Engineers Australia

Sydney sandstone and shale parameters for tunnel design

Robert Bertuzzi


Inherent in any set of rock mass parameters are various assumptions regarding, amongst other things, the depth of cover, the proportion of materials encountered and the scale of the proposed excavations. The factors typically used to define rock mass conditions are:

  • the rock type (lithology)
  • the strength of the rock substance (intact rock strength)
  • the fracturing of the rock mass by defects (bedding, joints, shears, etc.)
  • the persistence and spacings of defects
  • the number of sets of defects
  • the infill material
  • the roughness of defects
  • the groundwater pressures
  • the reactivity of the rock substance to environmental change (shrink/swell, slaking)

The designers’ rock mass parameters communicates their assumptions regarding the ground conditions to those in the field responsible for implementing the designs. It is vital that those in the field are vigilant in observing, documenting and interpreting geological structures that may dominate support requirements at a particular location, and which would render the “average” rock mass class irrelevant and feed back to the designers where conditions are different from those anticipated.

The intention of this paper is to present typical geotechnical characteristics for tunnel design in Sydney’s Hawkesbury Sandstone and Ashfield Shale following the Sydney classification system (Pells et al., 1998). It is suggested that for geotechnical parameters the Mittagong Formation can generally be considered in line with the Hawkesbury Sandstone classes. This paper also presents design parameters that can be used in numerical analyses for tunnel design.

Different sets of material properties may need to be provided to cater for different scales as design parameters are dependent on the scale of assessment. For example:

  • Overall tunnel scale – properties to be used in continuum analyses e.g. FLAC, Phase2, PLAXIS, Abaqus. Specific geological structures, one or two at most, can be included in the model.
  • Approximately 1m3 scale – properties to be used in discontinuum analyses where numerous geological structures are explicitly modelled, e.g. UDEC.

This paper updates Bertuzzi and Pells (2002) with data from recent tunnelling projects. The database now includes information from the Ocean Outfalls, Sydney Harbour Tunnel, Eastern Distributor, M5 East, Cross City, cables tunnels, Epping-Chatswood, Lane Cove, CBD Metro, M2 and the northwest rail projects. This paper also presents useful methods by which the designers can communicate their views of rock mass conditions, particularly to those in the field. Finally, the paper brings into consideration the rock mass behaviour types recommended by the Austrian Society for Geomechanics (2010). The example of a nominally 6 to 12 m diameter TBM at depths of up to 50 m is used.

It is hoped that practitioners will find this paper useful in their work in Sydney.