1 March 1997
for flexible pipe.
c. Pipe installation.
(1) Bedding. Figure 5-2 depicts various classes of bedding generally used when installing
sewers. A complete discussion of each class is contained in several engineering publications,
including WEF Manual of Practice FD-5 and the Clay Pipe Engineering Manual by National Clay
Pipe Institute (NCPI). The designer should refer to these when selecting a pipe bedding system.
When the class and type of bedding have been chosen, the bedding materials will be
coordinated with and identified in the specifications, and the types and sizes will be shown on the
(2) Backfill and compaction. Backfill materials and compaction requirements will be
included in the specifications. The possible use of locally available materials for backfill will be
investigated. Compaction requirements will be designated for the particular soil and moisture
content at the site.
(3) Installation manuals for the particular types of pipe to be specified will be reviewed to
ascertain that bedding, backfill and compaction are adequate for the existing subsurface
conditions at the site.
5-4. SPECIAL DESIGNS.
a. Unsatisfactory soil conditions. Information on subsurface conditions must be obtained
from borings, drill holes, or test pits prior to design of the sewer system. Soil considered too
unstable for use as pipe bedding or backfill consists of silt, quicksand, peat bog, muck and other
organic materials. Where these materials exist, the following procedures will be used to provide
a suitable pipe bedding.
(1) In situations where unstable materials occur at shallow depths, it will generally be
acceptable to over excavate native soil to just below the trench bottom, and replace with a layer
of crushed stone, gravel or other coarse aggregate. Concrete or wooden cradles can be used in
lieu of aggregates.