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<br /> <br />32 <br /> c. Design and Construction <br /> <br />1. The bottom trenches and beds shall be at <br />least 30 24 inches above the active mottling <br />zone or bedrock. <br /> <br />2. The trenches shall not be less than 24 <br />inches nor more than 36 inches wide. Any <br />trench wider than 36 inches shall be <br />considered a bed. <br /> <br /> 3. Trenches and beds shall not be more than <br />100 ft in length. <br /> <br /> 4. The bottom of the trench or bed excavation <br />shall be level. <br /> <br /> 5. The bottom and sides of the soil treatment <br />system to the top of the filter material shall <br />be excavated in such a manner as to leave <br />the soil in a natural, un-smeared, and <br />uncompacted condition. Excavation shall be <br />made only when the soil moisture content is <br />at least or less than the plastic limit. <br /> <br /> 6. When the percolation rate is slower than 15 <br />minutes per inch, excavation shall be by <br />back hoe or other means that allow the <br />equipment wheels or tracks to remain on the <br />surface soil. Excavation equipment or other <br />vehicles shall not be driven on the soil <br />treatment area. <br /> <br /> 7. There shall be a layer of at least 12 inches <br />but no more than 24 inches of filer materials <br />on the bottom of the trenches and beds. <br /> <br /> 8. Distribution pipes – gravity distribution <br />