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.
Coal mining in the Midwest and Northeastern United States has repeatedly been plagued by difficult ground conditions and roof falls. It was only in the last decade or so that the relationship between in situ stresses and mine opening instability has become widely accepted. Despite this recognition, mine layouts continue to be designed with little regard to the prevailing state of stress. The net result in high horizontal stress areas of the United States, such as the states of Pennsylvania, Kentucky, West Virginia, and Illinois, has been that some significant failures have occurred, many apparently related to the effect of in situ stress on the ill-designed mines. Mark et al (1998) report that In 1995 alone two longwall coal mines were forced to close because of high horizontal stresses and panels excavated without concern for their magnitudes and orientations. Once roof stability problems, especially at intersections, are encountered and suspected to be related to the local stress conditions, stress estimates are sometimes made based on ad-hoc and rather questionable equations (Mark & Mucho 1994). Actual stress measurements are rarely conducted, and When they are, mine openings have already been excavated, following which costly remedial work is often necessitated.
The U.S. National Institute for Occupation Safety and Health (NIOSH) has conducted extensive research that demonstrated the hazards of not incorporating the state of stress in the design of coal mines in Northeastern U.S. (Mark et al 1998). The authors conclude that the direction of the maximum horizontal stress in that area of the country is typically NE to E, but the magnitudes cannot be locally quantified a priori.
