TM 5-820-3/AFM 88-5, Chap. 3
age. The study revealed that buried conduits and
plastic hinge, and excessive deflection, as functions
associated storm drainage facilities installed from
of pipe size and stiffness, backfill conditions, fill
the early 1940's until the mid-1960's appeared to be
height, and live load were considered for flexible
in good to excellent structural condition. However,
pipes. Steel yield and concrete crushing, shear fail-
many failures of buried conduits were reported
ure and tensile cracking, as functions of pipe size,
during construction. Therefore, it should be noted
backfill conditions, full height, concrete strength,
that minimum conduit cover requirements are not
steel content, and live load were considered for re-
always adequate during construction. When
inforced concrete pipe. Nonreinforced concrete and
construction equipment, which may be heavier than
vitrified clay pipe design are based on the American
live loads for which the conduit has been designed,
Concrete Pipe Association's D-load design pro-
is operated over or near an already in-place
cedure based on a 0.01-inch crack.
f. The tables (B-I through B-23) in appendix B
underground conduit, it is the contractor's
responsibility to provide any additional cover
identify the recommended minimum and maximum
during construction to avoid damage to the con-
cover requirements for storm drains and culverts.
duit.
These cover depths are valid for the specified loads
d. Since 1940 gross aircraft weight has
and conditions, including average bedding and
increased twenty-fold, from 35,000 pounds to
backfill. Deviations from these loads and conditions
approximately 700,000 pounds. The increases in
significantly affect the allowable maximum and
aircraft weight have had a significant effect on
minimum cover, requiring a separate design
design criteria, construction procedures, and
calculation. Most pipe seams develop the full yield
material used in the manufacture and construction
strength of the pipe wall. However, there are some
of buried conduits. Major improvements in the
exceptions which occur in standard metal pipe
design and construction of buried conduits in the 2
manufacture. To maintain a consistent safety factor
decades mentioned include among other items
of 2.0 for these pipes, the maximum ring
increased strength of buried pipes and conduits,
compression must be one-half of the seam strength
increased compaction requirements, and revised
rather than one-half of the wall strength for these
minimum and maximum cover tables.
pipes. Table 2-2 shows cover height reductions for
e. For minimum and maximum cover design, H-
standard riveted and bolted seams which do not
20, 15-K, F-15, C-5A, C-141, C-130, B-I and B-52
develop a strength equivalent to fy = 33,000 pounds
live loads and 120 pounds per cubic foot backfill
per square inch. The reduction factors shown are
have been considered. Cover heights for flexible
the ratios of seam strength to wall strength. The
pipes and reinforced concrete pipes were based on
maximum cover height for pipes with weak
an analysis of output (Juang and Lee 1987) from
seaming as identified in table 2-2 can be determined
the CANDE computer program (FHWA-RD-77-5,
by multiplying the maximum cover height for a
FHWA-RD-77-6, FHWA-RD-80-172). Wall crush-
continuously-welded or lock seam pipe (app B) by
ing, seam separation, wall buckling, formation of a
the reduction factors shown in table 2-2.
2-3
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