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potential ground water level in the Lower ends of Che pits on the basis of topo- <br />a. graphy and the pitch of the coal. Figure 4.3-22 provides a conceptualization of <br /> Ch is determination. As is evident Erom the figure, spoil trough disposal is <br />• acceptable closer to the north end of the pits Chan is spoil bench disposal. In <br /> turn, spoil bench disposal can be safely used closer Co the l ower end of the pits <br /> than bottom of the pi[ disposal can, <br />Disposal sites will be selected on a cut by cut basis for each pit to ensure the <br />depth of disposal is at least ten feet less than the depth to the maximum poten- <br />tial water [able. 2iezometric surfaces within the pits will be evaluated during <br />future studies and disposal criteria may be modified to reflect this information. <br />Figure 4.3-22 depicts a situation where a backfiLled pit cut has filled with water <br />until the water level reaches the elevation of the pit crest at its northern end. <br />At [his point, the water will escape by a spring or seep. In actuality, the water <br />Cable may mound and somewhat exceed the pit crest elevation. The maximum poten- <br />tial extent of this mounding can be mathematically predicted. <br />A flow line passing from a more porous material (backfiLled spoils) to a Less por- <br />.~ ous material (contiguous undisturbed geologic strata) will obey the law of inci- <br />dence and refraction given by: <br />K, tan a, <br />KZ tan a: <br />where K = gpd/ft .2 <br />~ t +t 1'+ +{+ .F+ <br />+ f , + } t 'r' + + . <br />T ~, T + T , . <br />+ + '. <br />i- ++ } • . .. . . <br />+ ~• <br />K, > kz <br />Therefore, springs or seeps will only develop at .the north end of the pit cuts <br />where the recharge amount exceeds the conducting capacity of the high wall face. <br />4-98 <br />i~;~- <br />7-8-82 <br /> <br />~! <br />„ „ 215ow <br />~. <br />