonate content is mainly in the form of the skeletal remains of microscopic marine organisms (e.g., foraminifera) and the fossiliferous nature of the particles is thought to explain the fact that the effective friction angle is typically in the range of 43 to 46 degrees. The void ratio is relatively high (1 to 2) but the undrained shear strength is also high (140 kPa to 280 kPa) and the shear wave velocity (generally in the range of 400 m/s to 600 m/s) is relatively constant with depth; all of which indicate that the deposit is cemented.