Slope Stabilisation of Oxley Highway (HW11), Sites 4 and 22, Mount Seaview, NSW Australia

Oxley Highway (HW11) at Mount Seaview on the mid-north coast of NSW experienced extensive slope failures at several locations during the severe rain events in March 2021 and 2022. The Assessed Risk Level (ARL) of the slopes increased from ARL4 (Low Risk) to ARL2 (High Risk) causing one lane to be closed at the slip locations permanently until the remediation was completed. As part of the remediation program, Transport for New South Wales (TfNSW) engaged WSP to carry out the geotechnical assessment and slope stabilisation design of the slips. A team of geotechnical engineers and geologists from WSP was mobilised to inspect the slip sites and provided concept remedial options for TfNSW considerations for the detailed design. Among many sites, this paper talks about the slip remediation design for Sites 4 and 22. Site 4 has downslope failure which led to a tension crack in the pavement while Site 22 has upslope failure. Temporary measures using soil nails were undertaken at Site 4 to keep the road open while fallen rocks and debris were removed at Site 22. This paper talks about the causes of slip failures and the remedial design process considering value for money and constructability to restore the road and reduce the slope risk level to ARL4.