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BufldoAMine TaifinRS Pauis Evalualian <br />4.1 through 4.4 by the vertical lines labeled "current conditions." In three of the four samples, <br />• the current stress condition lies within the azea banded by the fully drained condition and the full <br />hydrostatic ponding condition. The fourth sample was consolidated to an effective stress slightly <br />higher than would occur if fully drained conditions existed. This appazent overconsolidation <br />could either be the result of past or present development of negative pore pressure (suction) or, <br />more likely, the result of slight cementation of the tailings. <br />Given the current state of stress in the tailings and the consolidation test results, further changes <br />in tailings volume due to consolidation are expected to be minimal as the effective stress <br />increases with continued pore pressure dissipation, even if the long-term pore pressures <br />eventually drop to fully drained conditions. The laboratory consolidation test results indicate <br />that if pore pressures within the tailings are conservatively assumed to dissipate completely, the <br />resulting void ratio changes would range from approximately 0 to 8 percent. This would result in <br />changes in overall tailings volumes ranging from 0 and 3.6 percent, with an average compression <br />of about 1.5 percent. For an average tailings thickness of 30 feet, an additional settlement of ~ to <br />• 6 inches would be expected if pore pressures dissipated completely. This would result in an <br />additional 5 to 6 inches of pore water per unit azea being expelled from the 30-foot zone of <br />tailings. This flow would occur over an extended period of time (20 to 50 years or more). <br />Therefore, the rate of pore fluid drainage due to consolidation is conservatively estimated to be <br />0.25 to 0.10 inches per unit area per year. <br />4.5 Tailings Flow Conditions <br />For steady-state, downwazd water flow in layered materials, porous-media flow theory indicates <br />that higher water saturations will occur in finer-grained layers (Corey, 1994). In most situations, <br />a fine-grained layer will become fully saturated if the infiltration rate (expressed as volume flux <br />per unit azea) exceeds the volume flux per unit azea (expressed as the product of the vertical <br />hydraulic conductivity of the layer and the gradient for flow through the layer). For a unit <br />gradient, the infiltration rate can be compazed directly with the vertical hydraulic conductivity. If <br />the infiltration rate is significantly greater than the vertical hydraulic conductivity of a layer, that <br />• layer will become saturated and perched ground water will likely occur in the overlying coarser- <br />grained layer (if one exists). Thus, if saturated or perched water conditions are found within <br />Homeslake ,4limrrg Company Shepherd it)rlfrr, /nc. <br />pil"-JlfibrllJug.rp~ I6 Aprd N, 199i <br />