"A research program has been carried out to facilitate incorporation of rubble formation and burnout phenomena in mathematical modeling of the production stage of underground coal gasification (UCG). Forward combustion in single channels in coal has been investigated by a computer model. The state of the coal ranges from virgin coal to completely charred coal. Usefully rapid regression of a UCG cavity wall is brought about by a sequence of roof or sidewall collapses. An extensive theoretical analysis of collapse mechanisms has been performed. Failure mechanisms include failure due to strength loss, formation of cracks which intersect bedding planes, and moisture induced stresses. These competing mechanisms are unified in a comprehensive model. Cavity growth by permeate flow of hot reactive gases from the cavity through the natural fissure pattern in the coal has been analyzed. It is shown that this mechanism is too slow to be of practical significance unless the fissures are much more open than would be anticipated in realistic seams. A cavity wall regression model has been developed and coded for application in a computer program for cavity system performance. This model synthesizes the comprehensive roof and wall collapse model with information describing thermal attack on the cavity wall and roof, obtained from a rubble consumption model."