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7.0 UNDERDRAINS <br />To minimize development of a potential perched water table beneath the <br />refuse pile, underdrains shall be constructed. As specified in Section 4.09.2 of <br />the Regulations of the Colorado Mined Land Reclamation Boaid for Coal <br />Mining (6/30/91 revision), the underdrain system shall consist of a main trench <br />16 feet wide by 8 feet deep, and four lateral trenches 10 feet wide by 4 feet <br />deep, backfilled with a hard, durable, non-toxic forming rock, such as <br />sandstone. The underdrain layout for the final refuse pile configuration is <br />presented in Figure 1. The gradation for the rock material used in the lateral <br />underdrains is listed in Table 5.0 <br />TABLE 5.0 <br />Gradation for Rock Material to be Used in Lateral Underdrains <br />Sieve Size (inch) Perceat Passing (°,'o) <br />12 100 <br />6 40-80 <br />3 10-30 <br />1-v2 0-l0 <br />For construction of the main underdrain, no more than 10 percent of the rock <br />may be less than 12 inches in size, and no single rock may be larger than 25 <br />percent of the width (0.25 s 16 ft = 4 ft). <br />16 <br />