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PERFORMANCE OF RETROFITTED DETERIORATED ARCH DAMBy Avinash C. Singhal, Fellow, ASCE, and Larry K. Nuss ABSTRACT: The Stewart Mountain Dam is a 212 ft (64.6 m) high multi-curvature thin arch dam. The structure was completed in 1930. It has experienced alkali-silica reactions within the concrete and exhibited no bond across horizontal construction lift surfaces. The dam could be subjected to upgraded maximum credible earthquake (MCE) or probable maximum flood (PMF) loadings. Alkali-silica reactions and expansions have caused visible surface cracking. Several measures for prevention of further deterioration and strengthening were considered. Post-tensioned cables were selected to provide seismic strengthening. Cable design uses the stiffness, and acceleration response spectra methods. Vertical post-tensioned cables were installed during 1990-92 construction phase. This paper summarizes various field and laboratory investigations, structural analyses, and design parameters needed for post-tensioning a deteriorated arch dam. Post-tensioned cables are found to be a viable solution for the dynamic stability of a thin-arch dam. Methodology presented in this paper is applicable to other deteriorated dams.