Mechanical performance evolution and size determination of strip coal pillars with an account of thermo-mechanical coupling in underground coal gasification

Abstract

In this study, underground coal gasification (UCG) technology with the strip mining method was recommended to avoid the instability of surrounding rock in the UCG cavity and possible groundwater pollution, in which mechanical performance and reasonable size of the strip pillar play the crucial role. The respective innovative calculation method considering the thermo-mechanical coupling effect was proposed and applied to size designs of pillars in the UCG projects of the Shanjiaoshu Mine and Mazino coal deposit. The proposed method feasibility was experimentally validated and yielded the following predictions of the coal pillar mechanical performance evolution during the UCG process. Heat-affected zones in the coal pillar and immediate roof gradually increased with the following decline. High temperatures above 250 °C affected the coal and rock masses only at a distance not exceeding 2 m from the surface, but their effect continued even after the UCG termination. High temperatures reduced the load-bearing capacity in the area near the coal pillar surface and induced long-term thermal stresses in coal pillars, which alternatively played a dominant role in affecting the ultimate strength of the coal pillar at different stages. The plastic zone width xp in the pillar is varied and will remain unchanged once it reaches its peak value. The width of strip pillars Wp is greatly influenced by high temperatures; it exhibits a W-shaped variation pattern during the UCG process and slowly drops after gasification. The strip pillar width of UCG can be determined using the maximum value of function Wp(t) considering a certain safety factor. While the wide pillar's total strength and size are slightly affected by the temperature, its width can be obtained from Wp(0) with a safety factor. This study can be used for predicting the mechanical performance evolution and size determination of pillars in the UCG process.