Page 327 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SITE UTILITY SYSTEMS

SITE UTILITY SYSTEMS 6.49

2. Pipe in an embankment condition. This is a classification for pipe installed on the sur-
face of the ground (positive projection) or in a trench (negative projection) with a layer
of earth placed above the original ground surface.
3. Pipe in a trench or ditch. This is a classification for pipe installed in an excavation below
grade, with the trench backfilled approximately to the original ground line. Trench load-
ing will be the only condition considered here, since it is the most common means of
installation.

Pipe Classifications
Piping is divided into two classifications, rigid and flexible. Rigid pipe material is concrete,
composite, metal, or clay. Rigid pipe fails when it breaks under a three-edge-bearing test.
Concrete pipe fails when the three-edge-bearing test produces a crack 0.01 in wide and 2 in
long. Flexible pipe is plastic, composite, or corrugated steel. Flexible piping is considered
to fail when loads produce a vertical deflection of 5 percent or more of pipe diameter.

LOADS ACTING ON A PIPE

Definitions
The earth load is the weight of all earth backfill over the pipe. The superimposed load
is produced by either a moving object (live load) passing over the pipe or a uniformly
distributed (dead) load placed at ground level over the pipe (in addition to the earth load).
A uniformly distributed load and a live load acting concurrently on a pipe will not be
considered.

Earth Loads
General. The earth load is the vertical force earth transmits to a pipe buried in a trench.
The pipe is subject to a very complex relationship among many factors. The type of fill
and the depth and width of a trench must all be considered. Original research at Iowa State
College under the direction of Professor Anson Marston produced a formula for determina-
tion of earth loads. This, and additional research, resulted in Eqs. (6.11) and (6.12.)
Earth Load Calculation. The earth load is obtained by selecting the lower result from
either Eq. (6.11) or (6.12). Actual tests have proven that the loads for a positive projection
condition should be used when it is the lower of the two even if the pipe is placed in a
trench.
L = C WD (positive projection condition) (6.11)
p
e
L = C WB (ditch condition) (6.12)
d
e
where L = earth load on the pipe (lb/ft of pipe length)
e
C = coefficient for load calculation of ditch condition
d
C = coefficient for load calculation of positive projection condition
p
W = weight of backfill, lb/ft 3
B = width of trench at top of pipe, ft
D = outside diameter of pipe, ft

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