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Western Dam Engineering Technical Note May 2016 21 Figure 1. Foundation Backward Erosion Piping Model (adapted from [8]) Deep Pervious Foundation => Heave The first condition is a relatively low permeability structure (dam) constructed on a homogenous, pervious, foundation. Figure 2 depicts a failure initiating for this idealized condition. Breaching of the dam could occur if sufficient sand were eroded from beneath the dam, and a pipe formed under the embankment large enough to empty the reservoir or cause the embankment to collapse into the pipe. Figure 2. Heave of a Cohesionless Soil (Vertical Seepage Exit) Confined Pervious Foundation => Uplift and Blowout The second idealized condition is sometimes termed a blanket-aquifer, which consists of a pervious foundation layer (e.g., sand) overlain by a significantly less permeable (e.g., clay) confining layer. Artesian pressure develops when the piezometric head (pore water pressure) within the pervious layer exceeds the downstream ground surface elevation. When the pressure is great enough to overcome the weight of the confining layer, it is known as uplift. This can cause a spongy condition at the toe due to a reduction in the effective stress. When this uplift pressure acting on the bottom of the confining layer results in rupture of the layer, it is known as blowout. If seepage through the pervious layer is sufficient, cohesionless soils within the pervious layer can be carried to the ground surface and deposited in a conical ring, known as a sand boil. This can also occur through defects existing or created through the confining layer. See further discussion below on causing defects to confining layers. Figure 3. Uplift and Blowout of a Confining Layer (Vertical Seepage Exit) Many alluvial valleys have a surface layer created by overbank flooding deposits (or remnant organic surface soils) that have hydraulic conductivities lower than the underlying alluvial soils. Foundation treatment for many small to medium sized dams was