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Monitoring a Cut-Off Wall

Upper Huia DamAuckland, New Zealand: Upper Huia Dam is one of ten water supply dams owned and operated by Watercare Services Ltd in the Auckland region. Designed and constructed in the late 1920s, the concrete gravity dam does not comply with present day practices regarding uplift. A mitigating factor, however, is a cut-off wall, which runs the full length of the dam and significantly reduces seepage pressure beneath the dam.

Safety reviews examined the behavior of the dam under different loading conditions and determined that catastrophic failure will not be a concern so long as the uplift pressures at the upstream heel of the dam remain below 75% of full reservoir head. The reviews emphasized the significance of the cutoff wall on the overall stability of the dam. Deterioration of the cutoff wall could lead to a reduction in its effectiveness, and the resulting rise in uplift pressures could jeopardize the stability of the dam. Thus the studies recommended the implementation of a monitoring program that would provide continuous and effective real-time surveillance of pore-water pressures beneath the dam.

Profile looking upstreamSlope Indicator's VW piezometers were selected for this purpose, since they are easily automated and offer long-term stability. The piezometers were connected to Watercare's SCADA system by a VWD500 interface. This interface provides the appropriate excitation to the piezometers and converts the returned vw signals to 4-20 mA signals. These are fed into a Moscad Telemetry PLC and transmitted by radio to the control room, where the data are displayed on a computer screen in graphic form.

Placement of packer and piezometerThe piezometers were installed in NQ cored boreholes that were drilled through the dam body into the foundation rock at four carefully chosen locations. The piezometers were placed as close as possible to the dam / rock interface so that they would measure pore-pressures immediately beneath the dam. To allow for future replacement and calibration capabilities, it was decided that the four boreholes remain ungrouted and that they be sealed with inflatable packers. A fifth piezometer is used as a barometer, so that readings can be corrected for changes in atmospheric pressure.

All piezometers show very close response to variations in lake levels and indicate that the cutoff wall is providing 46 to 68% reductions in uplift pressure. Should the instruments show a reduction in the effectiveness of the wall, prompt remedial work, such as drainage or anchorage, can be undertaken.

Thanks to Alaa S Ahmed-Zeki of Watercare Services Limited for this story, which was condensed from a paper originally published at the ASCE / Geo-Institute Specialty Conference titled: "Performance Confirmation of Constructed Geotechnical Facilities" by Ahmed-Zeki, Logan, McQuarrie, and Jaduram, Amherst, Massachusetts, USA, 9-12 April 2000..

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