Page 27 - Buried Pipe Design

Introduction and Overview 5

Figure 1.1 The effect of soil settlement on (a) rigid and (b) flexible pipes. S represents
settlement of backfill for a rigid pipe. D represents vertical deflection of a flexible pipe
as it deflects under earth pressure. (Reprinted, by permission, from AWWA Manual M-11,
Steel Pipe Design and Installation, American Water Works Association, 1964.)
pressure and external load is usually critical. For flexible pipes, stiff-
ness may be important in resisting ring deflection and possible buck-
ling. Each manufacturer or industry goes to great lengths to establish
characteristics of its particular product. These parameters are readily
available to the design engineer. The desire to have products with high
strength has given rise to reinforced products such as steel-reinforced
concrete and glass-reinforced thermal setting plastic. For such prod-
ucts, other performance limits often arise such as strain limit to pre-
vent cracking. For a thermal plastic pipe, such as PVC pipe, strength
is measured in terms of a long-term hydrostatic design hoop stress.
Thus, it can be seen that not all installations of all products will be
designed in exactly the same manner. The engineer must be familiar
with design criteria for the various pipe products and know where
proper design parameters can be obtained.

Pipe Hydraulics
The field of study of fluid flow in pipes is often referred to as
hydraulics. Designers of water or sewer systems need some knowledge
of pipe hydraulics.
Flow in pipes is usually classified as pressure flow for systems where
pipes are flowing full or open-channel flow when pipes are not flowing
full. Water systems are pressure systems and are considered to be
flowing full. On the other hand, sewer systems, for the most part, are
open-channel systems. The exception to this is forced sewer mains
where lift pumps are used to pump sewage under pressure. The relatively