Estimating the rock mass strength of Hawkesbury Sandstone
This paper presents nine documented case studies where stress induced failure has
occurred in tunnels excavated within Sydney’s Hawkesbury Sandstone. The case studies
cover a range of conditions:
construction dates from 1880s to 2006
spans from 3.2 to 12.5 m
depths from 17 to 124 m
drill & blast, roadheader and TBM excavation
Rock mass strength envelopes for typical Hawkesbury Sandstone conditions are derived
using the 2002 Hoek-Brown criterion and the Diederichs spalling criterion based on available
laboratory testing and mapping data.
Two-dimensional finite element modelling, based on geological mapping of the failure areas,
is used to assess the likely magnitudes of the principal stresses that were present at the time
of the failures. The finite element models are interrogated to provide the values of major and
minor principal stresses in and around the mapped failure zones.
The stresses at failed and stable points are then compared to the rock mass strength
envelopes. The ability of the Hoek-Brown rock mass strength envelope to differentiate
between the ‘failed’ and ‘stable’ zones is remarkable.

Estimating the rock mass strength of Hawkesbury Sandstone