Abstract：Abstract：The horizontal section top coal caving in steep seam is different from the gently inclined seam. Following the horizon of the coal seam thickness，the working face is arranged with short length. However，no roof but top coal and rock residues is found above the working face. The study shows that arch structure of spanning strata which parallels to its spanning coal seam，is formed above the working face in mining. The mechanical model of arch of spanning strata is established；and the inner forces calculation of arch，establishment of maximal arch span and arch shape are analyzed. The instability of arch of spanning strata can be cataloged as sliding instability and structure instability. The influence of instability on strata behavior of working face is analyzed and tested by field experiments. It shows that the analytical results of the proposed mechanical model agree well with those of in-situ tests measurement. The results indicate that the height of horizontal subsection can be increased. It is much meaningful to effectively control strata and to realize high yield and high efficiency for mining in the condition of short working face in steep seam.
