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I I I I I I I I I I I I I I I I I I I 11. The distribution pipes shall be laid level or on a uniform slope away from the distribution device of no more than four (4) inches per one hundred (100) feet. 12. The distribution pipes in beds shall be uniformly spaced no more than five (5) feet apart and not more than thirty (30) inches from the side walls of the bed. 13. The drainfield rock shall completely encase the disposal pipes to a depth of at least two (2) inches. The top of the drainfield rock in trenches, beds, and mounds must be level in all directions. 14. The fIlter material shall be covered with either a permeable synthetic fabric or a four (4) inch layer of compacted hay or straw covered with untreated building paper. Where a drop box distribution system is used to fill a trench to within two inches of the top of the drainfield rock, a permeable synthetic fabric must be used to cover the drainfield rock. 15. The trenches or beds shall be backfilled with the excavated material and slightly crowned above finished grade to allow for settling. The backfill shall not be completed to a density greater than that of the original soil. 16. The minimum depth of cover over the distribution pipes shall be a least eight (8) inches. The maximum depth of cover over the distribution pipes shall be no more than thirty-six (36) inches and preferably no more than twenty-four (24) inches. 17. A grass cover shall be established by the owner or the owner's agent over the soil treatment area. Dual Field Systems 1. Dual field systems shall be used only where the percolation rate is slower than five minutes per inch. 2. Dual field systems shall be sized, designed, and constructed as set forth above for standard systems except as follows: a. The soil treatment area shall be divided into two or more parts. b. Alternating soil treatment areas shall each be connected to a valve box outlet 3. A part of the soil treatment area shall be used no more than one year unless inspection of the effluent level indicates that a longer duration can be used. Slowly Permeable Soils 1. Excavation for the purpose of constructing a soil treatment system must not be made in any soil layer having a percolation rate slower than 120 minutes per inch. 2. Excavation for the purpose of constructing a soil treatment system must not be made in a soil layer having a percolation rate slower than 60 minutes per inch unless the moisture content is lower than the plastic limit of the soil. 3. Drainfield rock must not be placed in contact with original soil having a percolation rate slower than 60 minutes per inch. 53