A rockfall avalanche in a sandstone landscape, Nattai North, NSW

SUMMARY

Rockfall avalanches are commonly associated with the alpine regions of Europe, South America and north‐western Canada, but modern examples have only been reported very recently in Australia (Pells et al. 1987). The Nattai North rockfall avalanche is located on the Burragorang Walls escarpment in the sandstone landscape of the Sydney Basin. The volume of rock involved in the failure had sufficient magnitude to enable the resulting mass of debris to flow in the manner of a semiviscous fluid. The conventional models of rockslope evolution, involving undercutting followed by blockfalls, do not apply at this site. Indeed these models do not apply to most of the large‐scale rock collapses in the Sydney Basin. All such rockfalls have occurred in the vicinity of underground coal mines. Coal mining has affected the stability of nearby escarpments by altering stress distributions within the rock mass. The subsequent failures are typically larger and of a different form than those occurring naturally.