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Western Dam Engineering <br /> Technical Note <br /> <br /> May 2016 <br /> <br />24 <br /> <br />Figure 11. Sand boils on downstream toes are often slightly submerged, and sometimes detectable by water ripples <br />(a). The may start as very small deposits (b). Once observed they are often sandbagged to help limit progression (c). <br />Sand Boils <br />‘Boils’ will often appear at locations of concentrated <br />flow. While particle movement may be evident in <br />these boils, there may be no continuous removal of <br />soil if the material ‘rolls’ in a steady state condition <br />(i.e., particles simply rise and fall within the boil). The <br />soil stratigraphy also plays an important role in the <br />seriousness of boils. If the sand boil comprises fine- <br />grained soil being removed from a shallow foundation <br />layer that is underlain by a coarser layer, the erosion <br />will likely not progress through the coarser layer. If the <br />sand boil comprises fine-grained sand being removed <br />through a defect in an overlying clay layer (crack, <br />blowout, penetration, etc.) or through a coarse- <br />grained layer, the erosion may progress upgradient. <br />However, boils in any state can be an alarming <br />condition as a slight increase in gradient may result in <br />rapid development of backward erosion piping. [6] <br /> <br />Figure 12. A Sand Boil of Fine Sand being Removed <br />from an Upper Layer Underlain by a Coarse Sand <br />Layer has Limited Potential to Progress [9] <br /> <br /> <br /> <br />Figure 13. Finer Soil Being Removed through (a) a <br />Defect (crack, blowout, penetration) in a Confining <br />Layer, or (b) a Coarser Overlying Layer can Progress <br />[9] <br />Other Potential Defects <br />Other features that can increase the likelihood of <br />backward erosion piping through the foundation due <br />to damage to a confining layer include rodent holes, <br />deep roots, penetrations such as power or telephone <br />Finer Soil <br />Coarser Soil <br />Finer Soil <br />Coarse Soil <br />FLOW <br />Finer Soil being Removed <br />Likely not a problem <br />These conditions are likely a problem unless the sands are “rolling” <br />(a) <br />(b <br />Water <br />Ripples <br />(a) (b) (c)