The existence of the overlying compound key strata would cause stress distribution in surrounding rock, breaking of the key strata and the periodic weighting of a mining face, which are also related to variation of the mining height. Based on the compound key strata theory, the simplified mechanical model of the compound key strata before breaking is established for the fully mechanized top coal caving mining of thick coal seam. Through calculation, the computational expressions of equivalent load of the upper strata and the ultimate breaking step of the key strata is obtained. By using the FLAC3D numerical software, the abutment pressure under the compound key strata during fully mechanized caving of thick coal seam is simulated, and the calculated breaking of the compound key strata, periodic weighting, stress concentration area etc. agree with in situ observation.
When there are two or more key layers of hard rock bed in overlying strata whichever upper or lower, the interaction in terms of the compound effect will affect the deformation and breaking behavior during coal mining (Miu et al. 1999). Qian, Miao, Mao etc (Miu et al. 1999, Mao et al. 1999, Sun et al. 2005), defined the basic concept of the compound key strata the compound effect, and simulated the influence of compound effect by discrete element, which provided foundation of discrimination of compound effect. The study (Mao et al. 1999) showed that the ultimate breaking length L of the key strata had an inverse relationship with the mining height b, indicating that mining height had significant influence on the breaking of the key strata, stress distribution of surrounding rock and periodic weighting. Based on the compound key strata theory, the affecting scope of abutment pressure and periodic weighting in the fully mechanized top coal caving mining face of thick coal seam under the compound key strata in Binchang are analyzed by using theoretical calculation and FLAC3D numerical simulation, the result agreed with field test data. This study provides a theoretical and instructive guide to select the support type, and so on.