Seismic Deformation Analyses of the New Coquitlam Dam

Coquitlam Dam is a 30 m high embankment dam constructed in the early 1910s. The dam site is situated in a region of high seismic hazard in British Columbia, Canada. The existing dam's core and shells, and part of the dam foundation are deemed to be liquefiable under the design earthquake loading. Seismic upgrade to the Coquitlam Dam is currently being implemented by constructing a new compacted earth core rockfill dam at the downstream toe of the existing embankment. The new dam will be mostly founded on dense glacial deposits. Due to site and constructability constraints, a portion of the upstream slope of the new dam will sit on part of the downstream rockfill toe of the existing dam that is underlain by liquefiable sand and gravel alluvium along the original river channel. This paper describes the analyses carried out to conservatively assess the seismic performance of the new embankment dam. Deformation analyses using the 2-D plane strain finite difference program FLAC showed that displacements of up to about 1.1 m vertically and 0.4 m horizontally could occur at the dam crest under the design earthquake loading. Treatment of the liquefiable alluvium is thus deemed not necessary. The anticipated movements can be safely accommodated by the new dam that has been designed with sufficient freeboard, and suitably wide upstream and downstream filter and transition zones.