Occurrence and stress setting in steeply inclined and thick coal seams are totally distinct from the ones in gently inclined coal seams. The difference would result in different roof structures when the roof is unstable. Furthermore, it would lead to inappropriate selection of supports without correct understanding of dynamic evolution of roof structure. This paper adopted comprehensive methods including theoretical analysis, 3DEC numerical simulation, field data comparisons and analysis, and aimed at the appropriate selection of supports and safe mining of Urumchi coal field. The results indicated that parameters including length, position of crest section of the asymmetric flat-topped arch structure(AFAS) and its height kept decreasing with increment of coal seam angle, showing extremely asymmetrical characteristic. simultaneously, the total weight of caving coal and rock mass and the average loading of supports were reducing obviously, while the reduced rate tended to be slow. As a result, the safe mining was improved dramatically and it is very crucial to do intense research and master the dynamic evolution.
The appropriate selection of supports has always been a vital ingredient for safe mining (Liu Changyou, 2015, Liu Jinhai, 2012, Zhang Dongsheng, 2013, Wang Jiachen, 2009), while the empirical method that is employed by most coal mines proves to be unreasonable. Unlike the result that is inferred by empirical method, the field monitoring data of ground pressure suggests that the working resistance of supports do not increase obviously with increment of mining depth. For example, both the mean value of measured maximum working resistance and time-weighted average resistance of working face of different depth in Wudong coal mine are less than 8000KN/support. Data and examples indicate that the supports are protected by a temporary roof structure. Many researchers have successively tried to describe the roof structure and determine the average loading of supports with various methods (Lai Xingping, 2010 and 2013, Shao Xiaoping, 2006, 2008 and 2009, Yang Fan, 2006). Shao Xiaoping (2007) proves that the roof structure would change with coal seam angle and supports only bear the weight within roof structure. Niu Shaoqing (2014) believes that roof instability is caused by relative sliding at roof sliding interface and provides support method. In addition, a fitting logarithm function between safety factor of roof and thickness to span ratio and span of roof with strength reduction method has been achieved by Zhao Yanlin (2010). Surely, all of these researches have made great contribution to the study of roof structure. However, seldom study have focused on the dynamic evolution of roof structure. To solve the above mentioned problem, this paper adopted comprehensive methods including theoretical analysis, 3DEC numerical simulation, field data comparisons and analysis. And the results indicated that there do exist a AFAS in the roof of working face. It is precisely the AFAS that allow supports only to bear the weight of caving coal and rock mass within AFAS. It is necessary for appropriate support selection to master the dynamic evolution of roof structure.
