DWR_2952033 - Laserfiche WebLink

Home Search

Western Dam Engineering Technical Note August 2016 16 8. Breach – deformation from loss of material becomes large enough to result in embankment collapse and potential overtopping. For internal erosion to lead to failure, conditions must exist for the full sequence of events to occur. However, significant damage to the dam can still take place as the failure mode progresses through each step of the event tree, even if human intervention or natural events prevent failure from occurring. Mechanisms of Internal Erosion Initiation The term “internal erosion” is the industry’s generic term to describe erosion of soil particles by water passing through a body of soil. “Piping” is often used generically in literature, but actually refers to a specific internal erosion mechanism. There are four mechanisms by which internal erosion can initiate: 1. Backward erosion piping (BEP) 2. Concentrated leak erosion 3. Contact erosion 4. Suffusion/suffosion The conditions of each mechanism vary but all can result in sufficient migration of material from within the dam’s footprint leading to significant dam deformation or breach. Backward Erosion Piping (BEP) BEP is characterized by the detachment or erosion of particles at the exit of a seepage path and the propagation of that path upstream towards the reservoir. Movement occurs because of high gradients at the exit location, usually a free surface on the downstream slope or toe. The erosion is sustained because of a “roof” formation that maintains a small “pipe” that works backward from downstream toward the reservoir. BEP generally occurs in erodible non- plastic soils that are overlain by more cohesive materials or conditions that promote arching to sustain a “roof.” Sand boils at the downstream toe are the most common sign that BEP could be occurring. Figure 1. Backward Erosion Piping Through the Embankment [2] Figure 2. Sand Boil at Downstream Toe of Earth Dam Global backward erosion is the name given to the type of backward erosion in which the soil above or around a backward erosion pipe is unable to ‘hold a roof.’ This is also referred to by Reclamation as “Internal Migration.” Incipient backward erosion pipes form but soon collapse resulting in general movement of the soil from above. There are two subset mechanisms within “Global BEP,” depending on how the collapse of the pipe or void occurs: sloughing and stoping. Sloughing When BEP occurs at the downstream slope due to seepage breakout at the face, it can develop in the form of progressive sloughing of soil. Seepage exiting the free downstream face may be sufficient to begin removing and washing away particles at the face. The process works by gravity and the failure mode becomes one of unraveling of the downstream slope. The process may continue in a step-wise fashion toward the reservoir until a breach is formed through the dam as shown in Figure 3. Digging at the downstream toe of an embankment dam for investigation or remediation can initiate an unfiltered seepage exit. Particularly if the pool is, or was recently full. This can quickly trigger internal erosion failure modes and require quick response. Uncontrolled seepage can be triggered by simply removing the vegetation layer which can serve as a cap on pervious soils. Know where the water table is before you dig and have a pre-planned emergency response action ready to implement.