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USACE orders safety evaluation of Fern Ridge dam

The US Army Corps of Engineers (USACE) has announced that the potential for Fern Ridge dam to collapse in a large earthquake warrants an in-depth study of its structural integrity and the foundation on which it sits.

Built in 1941, Fern Ridge is an earth-filled dam 1929m long and 13.4m high at its river section. USACE says there are no apparent structural deficiencies at the dam, which has performed without problems for more than 60 years.

A recent, seismic evaluation of the earth dam on the Long Tom river in Oregon, had indicated that a large earthquake could cause liquefaction in the base of the dam.

The liquefaction threat emerged from information that suggests the possibility of a future large-magnitude earthquake off the Oregon Coast along the Cascadian Subduction Zone.

USACE says that Liquefaction at Fern Ridge may cause significant loss of foundation strength, possible damage to the dam, and the loss of freeboard.

In the next phase of the investigation, USACE will assess the extent of the sand zone beneath the dam. In addition, the agency will evaluate the amount of seismic shaking the dam can withstand and estimate the anticipated damages to the dam should a large quake hit.

If the expected damages are found to be significant, the corps would pursue design changes to strengthen the foundation.