Variation law and estimation model of permeability coefficient in unconsolidated confined aquifer

The coal measure strata in the concealed coalfields in North China are generally covered by the Quaternary loose soils. Many hydrogeological hazards, such as mine water inrush, surface subsidence and wellbore deformation failure, have occurred in mining under unconsolidated confined aquifer, which has caused huge economic losses and casualties. The permeability coefficient is an important parameter that reflects the permeability of soil medium in the unconsolidated confined aquifer. A reasonable estimation of its values has important guiding significance for the prevention and control of such hydrogeological hazards. At present, the permeability coefficient of the unconsolidated confined aquifer in the concealed coalfield in North China is often only determined by the pumping test, and there are few models to estimate the permeability coefficient of unconsolidated confined aquifer based on geological exploration borehole information and data. In the paper, the data of the existing pumping test boreholes in the unconsolidated confined aquifer in the Qidong coalmine of the Huaibei Coalfield was collected and the thickness of confined aquifer, the mud-sand ratio, the thickest sand ratio, the effective stress and the thickness ratio of bottom gravel were selected as influencing indices. The thickness ratio of bottom gravel, the effective stress and the mud-sand ratio were determined as the key influencing indices after analyzing the correlation with the permeability coefficient. Therefore, its influence laws on permeability coefficient are analyzed, and the results show that the permeability coefficient of unconsolidated confined aquifer increases with the thickness ratio of bottom gravel, and decreases with the effective stress and the mud-sand ratio. Then, using multiple linear regression analysis, a permeability coefficient estimation model was proposed based on the thickness ratio of bottom gravel, the effective stress and the mud-sand ratio. The estimation model has been successfully estimated the permeability coefficients of the unconsolidated confined aquifer in Qinan, Zhuxianzhuang and Qingdong coalmines of the Huaibei Coalfield, and the estimated results are consistent with the values of pumping test.