Civil Engineering Reference
In-Depth Information
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
) (20 ft) (2 ft)
5000 lb per linear foot of pipe length
For the embankment condition, use
B
D
2 ft,
H
20 ft, and
t
125 lb/ft
3
. Fig-
ure 11.9
a
is entered with
H
20 ft, the curve marked 24 in. (2 ft) is intersected, and the
value of
W
read from the vertical axis is about 7600 lb. Therefore,
125
____
W
7600
100
9500 lb per linear foot of pipeline length
Note this value of 9500 lb is greater than the minimum dead load (5000 lb), and the above
value (9500 lb) would be used for the embankment condition.
For the trench condition, use
D
2 ft,
H
20 ft, and
t
125 lb/ft
3
. Also assume
that the trench width at the top of the pipeline will be 4 ft (
B
4 ft), and the trench will
be backfilled with sand. Figure 11.9
a
is entered with
H
B
20
4
5, the curve marked
sands
is intersected, and the value of
C
W
of about 2.3 is obtained from the vertical axis.
Therefore,
W
C
w
t
B
2
2.3 (125) (4
2
)
4600 lb per linear foot
Note this value of 4600 lb is less than the minimum value (5000 lb), and thus 5000 lb would
be used for the trench condition.
For the jacked or driven pipelines, use
D
B
2 ft,
H
20 ft, and
t
125 lb/ft
3
, and
the pipeline will be jacked through a sand deposit. Figure 11.9 is entered with
H
B
20
2
10, the curve marked
sand
is intersected, and the value of
C
w
of about 1.6 is obtained
from the vertical axis. Therefore,
W
C
w
t
B
2
1.6 (125) (2
2
)
800 lb per linear foot
Note this value of 800 lb is less than the minimum load value (5000 lb), and thus the value
of 5000 lb would be used for the jacked or driven pipe condition. Basic soil mechanics indi-
cates that the long-term load for rigid pipelines will be at least equal to the overburden soil
pressure (i.e., the minimum design load).
Earthquake conditions:
Use
B
2 ft,
H
20 ft,
t
125 lb/ft
3
; and assume that for the
design earthquake, the peak ground acceleration
a
max
0.30
g.
Using
k
v
2
⁄
3
k
h
2
⁄
3
(0.30)
0.20, the pseudostatic forces are as follows:
Minimum pseudostatic force:
F
v
k
v
W
min
0.20 (5000)
1000 lb per linear foot
Embankment condition:
F
v
k
v
W
0.20 (9500)
1900 lb per linear foot
For the trench condition,
F
v
k
v
W
min
0.20 (5000)
1000 lb per linear foot
For jacked or driven pipeline,
F
v
k
v
W
min
0.20 (5000)
1000 lb per linear foot
