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vehicle or excavation equipment shall be allowed on the trench <br />or bed area. Excavation shall be by back hoe or other means <br />that allow the equipment wheels or tracks to remain on the <br />surface soil. <br />7. There shall be a layer of at least six (6), but no <br />more than twenty —four (24) inches of filter material in the <br />bottom of the trenches and beds. <br />8. Where disposal trenches are constructed within ten <br />(10) feet of trees six (6) inches or larger in diameter, or <br />dense shrubbery, or where it can reasonably be anticipated <br />that such vegetation will be present during the expected life <br />of the system, at least twelve (12) inches of filter material <br />shall be placed beneath the distribution pipe. <br />9. Distribution pipe used in trenches or beds for <br />gravity flow distribution shall be at least four (4) inches in <br />diameter and constructed of sound and durable material not <br />subject to corrosion or decay or to loss of strength under <br />continuously wet conditions. <br />a. Agricultural drain tile shall be in twelve (12) <br />inch lengths and laid with one — fourth (1/4) inch <br />open .joints on grade boards. All open joints shall <br />be protected on top by strips of asphalt treated <br />building paper at least ten (10) inches long and <br />three (3) to six (6) inches wide or by other <br />acceptable means. <br />b. Other devices may be used to distribute sewage <br />tank effluent over the soil treatment area upon <br />approval of the administrator. <br />10. Distribution pipes used in trenches or beds for <br />pressure distribution shall be at least one inch in diameter <br />and constructed of sound and durable material not subject to <br />corrosion or decay or to loss of strength under continuously <br />wet conditions. <br />a. Perforations shall be sized and spaced as shown <br />in Table I. <br />11. The distribution pipes shall be laid level or on a <br />uniform slope away from the distribution device of no more <br />than four (4) inches per one hundred (100) feet. <br />12. The distribution pipes in beds shall be uniformly <br />spaced no more than five (5) feet apart and not more than <br />thirty (30) inches from the side walls of the bed. <br />13. The filter material shall completely encase the <br />disposal pipes to a depth of at least two (2) inches. <br />