Numerical simulation of longwall top-coal caving with extra-thick and hard coal seam based on bonded particle model

Based on the mining conditions of longwall top-coal caving with super-large mining height and hard coal seam in Yushen mining area,this paper analyzed the difference between the actual situation of top coal caving and that of numerical simulation using unbonded particle model. By comparing the mechanical properties of bonded particle model and unbonded particle model,and investigating the applicable conditions of the two models,it was concluded that the bonded particle model is more suitable for the simulation of longwall top coal caving in the hard coal seam. This paper illustrated the process of modeling and simulation with bonded particle model. It was pointed out that the calibration of micro-parameters and macro-mechanical properties of particles should be carried out before modeling. The bonded particle model was used in the rock layer,while the smooth joint model was used to simulate the mechani- cal properties of the structural surface between layers. The simulation of various working conditions of hydraulic roof support,including setting load stage,resistance increasing stage and constant resistance stage,were realized through Fish language and servo control principle. Inverse kinematic was used to renew the pose of the whole hydraulic roof support according to the position of its canopy. Swing of different amplitude of rear canopy was realized through Fish language. The simulation investigation shows that the unstable voussoir beam was formed in the lower part of main roof and the stable voussoir beam was formed in the upper part of main roof,ant the roof weighting distance was about 10-20 m. The fragmentation and cavability of top coal presented double periodicity,which comprises strike periodicity and vertical periodicity. (The strike periodicity is corresponded with periodic caving span,and the top coal can be fully broken during the roof weighting period and the cavability is better than that of the non-weighting period. The vertical periodicity is related to the position of top coal,and the cavability of lower part of top coal is better than that of upper part. ) The overall stability of the coal face is relatively high,and the coal face spalling occurs during the weighting pe- riod,and the hydraulic roof support is in the state of high working resistance. The transient dynamic loose arch struc- ture,small unsteady arch structure and large stable arch structure can be formed during top coal caving with various fragmentation. The arch formed on the rear canopy can be broken by the method of “small arch small swing,large arch large swing” of the rear canopy,which is more efficient,while the arch formed on the caving shield can only be broken by the advance of the hydraulic roof support. Numerical results are consistent with the field test of longwall top coal ca- ving in Jinjitan coal mine,which indicates that the bonded particle model could be used to simulate the top coal caving and strata movement behavior of longwall top coal caving in extra-thick and hard coal seam. This research provides a reference for the numerical modeling of the top coal caving and the structure of overlying strata in hard coal seam with longwall top coal caving,which contributes to the selection of the mechanical model and the implementation of simula- tion process.