Engineers Australia

An overview of engineering geology and geotechnical challenges in the Newcastle Region

M. G. Delaney

Abstract

The cliff areas along the Newcastle coastline comprise an inter-bedded sequence of sedimentary rocks with layers of variable strength, weathering susceptibility and defect intensity that exert a strong control over cliff morphology and erosion. Coastal cliff line instability in the Newcastle region is predominantly a surface related phenomenon associated with ongoing erosion and degradation processes rather than a deep-seated instability problem. Risk is primarily associated with toppling failure of undercut sandstone blocks, fretting/sloughing of less resistant tuffaceous, shale and coal horizons and erosion of accumulated soil deposits (talus). Sandstone and conglomerate rock falls are predominantly associated with loss of support due to preferential erosion of weaker less durable materials and the presence of occasional low angle joint surfaces. This natural process has been going on for thousands of years and typically results in localised small scale instability problems.

The paper discusses three documented rock falls that have impacted on coastal infrastructure in the last 5 years and outlines the failure mechanisms involved, in particular the role of low angle joints.

Cliff regression along the Newcastle coastline is a process associated with long term slope degradation and erosion as well as periodic localised rock fall events. The paper assesses the rate of long term cliff line regression from literature review, analysis of historical photographs, direct survey and geological interpretation.