Alternative Title

Paper No. 2.25

Location

St. Louis, Missouri

Session Start Date

3-8-1998

Session End Date

3-15-1998

Abstract

Ochoco Dam, a hydraulic fill structure, was completed in 1920 and is located just east of Prineville, Oregon. Foundation units consist of a massive landslide, talus/alluvial fan complex, alluvium, and John Day Formation. Dam embankment right-abutment seepage occurred during first filling. Upstream right-abutment treatment in 1921 reduced this seepage from an estimated 43 cfs to approximately 28 cfs. Further modifications were completed in 1950 reducing seepage to approximately 12 cfs. As part of the U.S. Bureau of Reclamation's dam safety program, geotechnical investigations were started in 1985. Following this program, erratic piezometer readings in 1989 and sinkhole development along the upstream right abutment led to water surface restriction and construction of a right-abutment geomembrane liner in 1990. Further deteriorating conditions prompted draining of the reservoir in 1993. Extensive modifications, including an upstream zoned embankment with graded filters and drainage system exiting through maximum section of the original embankment, were completed in 1995. First-filling criteria were instituted following these modifications. On May 17, 1995, turbid flow was noted in the main drain outflow with drainage increasing from 1,770 gpm to 3,600 gpm, accompanied by a dramatic rise in pore pressure and increased flows from right-abutment weirs. Extensive underwater examinations and a fluorescein tracer dye testing program resulted in discovery of a ten-foot-diameter sinkhole in the upstream embankment. The sinkhole was backfilled and the reservoir was drained for repairs. Forensic investigations indicated complete sinkhole penetration of the newly constructed embankment with extension down to the 1918 hydraulic fill through an unexpected wedge of large rockfill from previous construction. Following these repairs, dam performance has been closely monitored through the 1996 and 1997 irrigation seasons with performance within expected parameters.

Department(s)

Civil, Architectural and Environmental Engineering

Appears In

International Conference on Case Histories in Geotechnical Engineering

Meeting Name

Fourth Conference

Publisher

University of Missouri--Rolla

Publication Date

3-8-1998

Document Version

Final Version

Rights

© 1998 University of Missouri--Rolla, All rights reserved.

Document Type

Article - Conference proceedings

File Type

text

Language

English

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Safety of Dams Modifications of Ochoco Dam − Crooked River Project, Oregon

St. Louis, Missouri

Ochoco Dam, a hydraulic fill structure, was completed in 1920 and is located just east of Prineville, Oregon. Foundation units consist of a massive landslide, talus/alluvial fan complex, alluvium, and John Day Formation. Dam embankment right-abutment seepage occurred during first filling. Upstream right-abutment treatment in 1921 reduced this seepage from an estimated 43 cfs to approximately 28 cfs. Further modifications were completed in 1950 reducing seepage to approximately 12 cfs. As part of the U.S. Bureau of Reclamation's dam safety program, geotechnical investigations were started in 1985. Following this program, erratic piezometer readings in 1989 and sinkhole development along the upstream right abutment led to water surface restriction and construction of a right-abutment geomembrane liner in 1990. Further deteriorating conditions prompted draining of the reservoir in 1993. Extensive modifications, including an upstream zoned embankment with graded filters and drainage system exiting through maximum section of the original embankment, were completed in 1995. First-filling criteria were instituted following these modifications. On May 17, 1995, turbid flow was noted in the main drain outflow with drainage increasing from 1,770 gpm to 3,600 gpm, accompanied by a dramatic rise in pore pressure and increased flows from right-abutment weirs. Extensive underwater examinations and a fluorescein tracer dye testing program resulted in discovery of a ten-foot-diameter sinkhole in the upstream embankment. The sinkhole was backfilled and the reservoir was drained for repairs. Forensic investigations indicated complete sinkhole penetration of the newly constructed embankment with extension down to the 1918 hydraulic fill through an unexpected wedge of large rockfill from previous construction. Following these repairs, dam performance has been closely monitored through the 1996 and 1997 irrigation seasons with performance within expected parameters.