Slope risk assessment of near vertical cuts in Hawkesbury Sandstone F3 Freeway, Wahroonga to Hawkesbury River, NSW

A slope risk assessment analysis was undertaken on a total of 83 high angle road cuts in Hawkesbury Sandstone along a 20 km length of F3 Freeway, north of Sydney.

Details of individual cut features and methods used to prioritise cuts for remedial works are given, together with a discussion on risk assessment as part of the design process for future freeway works.

Introduction

The F3 Freeway forms the major transport link between Sydney and the Central Coast and Newcastle regions located some 100 km to 150 km northwards, with traffic movements of the order of 15,000 vehicles per day (1989).

Within the study area, the route of the freeway lies within a geological setting of rugged terrain dominated by the near flat lying Hawkesbury Sandstone of Middle Triassic Age. Local relief is in the order of 100 m – 200 m.

Construction of the Freeway was largely undertaken in the late 1960s to replace the older, two lane Pacific Highway, with a further southern extension constructed during 1986-87. Excavation methods appear to have been largely by drilling and blasting.

The resulting roadway from both periods of construction is characterised by alternating cut and fill sections with the cuttings typically steeply excavated in medium to high strength sandstone with some mudstone interbeds and occasional volcanic intrusives.

Some cuttings have intermediate benches, although most are continuous with slope angles up to 80˚ and heights ranging up to 70 m.

During 1994, Douglas Partners were engaged by the Roads and traffic Authority of NSW (RTA) to undertake a Slope Risk Priority investigation on cuttings along the southernmost 20 km of the F3 Freeway, between Wahroonga and the Hawkesbury River.