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Western Dam Engineering <br /> Technical Note <br /> <br /> August 2016 <br /> <br />20 <br /> Figure 11. Seepage through the Embankment at Blackmon <br />Dam in Tasmania <br /> <br />Contact erosion can occur due to seepage through a <br />pervious zone above a core that does not extend to the <br />surface. Suffusion and suffosion can occur if <br />embankment materials are poorly or gap-graded. <br />Cloudy seepage out of the embankment or the <br />deposition of fine material at the toe can indicate <br />suffusion or suffosion is occurring. Larger than normal <br />settlement or deformation can be a sign of suffosion. <br />Internal Erosion through Foundation <br />BEP through the foundation is initiated at an unfiltered <br />exit downstream of the embankment toe. The exit can <br />exhibit heave or uplift/blowout as well as sand boils. <br />The foundation material must be erodible for BEP to <br />occur, but the cohesion necessary to form a pipe could <br />come from an overlying confining layer, producing a <br />failure path along the interface. Concentrated leak <br />erosion can occur due to arching through a crack in the <br />foundation created by differential settlement or <br />collapse, and arching across formations or <br />irregularities in the foundation. <br /> <br /> Figure 12. Internal Erosion through the Foundation <br />by BEP [2] <br /> Figure 13. Seepage through Foundation Exiting at <br />Downstream Toe of Earthen Dam <br />Contact erosion can occur through the foundation <br />when a pervious foundation layer underlies a fines <br />layer (e.g., the overlying embankment or a fine-grained <br />foundation layer) allowing for water flow in the <br />permeable layer to transport the fines of the adjacent <br />fine layer. <br />Suffusion or suffosion can occur in the foundation if <br />there are poorly or gap-graded layers allowing for the <br />fine material to move through and out of the coarser <br />material matrix. Sediment deposition in downstream <br />ditches or channels can be an indication of <br />concentrated leak erosion, contact erosion or <br />suffusion/suffosion. Larger than normal settlement or <br />deformation can be a sign of suffosion. See our <br />previous article for more information on mechanisms <br />for internal erosion through the foundation: “IS YOUR <br />EMBANKMENT DAM UNDER PRESSURE - UNDERSEEPAGE <br />IMPACTS“. <br />Internal Erosion of Embankment into <br />Foundation or Abutment <br />BEP, contact erosion, and concentrated leak erosion <br />can initiate at the interface between the embankment <br />and foundation (including the abutment) by <br />transporting embankment material into a void, joint, <br />or other opening in the foundation. BEP can occur <br />when gradients at the interface are large enough to <br />transport embankment material into a permeable <br />foundation or abutment and the embankment material <br />is cohesive enough to hold a roof. Concentrated leak <br />erosion can occur when a defect in the foundation or <br />abutment (e.g., a fracture) concentrates seepage, and <br />embankment material is scoured by the flow. Contact