Evaluation method of resistance to seepage failure due to mining near unconsolidated aquifers I: critical hydraulic gradient

The evaluation on resistance to seepage failure is an important fundamental work for the prevention and control of water-sand mixture inrush during coal mining near the unconsolidated aquifers. However, no normative methods and standards have been developed at present. The key to evaluate the resistance to seepage failure of unconsolidated aquifers and weathered zones is to determine their hydraulic gradient and the critical hydraulic gradient. Herein, this study reviewed the determination method of critical hydraulic gradient for soil flow and internal erosion, and pointed out the different characteristics between the critical hydraulic gradient for water-sand mixture inrush near unconsolidated aquifers induced by coal mining and the traditional critical hydraulic gradient. Besides, the calculation formula of the critical hydraulic gradient of resistance to seepage failure of the loose sand layers, clay layers and the weathered zones of bedrock were obtained for the concentrated channel, with consideration to the outlet size of the free-face and the physical and mechanical properties of the rock and soil layers. On this basis, this study verified the applicability of the critical hydraulic gradient formula for water-sand mixture inrush by the experimental results, and analyzed the sensitivity of single-and multiple-factor parameters to the critical hydraulic gradient. The analysis results show that the fracture aperture is the most important factor affecting the critical hydraulic gradient, and controlling the overburden failure induced by mining is a key measure to reduce the disaster of water-sand mixture inrush in actual engineering. Moreover, the critical hydraulic gradient of the coal-measure weathered zone is closely related to the content of clay components and the applied load. Definitely, the critical hydraulic gradient is ranged from 2.9 to 67.2 when the proportion (by mass) of clay in the weathered rock is 5%‒40%. Thus, a fitting estimation relationship using Gaussian function and the estimation formula considering the overlying load were given. Generally, the results will provide a reference for the calculation of critical hydraulic gradient for the evaluation of water-sand mixture inrush due to coal mining near the unconsolidated aquifers.