Abstract: The key stratum theory widely applied to coal mining area has provided a significant theoretical foundation for controlling the strata and reasonably explained the strata behaviours after excavating coal seam. The theory contends that hinge structure formed after roof failure and instability controls the movement behaviour of overburden strata, but it ignores the influence of shear strength between strata on the movement. Based on the hypothesis of interlaminar shear stress, this paper establishes the mechanical model of composite beam in overlying strata, and then derives the theoretical solution of shear stress of nonhomogeneous beam. Meanwhile, a key stratum decision step is proposed by considering the inter-layer shear stress, and the equation for calculating the breaking interval of the key stratum is corrected. The results show that overlying strata controlled by key stratum probably does not fracture with key stratum at the same time. According to the double-layer rock mining model, we propose a concept of flexible control layer (FCS) which can hinder the deformation of one or more overlying strata above it after the key stratum broke. Additionally, the discrimination method for FCS is further deduced, and the effects of FCS on the composite fracture and strata behaviours are discussed. The existence of FCS weakenes periodic roof weighting to some extent and generates a secondary weighting. Finally, the rationality of this hypothesis is proved with the examples in practical engineering.

Key words: sedimentary of coal series, key stratum, flexible control stratum (FCS)