2016 – Tullaroop Dam: Dealing with Filter Breakdown and Compaction During Construction

Jamie Cowan, Chris Kelly and Gavan Hunter

Dam safety upgrade works were undertaken at Tullaroop Dam in 2015/16 to reduce the risk of piping through the main embankment. Unexpected cracking and elevated pore water pressures were observed within this earthfill embankment over a period of 10 to 15 years. In 2005/06 a filter and rockfill buttress local to the embankment was constructed on the left abutment after a 60 mm wide diagonal crack opened up on the downstream shoulder from crest to toe.

Similar to the 2005/06 upgrade works, the 2015/16 embankment works were direct managed by Goulburn-Murray Water. Filter and rockfill materials were sourced from commercial quarries previously used for dam upgrade projects and for which significant testing of materials had been undertaken, especially on the fine filter.
Mid project it became clear that the fine filter was breaking down under handling and compaction such that several in-bank gradings fell outside the specified fine limit. Further testing of quarry surge piles, site stockpiles and in-bank placed filters was undertaken to understand the extent of the breakdown. It was assessed that the breakdown was occurring on the 0.5 to 2.0 mm fraction, generating finer sizes in the 0.1 to 0.6 mm fraction. The increase in fines content (minus 75 micron) was less than 1% and met specification. The in-bank material was accepted as placed and the specified filter envelope adjusted to allow for the observed breakdown.

Difficulties were experienced with compaction of the fine filter in the inclined chimney filter to achieve the target density in the range 65% to 80% Density Index when the layer width reduced to 0.75 m for a 0.5 m compacted lift thickness. No difficulties were experienced when the layer width was 1.5 m or in trenches. Further trials were undertaken on the embankment to better understand the compaction issues and used different roller types. It was assessed that an important factor was the arching effect of the adjacent coarse filter. Going forward thinner lifts were used and smaller width rollers to achieve the specified minimum density.

The paper provides details on the embankment construction works, focusing on the fine filter breakdown and compaction issues. Details of the testing undertaken, the actions to resolve the issues and interactions with the supply quarry and construction team are provided.