Stress measurement should be an integral component of the data collection for mine design and operation. It is seductive to suggest that using numerical methods to assess mine design and sensitivity analysis related to the expected in situ stress can obviate the need for stress management. There are probably regional and local tectonic influences on the current in situ stress; these should be understood in the context of a regional and local geological/structural environment.
The most commonly used and most widely accepted stress measurement method, the CSIRO HI cell, can only be used once access has been gained to the underground. The method, in its current state, assumes the rock mass is isotropic, homogeneous and behaves in a linear elastic manner. Typically two or three measurements are undertaken at each location, so the ability to see this method in a statistical sense is difficult. Under the circumstances the method is not appropriate for projects being considered at prefeasibility and feasibility level unless these are being conducted where measurements have already been undertaken. Other methods that require access include the ANZII cell which has the added capability of uniquely identifying the influence of anisotropy in the rock mass and its influence on stress measurement (both in magnitude and direction). There are several methods which measured by axial stress field and require different orientations to capture the in situ stress. These include the door stopper, the USBM cell and the borehole slotter. The appropriate technique to use is often dependent on the rock mass in which the stress measurement is being undertaken.
The hydraulic fracture method (HF) can provide stress measurements well ahead of development. This method has the advantage of being able to undertake several measurements in a single borehole. Knowledge of the fracture direction based on using an impression packer, provides the orientation of the minimum principal stress normal to the borehole direction. Using the assumption of linear elastic, isotropic and homogeneous behaviour, the maximum principal stress normal to the direction of the borehole can be determined. There is one overriding assumption in this method and that is one of the principal stresses is parallel to the bore hole direction. Around mineralised systems this may not be the case, however the assumption may not have a significant impact at design stage. In most cases an HF program would require a specifically drilled hole, and this becomes the most expensive part of the data collection.
Promise has been shown with stress memory techniques using existing oriented core. The assumption is that the rock can remember the stress to which it has previously been subjected. The common techniques include acoustic emission (AE), deformation rate analysis (DRA) and more recently seismic velocity (SV). Both DRA and SV can also be used to determine the rock mass anisotropy (with the former providing the magnitude). Proponents of the methods purport that they can measure the current in situ stress. This means that provided oriented core is recovered during exploration, the cost of obtaining statistically representative stress measurements can be dramatically reduced. With the added benefit of measuring the anisotropic behaviour of the rock mass, which can then be used in numerical modelling in addition to providing the in situ stress orientation and magnitude, more reliable mine designs can be undertaken if the major geological controls are also included. If the anisotropy is known this can be used to correct the stress measurements which assume isotropic behaviour when they become available.
No one technique should be used in isolation.
This workshop will have presentations from practitioners in the field, it will discuss the limitations of the methods and provide an up-to-date guide on which technique/s are suitable for your project.