Behaviors of strata movement in coal mining

Coal mining involves extracting the resources from the earth. It will disturb the strata in the mining area, which inevitably causes strata movement and the change of stress and fracture fields,thus affect safety and environmen- tal such as the occurrence of ground pressure, groundwater loss and surface subsidence. Therefore, mining-induced strata movement and its influence on safety and environment are the basic scientific problems of coal mining. The un- derstanding of these laws will improve the scientific nature of coal mining. Because of the complexity of strata move- ment,however,there are still several issues to be addressed. This paper introduces the mining-induced strata movement of the block structure,which is followed by the review of advances and outstanding issues in the influence of strata movement on longwall face support,mining-induced fractures evolution,groundwater,surface subsidence,and mining- induced stress field. It is found that mining-induced strata movement is a combination of stress concentration caused by the span of thick and hard strata and the mechanical behaviors of the rock blocks associated with the breakage of the strata. During the collapse of thick strata and the corresponding rock block movement,the characteristics of mutations and discontinuities typically occur. In such a process,a structure that can sustain large deformation may be formed af- ter the rock blocks are hinged. The “R-S” stability of the structure causes significant influence to such phenomena as the occurrence of ground pressure,mining-induced fractures and surface subsidence. Mining-induced strata movement is like a “black box” system. Currently it is just explained in the aspect of control principle,and reaches the level of a “gray box” system. Therefore,strata control refers to “macroscopic” control in many engineering cases. The Voussoir Beam model has established longwall support theory and the estimation principle of support working resistance. In a general case,the support resistance that is determined as the weight of the overburden with a thickness of 4-8 times the mining height could meet the safety requirements of most mining projects. Specially,8 times should be chosen for the longwalls with a large mining height or a thin immediate roof,a shallow cover depth,or the support crush risks. As for some particular conditions,however,support crush cannot be avoided by the single measure of increasing support resistance,but should be combined with other engineering precautionary measures. According to the stability of rock blocks of overburden key strata,three types were proposed to characterize surface subsidence for varied alluvium con- ditions in different areas of China. It is suggested that subsidence type should be considered during the selection of subsidence prediction,control and reclamation methods. The presence of key strata and the corresponding breakage characteristics play a vital role in the distribution of mining-induced stress. At present,the prevention of accidents caused by stress concentration is harder than that of methane accidents. With this in mind,it is suggested that only test-style mines could be established in the areas with tectonic stress concentration or a large depth. This paper points out the direction for the study of strata movement and the corresponding influence on safety and environment in the coal mining industry.