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Western Dam Engineering <br /> Technical Note <br /> <br /> August 2016 <br /> <br />23 <br />Stress Conditions <br />The stress conditions that most influence the potential <br />for internal erosion are those that create defects or <br />occur at naturally occurring defects. Cracking from <br />differential settlement can occur when there are <br />irregularities in foundation geometry, large differences <br />in embankment height, rigid elements within the <br />embankment (conduits), or differences in <br />compressibility of foundation or embankment <br />material. Foundation defects can include bedrock <br />joints, fractures, bedding planes, folation, shears, and <br />faults. The size and continuity of the foundation <br />defects, as well as the effectiveness of any foundation <br />treatment, are factors in the potential for internal <br />erosion to occur. Foundations with karstic or <br />solutioning properties show a history of sinkholes and <br />caves and can be a major concern for internal erosion <br />of the embankment into the foundation. <br /> <br />Hydraulic fractures can form when hydraulic pressures <br />exceed minor principal stresses between material <br />particles. This can be due to areas of low stress causing <br />arching, improper drilling methods being used in the <br />core of a dam, or when a slurry trench cutoff is <br />installed as a seepage barrier. However, hydraulic <br />fractures can sometime close as stresses redistribute <br />or materials saturate and expand. <br /> <br />High permeability or low stress zones can be caused by <br />a wide range of construction related issues. Low stress <br />zones around conduits and structure walls can occur <br />due to difficult or inadequate compaction especially <br />under a conduit, around cutoff collars, or next to a <br />steep or vertical wall. High permeability zones within <br />an embankment can occur due to poor compaction <br />from low density material, thick lifts, and too much or <br />too little water content. Poor quality control of <br />material can lead to segregation and layers or pockets <br />of coarse material. Poor treatment of foundation <br />bedrock prior to placement of the embankment can <br />create high permeability seams at the contact. Layers <br />exposed to freezing or significant precipitation can <br />experience a decrease in density, and desiccation <br />cracking can occur in those exposed to high heat. <br />Rodent burrows or root systems from vegetation can <br />also create defects in the embankment that can lead to <br />propagation of cracks or piping. <br /> <br /> <br /> <br />Material Properties <br />Stress Hydraulic <br />Load <br />Internal Instability Filter Incompatibility Void Space Free Surface <br />Arching Vibration Low Stress <br />Seepage Velocity Hydraulic Gradient Pore Pressures <br />Heave/Blowout Hydraulic Fracture <br />Soil Distress Fines Migration <br />Concentrated <br />Leak Erosion <br />Contact Erosion <br />Figure 20. Factors Affecting the <br />Initiation of Internal Erosion <br />(Adapted from [2], [5]) <br />