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Western Dam Engineering Technical Note August 2016 22 Figure 19. Sinkhole in crest of a dam in Montana that manifested after several years of undetected internal erosion into a damaged conduit [6] Conditions for Initiation of Internal Erosion For internal erosion to initiate, certain conditions need to exist within the dam. The material properties, hydraulic load, and critical stress conditions all play a part in the initiation, continuation, and progressions. Figure 20 below summarizes how different factors contribute to conditions for internal erosion to occur. Material Properties The erodibility of a soil is the major factor in the probability of internal erosion occurring and is a function of material properties including particle size, plasticity and gradation. Cohesionless materials (low plasticity) such as sands and silts are more likely to erode than those with cohesion (such as clays) as the particles have fewer internal forces keeping the particles together. Fine-grained sand and silt particles are more likely to erode than larger, coarse particles (as in coarser sands and gravels, cobbles, and boulders) because it takes more energy (seepage velocity) to move larger, heavier particles. The gradation distribution of a material is also important as a well-graded material may provide its own filtering in that the larger materials provide the weight needed to counteract movement, while the smaller materials fill voids and reduce permeability and therefore, seepage velocity. More uniformly graded soils are therefore more susceptible to erosion than well-graded soils. Broadly-graded soil (soils with a wide range of particle sizes; e.g. ranging from cobble to silt sizes) can be susceptible to internal instability (suffusion or suffosion). This is particularly true when the gradation distribution lacks certain particle sizes, known as gap- graded soils. In these soils, the coarser fraction of the soil is too large to filter the finer fraction. Table 2. Internal Erosion Potential of Soils (Adapted from [4]) Greatest Piping Resistance Category (1) 1. Plastic clay (PI>15), Well compacted 2. Plastic clay (PI>15), Poorly compacted Intermediate Piping Resistance Category (2) 3. Well-graded material with clay binder (6<PI<15), Well compacted 4. Well-graded material with clay binder (6<PI<15), Poorly compacted 5. Well-graded cohesionless material (PI<6), Well compacted Least Piping Resistance Category (3) 6. Well-graded cohesionless material (PI <6), Poorly compacted 7. Very uniform, fine cohesionless sand (PI<6), Well compacted 8. Very uniform fine cohesionless sand (PI<6), Poorly compacted Note: Dispersive soils may be less resistant than Category 3. Hydraulic Load Hydraulic gradients and seepage velocities are also key factors in determining the potential of internal erosion. For simplicity, average (horizontal) gradients along the entire suspected internal erosion pathway are usually estimated to evaluate the potential for internal erosion to occur, as it is difficult to measure in the field. Several researchers have measured erosion potential in the past and determined gradients at which certain soils may erode. However, conditions in the field contain many more uncertainties than laboratory environments used for testing, and BEP has been estimated to occur with gradients less than 0.05. Based on laboratory testing, the initiating gradient to erode sands is a function of the gradation, with poorly (more uniformly) graded sands being more susceptible to erosion at lower gradients than well graded soils. Upward, or vertical, gradients can be estimated in cases where a confining layer exists at the downstream exit point. The vertical gradients relate to the potential for heave or uplift to initiate erosion at the exit point and are impacted by the properties and thickness of the confining layer. (See the previous Tech Note article “IS YOUR EMBANKMENT DAM UNDER PRESSURE - UNDERSEEPAGE IMPACTS“)