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.

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-and-filling 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.