The safety of coal seam mining is affected by the evolution of vertical fissure in the overburden strata with ascending extraction of lower coal seam. In this paper, similar material simulation method was used in the experiment to research the overburden rock fissure about the ascending mining at Xichagou coal mine district of Northern Shaanxi. The result shows that: after the mining of the lower 5−1 seam at this coal, overburden presents the typical characteristics of "three zones"; after the sixth periodic weighting, ascending crack through strata, which is located above 3−1 seam at about 60 m, is formed in the process of later periodic weighting with the different pressure degree in the direction of caving angle nearly coal wall on one side of working face, the evolution process is that the crack opens when pressure comes and gradually closes when pressure has gone, the issue which crack through strata connects the surface can affect safety of the coal seam mining and its upper seam; Ascending mining of 3−1 seam working face haven't the sliding instability and bench convergence phenomena in the vertical crack through strata, the working face can finish the continuous advance. The study of ascending safety mining has significant reference to other similar coal mine.
Coal seam group mining usually adopts descending mining method. However, in some historical and specific conditions, it is likely to mine the lower coal seam firstly and mining its upper seam later, which is named ascending mining method. Ascending mining method, which began in the 20 or 30's of last century abroad, was studied in West Virginia and Pennsylvania in the USA, Silesia coalfield in Poland, Donbas coal in the former Soviet Union. By these studies, we accumulated much successful experience (Dunham & Stace 1978, Wang & Li 1995). Domestic practice of ascending mining began in 1970s, the domestic scholars also obtained some research results about ascending mining theories (Yuan 2006, Zhang et al. 2007, Ma et al. 2007). After years of research and practice, for ascending mining, domestic scholars preliminarily think that (Liu 2007): when only one seam is mined and its mining influence factor K > 7.5, the upper seam can be mined normally.