Analyses of Dam Failures in 1985 Chilean Earthquake

During the Central Chile earthquake of 1985, two small dams suffered substantial embankment deformations indicating liquefaction and major strength loss in the embankment or foundation soils. However, the configuration of the embankments after failure suggested that the soil had retained a small but significant residual strength after initial liquefaction $(r_u\approx 100\%)\text{.}$ Because of the scarcity of field values of residual strength after initial liquefaction, a study of these dams was undertaken. Standard penetration test (SPT) values were determined and normalized for effective confining stress and hammer energy. Silty sand layers of low density were identified near the base of both embankments. Subsequent analysis of the results of the field study gave upper and lower bounds of residual strength in these loose layers. Calculated values of residual strength were found to vary with the inferred pre‐earthquake densities of the materials. These values were compared with those determined from previous case studies and shown to be in reasonable agreement.