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Section 816.71 (b) Continued. <br />• (iii) Contain lateral drains to each area of potential drainage or <br />seepage. <br />(2) A filter system to insure the proper functioning of the rock underdrain <br />system shall be designed and constructed using standard geotechnical en- <br />gineering methods. <br />(3) In constructing the underdrains, no more than 10 percent of the <br />rock may be less than 12 inches in size and no single rock may be larger <br />than 25 percent of the width of the drain. Rock used in underdrains <br />shall meet the requirements of Paragraph (b)(4) of this Section. The <br />minimum size of the main underdrain shall be: (See Table as shown in <br />Office of Surface I-]fining Permanent Regulatory Program, Book 3, Disposal <br />of excess spoil: Valley fills for this information). <br />(4) Underdrains shall consist of non-degradable, non-acid or toxic <br />• forming rock such as natural sand and gravel, sandstone, limestone, or <br />other durable rock that will not slake in water and will be free of <br />coal, clay, or shale. <br />The underdrain system has been designed to handle all subdrainage <br />and seepage to maintain the stability of the fills. The durable <br />rock underdrains will extend along the natural drainage systems <br />from the toe to the head of the fills and will contain lateral <br />drains to handle any areas of potential drainage or seepage. In <br />order to prevent the clogging of the rock underdrain system, <br />filter systems will be utilized. The design of the subdrainage and <br />filter systems can be found in Exhibit 9, Excess Spoil. <br />A ten foot bed of competent sandstone is present above the Lennox <br />coal seam and slake durability tests indicate that this material will <br />be suitable for drain rockfill. It is non-degradable, non-toxic <br />forming, will not slake in water, and is free of coal, clay, or shale. <br />• <br />816-96 <br />