RESULTS AND IMPLICATIONS OF SEISMIC PERFORMANCE STUDIES FOR DUNCAN DAM

Screening-level studies indicated large zones of the foundation soil beneath Duncan Dam could be triggered to liquefy under the design earthquake, resulting in flow slides that could lead to breaching of the dam. In these studies, estimates of the key soil parameters were obtained from an indirect procedure using penetration tests and Seed's chart, which is based on field experience during past earthquakes together with empirical correction factors for high confining stress and static bias. This is state-of-the-practice procedure in North America and is used because of the difficulty in retrieving and testing undisturbed samples of sand. With the high confining stresses present at the site, the indirect approach gave very low values of the key liquefaction properties of the soil, and the use of such low values in analyses indicated that major and costly remedial measures were required. Thus a more direct approach was undertaken. This involved both direct and semidirect determination of the key soil parameters through an extensive laboratory study based on testing of undisturbed samples obtained by freezing in situ. The results showed that both the triggering resistance and the residual strength values of the soil were very much higher than those obtained from the indirect approach. Analyses carried out using direct measurement of the key soil parameters indicated that the dam, in fact, would be stable.