Structural Controls Of The Otway Ranges And Hazards For Road Users

ABSTRACT

The Otway Ranges have long been established as a region with significant landslide hazards to both private property and public asset owners. Owing to the folded nature of the interbedded sedimentary Cretaceous Eumeralla Formation, dipslope behaviour governs a large percentage of the slopes, and this is generally well documented by others. Along the southeast facing coastline of the ranges a significant proportion of the Great Ocean Road has been constructed sub-parallel to the strike of bedding and it is unsurprising that planar sliding on bedding is a common slope control that informs hazard assessments.

For inland routes and sections of roadway orientated outside of the kinematic window for dip-slope failure, structural controls on cut and buried slopes are often present. These are typically associated with shear and fault zones and in some cases the interaction with weaker siltstone and mudstone beds. The structural controls of two case studies are presented, each highlighting a unique set of hazards to road users. Each case study highlights the importance of establishing the structural trends, even when assessing slopes formed with fill and buried landforms. These trends then inform the engineering geological model and ultimately the assessment of hazards to road users.

1 INTRODUCTION

The Otway ranges in Victoria, Australia, are home to the Great Ocean Road (GOR), which adds significant value to the tourism industry. The State Government has active strategies in place to improve safety in the Otway Ranges (Great Ocean Road region strategy (planning.vic.gov.au) 2024) and was actively involved in the Wye River and Separation Creek rebuild following the Christmas 2015 bushfires. The Department of Transport and Planning (DTP) is tasked with management of roadside geotechnical hazards for the GOR and key inland routes.

The structural trends along the GOR have been well documented by Medwell (1968), Cooney (1982) and Edwards et al (1996). The works reported by the preceding authors are broadly focussed on major folds, faults and bedding trends with less detail on fault and shear systems, Figure 1. Furthermore, large planar failures on dip slopes are well established as a mode of failure, especially along the GOR where bedding is often undercut by wave driven erosion and the construction of the GOR itself (Williams & Muir, 1972). The extensive mapping of bedding trends is helpful as typical defect sets are consistent with literature in folded sedimentary rocks (Fookes, et al 2000). The present author’s approach to hazard mapping in the Otway Ranges places a priority on locating intact rock in the study area and establishing bedding trends as a minimum to begin understanding structural controls at each site.

The two case studies presented are located within deposits of the Cretaceous Eumerella Formation (Edwards, 1996). The formation is composed mainly of fine to medium grained sandstone and siltstone interbedded with thinner and less frequent mudstone. The quartz content is relatively low and the deposits weather rapidly to sands and clays. The siltstones are notably of lower strength, slake prone and more readily erodible in comparison to the sandstone (Gill, 1979).

Edwards (1996) outlines the broad composition and physiography of the Otway Ranges as follows:

• The “ranges are composed of uplifted and eroded Cretaceous Eumeralla Formation”
• Miocene compression activity has produced northeast trending anticlinoria
• The southeastern limb of these folds often forms dip slopes in proximity to the coastline
• Numerous folds are offset by faults. Typically streams run sub-parallel to these fault systems.

The two cases studies aim to highlight the structural controls of slopes with aspects orthogonal to bedding. The first case study highlights that structure may control landslide geometry but not the mode of failure. It includes a translational failure in a fill batter built across a gully. The gully is controlled by a buried fault and the identification of the structure assisted with limiting the zone of remediation. The importance of historical publications that identified last interglacial sea levels and how this can affect slope remediation is discussed. The second case study was a thin wedge failure in weathered rocks in proximity to major regional folds and faults, where the structure directly controlled the landslide.