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