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6.0 New Horizon 2 Mine Spoil Compactibility and Erodibility <br />Determining the compaction potential of disturbed overburden is important to assess post mine <br />spoil conditions including: hydraulic conductivity, revegetation success, landscape stability, and <br />equipment traffic. Generally, stability and/or compaction problems are related to soils high in <br />shrink/swell clays, soils with large proportions of sand and clay (sandy clay texture), and soils <br />with approximately equal proportions of the sand, silt, and clay -size particle fractions. These <br />three conditions are not highly desirable since a high shrink/swell clay content in post mine <br />landscapes causes poor stability and low permeability conditions, a sandy clay textured soil <br />develops surface crusting problems, and an equal particle -size distribution causes excessive <br />compaction which results in restricted water movement and root growth. <br />The first two considerations listed above were among those used for establishing the grading <br />system for texture. Unsuitable clay textures were identified within two thin lenses of the upper <br />overburden in the western half of the study area (Section 2.04.6). These clay lenses occur only <br />near overburden Drill Site 870E (see Map 2.04.6-1, Geologic Cross Section and Sample <br />Locations for New Horizon 2 Study Area). These clay lenses are in the projected mining <br />disturbance area and when they are being excavated, they will be thoroughly mixed with non - <br />clay type overburden. The mixing and dilution process will be accomplished by: 1) blasting, <br />which will start the mixing of clay lenses with non -clay formations above and below the clay <br />seam 2) the overburden shovel will be digging a full face through the clay lense from top to <br />bottom to load each truck for transportation to the backfill, 3) and finally, the trucks with the <br />diluted clay lenses on board will dump their load along a several hundred foot dump face. After <br />this process, the concentration in any one spot of clay like material will be minimal and <br />compatibility of the backfill will be negligible. <br />The New Horizon 2 post mine spoil textures will typically be loamy with an approximate particle <br />size distribution of 65 percent sand, 25 percent silt, and 10 percent clay (Section 2.04.6, Geology <br />Description). Surface crusting, which is critical to seedling emergence, is discussed in the Soil <br />Survey (Section 2.04.9), Revegetation Plan (Section 2.05.4(2)(e)), and Topsoil Management of <br />this Section. Since clay percentages and shrink -swell potentials are low, stability and <br />trafficability problems are not anticipated. <br />The resultant loamy textured spoil will possess physical qualities which are conducive to plant <br />growth. The available water holding capacity will likely be moderately low to moderate <br />(approximately 0.07 to 0.11 inches of water per inch of soil), permeability moderate to <br />moderately rapid (about 1.0 to 6.0 inches per hour), and cation exchange capacity low to <br />moderately low (5 to 10 milli -equivalents per 100 grams). <br />July 2018 (TR -84) 2.05.4(2)(d)-6 <br />