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 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.
Paper in Rock At Great Depth (Proc. ISRM-SPE Int. Symp.), Balkema, 1989, PP. 695-701