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<br />4. The bottom of the trench of bed excavation shall pe level. <br /> <br />5. The bottom and sides of the soil treatment system to the <br />top of the drainfield rock shall be excavated in such a manner as <br />to leave the soil in a natural, unsmeared, and uncompacted <br />condition. Excavation shall be made only when the soil moisture <br />content is at or less than the plastic limit. <br /> <br />6. When the soil percolation rate at the bottom of th~ trench <br />or bed is slower than fifteen (15) minutes per inch, no vehicle or <br />excavation equipment shall be. allowed on the trench or bed area. <br />Excavation shall be by back hoe or other means that allow the <br />equipment wheels or tracks to remain on the surface soils. <br />Excavation equipment or other vehicles shall not be driven on the <br />soil treatment area. <br /> <br />7. There shall be a layer of at least six (6) inches, but no <br />more than twenty-four (24) inches of drainfield rock in the bottom <br />of the trenches and beds. <br /> <br />8. Where disposal trenches are constructed within ten (10) <br />feet of trees six (6) inches or larger in diameter, or dense <br />shrubbery, or where it can reasonably be anticipated that such <br />vegetation will be present during the expected life of the system, <br />at least twelve (12) inches of filter material shall be placed <br />beneath the distribution pipe. <br /> <br />9. Distribution pipe used in trenches or beds for gravity <br />flow distribution shall be at least four (4) inches in diameter and <br />constructed of solid and durable material not subject to corrosion <br />or decay or to loss of strength under continuously wet conditions. <br /> <br />a. Agricul tural drain tile shall be in twelve (1.2) inch <br />lengths and laid with one-fourth (1./4) inch open joints <br />on grade boards. All open" joints shall be protected on <br />top by strips of asphalt treated building paper at least <br />ten (10) inches 1.ong and three (3) to six (6) inches wide <br />or by other acceptable means. <br /> <br />b. Other devices may be used to distribute sewage tank <br />effluent over the soil treatment area upon approval of <br />the administrator. <br /> <br />10. Distribution pipes used in trenches or beds for pressure <br />distributipn shall be at least one inch in diameter and constructed <br />of sound and durable material not subject to corrosion or decay or <br />to loss of strength under continuously wet conditions. <br /> <br />a. Perforations shall be sized and spaced as shown in Table <br />I. <br /> <br />5 <br />