Deformations About Excavations in Highly Stressed Rock

by Paul Nash, DJ Douglas & Partners, West Ryde,
Garry Mostyn, DJ Douglas & Partners, West Ryde,

Document Type: Proceeding Paper

Part of: Vertical and Horizontal Deformations of Foundations and Embankments


The central business district of Sydney, New South Wales (NSW), is dominated by the Hawkesbury Sandstone, a Triassic sedimentary formation. Measurements have confirmed the existence of a high in-situ horizontal (compressive) stress field. Average city basement excavations have deepened from 10 m to up to 30 m over the last two decades and rock stress problems have started to be encountered. This paper reports an elaborate and realistic investigation using UDEC (Universal Distinct Element Code) to consider the effects of rock mass defects which are such a dominant characteristic of Hawkesbury Sandstone. UDEC is an ideal modelling tool for this situation. The investigation showed, as expected, that the deformations due to stress relief were influenced by the properties of the rock substance and the properties of the defects. The deformation was found to be a two stage process: firstly, the initial elastic dilation of the rock due to stress relief, which appear to be linear with depth; and, secondly, opening of vertical defects by rafting on horizontal defects near the excavation face, which is negligible at shallow depths (less than 10 m) and increases rapidly with depth. Other conclusions are presented. The work has shed considerable light on behavior of basement excavations in highly stressed rock masses.

Subject Headings: Rock masses | Excavation | Defects and imperfections | Sandstone | Rocks | Rock mechanics | Lateral stress | Australia | Sydney | New South Wales

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