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Lowest Zone : The lowest zone is backfilled for deep undercuts up to within 4 inches of the bottom of the pipe . Bedding Zone : The zone above the Lowest Zone is the Bedding Zone . Usually it will be the backfill which is the 4 inches of soil below the bottom of the pipe . When rock or other hard material has been removed to place the pipe , the Bedding Zone will be the 12 inches of soil below the bottom of the pipe . i Cover Zone : The next zone is backfill that is placed after the pipe has been laid and will be called the Cover Zone . This zone extends to 12 inches above the top of the pipe . The Cover Zone and the Bedding Zone are considered the Soil , Envelope for the pipe . Top Zone : The Top Zone extends from 12 inches above the top of the pipe to the base or final grade . 120 -8 . 3 . 2 Material : 120 -8 . 3 . 2 . 1 Lowest Zone : Backfill areas undercut below the Bedding Zone of a pipe with coarse sand , or other suitable granular material , obtained from the grading operations on the project , or a commercial material if no suitable material is available . 120 -8 . 3 . 2 . 2 Soil Envelope : In both the Bedding Zone and the Cover Zone of the pipe , backfill with materials classified as A- 1 , A-2 , or A- 3 . Material classified as A- 4 may be used if the pipe is concrete pipe . 120 -8 . 3 . 2 . 3 Top Zone : Backfill the area of the trench above the soil envelope of the pipe with materials allowed on Design Standard , Index No . 505 . 120 -8 . 3 . 3 Compaction : 120 -8 . 3 . 3 . 1 Lowest Zone : Compact the soil in the Lowest Zone to approximately match the density of the soil in which the trench was cut . 120 -8 . 3 . 3 . 2 Bedding Zone : If the trench was not undercut below the bottom of the pipe , loosen the soil in the bottom of the trench immediately below the approximate middle third of the outside diameter of the pipe . If the trench was undercut , place the bedding material and leave it in a loose condition below the middle third of the outside diameter of the pipe . Compact the outer portions to meet the density requirements of the Acceptance Criteria . Place the material in lifts no greater than 6 inches (compacted thickness ) . 120 -8 . 3 . 3 . 3 Cover Zone : Place the material in 6 inches layers (compacted thickness ) , evenly deposited on both sides of the pipe , and compact with mechanical tampers suitable for this purpose . Hand tamp material below the pipe haunch that cannot be reached by mechanical tampers . Meet the requirements of the density Acceptance Criteria . 120 -8 . 3 . 3 . 4 Top Zone : Place the material in layers not to exceed 12 inches in compacted thickness . Meet the requirements of the density Acceptance Criteria . 120 -8 . 3 . 4 Backfill Under Wet Conditions : Where wet conditions are such that dewatering by normal pumping methods would not be effective , the procedure outlined below may be used when specifically authorized by the Engineer in writing . Granular material may be used below the elevation at which mechanical tampers would be effective , but only material classified as A- 3 . Place and compact the material using timbers or hand tampers until the backfill reaches an elevation such that it' s moisture content will permit the use of mechanical tampers . When the backfill has reached such elevation , use normally acceptable backfill material . Compact the material using mechanical tampers in such manner and to such extent as to transfer the compacting force into the material previously tamped by hand . 28 Technical Specifications F :\Public Works\ENGINEERING DIVISION PROJECTS\0744-8th St-US1 to 6th Ave\Admim\bid documents\0744 CONTRACT DOCUMENTS\Technical Specifications . doc