Seismic stability analysis of a large slurry impounding embankment dam

A large slurry impounding embankment dam in the eastern USA was built in the 1970s. In 1992, regulation changes led to 1993 seismic re-analysis of the upstream slope. Stability analysis programs from the 1970 to 1993 period (and most today) do not properly handle seismic analysis of a multi-fluid reservoir involving water over sludge or just water (if the water is treated as more than one layer). To avoid unneeded and expensive remedial repairs, better modeling of the 2-fluid reservoir problem was required. Only one of the 3 programs used through 1993 could do this and that one only approximately; but, it yielded an acceptable factor of safety for significant deep circles of about 1.2. Since 1993, a program correctly treating a multi-fluid reservoir has been developed. If it was available in 1993, re-analysis would have cost less than a third of the roughly $75,000 required. This paper discusses the seismic analyses, including comparisons of results using the 4 programs.