Structural Characteristics of the Overlying Strata in a Fully Mechanized Longwall Face with the Low-Position Thick-Hard Roof

Strata activity is enhanced in the condition of the thick seam and large mining height, which increases the difficulties in roof management. A detailed understanding of roof structure characteristics and its movement law is a key to control the strata behavior. Since the existing formula hardly applies to some specific cases of geological conditions, such as the condition of a fully-mechanized longwall face with the low-position thick-hard roof in Suancigou Mine, a new formula of distinguishing structural characteristics of key strata based on a mechanical model of rock beam in periodic motion was proposed. Considering the relationship between the allowed rotating space of key strata and its limited rotation, the judgment criterion whether a cantilever beam can be formed was established. Also, a method for calculating the reasonable working resistance of hydraulic supports under these special roof structure was deduced from the angle of preventing rotating instability of the main roof. A field application was performed at 6-105-2 working face, and the proposed judgment criterion indicated that the LPTHR would break in the form of a voussoir beam structure at the Z1 drilling area, while at Z2 and Z3 drilling area, the broken LPTHR would behave as the cantilever structures. Field monitoring results showed that the limit-rotation-based cantilever structure criterion could accurately discriminate the structural characteristics of LPTHR. According to the calculation formula of support resistance of hydraulic support, the reasonable working resistance of hydraulic supports in 6-105-2 working face was determined to be 16,245 kN.