Geomechanical parameters of in situ rock masses, including rock stress, deformation modulus, Poisson's ratio, and strength have been measured or determined simultaneously utilizing a combination of three U.S. Bureau of Mines hydraulic borehole pressure cells installed in one hole. Two case studies were conducted at a western U.S. coal mine in two deep-seated and adjacent seams under high horizontal stresses. Measured vertical rock stresses generally agree with the theoretical values derived from the respective overburden loads. Deformation modulus is highly stress dependent, and the plot of transient deformation modulus versus normalized axial stress increment is a parabolic function of the form y=ax2, up to the yield point. Poisson's ratio decreases during the initial loading period, then stabilizes with small fluctuations until failure. Coal strength increases in accordance with increasing load and confinement even beyond the yield point and up to ultimate failure. Failure strength under highly confined conditions is substantially higher than the unconfined strength. The failure strength is a function of the intermediate confining stress, as well as of the minimum confining stress.
Knowledge of the geomechanical parameters of rock masses is needed for both design and stability assessment of the underground excavation. These parameters include, among others, the state of stress and the strength and deformability of the rock mass. Strength is of particular importance if underground excavations are located at great depth or exposed to high confining stress fields for which the failure mechanisms are stress-induced. Except in a few instances, (Mogi, 1971, 1972; Amadei and Robison, 1986; Lu, 1988) the effects of general triaxial stress states on the in situ strength of rock masses have not been extensively studied because of the complexities and difficulties in determining general or true triaxial strength. In order to assess the stress-induced instability of an underground structure, the rock mass strength must be determined under in situ loading conditions or, if determined in the laboratory, under simulated in situ conditions. This procedure was suggested by Lu (1986a) to assess the three-dimensional stability of mine pillars with the "integrity factor" model. For rock stress determination, four methods, "Flat Jack," "Hydraulic Fracturing," "USBM Deformation Gage," and "CSIR or CSIRO Cell", have been recently suggested by the ISRM Commission of Testing Methods (Kim and Franklin, Joint Coordinators, 1987). However, besides the fact that they are indirect stress measuring methods, their field operations are tedious and expensive. For rock deformability determination alone, the ISRM Commission on Testing Methods have also suggested two methods: "Large Flat Jack" technique (Loureiro-Pinto, J., Coordinator, 1986) and "Flexible Dilatometer" technique (Ladanyi, B., Coordinator, 1987). During the last several years, the U.S. Bureau of Mines has developed techniques for the geomechanical characterization of in situ rock masses using hydraulic borehole pressure cells (Lu, 1981, 1986b, 1987). Lu (1988) proposed that comprehensive geomechanical characterization of a rock mass can be made using a single installation of a combination of three USBM hydraulic borehole pressure cells installed in one borehole (Fig. 1).
