Emergency Embankment Stabilisation Works, Pacific Highway, Eungai Creek

M. Lecina, J.C.W. Toh, S.J. Clarke, A. Rosnell and R. Firmer

The Pacific Highway stretches 900 km from Sydney to Brisbane and has been progressively upgraded and realigned over the last 30 years to reduce travel times and improve safety to road users. The northbound embankment of the highway near Eungai Creek was constructed around 1991 and less than a decade later the highway was upgraded with a second embankment to provide dual carriageway conditions.

Cracking of the northbound pavement occurred as early as 2007. Sealing of the cracks was undertaken, though the cracking continued. In 2017 geotechnical site investigations were undertaken and instrumentation installed in the northbound embankment. Substantial displacement between the concrete pipe sections which formed the culverts beneath the embankment was observed. Monitoring over the subsequent four years indicated relatively minor movement of the toe and crest of the embankment.

Site investigations indicated variable subsurface conditions, with localised zones of firm alluvium within and beyond the embankment footprint, underlain by residual soil and phyllite bedrock. Standpipe piezometers consistently indicated significant artesian groundwater pressures.

A review of the site was undertaken in early 2021 to identify potential deformation and failure mechanisms and provide concept options for the permanent stabilisation of the northbound embankment. Additional investigation, detailed field mapping and survey were undertaken to gain a better understanding of the site and assess whether further deterioration had occurred over recent years. Observed conditions were significantly worse than anticipated such that urgent maintenance and repair works were initiated immediately.

Shortly after completion of the field work, wet weather triggered an embankment failure which required partial closure of the highway. Significant contributors to the failure were the poor condition of the surface drainage which led to saturation and softening of the embankment fill, and artesian groundwater conditions that weakened the foundation soils.

Emergency stabilisation measures were developed for rapid implementation and to allow for the future permanent works. These works comprised the construction of soldier piles along the embankment crest and placement of a rockfill toe berm.

Construction of the emergency stabilisation works has progressed smoothly and allowed the highway to fully reopen and continue to provide its key function as an essential transport artery connecting Australia’s eastern seaboard.