The state of periodic loading and the interval of periodic roof weighting have an important role in geomechanical stability, and hence in the continuity of longwall mining operations. In this paper, the mechanism of roof caving in longwall mining -together with the effect of engineering and geomechanical properties of surrounding rock masses on the magnitude and timing of periodic loading -is studied. For this purpose, a longwall mine is first modeled using Phase2 software, and then, by simulating the roof caving process, the periodic roof weighting intervals are calculated. Sensitivity analysis is then applied in order to determine the effect of changes in the mechanical properties of the rock mass, especially in the main roof and immediate roof. Results show that the applied algorithm in this study can effectively calculate the periodic roof weighting interval in the longwall mining method. Also, due to the flexible nature of numerical modeling, a reliable application of this approach can be envisaged in most mining conditions.

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Nowadays along with population growth, industry development, consumption of mineral resources and the fact that the reserves on hand are running out, the depth of surface and underground mines for further exploitation are increasing. The block caving method for low-grade and large-scale deposits has shown a growing rate of application. The dimensions of blocks are one of the most important parameters which should be taken into account since it has been proved to have a great deal of effect on technical issues such as commencement of caving and mine design. In this study, firstly for the purpose of facilitation, some assumptions were considered and having used these assumptions for estimation of optimized length and width of block, a relationship based on rock mechanics and physics was explored and finally, it was transformed into an inequality. Solving this inequality provides us with the optimized length and width of the block. The explored relationship was analyzed and the graphs thereof were drawn. The analysis showed that mineral thickness and density had the least effect. The impact of hydraulic radius on caving showed that square blocks are best suited for caving. While using square blocks, the relationship becomes a quadratic inequality with one unknown which is easy to be solved. Proved with a slight decrease in the safety factor of about 5%, we can increase the block length about 2 times while safety remains unchanged. Considering the capacity of transportation systems and the production of the interfaces, presented that the block length is measured in terms of production. Regarding to provide relationship for the production and manufacture of the blocks and parameters. Gangue Density and Distance have the most negative impact on the block length and fallowed by the Safety Factor and Width/Length (Y/X) and Depth respectively. Angle of Internal Friction and Ore Density and Work Hours have the positive impact on the block length. Ore Hight and Ore Density and Job Efficiency have the least impact on the block length. In 90% of cases, the block length is between 32.9 m and 91.3 m. It is clear from the graph that approximately 60 m for block length has the most Probability Density which is more than 0.025. In terms of transportation, in 90% of cases, the block length is between 85.76 m and 109.24 m and approximately 97 m for block length has the most Probability Density which is more than 0.057. In the simulation by Phase2 software, the results compared with the different modes of the block. In blocks of 55, 60 and 65 m, Total Displacement, Yielded Elements (percent) and Yielded joints reached to a satisfactory condition and perfect caving will occur. In block 70m, these values reached to maximum. As the purpose of this paper is obtaining the optimal block length Thus, blocks which are between 55 and 65 meter, can represent the optimal size.

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