To use this table, find the chart that applies to the particular aircraft wheel/gear load and landing gear type. When
the proper chart is located, read down the left column of the table to determine the appropriate row for the gauge
concerned. When the proper row has been found, read horizontally to the right and under the column headed with
the pipe diameter in question, the minimum cover, in feet, will be found. Examples are shown below:
60,000-lb Single Wheel - 8-gauge pipe
Pipe Diameter
36"
42"*
48"
54" 60"
Cover Required
2.5'
2.75'
3.0'
3.25' 3.5'
27,500-lb Single Wheel- 12-gauge pipe (use 25,000-lb Single Wheel)
Pipe Diameter
24"
30"*
36"
42"
48"
Cover Required
1.5'
1.75'
2.0'
3.5'
3.0'
*Linear interpolation between sizes listed.
BEDDING
Culverts are constructed on a firm, well-compacted soil foundation, except box or arch culverts which may be
placed on a rock foundation when suitable rock is encountered. The foundation is always shaped to fit (or to bed)
about one-fourth of the outside circumference of the pipe or to a minimum depth of one-tenth the diameter. In
addition, foundations for pipe culverts are generally cambered or convexed upward.
The bed is laid so that the invert of the pipe is at grade. The culvert site should be checked to determine the
composition of the soil. If the bedding is composed of a good granular material, only enough will be required to
form a properly compacted and shaped bed. A poor bottom, composed of organic material, muck, silt, or material
large enough to puncture the pipe, will require removal and replacement. The depth of the material that must be
removed will depend upon the actual site conditions.
BACKFILL
Care must be taken in the compaction of the soil in the backfill operation of culvert placement, because the fill
must support the loads to prevent the culvert from crushing. Compaction is accomplished in six-inch layers if
done by hand mechanical tampers and not over four inches if by hand tampers. Backfilling and compaction are
done by hand to a minimum of one foot above the top of the culvert. During this operation the layers should be
kept at the same elevation on both sides of the pipe. When compacted fill is 12 inches over the top of the pipe, fill
and compaction can be made using mechanized equipment. Special attention must be given to the backfill and
compaction beneath the diameter of the pipe. This is the most difficult area to fill and compact, and the most
often neglected. Before compacting the backfill, CMP of 48 inches and over should be internally strutted, as
shown in figure 5-7, page 5-12, to elongate the vertical dimension. The struts are removed after the compaction
operation has been completed.
5-11
EN5465
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