Abstract: of water level in this paper. Including seepage in a 3-dimensional theoretical analysis,the study first shows how the critical slope or angle of repose of sediment is related to the hydraulic gradient of seepage. The measured laboratory data rea-sonably fit well with the theoretical relationship in the case of injection and suction. The data reveal that the slope is reduced and increased with injection and suction,respectively. Additionally,the critical stable slope is shown to be related to both the hydraulic gradient and the seepage direction. The study identifies the seepage direction which results in a minimum critical stable slope for a certain hydraulic gradient of seepage.