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each year. <br />6.0 New Horizon #2 Mine Spoil Compactibility and Erodibility <br />Determining the compaction potential of disturbed overburden is important to assess postmine spoil <br />conditions including: hydraulic conductivity, revegetation success, landscape stability, and equipment <br />traffic. Generally, stability and/or compaction problems are related to soils high in shrink/swell clays, soils <br />with large proportions of sand and clay (sandy clay texture), and soils with approximately equal proportions <br />of the sand, silt, and clay -size particle fractions. These three conditions are not highly desirable since a <br />high shrink/swell clay content in postmine landscapes causes poor stability and low permeability conditions, <br />a sandy clay textured soil develops surface crusting problems, and an equal particle -size distribution <br />causes excessive 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 system for <br />texture. Unsuitable clay textures were identified within two thin lenses of the upper overburden in the <br />western half of the study area (Section 2.04.6). These clay lenses occur only near overburden Drill Site <br />870E (see Map 2.04.6-1, Geologic Cross Section and Sample Locations for New Horizon 2 Study Area). <br />These clay lenses are in the projected mining disturbance area and when they are being excavated, they <br />will be thoroughly mixed with non clay type overburden. The mixing and dilution process will be <br />accomplished by: 1) blasting, which will start the mixing of clay lenses with non clay formations above and <br />below the clay seam 2) the overburden shovel will be digging a full face through the clay lense from top <br />to bottom to load each truck for transportation to the backfill, 3) and finally, the trucks with the diluted clay <br />lenses on board will dump their load along a several hundred foot dump face. After this process, the <br />concentration in any one spot of clay like material will be minimal and compatibility of the backfill will be <br />negligible. <br />The New Horizon 2 postmine spoil textures will typically be loamy with an approximate particle size <br />distribution of 65 percent sand, 25 percent silt, and 10 percent clay (Section 2.04.6, Geology Description). <br />Surface crusting, which is critical to seedling emergence, is discussed in the Soil Survey (Section 2.04.9), <br />Revegetation Plan (Section 2.05.4(2)(e)), and Topsoil Management of this Section. Since clay <br />percentages and shrink -swell potentials are low, stability and trafficability problems are not anticipated. <br />The resultant loamy textured spoil will possess physical qualities which are conducive to plant growth. The <br />available water holding capacity will likely be moderately low to moderate (approximately 0.07 to 0. 11 <br />inches of water per inch of soil), permeability moderate to moderately rapid (about 1.0 to 6.0 inches per <br />August 2013 (PR -08) 2.05.4(2)(d)-8 <br />