Mechanical properties of Blair dolomite

Pressure-volume, uniaxial stress loading to failure, uniaxial strain, and acoustic velocity determinations were made on Blair dolomite at confining pressures ranging to 3.5 GPa (Pa = Paschals where 10/sup 5/ Pa = 1 bar or 0.1 GPa = 1 kbar). The bulk modulus K, rapidly rises from an initial 10.4 GPa (at atmospheric pressure) to 102.0 GPa at 1 GPa pressure. At higher pressures, K remains essentially constant (110 GPa). The maximum volume change on loading is 3.9% at 3.5 GPa; the unloading closely follows the loading path. Comparison of uninxial stress tests in compression to 0.7 GPa and extension to 2.1 GPa confining pressure demonstrates that the characteristic shear stress at failure as well as the transition from brittle fracture to ductile flow is strongly dependent upon both the value of the intermediate principal stress sigma /sub 2/ and the rate of strain. The onset of dilatancy as determined in uniaxial compression occurs at about two-thirds of the failure stress. The uniaxial strain loading path is well below the failure envelope in compression. In uniaxial stress loading (compression), Young' s modulus (E) and shear modulus ( mu ) are demonstrated to be very sensitive to both confining pressure and to the level of shear stress. For example, at pressures of 0.1 MPa to 0.5 GPa, both E and mu first increase up to shear stresses of 0.05 to 0.15 GPa and then decrease at all higher stress values. These moduli are shown to be very sensitive indicators of the onset of dilatancy. Elastic moduli as derived from acoustic velocity measurements also increase with confining pressure (to 1 GPa), with the major change occurring below 0.1 GPa. All of the observations made at nonhydrostatic conditions are consistent with the closure of preexisting cracks at low pressures and low shear stresses followed by an increasing rate of crack growth as stress is increased, even at the higher corfining pressures. However, some cracks, which would normally close with hydrostatic pre ssure, remain open under uniaxial stress loading at similar mean pressures. (auth)