According to the typical structure of the hanging roof blocks after shallow horizontal mining, taking the fully mechanized mining face with large mining height in Shendong Mining Area of China as engineering background, theoretical analysis and numerical simulation were used to analyze the arching law of the principle stress and boundary distribution of the horizontal stress in the roof block structure. Three typical mechanical models of the roof structure were proposed, such as the symmetrical stress arch of two key blocks, the semi arch of multiple key blocks and the squeezed arch of multiple key blocks. The results indicated that the horizontal stress displayed a nonlinear distribution at the abutments of the symmetrical stress arch and there was a linear stress distribution with higher peak value at mid-span of the arch; the high horizontal stress at the arch abutment was necessary to form the squeezed arch of multiple key blocks; the horizontal stress was relatively less at the arch abutment of the semi arch structure; the main key block was easier to slide in this structure for the boundary horizontal stress displaying the nonlinear distribution. The causes of step subsidence and strong roof weighting of shallow large height mining face were revealed by using the semi arch model. Moreover, the instability of the whole compressive arch of the multilayer bedrock would lead to long periodic roof weighting. The results are of instructive significance for roof weighting forecast and strata control during shallow horizontal mining for thick coal seam.

1. Introduction

The Shendong Mining Area was characterized by large thickness, shallow buried and flat dipping coal seam, which was a typical high efficiency mining district of shallow thick coal mining in China. The large-scale shallow mining for full thickness of coal increased the failure range of the thin bedrock roof, resulting in strong strata behavior, and the bedrock and thick alluvial sand layer being suffered seriously damaged. So it was an important problem of the roof weighting and ground subsidence induced by shallow mining to restrict the development of similar mining in the west of China (L.Q. Ma et al., 2009; D.S. Zhang, et al., 2011).

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