Research conducted by the National Institute for Occupational Safety and Health (NIOSH) has shown that knowledge of the direction and magnitude of the in situ horizontal stress can be critical to the success of many underground mining operations. One techniquemployed by NIOSH for determining in situ stress has been hydraulic fracturing from boreholes drilled in mine. However, in some cases the combined rock strength and stress magnitude have been too high to allow successfid stress determination using available hydraulic fracturing equipment. NIOSH developed high pressure straddle and impression packers capable of operating at pressures up to 103 MPa in a 51 mm diameter borehole. The high pressure hydraulic fracturing system was used to measure horizontal and vertical stresses in a western Pennsylvania limestone mine with a high incidence of stress related roof failures. Breakdown pressures ranged from 43.9 to 95.7 MPa and in two cases the rock could not be broken down even at the maximum system pressure, 103 MPa. The estimated ranges for the maximum horizontal principal stresses were 27.2 to 65.9 MPa. The measured vertical stress was 3.8 MPa. The calculated K ratio was betweeh 7 and 17. The stress directions, determined from the crack directions in roof and floor holes, were N76øE and N66øE, respectively. Because of the high breakdown pressures required to initiate the fractures, neither of the horizontal stress measurements could have been made with any other existing hydraulic fracturing equipment. The author believes the packers represent a significant improvement in the state of the art of in situ stress measurement under near mine conditions.
Research conducted by the National Institute for Occupational Safety and Health (NIOSH) over the last decade has shown that horizontal stress can have a significant effect upon the success or failure of underground mines (Molinda et al. 1992, Mark & Mucho 1994). Recent studies by NIOSH have documented cases in both coal (Dolinar et al. 2000) and limestone (Iarmacchione et al. 1998) mines where understanding and mitigating the effects of horizontal stress have been critical to the safety and continued operation of mines. Research (Agapito et al. 1980, Mark & Mucho 1994) has also shown that horizontal stresses in the range of 10 to 30 MPa, at mining depths from a few tens of meters to well below 1000 m, once characterized as "high" are actually a predictable consequence of plate tectonics (Zoback & Zoback, 1989).
An important part of NIOSH's recent research efforts has been the development of methods to identify the conditions under which horizontal stress adversely affects mining operations, and making the mining industry aware of both the effects of horizontal stress and the means of mitigating these effects. NIOSH and others have used several techniques to measure horizontal stress and assess its potential for causing ground control problems, including the "stress mapping" technique (Mucho & Mark 1994), direct measurement of strain, generally through overcoring and the technique of interest in this paper, the hydraulic fracturing method. This technique is sufficiently well documented to be standardized under ASTM Designation D 4645-87.
Although ASTM D 4645-87 is applicable to any depth, the standard is primarily concerned with stress measurements conducted in vertical boreholes drilled from the surface, rather than from underground mine sites, from which most of NIOSH's stress testing has been conducted. Most of the equipment used for conducting hydraulic fracturing stress measurements has been developed as an outgrowth of the oilfield technique of hydraulic fracturing to increase oil and gas production and is of