Collapse of the Old Pacific Highway, Piles Creek, SomersbyBest paper (overall) from the 13th ANZ Conference on Geomechanics, Perth 2019

Strath J. Clarke, Garry Mostyn and Bernard Shen

On 8 June 2007 a section of the embankment collapsed along the Old Pacific Highway at Somersby, New South Wales, Australia. The collapse occurred during a heavy rain storm, at a location where three corrugated steel pipes conveyed Piles Creek through the base of the road embankment. Not long after the collapse, a car drove into the resulting void and all the occupants drowned in the flooded creek.

A detailed forensic investigation was undertaken of the Piles Creek site, and also a similar culvert located further along the highway at Leask Creek.
The inverts of the three pipes at Piles Creek had rusted away over their full length.

The Leask Creek culverts were displaying some features which were known to have been present prior to the failure at Piles Creek, including loss of the pipe inverts and a depression in the road surface above the culverts. The Leask Creek embankment contained substantial erosion gullies, an active landslip and several voids. These conditions were inferred to be indicative of those likely to have been present at Piles Creek prior to its collapse.

The investigation revealed that the Piles Creek culverts were likely to have been perforated at least six years prior to the final catastrophic failure.

The causes of the failure were the loss of the pipe inverts, and subsequent ground loss into the pipes leading to increased erodibility of the loosened and voided fill, with the collapse triggered by the flood event.