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Section 816.71 (1) Continued. <br />period and periods following heavy precipitation. Some of these <br />drainages are fairly steeply incised. The design and construction <br />methods planned for the durable rock underdrains and the filters <br />for the fills are outlined in Exhibit 9, Excess Spoil. These <br />drains will prevent any seepage of water into the actual spoil <br />material which might affect the fill stability. The ten foot <br />bed of competent sandstone above the Lennox coal seam will be used <br />for the construction of underdrains. This sandstone is non- <br />degradable, non-toxic forming, will not slake in water, and is <br />free of coal, clay, or shale. See Section 774.14 (b) (1) (iv) <br />with respect to the chemical characteristics of the overburden and <br />Item 4 of the recommendations in Exhibit 9, Excess Spoil, for the <br />physical characteristics of the sandstone. <br />(m) The foundation and abutments of the fill shall be stable under all <br />conditions of construction and operation. Sufficient foundation investi- <br />• gation laboratory testing of foundation materials shall be performed in <br />order to determine the design requirements for stability of the foundat- <br />ion. Analyses of foundation conditions shall include the effect of <br />underground mine workings, if any, upon the stability of the structure. <br />Detailed foundation investigation and <br />foundation materials was performed by <br />of CTL/Thompson, Inc. The results of <br />porated into the foundation design so <br />assured. These tests and designs are <br />Excess Spoil. There has never been, <br />laboratory testing of the <br />the consulting engineer firm <br />these tests have been incor- <br />that fill stability can be <br />presented in Exhibit 9, <br />for is there planned to be, <br />any underground mining in the area of these fills. Thus, there <br />will be no effect of underground mine workings upon the <br />stability of the structures. <br />(n) Excess spoil may be returned to underground mine wrorkings, but only <br />in accordance with a disposal program approved by the regulatory authority <br />• <br />816-94 <br />