Page 153 - Buried Pipe Design

Design of Gravity Flow Pipes 127

1. Find the maximum ring compression stress in the pipe wall.
2. Evaluate the soil stress at section AA.
The pressure on pipe due to dead load is
P d H 120 1.5 180 lb/ft 2

or
P d D 180(6) 1080 lb/ft

For live load on pipe, use the Boussinesq equation:

0.477WD 0.477 20,000 6
w l 25,440 lb/ft
2
2
H (1.5)
25,440 2
w l
P l 4240 lb/ft
D 6
Total vertical load on pipe
V w d w l 1080 25,440 26,520 lb/ft 2210 lb/in

or
V 26,520 2
P 4420 lb/ft
D 6
Ring compression stress

PD V 2210 2

t 1637 lb/in
2t 2t 2 (0.675)
As in the previous example, the pipe wall will not crush, but this stress
is more than one-half of the compressive strength. There may be local
2
buckling. It is interesting to note that the total load of 4420 lb/ft is equiv-
2
alent to a static load of about 37 ft of cover for soil weighing 120 lb/ft .
Thus the soil in the pipe zone should be compacted to a density required
for 37 ft of cover.
For the dead load on section AA, use Eq. (3.26):
r 2
w d (X 2r) (H 3)
2

9
(2 6) (1.5 3) 120
2
2624 lb/ft (dead load on section AA)