Millbrook Dam - Forensic Investigation
Abstract
Millbrook Dam is a 32m high zoned earthfill dam located 20km north-east of Adelaide that provides the highest elevation of any of the water storage servicing the city. Downstream is the larger Kangaroo Creek Dam. The dam is owned and operated by South Australia Water Corporation (SA Water). Stage 1 of the 2001 Dam Safety Review identified a bulge in the downstream slope that had been observed over many years by SA Water to be stable. Stage 2 of the Dam Safety Review set out to forensically investigate the origins of this bulge.
The dam embankment was constructed over 4 summer construction seasons between 1914 and 1918 using horse and dray to cart the material and bullock teams to compact the clay. Although there had been regular surveys of the construction progress up until 1918 an extensive search of the SA Water archives found little information between April 1918 and December 1918 when the dam was opened. There were local newspaper reports of a failure at the dam site in May 1918 which were denied by the government. However the SA Water and the State Library held historic photographs of remedial works being performed in the central third of the embankment in July 1918 indicating the crest had subsided 2m over a length of 100m
In 2003 Sinclair Knight Merz (SKM) inspected the dam and supervised the excavation of test pits on the crest and downstream slope and arranged surveys to be carried out on buried crest pins to develop a forensic history of the embankment construction and performance. From these studies a geotechnical model was developed of the failure in 1918 and estimates made of the horizontal and vertical movement. Subsequent uncovering of an original survey plan prepared during this crucial six month period confirmed the model and estimates of movement.
Authors
D.Raisbeck & A.Parsons