Abstract: To explore kinds of similar materials and the materials proportion that suitable for soft rock roof employed in underground coal gasification(UCG) model test, mechanical parameters of soft roof taken form field are obtained by rock mechanics property experiment. Based on similarity principle , river sand are selected as aggregate, with clay and gypsum for cement. Some different ratios of materials are made by means of orthogonal experiment and comprehensive experiment, and the uniaxial strength of similar materials are used to make compare with the uniaxial strength processed by similarity principle. The results show that the gypsum can not serve as a cement for soft rock roof used for UCG model experiment, as the gypsum has a significant influence on the strength of similar materials, and is prone to pulverize under high temperature. While river sand and clay are selected as raw materials for making similar materials, and the uniaxial strength of similar materials made at the river sand/clay ratio of 3:1 is consistent with the uniaxial strength by calculated according to similarity principle, and meet the mechanical requirements of soft roof used for the UCG model experiment.

Key words: underground coal gasification, model experiment, roof simulation, similar materials, orthogonal experiment