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J Western Slope Carbon, Inc. Page 11 <br />December 15, 1981 <br />In our opinion, the Pit No. 6 area is geotechnically suited for development <br />J of a coal process waste disposal site. However, the existing retention em- <br />bankment slopes are over-steep, marginally stable, and will need to be <br />flattened. Additionally, temporary and permanent surface runoff diversions <br />will need to be provided to prevent saturation and surface erosion of the <br />7 exposed waste within the disposal area. <br />The following are our comments and recommendations regarding site grading <br />criteria and fill placement, embankment stability, surface runoff control, <br />and the potential for groundwater contamination. <br />SITE GRADING CRITERIA <br />In our opinion, site materials can be excavated to the required depth by <br />modern heavy-duty grading equipment. As used in this section, the term <br />"compact" and its derivatives refer to ASTM designation: D 1557-78 and <br />means at least 90% compaction at a moisture content near optimum. On this <br />basis, we anticipate between a five and ten percent loss in volume of the <br />native soils in the process of excavation and recompaction; we anticipate <br />between 10 and 15 percent gain in volume (bulkage) when excavating and <br />recompacting the shale bedrock. <br />The following recommendations apply to general grading of the site for <br />preparation of fill placement, placement of waste material, and application <br />of final cover: <br />1. In areas to receive fill, strip all grass, brush, and root- <br />filled organic topsoil (estimated thickness six to eight inches) <br />and any other deleterious substances. The stripped material <br />should not be used in engineered fill, but may be stockpiled <br />for reuse as topsoil. <br />2. Excavate the residual soils and near surface rock to the <br />desired depth and stockpile for subsequent use in engineered <br />fills and as final cover. <br />3. In areas to be filled, remove all soft or loose soils exposed <br />