Seismic upgrade of Coquitlam Dam

The Coquitlam Dam,constructed in 1913,is a 30 m high hydraulic fill embankment dam.The dam is situated in a region of high seismic hazard in British Columbia,Canada.The population at risk is in the tens of thousands.The dam does not meet the present day seismic safety standards.The hydraulic fills in the dam and some foundation soils are loose and susceptible to liquefaction.Under a moderate to severe earthquake,the dam would suffer severe damage with a possibility of dam breach.Seismic upgrade to the dam is required.The maximum design earthquake is a M7.5 event with a peak ground horizontal acceleration of 0.66g.A range of seismic upgrade options was evaluated,ranging from structural remediation,rebuilding of the dam,restricting reservoir operation to decommissioning.On the basis of technical,social,environmental and economic considerations,constructing a new downstream embankment dam was selected as the seismic upgrade option to reduce the seismic risk of the Coquitam dam.The new dam consists of an earth core rockfill embankment and a concrete transition at the left abutment.The new embankment is mostly founded on competent silt while the concrete transition is entirely founded on bedrock.Due to site and constructability constraints,a small portion of the upstream slope of the new dam sitting on part of the downstream shell of the existing dam is underlain by liquefiable sand and gravel alluvium along the original river channel.Advanced seismic stability and deformation analyses demonstrate that the resulting deformations of the dam are acceptable due to large freeboard and thick filter and transition zones.To control foundation seepage and exit gradient,a plastic concrete cut-off wall,a grout curtain,and a set of downstream pressure relief wells are incorporated in the design.To monitor the behaviour and performance of the new dam,a failure mode based approach has been taken in the instrumentation design for both construction and long term monitoring.This paper describes the design of the new Coquitlam Dam,including the design of the instrumentation for dam safety monitoring.