Observations on the behaviour of surface and underground excavations in the shales of the sydney region: Geotechnical Lessons

This paper presents the authors’ combined experience of the behaviour of Sydney Shale during the construction of surface and underground excavations in the Ashfield Shale and the Bringelly Shale in the Sydney region. It includes observations made during the excavation phase of large-scale infrastructure projects, such as WestConnex M4 East, Sydney Metro Northwest, Sydney Metro – Western Sydney Airport Station Boxes and Tunnelling and Sydney Metro West – Western Tunnelling Package. These projects involved cut and cover and mined tunnel and cavern excavations for railway stations, rail tunnels and motorways. The vertical surface excavations discussed here are up to 38 m deep and 150 m long. The largest cavern described is 26 m wide, 27 m high and has a cover of 15 m.

Rock mass classification systems used in Sydney frequently classify shale and sandstone according to the same parameters: defect spacing and intact strength. This paper highlights significant differences in the engineering geological properties and rock mass behaviour of Sydney shales when compared to Sydney sandstones. Examples of observed rock mass behaviour and associated geotechnical risks relevant to Sydney shales, such as wedge failure in vertical walls, tunnel face instability and overbreak in the tunnel crown are included. Comments are made on the nature of geological defects in shale relevant to the construction of both surface and underground excavations. The authors highlight items for consideration in the design of excavations in shale, in contrast to sandstone, in terms of ground support, excavation geometry, excavation sequence, construction monitoring and field verification of expected ground conditions. Photographs documenting observed rock mass behaviour, and the character of typical geological defects are presented. Monitoring data quantifying observed movements are included.