Summary

Safe design of underground excavations in mining requires good knowledge of the state of natural stresses in the rock mass.

So far, the only way to determine those stresses with an acceptable degree of accuracy has been to carry out in-situ measurements. Yet the stress measurement techniques commonly used in Quebec mines have significant limitations in terms of reliability and precision, which always leave doubts about the validity of the measurement results, thus exposing miners to potential risks that are hard to assess following a local release of stresses.

The stress measurement techniques most commonly used in Quebec mines are an application of the recovery principle, in which strains measured on diamond drill borehole walls following local stress release are converted into stresses by means of reasonably sophisticated calculation models. For all these techniques, the recovered strains are obtained by working out the difference between an initial reading (before the stress release) and a final reading (when the local stresses have been fully released). Until very recently, the situation was the same for the modified doorstopper technique developed at Polytechnique Montréal. However, the two lead researchers have now come up with an innovative method based on the inverse problem approach, whereby the change in the strains at the measurement point during the stress release is estimated by the least squares method, using the entire recovery curve obtained when taking a measurement with a device they developed some 20 years ago.

The method proposed in this study radically changes the way that stress measurements can be interpreted, not only using the modified doorstopper technique, but also with all techniques based on the recovery principle. This change does not involve any additional costs over current approaches.

With these new tools, all Quebec mining companies now have access to equipment and interpretation models that are among the best in the world for the measurement of in-situ stresses. Combining cutting-edge electronic field equipment with a new interpretation model based on all available measurement data leads to a significant improvement in the quality of measurement results, thus making working conditions safer for miners.