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Mr. Lee Sampson <br />September 22, 2022 <br />Page 3 <br /> <br />Agapito Associates, Inc. <br />PREVIOUS OBSERVATIONS OF NATURAL SORTING OF SPOIL AT COLOWYO MINE <br />Previous work carried out by Shannon and Wilson in 20091 concluded that a constructed <br />underdrain extending the length of the valley fill in Little Collom Draw was not required for either <br />temporary or permanent conditions. The assessment stated the following: <br /> <br />In our opinion, a drain extending the full distance from the downstream toe to the <br />upstream toe is not required, particularly for a temporary spoil fill. Our experience <br />and observations indicate that the spoil and placement methods at Colowyo <br />produce a relatively free draining layer at the base of each lift of spoil. <br /> <br />The natural sorting of the spoil after it is end dumped over the crest of each lift has been <br />studied and cataloged extensively at the mine. Shannon and Wilson in 20091 made the <br />following observations: <br /> <br />The gradational characteristics of spoil vary due to sorting as the spoil is end <br />dumped over the crest of each lift. The relatively high lift thicknesses (30 to <br />100 feet) and end-dumping over the edge of the lift produce significant sorting of <br />the spoil. <br /> <br />The natural sorting process results in boulder-sized sandstone blocks at the base of each lift. <br />Shannon and Wilson in 20091 made the following observations: <br /> <br />Very coarse spoil (median grain size D50 of 10-inches or more) is typically present <br />at the base of each lift of spoil. <br /> <br />Agapito has also made several observations of natural sorting of spoil material from end <br />dumping at the mine. At dump heights of greater than 100 ft, the toe of the slope is <br />comprised of sandstone and siltstone cobbles (3 to 12 inches in diameter) and boulders <br />(>12 inches in diameter), while the upper slope is dominated by finer sand and gravel <br />material. The most recent example observed is shown in Figure 2, a photograph of active <br />end dumping of spoil into West Pit at a height of greater than 100 ft. <br />CHARACTERISTICS OF THE DUMPED SPOIL IN LITTLE COLLOM DRAW <br />The spoil placed into Little Collom Draw consists of excavated bedrock composed of sandstone, <br />siltstone, and mudstone from the Williams Fork Formation. Through the process of natural sorting, <br />as shown in Figure 3, the base of the fill consists of sandstone and siltstone cobbles (3 to 12 inches <br />in diameter) and boulders (>12 inches in diameter). Figure 4 shows the sorted spoil material at the <br />northernmost end of the valley observed during the site inspection. <br /> <br /> <br /> <br />1 Shannon & Wilson, Inc (2009), “Geotechnical Study Collom Temporary Soil Fill, KCCC – Colowyo Mine,” Report <br />Revision PR-03, August 5th.