Internal erosion

Internal erosion of soil particles from within a dam by water that seeps through the dam is one of the most common causes of failure of levees and earth dams.

Internal erosion is especially dangerous because there may be no external evidence, or only subtle evidence, that it is taking place. Usually a sand boil can be found, but the boil might be hidden under water. A dam may breach within a few hours after evidence of internal erosion becomes obvious.