Geotechnical Parameters Of Sydney Sandstone And Shale

Robert Bertuzzi and Philip J.N. Pells

The classification system for Sydney sandstone and shales, through the Australian Geomechanics Society (Pells et al, 1978; Pells, Mostyn and Walker, 1998) was intended to assist in the design of foundations on rock in the Sydney area. The five class system has proved to be a good tool for communicating rock mass quality for other geotechnical projects such as tunnels and deep basement excavations. However, the classification system is not a design tool for works other than foundations on rock. Tunnels, slopes, deep basements and retaining walls should be designed using normal methods of applied mechanics. However, such methods, whether hand stability calculations or complex analyses using programs such as UDEC, require engineering parameters covering strength and deformation characteristics. In some instances, such as rock substance strength and modulus, the parameters may be measured by laboratory testing. However, when it comes to rock mass parameters use has to be made of parameters back figured from monitoring of actual excavations and retaining structures; published correlations from other geological environments, such as mass modulus versus RMR; or semi-theoretical approaches such as Hoek’s approach of estimating mass modulus from Hoek- Brown parameters. Unfortunately, when one scratches the surface many of these correlations and guidelines are based on scant data and they must be used with great caution.

This paper summarises the deformation and strength parameters the authors currently use for rock mechanics computations in the Sydney shales and sandstones. It is not intended to provide a detailed lithological or petrographic description of Sydney rocks. The reader is directed elsewhere for that information, for example Packham (1969), Chestnut (1983), Pells (1985), Pells (1993), Pells et al (1998), McNally & McQueen (2000), McNally & Franklin (2000), etc. Rather, the purpose of this paper is to improve the communication between engineering geologists, geotechnical engineers and the construction industry, in particular the tunnelling fraternity, when referring to Sydney rocks.

The paper is divided into four parts.

  1. The first is a recapitulation of the appropriate process of classification using the Sydney Classification System.
  2. The second presents typical insitu engineering parameters, which may be appropriate for engineering design once the rock mass has been classified. The tables should not be used to back-figure the rock mass class.
  3. The third presents typical Q and RMR values for sandstone and shale, as the authors have found that these may help in communicating conditions to practitioners unfamiliar with the Sydney Classification System. However, please note that the authors do not recommend using either the Q or RMR system, or the Sydney Classification System, for the design of tunnel support within these rocks. Several publications highlight the difficulties in using the Q and/or RMR system in Sydney, eg Asche & Cooper (2002), Pells (1997).
  4. The fourth presents six colour sheets describing the typical engineering geology of Class I/II, Class III and Class IV/V sandstone and then of Class I/II, Class III and Class IV/V shale. Photographs of example rock exposures are included on the sheets to further assist communication. The authors note that there are several locations around Sydney to observe these exposures, including:
    1. West Pymble Bicentennial Park – Class II to V sandstone;
    2. M2 tunnel and the Tarpian Cliff at the Opera House – Class I and II sandstone;
    3. c. Eastwood Brick Pit – Class V to II shale;
    4. M2 motorway – Class V and IV shale.

The authors hope that practitioners will find this paper useful in their work in Sydney.