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~i <br />3.4 Underdrains <br />To minimize development of a potential perched water table <br />underneath the refuse pile, underdrains will be constructed. <br />Since the existing ground surface slopes sufficiently down <br />northward, and the refuse pile benches will be constructed <br />with a back slope of two percent, surface runoff will drain <br />into the collection channel and perimeter channels. This <br />situation will minimize infiltration into the colluvium and <br />refuse material. However, considering the fact that there <br />will be some stripped area that will remain uncovered from <br />time to time, and spreading and compaction of refuse material <br />may not be carried out on a daily basis, underdrains will be <br />constructed to minimize potential development of pore pres- <br />sure in the soils. A further discussion of the necessity for <br />underdrains is presented in Section 5.4 of this report. <br />The underdrain system will consist of lateral trenches four <br />feet wide by two feet deep backfilled with a hard, durable, <br />non-toxic forming rock, such as sandstone. <br /> <br />i <br />i <br />• <br /> <br />• <br />9 <br />