Underground coal gasification (UCG) has received renewed interest as a potential technology for producing hydrogen at a competitive price particularly in Europe and China. The Lawrence Livermore National Laboratory (LLNL) played a leading role in this field and continues to do so. It conducted UCG field tests in the nineteen-seventies and -eighties resulting in a number of publications culminating in a UCG model published in 1989. LLNL successfully employed the ''Controlled Retraction Injection Point'' (CRIP) method in some of the Rocky Mountain field tests near Hanna, Wyoming. This method, shown schematically in Fig.1, uses a horizontally-drilled lined injection well where the lining can be penetrated at different locations for injection of the O{sub 2}/steam mixture. The cavity in the coal seam therefore gets longer as the injection point is retracted as well as wider due to reaction of the coal wall with the hot gases. Rubble generated from the collapsing wall is an important mechanism studied by Britten and Thorsness.