عنوان مقاله [English]
Internal erosion refers to the seepage-induced movement of a fraction of soil particles in such a way that initial state of the soil structure changes. Accordingly, severity and mechanism of the particles movement depend upon the geometric, mechanical and hydraulic conditions, which is of a higher probability in the case of concave upward and gap-graded soils. Internal erosion is one of the long-lasting challenges in design of earthdams and engineering fills to which special attention has been paid by the researchers. To date, extensive studies have been conducted on the four regular types of internal erosion such as concentrated leak erosion, contact erosion, backward erosion and suffusion, and subsequently, various techniques and criteria have been proposed for identification and assessment of this phenomenon. Hence, the previous studies are majorly focused on the geometric parameters (e.g. gradation, void ratio and etc.) whereas the mechanical and hydraulic factors have gained less attention. However, most of the studies and experimental apparatuses for evaluating the occurrence of these phenomena in susceptible soils, have been in terms of one-dimensional (upward or downward) flow applied perpendicular to the soil layers. There is also no mention of the effect of the flow direction and the direction of the layers in the existing criteria for evaluating internal stability. Whereas in natural deposites and even engineering fills are not necessarily perpendicular to the direction of the layers. Also, the direction of flow in the direction of gravity is not necessarily zero or 180 degrees and may be present in any case Therefore, in this study, a special physical model was designed and developed to study samples of gravel size and different directions of layers and inflow. The results of experiments carried out on gap-graded soils show that the the critical gradients are higher in soils with perpendicular layers than in parallel layers. Also, as the angle of flow relative to the direction of gravity increases, the critical gradients generally increases. in addition to, the direction of the layers relative to the inlet flow to the soil specimrnt completely effects on the shape of the erosion pipes in the soil.