Abstract: A physical model has been established on quick sand of complete wells and at the bottom of semi-spherical wells for dyke foundations of double-layer structure, and the dynamic progress of quick sand based on critical hydraulic gradient is simulated in calculation examples by using the time-division stable flow theory. The concentrated permeation channel resulted from osmotic deformation in the dyke foundation will develop from the sand gushing opening along the vertical axis of the dyke, and keep on expanding in horizontal direction from the opening to the river bed, with increasing depth. The size of the channel depends on the flow and water level at the sand gushing opening, and in particular, maximum channel size is possible at low water level. A relief well can lower the head in its adjacent area, but at the same time increase the hydraulic gradient, thereby making it more likely to develop osmotic deformation. Therefore special attention should be paid to the design and construction of the reversed filter of the relief well, so that the soil body can be kept at the original position without causing osmotic deformation.

Key words: quick sand model, critical hydraulic gradient, osmotic deformation , concentrated permeation channel