1978-09-28 Septic System Design Report

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C. Mounds shall not be located on natural slopes of more than 6 percent if the percolation rate is slower than 30 minutes per inch in all layers of natural or fill soil to a depth of at least 24 inches below the sand layer of the mound. D. Mounds shall not be located on natural slopes exceeding.12 percent tinder any soil percolation rate condtditions. E. Mounds shall not be located on any soil where the percolation rate in any layer of natural or fill soil to a depth of at least 24 inches below the sand layer of the mound is slower than 120 minutes per inch. 2. CONSTRUCTION EQUIPMENT , rubber -tired tractor may be used for plowing or disking to prepare the soil surface but in no case shall a rubber -tired tractor be uses after the surface preparation is completed. A crawler or track -type tractor shall be used for mound const..:uc- tion. 3. SOIL SURFACE PREPARATION A. The discharge pipe from the pump to the mound area shall be installed prior to soil surface preparation. The trench excavated to install the discharge pipe shall be carefully r3ckfilled and compacted to prevent seepage of effluent. B. The total area selected for the mound, includii,g that under the dikes, shall be roughened in order to thoroughly break up any existing sod layer and to provide a suitable transition zone between the original soil and the sand layer of the mound. The area shall be roughened only when the moisture content of the soil 8 inches below the surface is drier than the plastic limit. Plastic limit is the soil moisture content below which the soil may be manipulated for purposes of installing a soil treatment system, and above which manipulation will cause compaction and puddling. If. a fragment of soil can easily be rolled into a w.re 1/b inch in diameter, the moisture content is above the plastic limit. if the soil is dry t_ugr to be friable and falls apart when rolling it into a wire, the moisture content is below the limit, and the soil may be man- ipulated. The standard method of determining the plastic limit is specified in American Association of State Highway Officials (AASHO) Desionation: T 90-61.