Stress Fields in Deep Mines

Knowledge of stresses is an important component of any geomechanical design task. This requirement is accentuated in deep mines, where stresses not only lead to fracturing around excavation boundaries, but include induced seismicity and the associated problems of strainbursting and rockbursting. The conventional approach to measuring stresses in mines is to use overcoring in relatively few locations and fit depth dependent linear profiles to principal stresses. Use may also be made of regional stress data compilations. Both approaches assume that the stress field is relatively simple, and smoothing to remove or reduce scatter in the data is part of the fitting process. A similar approach is typically used to model faults and other geological structures, whereby they are created in an initially uniform stress field. It is shown that these methods are not consistent with evidence of stress field characteristics found in the typically complex geological environments of deep mines. Interaction between the stress field and geological structures results in a natural scatter in both stress magnitude and orientation. In some mines, these variations can be significant, and are important to characterize for design purposes. Conventional measurement and analysis methods are not well suited to either quantifying or reproducing the behaviour of such stress fields. Induced seismicity is a common characteristic of deep mines, and seismic data is normally abundantly available. The seismic stress inversion method, developed originally for the analysis of crustal earthquakes, provides information about the orientation of principal stresses and their relative magnitude. Its application to mining seismicity has been sparse, but it is shown that the method has potential to map stress fields both spatially and temporally. An example shows its ability to detect significant modification to the local stress field in the vicinity of a large seismic event. The method can only be used if there is induced seismicity, so it is not a substitute for conventional stress measurement. However, in an operating mine it can provide valuable information for mine planning, and could lead to an improved understanding of the relationship between faults and stress fields in the typically complex geological environments of deep mines.