Following the investigation of the US Army Corps of Engineers into the dams affected during the Chi Chi earthquake in Taiwan, it now appears that the worst damage could have been prevented.
A field reconnaissance team from the Engineer Research and Development Centre of the US Army Corps of Engineers, led by Mary Ellen Hynes, arrived in Taiwan just days after the earthquake struck on 20 September 1999. The team was sent to evaluate how earth and concrete dams perform during earthquakes, before repairs are started.
The Shih-Kang concrete gated dam received the greatest amount of damage from the earthquake, which measured 7.3 on the Richter scale. A fault offset caused the main part of the dam to be upthrusted by approximately 8m, while the other side of the dam experienced 2m of uplift. The fault uplift was not on the main fault rupture but a secondary one which was near the main fault while it was bending. Lateral shear at the dam was less than 1m. Spillway gates at the fault location were also heavily damaged.
Richard Olsen, a member of the reconnaissance team, said: ‘The river course will probably be changed because of the fault upthrust, and it is likely that a new dam will have to be made at a new location.’ In the case of the Shih-Kang dam, it is not known whether its constructors mapped the fault lines during the site investigation for the dam in the early 1970s. Olsen believes that the fault should have been observable during construction when the foundation rock was exposed, and before concrete was poured.
Major damage to the dam could have been prevented if the faults had been mapped and construction had not been allowed on, or near, known faults.
Other concrete dams, and new earth dams that did not have fault offset problems, performed well during the event, according to Olsen.