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This paper presents an investigation on the interactions of overburden failure zones induced by the mining of adjacent coal seams using the longwall caving method. Overburden failure is an important factor in safety assessments in the fields of mining engineering geology and safety geology, especially when mining under water bodies. In this study, the influence of the thickness and properties of the interburden between seams on the development and interactions of caving and fractured water flow zones are investigated by using in situ measurements, scale model testing and numerical simulations. The height of the fractured water flow zones in the scale model tests and numerical simulations are basically in good agreement with measurements after mining of the upper and lower seams of Seam No. 3 in the Cuizhuang Coalmine. Therefore, the scale tests and numerical simulations in the study are verified. The results show that interaction and superposition between two close distance seams cannot be ignored when the ratio (h/M) of the interburden thickness (h) to the cutting height of the lower seam (M) is less than a defined critical value. A dividing line, Line D, has been proposed to judge whether the interactions exist. When the (M, h/M) points are located above Line D, the caving zone induced by excavation of the lower seam will not propagate to the caving zone induced by the upper seam. Otherwise, for the (M, h/M) points below Line D, the interactions and superposition of the overburden failure must be considered when predicting the heights of the caving and fractured water flow zones.

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This paper is concerned with the evaluation of permeability of fractured rock mass due to the cover stress re-establishment, which is a major factor in controlling water and gas flow rate induced by mining operations in fractured rock. The case study considered in this paper is based on the results of observations of groundwater inrush and a decrease in water inflow from the fractured roof strata due to mining advancing in the Taiping Coalmine, Shandong Province, China. A conceptual model of an effective porous media was used to assess the permeability distribution in the fractured zone induced by coal mining. The cover stress re-establishment in gob fractured rock mass was evaluated using an empirical formula based on the surface subsidence. A simplified conceptual model of the fractured zone was used to evaluate the deformation of fractured zone along with the evaluation of changes in the rock permeability above the gob due to the cover stress reestablishment. These data were then used to calculate the water inflow rate into the panel. Predicted water inflow rates have been found to be in good agreement with those from monitoring data. This study improved the understanding of the mechanisms of the post-mining cover stress reestablishment on permeability change of the overburden fracture rock strata. These results can then be applied for numerical simulations of the process of overburden failure and consequent groundwater inrush due to coal mining.

This paper presents an investigation on the effects of intermittent cut-and-fill mining on the overburden failure under sand aquifers. In the intermittent cut-andfilling method, the panel is divided into long pillars, and then entries in every long pillar into a narrow strip; every narrow strip is then cut and backfilled. Field measurements, scale model tests and numerical simulations are used to compare overburden failure and subsidence due to three mining methods: intermittent cut-and-fill mining, longwall excavation and continuous filling. These methods were applied in a case study of the Taiping Coal Mine, in Shandong, China. The results of the numerical simulation and scale model testing reveal that the height of the maximum caving and water-conducting fractured zones, and the maximum subsidence due to intermittent cut-and-fill mining are much smaller than those with the use of longwall excavation and continuous filling and well verified by the field measurements. Additionally, the numerical simulation results of the stress distribution and variation during intermittent cut-and-fill mining are in close agreement with those of the field measurements. It has been proved that the intermittent cut-and-fill mining can successfully mitigate the overburden failure and subsidence, which contribute to preventing water and sand inrush into coal mines under sand aquifers. This provides a less costly and more effective approach to address surface subsidence and mine caving problems.