Rockfall risk assessment at North Head, Sydney Harbour National Park, NSW

A large rockfall in excess of 9,000 m3 occurred on 10 August 2016 near Sydney’s iconic North Head lookouts, within the Sydney Harbour National Park, NSW. JK Geotechnics (JKG) was engaged by NSW National Parks and Wildlife Service (NPWS) to carry out a detailed geotechnical assessment of the existing sandstone cliff faces and cliff top areas. The assessment included a walkover inspection of the cliff top, an aerial drone survey of the cliff faces and targeted inspections of selected portions of the cliff faces using abseiling techniques. Following the inspections and a subsequent risk assessment, NPWS partially restricted access at one of three cliff top lookouts, with the remaining two lookouts permanently closed. Following a hazard reduction burn within the national park on 17 October 2020, several open and persistent joints became exposed across the ground surface immediately behind the cliff faces. JKG reinspected the cliff top areas for additional signs of instability, reviewed historical information and provided updated geotechnical advice for two proposed lookouts located near the cliff edges (namely, Burragula and Yiningma). The results of the inspection confirmed that the observed open joint traces generally corresponded to structural features identified during the previous JKG geotechnical assessment completed in 2016 following the rock fall. An updated risk assessment informed the minimum set back distances of the proposed lookouts from the cliff edges. JKG also carried out numerous inspections during the demolition and construction phases and the lookouts were safely and successfully constructed circa June 2023.