TM 5-820-3/AFM 88-5, Chap. 3
CHAPTER 2
DRAINAGE PIPE
b. Factors which should be considered in select-
2-1. General.
ing the type of pipe include strength under maxi-
A drainage pipe is a structure (other than a bridge)
mum or minimum cover, bedding and backfill con-
used to convey water through or under a runway
ditions, anticipated loadings, length of sections,
fill or some other obstruction. Materials for
ease of installation, corrosive action by liquids car-
permanent-type installations include plain or
ried or surrounding soil, jointing methods, expected
nonreinforced concrete, reinforced concrete, corru-
deflection, and cost of maintenance. Although it is
gated steel, asbestos cement, and day and alumi-
possible to obtain an acceptable pipe installation to
num corrugated pipe.
meet design requirements by establishing special
provisions for several possible materials, ordinarily
2-2. Selection of type of pipe.
only one or two alternates will economically meet
a. The selection of a suitable construction con-
the individual requirements for a proposed drainage
duit will be governed by the availability and suit-
system.
ability of pipe materials for local conditions with
due consideration of economic factors. It is desira-
2-3. Selection of n values.
ble to permit alternates so that bids can be received
Whether the coefficient of roughness, n , should be
with contractor's options for the different types of
based on the new and ideal condition of a pipe or
pipe suitable for a specific installation. Allowing
on anticipated condition at a later date is a difficult
alternates serves as a means of securing bidding
problem. Sedimentation or paving in a pipe will
competition. When alternate designs are
affect the coefficient of roughness. Table 2-1 gives
advantageous, each system will be designed eco-
the n values for smooth interior pipe of any size,
nomically, taking advantage of full capacity, best
shape, or type and for annular and helical
slope, least depth, and proper strength and instal-
corrugated metal pipe both unpaved and 25 percent
lation provisions for each material involved. Where
paved. When n values other than those listed are
field conditions dictate the use of one pipe material
selected, such values will be amply justified in the
in preference to others, the reasons will be clearly
design analysis.
presented in the design analysis.
2-1
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