Civil Engineering Reference
In-Depth Information
mass supported by the pipeline will usually cause a more critical design condition than the
addition of a horizontal pseudostatic force acting on the sides of the pipeline.
As indicated in Eq. (11.1), the only unknowns in the pseudostatic method are the weight
of the soil mass bearing on the top of the pipeline
W
and the seismic coefficient
k
v
.
As pre-
viously mentioned, the seismic coefficient
k
v
can be assumed to be equal to
2
3
(
a
max
/
g
)
.
The
determination of
W
is described in the next section.
11.4.2 Static Design
For static design, the external load on a pipeline depends on many different factors. One
important factor is the type of pipeline (rigid versus flexible). Another important factor is
the placement conditions, i.e., whether the pipeline is constructed under an embankment,
in a trench, or is pushed or jacked into place. Figure 11.8 illustrates the three placement
conditions of trench, embankment, and tunnel (or pushed or jacked condition).
Other factors that affect the external load on a pipeline for the static design include the
unit weight and thickness of overburden soil, the surface loads such as applied by traffic,
compaction procedures, and the presence of groundwater (i.e., buoyant conditions on an
empty pipeline).
Rigid Pipeline Design for Static Conditions.
Examples of rigid pipelines include precast
concrete, cast-in-place concrete, and cast iron. Design pressures due to the overlying soil
pressure are as follows:
Minimum Design Load.
In general, the minimum vertical load
W
on a rigid pipeline is
equal to the unit weight of soil
t
times the height
H
of soil above the top of the pipeline
times the diameter of the pipe
D,
or
W
min
t
HD
(11.2)
As an example, suppose the pipeline has a diameter
D
of 24 in (2 ft) and a depth of over-
burden
H
of 10 ft, and the backfill soil has a total unit weight
t
of 125 lb/ft
3
. Therefore, the
minimum vertical load
W
min
acting on the pipeline is
W
min
(125 lb/ft
3
) (10 ft) (2 ft)
2500 lb per linear foot of pipe length
Embankment Condition.
Different types of embankment conditions are shown in Fig.
11.8. In many cases, compaction of fill or placement conditions will impose vertical loads
greater than the minimum values calculated above. Also, because the pipe is rigid, the arching
effect of soil adjacent to the pipe will tend to transfer load to the rigid pipe.
Figure 11.9
a
shows the recommendations for a pipeline to be constructed beneath a fill
embankment. In Fig. 11.9
a
,
W
vertical dead load on the pipeline,
D
diameter of the
pipeline, and
B
width of the pipeline (that is,
B
D
). Note that Fig. 11.9
a
was developed
for an embankment fill having a total unit weight
t
100 lb/ft
3
and an adjustment is
required for conditions having different unit weights.
As an example, use the same conditions as before (
B
D
2 ft,
H
10 ft, and
t
125 lb/ft
3
). Figure 11.9
a
is entered with
H
10 ft, the curve marked 24 in (2 ft) is intersected,
and the value of
W
read from the vertical axis is about 3800 pounds. Therefore,
W
3800
125
____
100
4750 lb per linear foot of pipeline length
Note that this value of 4750 pounds is greater than the minimum dead load (2500 lb), and
the above value (4750 lb) would be used for the embankment condition.
