Design and Development of the Experimental Apparatus for Study of Soil Internal Erosion in 2D Seepage

Internal erosion refers to the seepage-induced movement of a fraction of soil particles in such a way that the initial state of the soil structure changes. Accordingly, severity and mechanism of the particle 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 the design of earth dams and engineering fills to which considerable attention has been given. 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; subsequently, various techniques and criteria have been proposed for identification and assessment of this phenomenon. Hence, the previous studies are mainly focused on the geometric parameters (e.g., gradation, void ratio, 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 flow direction and direction of the layers in the existing criteria for evaluating internal stability, whereas natural deposits and even engineering fills are not necessarily perpendicular to the direction of the layers. Also, flow direction towards 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 critical gradients are more in soils with perpendicular layers than those with parallel layers. Also, as the angle of flow relative to the direction of gravity increases, the critical gradients generally increase; in addition, the direction of the layers relative to the inlet flow to the soil specimen completely affects the shape of eroded pipes in the soil.