Page 315 - Buried Pipe Design

286 Chapter Six

preferred gage was designated which would produce a reasonably stiff
pipe for handling and installation.
Later, fill height tables were developed on the basis of favorable expe-
rience on actual installations and then were extended as pipes were
placed under higher fills. Limits were imposed when trouble developed.
These tables were sometimes shown in manufacturers’ literature and
were often reproduced in highway design standards. This procedure
was sometimes reversed when an agency examined its experience on a
large number of installations made under controlled conditions.
Development of the art in this fashion was appropriate for the rela-
tively low fills used in highway construction and the conservative per-
formance limits that evolved were of little consequence from an
economic standpoint. However, as pipe applicability expanded and as
depth of cover increased, greater attention was focused on significant
bases for design. At least three emerged:

1. Excessive ring deflection or flattening of the pipe
2. The strength of longitudinal and helical seams
3. Ring compression stress in large pipes (wall crushing or elastic
buckling of the pipe wall)

Champions of each of the design criteria could cite examples and
propose installation procedures which tended to limit the investigation
to one basis and obviate the others. With the advent of modern high-
way design and construction methods, larger culverts and higher fills
demanded more rigorous design procedures in order to provide safe,
economical installations. With a considerable amount of cooperation,
effort, and compromise, design factors and other considerations have
been established. Design factors should be verified and modified if nec-
essary. Maximum allowable limits of performance should be reviewed.
Various simplified theories have been proposed for the design of
buried, corrugated steel pipes. Each may be valid, but only within lim-
itations. One theory is based on ring deflection y/D. (See Fig. 6.1.)
The external soil load on a buried pipe generally causes the cross-
section (or ring) to deflect such that the vertical diameter decreases
and the horizontal diameter increases. According to the ring deflection
theory, to design the pipe, some maximum allowable ring deflection is
specified, then the actual ring deflection is predicted by one of several
available equations. The vertical deflection Δy is usually more pre-
dictable and more meaningful than the horizontal deflection x. How-
ever, y and x are approximately the same for steel pipe (although
opposite in sign) with y usually the larger.
Other design theories are based on the compressive ring stress in
the pipe wall. The ultimate (or maximum allowable) compressive