The behaviour of rock surrounding an underground opening depends primarily on the strength of the rock mass and the level of stress existing in proximity to the excavation. As the excavation is made, a redistribution of the virgin stress
field occurs, leading generally to a concentration of stress parallel to the wall, and a relief of stress normal to the wall. If the concentrated stress magnitude is high enough relative to rock strength the rock mass can dilate or fracture.
Likewise, the relief of stress normal to the wall can lead to loosening and unravelling of blocks of rock bounded by discontinuities in the wall.
The role of the virgin in situ stress field has become well recognised in the Sydney area over recent years. Most geotechnical investigations for mining and civil engineering projects in the Sydney Basin will include a consideration of this factor, often involving direct stress measurements. This paper gives a brief summary of some of the factors controlling in situ stress, the historic evidence for the role of stress in the Sydney area, a discussion of the results of a large number of stress measurement campaigns, and a description of a number of projects undertaken in recent years that illustrate the importance of stress in the Sydney Basin.