In general, bedding for a conduit should comprise select, compact material that conforms to the external curvature of the conduit it supports. This is important for both flexible and rigid conduits.

For a flexible conduit, irregularities or imperfections in the bedding usually can be accommodated by minor shape deformations in the conduit without damage to the structural integrity of the pipe.

For a rigid conduit, such irregularities or imperfections in the bedding cannot be accommodated because the conduit cannot reshape itself without structural failure. Due to the compressive/tensile characteristics of rigid pipe under a load, critical shear zones can fail if bedding geometry is not in conformance with specifications. See Figure 14-3 for a schematic illustration of this characteristic.

Planned bedding should be supported thoroughly by specifications.

Bedding affects required reinforced concrete pipe strength. The four recognized classes of bedding are shown in Figures 14-4 through 14-7. The most common classes of bedding are Class B and Class C. Class C is the most economical and Class A the most expensive. However, for a given fill height, Class A bedding requires the lowest reinforced concrete pipe strength, and Class C requires the greatest strength. Base selection of bedding on designing the most cost-effective facility.