Page 340 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SITE UTILITY SYSTEMS

6.62 CHAPTER SIX

BEDDING

General

Bedding is the contact between pipe and earth and is used when pipe is installed in a trench,
prior to the major backfilling operation. The type of bedding used has an important influ-
ence on the total load any pipe can support.

Bedding Methods

Rigid Pipe. The four most popular bedding methods are shown in Fig. 6.33. The load
factors are indicated in each detail. The most common are the Class B methods. The least
desirable is Class D, which is not recommended. In all cases, the bell holes are dug out
prior to placement of pipe.

Flexible Pipe. The recommended bedding method for flexible pipe is shown in Fig. 6.34.

Pipe Bedding Material Classification

ASTM D 2321 presents a method of classifying soils and aggregates that are used as bed-
ding and backfill around pipes. Refer to Table 6.20 for the classification of embedment and
backfill material.

Selection of Bedding and Backfill Material
General recommendations for the selection and installation of soils for bedding and backfill
are given in Table 6.21.

Rigid Pipe

The load factor for a bedding condition is used to determine the actual supporting strength of a
pipe. As will be seen, the load factor increases the total load a pipe can support. The laboratory-
calculated pipe strength multiplied by a load factor will give the field supporting strength
of a pipe. These load factors have been determined experimentally for the various bedding
methods described and are indicated in Fig. 6.33 in each bedding diagram. However, they
do not contain a safety factor. The result of the calculations will determine whether the field
supporting strength of the pipe is great enough to resist the imposed load.

Flexible Pipe
Pipe Deformation (Deflection)

1. Radial deflection. Commonly called ovalling, radial deflection is a change in pipe diam-
eter caused by an exterior load on a portion of the pipe. This will occur in buried pipe
from a load on the top portion of the pipe if the support of the surrounding soil is not
enough to resist the force.
2. Longitudinal deformation. Often referred to as sagging, longitudinal deformation is a
change along the length of a pipe that causes the pipe to bend.
3. Deformation at structures. Not limited to flexible pipe, this type of deformation is
caused by uneven settlement at wall penetrations and manholes.

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