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Western Dam Engineering Technical Note May 2016 22 limited to stripping of vegetation, leaving the low permeable, organic soils to serve as the dam’s foundation. This layer of soil creates a low permeable blanket, confining pore pressures in more permeable underlying layers. These dams continue to function, but activities such as digging or drilling at the toe can puncture or reduce the thickness of the confining layer, leading to an elevated risk of blowout. Figure 4. “Blowout” Caused by Underseepage of a Dam in Montana (MT DNRC) Critical Gradient and Seepage Exit Direction Seepage in an upward direction reduces the effective stress within the soil. When the water pressure at a point in the soil is equal to the total vertical stress at that point, the effective stress is zero and the soil has no frictional resistance to deformation. When the effective stress is zero, the gradient is sometimes referred to as the critical gradient, which is defined b/ w, where b = the buoyant unit weight of the soil and w = the unit weight of water. For typical soils b/ w 1.0. This is the point that heave would occur. However, this is not to be confused with the seepage gradient that initiates internal erosion. Internal erosion of certain soils under flow conditions other than vertical can initiate at gradients significantly less than 1. Making the distinction between vertical and horizontal exit conditions is important, because the direction of the seepage exit influences the gradient that will initiate erosion. A foundation seepage exit may be in a ditch or other excavation penetrating into the erodible non-plastic (cohesionless) pervious soil, resulting in a horizontal or near horizontal seepage exit. Figure 5. Horizontal Seepage Exit into a Drainage Ditch along the Downstream Toe of an Earth Dam Vertical seepage exits need to overcome gravity and, in some cases, overcome the weight of a confining layer. Therefore vertical seepage exits typically have a higher gradient required to initiate erosion as compared to a horizontal exit. There are several methods that have been proposed to calculate a factor of safety against heave and uplift/blowout, which apply to vertical seepage ([5], [6]). In the case of horizontal seepage exits, heave and uplift/blowout computations do not apply. There is no generally accepted methodology for calculating a factor of safety against internal erosion for horizontal exits. In practice, horizontal gradients as low as 0.05 in fine, uniformly graded sands have been observed to initiate erosion in some particular soils [6]. Schmertmann [7] and Sellmeijer [8] offer some general guidance on critical gradients based on lab tests; however, the equations include a number of correction factors to apply them to field conditions and they were developed for a limited range of cohesionless soils. Caution should be used in applying these equations. Critical horizontal gradients are a topic of much ongoing research and currently remain a relatively subjective assessment. Field Detection Foundation internal erosion failures have occurred during first filling at some dams and at others, only after years or even decades of successful performance. This is one of the main reasons to regularly inspect dams with a trained, curious, and non-complacent eye. Visual observations are the first line of defense in detecting an initiating internal erosion mechanism in sufficient time to intervene. The initiation of dam safety incidents are often first observed by routine daily site visits by operators, dam owners, or the public noticing an unusual condition, especially since many Clay Horizontal Seepage Exit