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Western Dam Engineering <br /> Technical Note <br /> <br /> August 2016 <br /> <br />18 <br /> <br />Figure 6. The Result of Concentrated Leak Erosion along a <br />Conduit at a Dam in Montana <br /> <br /> <br />Figure 7. Common Crack Locations for Concentrated Leak <br />Erosion along Abutting Structures [2] <br />Contact Erosion <br />Contact erosion is often confused with concentrated <br />leak erosion; both are scour type mechanisms in which <br />seepage scours the material from a surface. In <br />concentrated leak erosion, the seepage occurs through <br />cracks, low stress zones, or voids; in contact erosion, <br />the seepage occurs through a pervious, coarse stratum <br />that is in contact with an erodible layer. Contact <br />erosion occurs when coarse material is in contact with <br />finer material and the flow path is parallel or along the <br />interface of the materials. The larger flow through the <br />more pervious coarse material scours or erodes the <br />adjacent finer material, transporting it through the <br />void space of the coarser materials. <br />This failure mode is most common at the interface <br />between the embankment and foundation, where <br />gravity assists in moving the finer materials down and <br />into the coarser foundation. It can also occur within <br />the foundation between geologic layers or between <br />embankment layers that occurred due to segregation <br />during construction. Contact erosion does not refer to <br />preferential seepage along contacts with structures <br />such as concrete sections, retaining walls, or rock <br />abutments (see Concentrated Leak Erosion). Sand boils <br />at the downstream toe, particles in downstream <br />channels and irregular settlement of the crest are the <br />most common signs that contact erosion could be <br />occurring. <br /> <br /> Figure 8. Contact Erosion Process (adapted from [1] [2]) <br />Suffusion/Suffosion <br />In materials that are widely graded or gap graded, fine <br />particles can erode from within the matrix of the <br />coarse particles when subjected to seepage flows. The <br />materials are considered internally unstable and it can <br />lead to an increase in permeability, greater seepage <br />velocities and potentially higher hydraulic gradients, all <br />resulting in an accelerating rate of suffusion. When the <br />coarse particles are densely packed and in point-to- <br />point contact with each other, the transport of fines <br />out of the matrix results in little to no volume change. <br />This is referred to “suffusion” and is depicted on Figure <br />9 (a). Particles in downstream channels or low points, <br />leakage on the downstream slope, and irregular <br />settlement of the crest are the most common signs <br />that suffusion could be occurring. <br />When the coarse particles are more loosely packed, <br />the transport of fines out of the matrix results in a <br />reduction in total volume, and the process is referred <br />= Possible location <br />for contact erosion