Pre-excavation in situ stress measurements and rational design of coal mines

Measurements of in situ stress should be conducted early, as part of the pre-excavation design of underground caverns. With the advent of the hydraulic fracturing stress determination technique, measurements can be conducted at great depths in exploratory holes during the site characterization stage. Such was the case at a pumped storage scheme, where hydraulic fracturing was used to estimate the state of stress prior to excavation and before finalizing the underground design. That brought about huge savings, because stress-related modifications of the original layout were carried out well in advance of going underground. Few mine operators have recently begun to follow this route. In one case, we conducted a set of stress measurements as part of the pre-excavation design of a coal mine. Hydraulic fracturing tests carried out in one strategically located borehole revealed that at the depth of the future mine the horizontal principal stress magnitudes were rather high, coupled with a much lower vertical stress. This condition required special attention from the designers with respect to both cavern orientation and geometry in order to insure that the stress concentration around the mine openings was kept at a minimum, and that any needed support and reinforcement was anticipated in advance of excavation.