Improving Dam Safety Using a Fin Drain

by B. E. Hall, Knight Hall Hendry and Associates, Sandton, South Africa,
C. F. Watermeyer, Knight Hall Hendry and Associates, Sandton, South Africa,
J. A. Frame, Knight Hall Hendry and Associates, Sandton, South Africa,

Document Type: Proceeding Paper

Part of: Geotechnical Practice in Dam Rehabilitation


Bloemhoek Dam is an embankment dam with a maximum height of 21 m, which leaked severely after first filling in 1978. This leakage resulted in the development of `boils' over a portion of the left abutment where the vertical clay core had been stopped at shallow depth. Although subsequent grouting below the completed embankment was successful in substantially reducing the leakage, concerns regarding the longterm safety of the dam were raised following a dam safety inspection carried out by the authors in 1987. In particular, exploration holes drilled into the foundation soils along the toe of the dam encountered strong seepage, some of which was concentrated in conduits which were apparently the remnants of termite galleries and runways. As the soils in the area were known to be dispersive it was considered vital to improve the safety of the dam. In order to control the seepage and counter the potential piping a 4 m deep fin-type toe drain was constructed along the section of the wall where the core had not penetrated to sufficient depth. Despite the shallow watertable which gave rise to unstable excavations, the drain was constructed successfully in an open trench by the simple expedient of maintaining a positive head of water in the trench.

Subject Headings: Embankment dams | Dam safety | Drainage | Seepage | Structural safety | Soil analysis | Soil structures | Soil-structure interaction

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