Due to the tunnel being designed and constructed in the 1950s, very little consideration was given to seismic factors. The river bed is a thick layer of sediment on top of bedrock. This sedimentary layer may liquefy during a major earthquake, leaving the tunnel with nothing to rest on, and thus vulnerable to total collapse. In recent years, as the awareness of the effect of serious seismic activity developed, an engineering assessment and subsequent retrofit project was initiated to increase the survivability of the tunnel in the event of a significant earthquake. This retrofit project started in the fall of 2004 and had been completed by the spring of 2006.