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Mr. Jim Matters <br />July 6, 2006 <br />Page 5 <br />illustrates the 8-ft-high dam regions where soil is expected to undergo inelastic behavior or <br />irrecoverable deformations. Figure 7 illustrates the 8-ft-high dam azeas within the embankment <br />where stresses aze greatest. The contours of effective sheaz stress indicate the lazgest shear <br />stresses aze at the upstream and downstream toes, as expected The effective sheaz stresses are <br />proportionally higher than for the 12-ft dam, but little additional inelastic behavior is indicated. <br />Strength of the compacted embankment material was incrementally decreased until the <br />dam no longer came to an equilibrium condition. The potential failure mode at this condition is <br />examined from changes in effective sheaz stress and rates at which sheaz strain changes. <br />Figure 8 show contours of effective sheaz stress for 8-ft and 12-ft-high dams after strengths are <br />reduced. As expected, the figure indicates that shear stresses aze simihrr to those m Figure 7. <br />Contours of sheaz strain rate indicate potential slope instability on the upstream embankment <br />(Figure 9). This figure suggests the most likely failure mode would be slumping of the upstream <br />face back into the pond and sudden breach would not be expected from shearing (piping <br />behavior was not examined). <br />The strength properties used for the condition when the model no longer came to an <br />equilibrium condition can be compared to the original strength properties to define the factor of <br />safety (i.e., FOS = So,;/Saa,ue). The computed safety factors aze FOS = 2.3 for the 8-ft-high dam <br />and FOS = 2.0 for the 12-ft-high dam. Both of these values are above the required 1.3 factor of <br />safety for impoundment darns with steady-state seepage saturation conditions. <br />RECOMMENDED CONSTRUCTION VERIFICATION <br />The actual factor of safety of the dam embankment may vary depending on actual soil <br />strength properties. This modeling was performed using our best estimate of expected average <br />material properties from limited site information. To ensure that the actual performance of the <br />dam embankment is not significantly different the following requirements aze recommended: <br />• The keyway trench should be inspected by an on-site engineer prior to placing the <br />compacted material <br />• The inspection should verify whether the keyway excavation is founded on alluvial <br />sediments or directly on bedrock If founded on alluvial sediments the inspection should <br />verify that the foundation soils are free of organics and qualitatively characterize the soil <br />and ground conditions. <br />• The inspection should include photographs of conditions and various construction stages <br />at both the dam site and borrow azea. Brief field notes should be included to documem <br />as-both changes. <br />• Water will need to be added to the embankment borrow material prior to compaction so <br />that the water content is close to 13%-14%. There should not be dry spots or large <br />vaziations in moisture content as placed. <br />• Cleazing the embanlmrent site should include removal and complete excavation of <br />significam roots from deciduous trees and brush. <br />Agapito Associates, Inc. <br />