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Attachment A <br />Groundwater Barriers <br />The use of grout or polyurethane seals to reduce or stop groundwater flow into the <br />underground retorts has been considered at the Logan Wash Mine. Limiting contact of <br />groundwater with the former retort area by filling the retort with grout or by constructing <br />a grout curtain around the perimeter of the retort area could potentially reduce the <br />quantity of groundwater seepage from the retort area. However, it is unlikely that either <br />of these systems could completely eliminate seepage from the retort azea. <br />Attempting to solidify the entire retort area by filling all of the voids with grout would be <br />enormously expensive, requiring tens of thousands of cubic yazds of grouting material. <br />Even if cost of this alternative was not prohibitive, it is unlikely that such a grouting <br />project would be able to eliminate all groundwater contact and flow through the retort <br />area. <br />The effectiveness of a grout curtain containment system depends on the adequacy of <br />penetration of the grout into the voids in the rock. The distance that grout will penetrate <br />from the injection point is limited by factors relating to both the grouting material and the <br />media being grouted, including: injection pressure, viscosity of the grout, and size of the <br />openings (Persoff and Fox 1979). Even in a very good grouting program, there would <br />likely be zones where the grout would not be able to penetrate the fine pores and fractures <br />in the rock. Karl Terzaghi, widely recognized as the father of modern soil mechanics, <br />investigated physical models to study the effects of a 95% effective grouting program. In <br />this study, he determined that the remaining 5% of openings greatly reduced the overall <br />effectiveness of the grout curtain (Goodman 1998). Even if a grouting program was <br />100% effective, the presence of soluble minerals, such as gypsum or limestone, either as <br />primary deposits within the rock sequence, or as secondary fracture filling could later <br />reduce the effectiveness of grout curtain as a barrier to groundwater flow. <br />Considerable effort was used to construct a grout curtain of high quality in the <br />construction of Teton Dam, in Idaho; however openings, or windows, in the grout curtain <br />were believed to be a contributing factor in the failure of this structure in1976 (Solava <br />and Delatte 2003). A similar problem with a grout curtain in jointed shale material was <br />believed to be one of the causes of the near-failure of Fontenelle Dam, in Wyoming, in <br />1965 (U.S. Bureau of Reclamation 2002). At Horsetooth Dam, in Colorado, increased <br />seepage and rising piezometric levels in monitoring wells were believed to be due to <br />possible deterioration of the grout curtain and/or dissolution of gypsum in the foundation <br />bedrock (U.S. Bureau of Reclamation 2002). <br />Intercept Drains <br />An alternate method of keeping groundwater from entering the retorts is to construct a <br />drain system that would intercept ambient groundwater upgradient of the retorts. <br />Conceivably, such a system would consist of a number of angled boreholes that may <br />capture groundwater and discharge it by way of gravity to lower mine workings or a <br />e <br />