Prevention of water and sand inrush during mining of extremely thick coal seams under unconsolidated Cenozoic alluvium

Mining extremely thick coal seams under layer of loose unconsolidated Cenozoic alluvium is common in north China. However, this may cause serious consequences including breaking down workface, submerging equipment, even serious flooding accident and casualties, because there is an alluvium aquifer composed of sand, clay, and gravels directly overlying above coal seams. Formulating preventive and control measures in advance is extremely important to a mine. In this paper, case study of water and sand inrush prevention is taken in Xiexin coal mine. A 15.5-m thick coal seam is threatened by unconsolidated Cenozoic alluvium water hazard during mining. By the research and exploitation of hydrological characteristics and strata lithology of unconsolidated Cenozoic alluvium, the lowermost layer in some areas is the layer of clay with good water resistance, and part of areas is permeable water-bearing sand mixed with gravel belonging to weak aquifer, which provide an advantageous condition to possibly set and optimally design sand-proof and caved-proof pillars. Considering its geological and hydrogeological characteristics, slice long-wall mining technology which extracts thick coal seams from top to bottom by dividing it into two slices is employed. Based on observations carried out in the field and physical model testing in the lab, strata failures induced by slice long-wall mining were investigated. The study indicated that the maximum height of the caved zone for upper slice was about 5.62 times of the upper slice mining height and the comprehensive height of the caved zone for two slices was about 4.71 times of the total mining height of two slices. The height of caved zone derived from physical model testing was consistent with the height obtained from field measurements. According to the hydrogeology of alluvium, overlying strata lithology, and mining conditions, the optimal design of sand-proof pillar at areas with weak water rich aquifer, and caved-proof pillar at areas with the lowermost layer of clay for workface 1702, workface 1704, and workface 1706 were given for safe production. The optimal design and technique measures, such as slice mining and reducing the thickness of extractions within the upper or lower slice, were successfully practiced. Overall, the conclusions are of engineering significance for coal mines suffering from water and sand inrush of unconsolidated Cenozoic alluvium.