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Seismic Cutoff

by Peter M. Byrne, Prof.; Univ. of British Columbia, Dept. of Civ. Engrg., 2324 Main Mall, Vancouver, BC V6T 1W5,
Alan S. Imrie, Manager; B.C. Hydro Geotechnical Dept., Vancouver, BC,
William F. Marcuson, III, P.E., CEWES-GV-Z, P.O. Box 631, Vicksburg, MS 39180-0631,


Serial Information: Civil Engineering—ASCE, 1988, Vol. 58, Issue 12, Pg. 50-53


Document Type: Feature article

Abstract:

The John Hart Dam, which is a 40-year old dam in British Vancouver, Canada, has been rehabilitated to meet current seismic codes. BC Hydro, the utility that owns the dam, conducted a study of the over 50 dams it owns and has undertaken a $50 million rehabilitation program. The John Hart Dam rehabilitation is a $17 million portion of that total. The dam was not designed to withstand earthquakes of the magnitude now thought possible in the area. In addition, the dam's earth embankment and foundation were found to be saturated and susceptible to liquefaction. Much less was known about liquefaction when the dam was designed and built. The rehabilitation design was complicated by the fact that the utility had to keep operating the dam and could not lower the reservoir during remedial work. Engineers looked at several alternative solutions to this problem and opted to build a cutoff wall by means of a jet grouting technique used in Europe. The cutoff wall could thus be constructed while the reservoir was full. A cement-bentonite slurry, 12% cement and 4% bentonite by weight, was chosen to provide a flexible material less prone to cracking during seismic loading. The cutoff was supplemented by a soil replacement and in situ densification program. These measures are designed to result in desaturation of soils in the area. The jet grouted cutoff wall is probably the first in the world constructed while a dam is operating. The two year remedial project was scheduled to be completed by the end of 1988.

Subject Headings: Dams | Rehabilitation | Seismic tests | Diaphragm walls | Utilities | Soil liquefaction | Seismic loads

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