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C. Mounds shall not be located on natural slopes of more than six percent if <br />the percolation rate Is slower than 30 minutes per inch In all layers of <br />natural or fill soil to a depth of at least 24 inches below the sand layer <br />of the mound. <br />D. Mounds shall not be located on natural slopes exceeding 12 percent under any <br />soil percnlation rate conditions. <br />E. Mounds shall not be located on any soil where the percolatino rate In any <br />layer of natural or fill soil to a depth of at least 24 Inches below the <br />sand layer of the mound is slower than 120 minutes per Inch. <br />2. CONSTRUCTION EQUIPMENT <br />A rubber -tired tractor may be used for plowing or disk4ng to prepare the soil <br />surface but in no case shall a rubber -tired tractor be used after the surface <br />preparation is completed. A crawler or track -type tractor shall be used for <br />mound construction. <br />3. SOIL SURFACE PREPARATION <br />A. The discahrge pipe from the pumo to the mound area shall be installed prior <br />to soil surface prvpxratiOP The trench excavated to install the discharge <br />pipe shall be careiviiy backfilled and compacted to prevent seepage of <br />effluent. <br />B. The total area selected for the mound, including that under the dikes, shall <br />be roughened in order to thoroughly break up any existing sod layer and to <br />provide a suitable transition zone between the original soil and the sand <br />layer of the Hound. The area shall be roughened only when the moisture <br />content of the soil eight inches below the surface is drier than the plastic <br />limit. <br />Plastic Ilett is the soil moisture content below which the soil may be <br />manipulated for purposes of installing a soil treatment system, and above <br />which manipulation will cause compaction and puddling. If a fragment of <br />soil can easily be rolled into a wire 1/8 -inch in diameter, the moisture <br />content is above the plastic limit. If the soil is dry enough to be f-i'hle <br />W. falls apart when rolling it into a wire, the moisture content I <br />the limit, and the soil may be manipulated. The standard method of <br />determining the plastic limit Is specified in American Association of ',Lace <br />Highway Officials (AASHO) Designation: T 90-61. <br />C. Surface preparation or roughening may be performed with a mold board plow, <br />a disk plow, ur a backhoe using only the teeth. Mold board plow furrows <br />shall be at least 8 inches deep, shall be thrown upslope, and Shall run <br />perpendicular to the slope. There shall be no deadfurrow under the rnund. <br />Disking may be used for surface preparaton as a substitute for mold board <br />plowing in sols having percolation rates faster than 15 minutes per inch <br />(sandy loam) In the top 8 -inch depth.. Disking shall be performed <br />perpendicular to the slope. <br />-7- <br />