Engineers Australia

Collapse of the Old Pacific Highway, Piles Creek, Somersby

Strath Clarke

On 8 June 2007 a section of the embankment collapsed along the Old Pacific Highway at Somersby, NSW. 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 the five 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 purpose of the investigation was to gather information to enable the cause of the failure to be determined.

Initial observations indicated that the inverts of the three pipes had rusted away over their full length. Detailed geotechnical investigations were undertaken of the subsurface conditions during the deconstruction of the remnants of the embankment. These investigations indicated that the fill was properly constructed, with no unusual properties. 

The culvert at Leask Creek was constructed as part of the same contract as Piles Creek, and was displaying signs of distress which were known to have been present prior to the failure at Piles Creek. The Leask Creek embankment contained substantial erosion gullies, an active landslip, and voids around the culvert headwalls and beneath the pavement. Additional voids and zones of loosened fill encountered during deconstruction of the Leask Creek embankment were inferred to be indicative of those likely to have been present at Piles Creek prior to its collapse. 

A literature review and inspection of other steel culverts indicated that the extent of pipe corrosion observed was not unusual. Hydrological assessment also showed that the creek catchments at both Piles Creek and Leask Creek culverts were not abnormal.

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

The causes of the failure were the loss of the pipe inverts, and elevated erodibility of the loosened and voided fill, with the failure triggered by the flood event.

About the speaker

Strath ClarkePells Sullivan Meynink Engineering Consultants

Strath Clarke is a Principal Engineer with over 27 years of consulting experience.

Upon graduating from the University of New South Wales in 1992, he joined Coffey Geosciences as a Geotechnical Engineer and worked on a wide range of projects for nine years. This included site investigation and geotechnical design for numerous major projects in Australia, such as the Esso CGS oil rigs project in Port Kembla. Overseas projects include the Emirates Twin Towers in Dubai, and specialist site investigations for Taipei 101 in Taiwan and the My Thuan bridge in Vietnam.

He attained his Masters of Engineering Science from UNSW in 1997, and joined PSM in 2001, where he extended his experience to the design and construction of complex civil structures.

He has extensive experience in the investigation, design, and construction of road and rail infrastructure tunnels in Sydney, including key tunnel design roles for the widening of the M2 tunnels, the award-winning Wynyard Walk tunnel, and North West Rail Link projects. He is currently the lead designer for the permanent lining of the huge Sydney Metro Victoria Cross Station cavern.

He is providing Independent Verifier services for the Transmission Gully project in New Zealand and has undertaken numerous review roles for public and private organisations in relation to the design of major infrastructure projects.

Strath has undertaken various forensic investigations, including Melbourne’s beleaguered Burnley Tunnel, the Somersby collapse, and a deep cut & cover rail tunnel in Sydney.

He has authored over 25 technical papers covering a range of topics, including in situ testing, monitoring, research projects, case studies and back-analysis, and new techniques and approaches to the assessment and design of tunnels.

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