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8.3 0 Sewage d- s; � <br />condition. Excavation shall be made only when the soil <br />moisture content is at or less than the plastic limit. <br />6. <br />When the soil percolation rate at the bottom of the trench or <br />bad is slower than 15 minutes per inch, no vehicle or <br />excavation equipment shall be allowed on the trench or bed <br />area. Excavation shall be by back hoe or other means that <br />allow the equipment wheels or tracks to remain on the <br />surface soils. Excavation equipment or other vehicles shall <br />not be driven on the soil treatment area. <br />7. <br />There shall be a layer of at least 6 inches, but no more than <br />24 inches of drainheld rock in the bottom of the trenches and <br />beds. <br />8. <br />Where disposal trenches are constructed within 10 feet of <br />_ <br />trees 6 inches or larger in diameter, or dense shrubbery, or <br />where it can reasonably be anticipated that such -vegetation <br />will be present during the expected life of the system, at least <br />12 inches of filter material shall be placed beneath the <br />distribution pipe. <br />9. <br />Distribution pipe used in trenches or beds for gravity flow <br />distribution shall be at least 4 inches in diameter and <br />constructed of solid and durable riaterial not subject to <br />corrosion or decay or to loss of strength under continuously <br />wet conditions. <br />a) Agricultural drain tile shall be in 12-inch lengths and <br />laid with .25-inch open joints on grade boards. All open <br />rr <br />joints shall be protected on top by strips of asphalt <br />I, <br />treated building paper at least 10 inches long and 3 to 6 <br />inches wide or by other acceptable means. <br />r <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 />10. <br />Distribution pipes used in trenches or beds for pressure <br />distribution shall be at least 1 inch in diameter and <br />constructed of sound and durable material not subject to <br />, <br />corrosion or decay or to loss of strength under continuously <br />wet conditions. <br />a) Perforations shall be sized and spaced as shown in <br />f <br />Table I. <br />11. The distribution pipes shall be laid level or on a uniform <br />slope away from the distribution device of no more than 4 <br />inches per 100 feet. <br />12. The distribution pipes in beds shall be uniformly spaced no <br />more than 5 feet apart and not more than 30 inches from the <br />side walls of the bed. <br />62140 1 <br />Page 252 Printed 12/28/90 ' <br />