CS11B - Concurrent Session 11B: Case History of 2002 Swift No. 2 Dam Failure

Swift No. 2 failure suddenly in the early morning of April 21, 2002, following 44 years of operation. Swift No. 2 Dam is an 83 foot tall zoned earth embankment dam at the head of a 16,700 ft long earth lined canal. The canal is fed by discharge from the Swift No. project. The project is located on the Lewis River in Southwest Washington, approximately 10 miles south of Mount Saint Helens. On April 22, 2002, the Swift No. 2 dam embankment failed in under 4 hours, releasing 9,000 cfs of flow resulting in damage to the adjacent highway and powerhouse. The failure resulted in a scour hole that extended about 50 feet below the original embankment dam foundation and was approximately 150 feet by 150 feet in dimension.

Following failure of the embankment, a forensics investigation was completed to understand the cause of the failure. It was found that the failure originated from an internal erosion mechanism that connected to the forebay about 600 feet upstream of the dam, and connected to the tailrace about 700 feet downstream of the dam. This seepage path, once formed, was able to continue due to a roof formed by the overlying basalt flow as the underlying alluvium eroded. The void was ultimately estimated to be approximately 7 acres in size and up to several feet in height, that eventually collapsed and resulted in breaching of the embankment.

The geology of the embankment dam foundation includes a very irregular and undulating basalt formation originating from Mount Saint Helens, referred to as the Cave Basalts, which is underlain by alluvium. The basalt formation includes many cavities, with over 5 ½ miles of documented lava tubes in the overall formation, and several large lava tubes identified under the forebay. During original construction in 1957-1958, the voids in the ballast were recognized as a concern and were filled with granular material using compaction equipment and water sluicing, and covered with a low permeability silty sand.

This paper summarizes the history of the project, the causes that led to the failure as determined form the forensics investigation, the impacts of the failure, and lessons learned following the forensic investigation. This significant failure has not previously been documented the literature.