Abstract: Based on the derived equations of overlying strata subsidence and infinitesimal surface stretching, two-dimensional and nonhomogeneous distribution model of delamination fracture rate, broken fissures rate and total fracture rate of the overlying strata in the combustion space area of underground coal fire is constructed. And through a case study, it is found that the fracture field of overlying strata is the vertical discontinuous, anisotropy and heterogeneous porous media composed of delamination fracture field and broken fissures field. Some conclusions are drawn as follows: (1) The fissure rate in the surrounding of overlying strata is large; and the internal region is small. (2) With the decrease of depth of overlying strata, fissures growth extent reduces gradually. The maximum value of delamination fissure rate successively reduced from 0.36 to 0.20, 0.14, 0.12; and the maximum value of broken fissure rate successively reduced from 0.32 to 0.11, 0.05, 0.02. (3) There is a mutational point of the vertical variation of fissure rate in the central area of overlying strata. (4) The distribution of fissure rate in the surrounding of overlying strata is led by broken fissures, but the internal region is led by delamination fissures. Finally, an indirect measurement method of overlying strata fissures has been proposed on the basis of the infrared thermal image analysis technology. And the high reliability of the model was confirmed by this method.

Key words: underground coal fire, delamination fracture, broken fissures, fissure rate, distribution model