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A two-dimensional crack/cavity thermo-mechanical-elastic loading model for characterizing the response of porous—permeable media is developed. The stress and displacement field expressions for this formulated model, with a general elliptic crack/cavity shape, are derived by using the complex variable approach for twodimensional thermo-elastic problems. The critical loading criteria for crack-closure are expressed in terms of the applied stresses and temperature gradient and the mechanisms for line crack-closure are determined from these conditions. Expressions for the contact normal stress and frictional shear stress are analytically developed by extending McClintock and Walsh's crack-closing concept to thermo-mechanical loads. The stress intensity factors for a partially closed crack are determined in terms of the thermo-mechanical loads from basic fracture mechanics concepts. Analytical expressions for volume change and effective water permeability for cleated media, such as coal, are derived in terms of thermo-mechanical loading changes, as well as stress intensity factors. The effects of stress and temperature gradient on the non-dimensionalized permeability are demonstrated. A two-dimensional thermo-mechanical failure criterion, including the effects of crack-closure, is developed by deriving crack-tip stress magnitudes and applying Griffith's approach. A generalized form of two-dimensional Griffith failure criterion is viii obtained by combining the critical stress Intensity factor concept in fracture mechanics with the two-dimensional thermo-mechanical failure criterion. In addition, the stress intensity factors for a partially closed three-dimensional crack and a three-dimensional thermo-mechanical failure criterion are developed by applying the crack-closure concepts and the methodologies used in the two-dimensional problem. Also, a generalized form of three-dimensional Griffith failure criterion is formulated. The theoretical derivations are applied to several fundamental problems pertaining to underground coal gasification (UCG). These problems include prediction of volume changes and effective water permeabilities of coals under various loading conditions. The effects of pore pressure, temperature gradient and temperature on coal permeabilities are studied. Estimates of tensile strengths, critical permeabilities and critical stress intensity factors for Pittsburgh coal are made from the existing measured uniaxial compressive strengths under elevated uniform temperatures and the derived two-dimensional failure criterion. The similar applications given to the coal are also made to Pittsburgh coal overburden shale. The generalized form of the two-dimensional failure criterion is compared with existing experimental data and empirical equations by means of failure envelopes in terms of stress Intensity factors. Failure envelopes for Pittsburgh coal for combined thermo-mechanical loading are formulated. The thermo-elastic response of UCG linking channel and water influx or gas leakage are also considered.