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This article presents an integrated approach for theoretical analysis and numerical modeling to investigate the mechanism of roof presplitting and its reasonable parameters in the hard roof and longwall top-coal caving face. The test site is located in the city of Jinzhong, Shanxi Province, China. The theoretical analysis results indicated that when the presplitting angle remains constant, the tensile stress at the unpenetrated surface is approximately exponentially distributed with the presplitting height. When the presplitting height remains constant, the variation curve of tensile stress at the unpenetrated surface with the presplitting angle is parabolic. As the presplitting height increases, the presplitting angle required for the tensile stress at the unpenetrated face to exceed the ultimate tensile strength of limestone first increases and then decreases. The ground response of retained entry obtained with numerical modeling confirmed the theoretical analysis results. Meanwhile, numerical modeling results also showed that presplitting height affects the volume of the collapse gangues and the structure of the overlying strata above the retained entry. The presplitting angle is effective in reducing the frictional resistance between the collapse gangue and roof structure of retained entry. Moreover, based on the ground response of the retained entries, the reasonable parameters of the roof presplitting were determined at 17 m and 15°. The field application showed that reasonable roof presplitting parameters could achieve satisfactory effects at pressure relief and retained entry in the longwall top-coal caving panel. Furthermore, the proposed design principle of the roof presplitting technique can be applied to other projects. [ABSTRACT FROM AUTHOR]

Cemented paste backfill (CPB) is an emerging mine backfill technique that allows environmentally hazardous tailings to return to the underground openings or stopes, thereby maximising the safety, efficiency and productivity of operation. Uniaxial compressive strength (UCS) is one of the most commonly used parameters for evaluating the mechanical performance of CPB; the prediction of the UCS of CPB structures from early to advanced ages is of great practical importance. This study aims to investigate the predictability of the UCS of CPB during the hydration process based on electrical resistivity (ER) measurement. For this purpose, the samples prepared at different cement-to-tailing ratios and solid contents were subjected to the ER test during the whole hydration process and UCS tests at 3, 7, 28 days of curing periods. The effect of cement-to-tailing ratio and solid content on the ER and UCS of CPB samples was obtained; the UCS values were correlated with the corresponding ER data. Microstructural analysis was also performed on CPB samples to understand the effect of microstructure on the ER data. The result shows that the ER of CPB decreases first and then increases with the speed which is faster in the previous part than the latter. The ER and UCS of CPB samples increased with increasing cement-to-tailing ratio and solid content and curing periods. A logarithmic relationship is established for each mixture in order to predict the UCS of CPB based on ER. Scanning electron microscope analyses have revealed that the microstructure of the CPB changes with the age from the initial floc to honeycomb, and eventually to the compact clumps. The ER properties of CPB samples were highly associated with their respective microstructural properties. The major output of this study is that ER test is effectively capable for a preliminary prediction of the UCS of CPB. [ABSTRACT FROM AUTHOR]