Abstract：Concrete blocks with fissures of different sizes are adopted to simulate mining fractured rock mass，and the clay，silt，coarse sand and gravel are used to compose 7 kinds of soil samples with different particle size distributions. Then，the types and mechanisms of the seepage deformation and failure of the unconsolidated soil layers across the caving zone and water flowing fractured zone within overburden，and the relationship among the critical percolation gradient of unconsolidated soil layers while seepage failure across mining-induced fissures from above to bellow vertically，soil particle size distributions，physico-mechanical properties and the widths of fissures are studied in laboratory tests by reequipped seepage meters. The results show that the silt，coarse sand and gravel with small amount of clay are easier to quicksand，and pipe and quicksand occurs frequently while d50 is less than 1/10 of the width of fissure. When the critical percolation gradient is larger than 1，the critical percolation gradient of soils with same particle size distributions is in direct proportion to the unit weight and cohesion，in inverse to the liquid index. The wider the fissures are，the smaller the critical percolation gradient for the same soil sample is when seepage failure deformation occurs across fissures，and the critical percolation gradient decreases exponentially with the extension of the width of fissures. Moreover，the relationship among the volume of quicksand and the width of fissure，and the initial water level are also studied in tests，and the results indicate that the volume of quicksand increases linearly with the width of sand inrush mouth in the same initial water head，and increases with the initial water head in the same size of sand inrush mouth，but the sand content in the quicksand decreases with time. So it can be deduced that the initial water level and width of fissures are the key factors controlling the amount of the quicksand in mine working face.
