Liquefaction Potential Assessment of Gravelly Foundation Soils at Clearwater Dam

The potential for soil liquefaction in a gravelly alluvium beneath a large embankment dam was assessed in this case history. Clearwater Dam is located in southeastern Missouri, near the New Madrid seismic zone. The 154-ft high dam is built on a 30-ft thick deposit of saturated, gravelly, alluvial soils; previous studies indicated that this foundation soil was susceptible to liquefaction. However, gravelly soils are difficult to characterize using conventional field test methods. In 2005, the potential for liquefaction during an Operating Basis Earthquake was re-evaluated using short-interval SPTs and large diameter samples. The results showed that although gravel was present throughout, characterization of the deposit based on data from historic site borings was representative. Because the gravel-sized particles appear to be sufficiently dispersed in the finer matrix and small enough relative to the sampler diameter, properly interpreted SPT data can be used to assess liquefaction potential in this particular deposit. For the design earthquake used in the 2005 study, significant liquefaction of the foundation soils is judged to be unlikely.