Abstract: Based on the improving the derivation process on the yielding criterion of Drucker-Prager incircle under the condition of plain strain, the direct coefficient of transforming from Drucker-Prager incircle to circum-circle and the suitable application scope of strength increase on the internal friction angle have been deduced. Then based on the strength increase FEM, the limit analysis method of discrete medium has been put forward. The FEM model of the fully mechanized caving mine formed by coal seam and overlying strata has been built up. By the over-stratum and coal strength increasing, the model reaches its critical state in the roof and overlying strata under the periodic weighting. And the roof caving surface shape and the fissure distribution have been obtained automatically. The result of caving and fissure height by strength increase FEM has been confirmed by empirical method, resulting from which, a new method to analyze the features of discrete media of critical state has been provided.

Key words: strength increase FEM, yielding criterion matching with Mohr-Coulomb, fully mechanized longwall top coal caving, roof caving surface, fissure distribution