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and concrete lines in dry, neutral pH soils. The very long time All of the pipe materials discussed here have viable applica-
periods shown in this table for higher corrosion index scores tions, but not all materials will perform equally well in a given
may at first appear excessive. However, they are not inconsis- service. Some materials are better suited for postinstallation
tent with previously cited research including one study that inspection. Although all pipelines can be inspected to some
uses 22ot years as a median life expectancy for the normally extent by direct observation and remotely controlled video
corrosion-vulnerable material of cast iron [2]. cameras, steel lines benefit from maturing technologies
Also illustrated by Table 1 1.4 is the other extreme, where low employing magnetic flux and ultrasound inspection devices
corrosion index scores indicate aggressive corrosion condi- (see Chapter 5).
tions. Examples include acidic, contaminated soils; steel pipe Because there is no “miracle” material, the material selec-
with a high potential to become anodic to other buried struc- tion step ofthe design process is partly aprocess ofmaximizing
tures; and concrete pipe in high chloride soils. In these cases, the desirable properties while minimizing the undesirable
a high corrosion rate can lead to through-wall corrosion in a properties. The initial cost ofthe material is not an insignificant
matter of months. property to be considered. However, the long-term “cost of
In producing this table for a specific study, it was recognized ownership” is a better view of the economics of a particular
that these hypothesized corrosion rates will not likely prove to material selection. The cost of ownership would include ongo-
be accurate in the real world, because they are not based on any ing maintenance costs and replacement costs after the design
empirical data. Nevertheless, an estimated relationship life has expired. This presents a more realistic measure with
between the corrosion scores and corrosion rates may be useful which to select a material and ultimately impacts the risk pic-
when applied consistently in this relative model. As databases ture more directly.
become more populated and engineers specifically seek data The evaluator should check that pipe designs include appro-
that demonstrate the relationship sought, the equations can be priate consideration of all loadings and correctly model pipe
better established to increase the ability of the model to predict behavior under load. Design calculations must always allow for
actual failure rates. the pipe response in determining allowable stresses. Pipe mate-
rials can be placed into two general response classes: flexible
and rigid. This distinction is a necessary one for purposes of
VIII. Design index design calculations because in general, a rigid pipe requires
more wall thickness to support a given load than a flexible pipe
This index captures much of the system strength or failure- does. This is due to the abiiity ofthe flexible pipe to take advan-
resistance considerations and is fully discussed in Chapter 5. tage of the surrounding soil to help carry the load. A small
The emphasis of the described assessment is to identify and deflection in a flexible pipe does not appreciably add to the pipe
rank the presence and severity ofpotential failure mechanisms. stress and allows the soil beneath and to the sides to carry some
When failure resistance is coupled with the measurement of a of the load. This pipe-soil structure is thus a system of high
failure mechanism’s aggressiveness, time-to-failure estimates effective strength for flexible pipes [60] but less so for rigid
can be made. For example, a corrosion index score indicating pipes.
aggressive corrosion, coupled with a design index indicating Some common pipe materials, often found in distribution
low pipe strength and higher stress states, suggests a short time systems, are discussed below.
to failure.
Rigidpipe
Pipe materials, joining, and rehabilitation
Asbestos cement pipe is generally viewed as a rigid pipe
A basic understanding of common pipe materials is important although it does have a limited amount of flexibility. Because
in assessing the risks in this index. Although transmission asbestos fibers and dust are hazardous to health, special care is
pipelines are overwhelmingly constructed of carbon steel, dis- warranted in working around this material if airborne particles
tribution lines have historically been built from a variety of are generated. This pipe has been used in both pressurized and
materials. Because a distribution system will often be a com- gravity-flow systems.
posite of different materials, it is useful to distinguish between Clay pipe is a low-strength material historically used in non-
materials that influence the risk picture differently. The mater- pressure applications. The advantages of the material include
ial’s behavior under stress is often critical to the evaluation. A high abrasion resistance and high resistance to corrosion.
more brittle material has less impact resistance. Impact resist- Concrete pipe includes several designs such as prestressed
ance is particularly important in reducing the severity of out- concrete cylinder pipe, reinforced concrete cylinder pipe, rein-
side force loadings. In regions of unstable ground, materials forced concrete noncylinder pipe, and pretensioned concrete
with higher toughness will better resist the stresses of earth cylinder pipe. These pipes are available in medium to large
movements. Traffic loads and pipe handling activities are other sizes and are typically used in nonpressure to moderately pres-
stress inducers that must be withstood by properties such as the surized systems. In recent years, large leaks have resulted from
pipe material’s fatigue (cracking) and bending (tensile) failed concrete pipe where the steel reinforcement has corroded
strengths. Stresses resulting from earth movements and/or tem- and the pipe has failed in a brittle fashion [60].
perature changes may be more significant for certain pipe Cast iron pipe, also called gray cast iron, is a part of the
materials. In certain regions, a primary ground movement is pipeline infrastructure in many countries. The first gas distribu-
caused by the seasonal freedthaw cycle. One study shows that tion systems installed in the United States were almost entirely
in some pipe materials, as temperature decreases, pipe breaks of cast iron pipe. More than 50,000 miles of cast iron pipe
tend to increase exponentially [5 11. remain in the U.S. distribution systems [15]. Cast iron pipe is