EVALUATION METHOD OF ANTI-SEEPAGE FAILURE DUE TO MINING NEAR UNCONSOLIDATED LAYERS Ⅱ: HYDRAULIC GRADIENT ANALYTICAL METHOD

The calculation of the actual hydraulic gradient in the overlying water-bearing sand layer of the roof aquifer during the mining process has always been one of the key bases in the judgment of roof sand inrush. However,so far,it has mainly used the expression of steady flow hydraulic gradient under constant rate. In fact,in the process of sand inrush,the hydrodynamic conditions and water inflow in the roof aquifer are unsteady. In this paper,based on the unsteady flow,we derived the analytical formula of hydraulic gradient J under the two-dimensional unconfined water and confined water single well fixed-drop model,and also analyzed the evolution characteristics of J with time and distance. The results show that for unconfined water,J decreases with the increase of r,and the rate of decline decreases with the increase of r. There is an inflection point r′,when r<r′,J decreases with time,when r>r′,J increases with time; with the increase of H₀,J increases. This paper obtained J under the unsteady flow of unconfined water is larger than that under the steady flow of unconfined water. For confined water,J decreases with the increase of r,and the decrease rate decreases with the increase of r. Different from unconfined water,at different r,J decreases with time,and there is no inflection point; with the increase of H₀,J increases. This paper obtained J under unsteady flow of confined water is not always larger than that under steady flow of confined water. Finally,this method was applied in the evaluation of water-sand mixture inrush assessment for a coal mine in the Yili River Valley,Xinjiang,China.