Abstract: Suspended cut-off wall is a common method for seepage control in deep alluvium foundations. To investigate seepage failure characteristics of foundations with suspended cut-off walls, a model test of seepage failure was carried out by using uniform sand and an unstable sandy gravel to make the foundation. The seepage failure patterns of the two foundations were analyzed based on the test results. It was revealed that the seepage failure pattern of foundation was related to the internal stability of the soil. Then a three-dimensional finite element model was established based on the model test, and the reliability of the numerical model was verified by comparing with the test results. Finally, the effects of the depth, thickness and defects of the wall on the impermeability of the foundation were analyzed by the numerical model. The results show that the seepage failure patterns of uniform sandy foundation and unstable sandy gravel foundation were backward piping failure and internal erosion failure respectively. The impermeability of the foundation mainly depended on the wall depth and defects in the wall and is irrespective of the wall′s thickness. By analyzing the change law of seepage field with the increase of wall depth, it was concluded that the wall had an optimal depth that was about 0.7 times of the total foundation depth.