A HARD LOOK AT SOFT ZONES: GEOTECHNICAL SIGNIFICANCE OF EOCENE STRATA BENEATH THE UPPER SOUTHEAST ATLANTIC COASTAL PLAIN

At the Department of Energy's Savannah River Site near Aiken, SC, underconsolidated soft zones occur in mixed carbonate/clastic strata of middle Eocene age. The existence of soft zones was identified in the early 1950s, prior to construction of critical nuclear facilities; subsequently, various approaches have been employed to resolve or accommodate the potential for undesirable deep foundation conditions. Investigation into their distribution, occurrence, origin, and behavior suggests that:

• Soft zones result from early diagenetic carbonate dissolution.
• Soft zones are not cavernous voids, but are isolated, poorly connected three-dimensional features filled with loose, fine-grained sediment.
• In spite of their underconsolidated nature, soft zones have survived through geologic time (tens of thousands of years), withstanding numerous seismic events and maintaining structural competence under significant overburden stresses.

Elsewhere in the region, solution tunnels and small caves have been discovered in equivalent or similar middle Eocene strata. The existence of soft zones and karst features has important land use, construction, and groundwater hydraulic implications. The evidence collected during more than 60 years of soft zone research suggests that, in most scenarios, soft zones and even some karst conditions can be accommodated with adequate geotechnical characterization, conservative engineering calculations, and appropriate design elements.