Abstract: Theoretical analysis and field measurement were adopted to analyze the hard and thick roof breaking characteristics under the different support boundary conditions in Shigetai coal mine. Steps of determining the roof broken location and size include: 1) the rock stress expressions was solved according to the different roof support boundary; 2) determining the broken and dangerous size according to the characteristics of the stress distribution between the general long and deep beam structure; 3) adjusting the broken and dangerous size and obtaining the distribution of the maximum principal stress and shear stress in the hard and thick roof strata; 4) selecting and determining the safe limit size of the hard roof when the maximum tensile stress or shear stress values close to the limited strength. Results show that the hard roof mainly presented shearing characteristics in Shigetai coal mine, and the first and periodic weighting length equals 35.0 m and 9.8 m respectively.

Key words: hard and thick roof, safe span length, broken position, periodic weighting