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1 MA Li-qiang * ZHANG Dong-sheng School of Mineral and Safety Engineering School of Mineral and Safety Engineering China University of Mining shallow coal seam; water protection mining technology; adaptable conditions; fractured zone; protective layer 1Introduction At the same time of safe and efficient mining, how to maximally protect the water resource in the overlying loosen water-bearing sand remains a typical problem in the mining process of the shallow coal seam in China and abroad[1-4]. The Shendong coal field in the west of China is characterized by shallow coal seam with thin bedrock, which makes it possible for the precious water in the overlying loosen water-bearing sand to lose to a large extent during the process of mining. What’s worse, this 1 Support by NECT-05-0480. [5-7]. At present, intermittent longwall mining , that is, fully mechanized continuous shortwall mining is generally adopted to reduce water loss. However, the recovery rate of coal resource is low by this means. On the basis of rational choice of the powered support, Shendong mining field is always trying to realize water protection mining by adopting the of rapid advancing ,combined with sectional measures reducing the sectional mining height or filling, but all these adopted s are not systematic, so the effect is not satisfactory and safe production cannot be assured.[8-9] Therefore, in this paper, the predictive ula of the height of Fractured zones as well as the protective layer for shallow coal seam with thin bedrock were studied and determined, and the overlying water body of loosen water-bearing sand was classified. Based on these, the necessary bedrock thickness of longwall coalface with different mining height for shallow coal seam with thin bedrock was confined. Combined with the present mining status, a set of new s of adaptable conditions classification of water protection mining technology for shallow coal seam with thin bedrock was put forward. 2 The determination of the height of fractured zone At present, the generally adopted ula for the fractured zone height of longwall coalface is as followsb M a M H L ⋅100 or L H cM d H L Height of fractured zone ,m ; M Mining height ,m ; a , b , c , d Corresponding parameters ; However, the above ula was not applicable to the shallow coal seam with thin bedrock. According to the comprehensive consideration of the structural features of Shendong mining field as well as the influence law of hard and soft coal strata on the growth of the fractured zone, the height of fractured zone with different mining height for shallow coal seam with thin bedrock was analyzed and calculated by using the FLUID-MECHANICAL function of the UDEC numerical software, combined with the experiential data generalized from the shallow coal seam mining [10-14]. Table 1 shows partial calculation results. Table 1 fractured zone Height corresponding mining height Mining height /m 2 2.5 3 3.5 4 4.5 5 5.5 Fractured zone Height /m 2935394349545861 Based on the results, the ula of the height of fractured zones for Shendong mining field was achieved 1 6.1029.9M H L In addition, the height of the caving zone might be calculated with reference to “Standards of pillar setting mining under the surface construction, water body, railway and the main shafts entries”. For the sake of safety, it should be regarded as strong cover and the ula is 16 1.2100M M H m 2 3 Determination of the protective layer thickness To assure that water inrush doesn’t occur in the coalface is the primary target of water protection mining. The protective layer lies right above the fractured zone, which can prevent water rushing from the overlying water body of loosen water-bearing sand to the coalface through the fractured zone. The generally adopted mining height for the shallow coal seam is big, so the mining height is usually determined from 3.5m to 5.5m. If water-proof coal pillar is set as stipulated in “Standards of pillar At the same time, under proper conditions, if there exists a clay blanket with the minimal thickness more than 3m to 5m between the fractured zone and the water-bearing sand, the water loss of the overlying water-bearing sand can be effectively prevented, which has been proved both theoretically and practically. As to other water-proof layers, such as mudstone, weathered rock and so on, their thickness can be calculated according to the corresponding water-proof ability with that of the above-mentioned clay blanket[15]. Above the bedrock of Shendong coal field is a belt of weathering, whose main lithology is mudstone, silt mudstone and siltstone. Based on the lithology and its weathering degree for shallow coal seam, combined with theoretical research and industrial test, it is concluded that the water-proof ability of upper bedrock with the thickness of 4m to 10m is identical with that of the clay blanket with the thickness of 3m to 5m. Therefore, the protective layer of longwall coalface for shallow coal seam with thin bedrock is determined as 10m. 4The classification of loosen water-bearing sand There generally exists loosen water-bearing sand above the top bedrocks of shallow coal seam in Shendong coal field. According to the experiences of mining under loosen water-bearing sand of domestic coal mine, the overlying water body of loosen water-bearing sand for shallow coal seam with thin bedrock was divided into five types[16-17], as is shown in table 2. It is advisable to adopt corresponding water protection mining according to different water bodies and certain mining conditions. Table 2 Classification of loosen water bearing layer Water Bearing sandr Degree Thickness /m Rate of flow /L.s-1.m-1 ⅠVery weak30 10 Ⅱstrong 1530 510 ⅢMedium 515 0.15 ⅣWeak 15 0.0050.1 ⅤVery weak15 15 10 10 10Mining shortwall Longwall Mining height/m 2.53 35 5.5 Mining speed/m.d -1 15 15 15 10 Medium water yield nature Measures Sectional measures Mining shortwall Longwall Mining height/m 2.5445.5 5.5 Strong water yield nature Mining speed/m.d -1 15 15 15 5 When the height of the bedrock is between 45m and 60m, and the water yield nature is strong, the longwall mining can be adopted after reducing the mining height. The corresponding mining height might be selected according to table 3, but the productivity is obviously lower and many resources are wasted. Therefore, shortwall or strip mining may be adopted to improve th
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