Page 26 - Buried Pipe Design

4 Chapter One

soil stiffness. Therefore, soil density is usually given special impor-
tance in piping system design.
Economy in any design is always a prime consideration. The engi-
neer must consider the cost of compaction compared to the cost of bring-
ing in a select material such as pea gravel which will flow into place in
a fairly dense state. For piping systems, a compacted, well-graded,
angular, granular material provides the best structural support.
However, such is not always required. In selecting a backfill material,
the designer will consider such things as depth of cover, depth of water
table, pipe materials, compaction methods available, and so forth.

Strength of Materials
There are many types of piping materials on the market today ranging
from rigid concrete to flexible thermal plastic. Proponents of each lay
claim to certain advantages for their material. Such things as inherent
strength, stiffness, corrosion resistance, lightness, flexibility, and ease
of joining are some characteristics that are often given as reasons for
using a particular material.
A pipe must have enough strength and/or stiffness to perform its
intended function. It must also be durable enough to last for its design
life. The term strength as used here is the ability to resist stress.
Stresses in a conduit may be caused by such loadings as internal pres-
sure, soil loads, live loads, differential settlement, and longitudinal
bending, to name a few. The term stiffness refers to the material’s abil-
ity to resist deflection. Stiffness is directly related to the modulus of
elasticity of the pipe material and the second moment of the cross sec-
tion of the pipe wall. Durability is a measure of the pipe’s ability to with-
stand environmental effects with time. Such terms as corrosion
resistance and abrasion resistance are durability factors.
Piping materials are generally placed in one of two classifications:
rigid or flexible. A flexible pipe has been defined as one that will deflect
at least 2 percent without structural distress. Materials that do not
meet this criterion are usually considered to be rigid. Claims that a
particular pipe is neither flexible nor rigid, but somewhere in between
have little importance since current design standards are based either
on the concept of a flexible conduit or on the concept of a rigid conduit.
This important subject will be discussed in detail in subsequent chap-
ters. See Fig. 1.1.
Concrete and clay pipes are examples of materials which are usually
considered to be rigid. Steel and plastic pipes are usually considered to
be flexible. Each type of pipe may have one or more performance limits
which must be considered by the design engineer. For rigid pipes,
strength to resist wall stresses due to the combined effects of internal