Accumulative effect of overburden strata expansion induced by stress relief

Strata movement caused by underground coal mining is the primary cause of a series of safety and envi- ronmental problems. Studying the movement law of mining-induced strata provides an important basis for safe,ef- ficient and green mining. Through studying the process of strata movement,this paper reveals the cumulative effect of overburden strata expansion induced by stress relief and its influence on strata movement. The results show that the overburden strata undergoes a dynamic process of stress relief induced expansion and re-compaction. Under the control of the key strata structure,the overburden strata move in groups from bottom to top. Before a key stra- tum break,the transfer of its overburden load to the underlying strata is blocked,resulting in their expansion,including bulking and elastic expansion. As the breaking height of key strata increases,the unloading height of over- burden increases. At the same time,the unloaded coal and rock strata are also compacted by the load of the bro- ken key layer,which results in the dynamic change of the total amount of overburden unloading expansion. The phenomenon that the total amount of overburden unloading expansion changes with the height of unloading over- burden is defined as the cumulative effect of overburden strata expansion induced by stress relief. A theoretical model was then developed and verified with the measurements of the bedding separation aperture under an ex- tremely thick igneous sill in the Haizi Coal Mine,China. The results also show that this cumulative effect has an important impact on the movement law of mining-induced strata,such as the amount of bedding separation under key strata,the height of interconnected fractured zone of key strata,and the surface subsidence coefficient under different mining conditions. The cumulative effect has changed the traditional understanding of bedding separation by resulting into a small bedding separation aperture that is usually less than 10% of the mining height. The vol- ume of the injectable zone in the overburden was found to be not that of the traditional bedding separation,but the injection-induced volume that was transformed from the expansion zone because of the high injection pressure. This finding has guided the innovation of the green mining technology of isolated overburden grout injection and its successful application in mining under surface infrastructures.