"Geotechnical and lithological characterization of the unaltered and altered overburden rock from the Hanna IV Underground Coal Gasification (UCG) site shows that the uniaxial compressive strength (qu) of the rocks has a positive linear correlation with percent calcite cement (r = .60). An inverse, non-linear relationship exists between uniaxial compressive strength and porosity (r2 = .60) and a positive non-linear relationship exists between uniaxial compressive strength and compressive wave velocities (r 2= .71). Comparison of these data with previous work on overburden from the Rawlins, North Knobs UCG site (Burns, et a1., 1983) shows that these relationships are essentially the same [or both sites. Correlations using data from the North Knobs site the somewhat better than those for Hanna IV. Analyses of the overburden rock units directly overlying the cavity show that roof rock strength , lateral and vertical homogeneity, thickness of the overburden, well spacing and span of the unsupported roof rock control the collapse of overburden into the reactor cavity . The Hanna II, Hanna IV and North Knobs VCG sites, which experienced little overburden collapses, are characterized by roof rocks with abundant calcite cement, low porosity high uniaxial compressive strength and an overburden thickness to maximum span of unsupported roof rock ratio greater than one. The Hoe Creek III site, which experienced overburden collapse to the ground surface , is characterized by a laterally discontinuous roof rock unit with high porosity, little calcite carnet and an overburden thickness to maximum span of unsupported roof rock ratio of less than one. The distribution of altered rocks in both cavities at the Hanna IV VCC site is, from bottom to top, altered coal (char) , paralava , paralava breccia and buchite, and hornfels . The highest temperatures (1200""-1500oC), occurred in the lower half of the cavity where char and para lava rock are found ."