In-situ rock stress measurement by overcoring from the surface, using a specialised equipment commonly referred to as an IST (In-situ Stress Testing) tool, provides a means of stress measurement technique on virgin or exploration grounds. Stress measurement depths of up to 750 meters from surface have been achieved, where the rock strata may have differing stiffness. A simplified tectonic strain model which examines the nature of loading within such rock strata is described in this paper. Stress interpretation using the in-situ measurement technique and tectonic strain analytical model provides a means to derive principal effective stress magnitude and direction for underground mine planning and roof control.
This paper describes the operation of the Sigra insitu stress measurement tool and the concept of tectonic strain used to examine the distribution of strain, and as a consequence stress, through layered strata. This method provides a consistent basis for examining the rock stress behaviour through rocks of varying stiffness. The results of stress measurement at three sites with widely varying characteristics are presented. These are located at Northern and Central Bowen Basin of Queensland, and Hunter Valley of New South Wales, Australia.
2 TYPES OF STRESS MEASUREMENTS
Stress measurements by overcoring have been undertaken for many decades (Leeman 1969, Hooker et al. 1974). It involves creating a borehole, placing some device that measures strains or dimensions in the hole, and then drilling over the top of that hole to relieve the stress and thus cause a dimension change. This dimension change is measured and so is the rock modulus and Poisson’s Ratio. By the use of mathematical formulae it is possible to calculate the magnitude and directions of the stresses existing in the rock.