Prediction and evaluation study on coal seam roof waterinrush based on multi-type four-double method

Water-rich and conductivity are two key criteria for evaluating the prediction of water inrush from the roof. The method of multi-source information superposition method based on GIS is more advanced, but there are problems such as insufficient data sources,complex evaluation process and over-professional evaluation methods. The difference between thickness of aquifer and height of water conducting fracture is used to evaluate the connectivity between aquifer and stope, but evaluation accuracy is low because of the large extreme values. It is necessary to pre-drain roof water under the conditions of water-bearing aquifer, but there is no quantitative criterion for pre-mining safety evaluation after drainage. In practice,the water inrush conditions of the roof are very different. At present, a set of technical evaluation system suitable for various geological conditions is used to predict the water inrush from the roof. For this reason, the ‘multi-type four-double’evaluation and prediction technology is proposed, which is referred to as the MTFD working method. Firstly, according to spatial combination relationship between the coal seam and the upper aquifers and water-bearing characteristic, the water inrush from the roof can be divided into evaluation types of A, B, C and D , and each type has specific applicable conditions and evaluation criteria, and it forms an integrated evaluation technical route. Secondly, basic data table and worksheet (Double Tables) can be established based on the borehole histogram information, and data sources for evaluation and prediction also can be obtained, which is used to complete the calculation tasks required for the evaluation. Thirdly, the water abundant index and danger index of water inrush(Double Indexes)are introduced and calculated in worksheet through calculation formula. In the meanwhile, contour map of water abundant and of water inrush danger (Double Diagrams) can be obtained correspondingly. Water abundant index is used to show the aquifer of water-bearing, including two kinds of extensive and dependable geoscience information which are the thickness of sandstone and the ratio between the thickness of sand and mud, eliminating the need for complex multi-source information normalization.The water inrush risk index is used to measure the possibility of water diversion from indirect aquifer into stope. Because the range of the two-pole interval is small, it is conducive to improve the accuracy and sensitivity of the evaluation, and at the same time overcome the deficiency of the empirical formula. Finally, the double predictions, namely the prediction of the dewatering volume or the prediction of the water inflow, the amount of the dewatered water predicted by the water-rich index as the main controlling factor, can be used as the quantitative criterion for the pre-harvest safe mining evaluation. The new estimation method of water inflow quantity based on water abundant index can be seen as a useful supplement of traditional evaluation method. In recent years, the MTFD work method has been widely used in Shanghaimiao Mining Area in Inner Mongolia, the Ningxia Wuhu Mining Area, and the Jining Mining Area in Shandong. It has guided the design and construction in 60 drilling holes in coal mining face. The application results show that MTFD method predicted by water abundant index is positively correlated with the measured water discharge, and the deviation rate of water inflow is about 4%. Furthermore, some typical mechanisms of water-sand inrush have been revealed by contour map of water abundant and of water inrush danger.