Stress and Open Pits


All rock masses are subjected to in situ stresses resulting from the weight of the overlying strata and from locked in stresses of tectonic origin. The excavation of an open pit results in disruption of the existing stress field with some of the stresses being relieved, while others are intensified. These changes in the in situ stresses can have very important consequences in terms of the behaviour of the rock mass surrounding underground excavations. However, the question to be considered is whether these stress changes are important in the design of the slopes of an open pit mine. The objective of this research work was therefore to understand the influence of in situ stresses on open pit design.  

In a typical open pit mine the slopes are designed on the assumption that the stresses are primarily gravitational acting vertically downward. However, horizontal stresses are induced as a result of constraints on displacement imposed by the surrounding rock mass and it is generally assumed that these stresses are isotropic and that their magnitude is either lower than or equal to the vertical stresses (depending upon the type of input required by the model). The fundamental question then needs to be answered is: if the horizontal stresses are significantly higher than the vertical stresses and the in-situ stress field is anisotropic (due to the tectonic history of the area) are the horizontal stresses induced in the slopes significant in terms of the behavior of the rock mass?  Do these induced stresses influence either the failure criterion or the permeability of the rock mass to such an extent that they have to be considered in an overall open pit design?  Alternatively, is the impact sufficiently small that we can afford to ignore the influence of the horizontal in-situ stress and proceed as we have always done in slope design? In this research work, the nature of these situations are outlined the practical suggestions on how they should be dealt with are offered, also presented in Appendix 3 of the open pit slope design guidelines book.