2021 – Analysis of an embankment dam on permeable rock foundations

Jonathon Reid, Brendan Trebilco

The dam reviewed was designed and constructed in two stages, with the embankment completed in 1965. The dam comprises a 37 m high earth and rockfill maximum section on the creek alignment and zoned earthfill embankments of varying arrangements on the abutment flanks with a total crest length over 2km.

A Dam Safety Review was undertaken as part of the owners on-going commitment to maintain its portfolio of dams in a safe and functional state. The dam has suffered from high seepage rates that were first observed in 1971 after the reservoir rose to a historic high level, which was then exacerbated in 2011 after the reservoir rose a further 10m to reach the Full Supply Level for the first time. Reviews of the embankment stability at this time resulted in operating restrictions being placed on the reservoir level.

Detailed instrumentation data collected over a range of filling events showed the rock foundations to be highly responsive in the areas of observed seepage. This resulted in rapid pore pressure responses in foundation soils and the lower portion of the embankment after a rise in reservoir level, but a much slower pore pressure response in the upper parts of the embankment.

Seepage and stability analyses were undertaken based on the high quality instrumentation data to review the stability of the sections for various operating levels and with projected pore pressure increases for rapid flood loading scenarios. The paper explores the sensitivity of the analyses completed and how different construction standards applied to varying sections on the same embankment resulting in acceptable and undesirable outcomes.

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