Page 154 - Buried Pipe Design

128 Chapter Three

or
w d 2624 2
P d 1312 lb/ft
X 2
The Live load on section AA can be evaluated by the Boussinesq equa-
tion (3.25):

0.477W
P l total live load on section AA
H 2
W wheel load on the surface
H 4.5 ft (which is the depth to section AA) 1.5 ft 3 ft

0.477W 0.477 (20,000) 2
so P l 471 lb/ft
2
2
H (4.5)
The soil pressure as section AA is
P′ P H P d P l H 1312 471 120(1.5) 1603 lb/ft 2

And H is the load supported by the pipe walls. Vertical soil stress on sec-
2
tion AA is
y P′ 1603 lb/ft .
The maximum pressure
x at the side of the pipe should be no greater
than the vertical load at the top of the pipe. That is,
x should be less
2
than or equal to H 180 lb/ft . If
x is greater than H, the pipes may
collapse inward from the sides. And
x is related to
y by the following
equation:

y 1 sin

x where K 3
K 1 sin

Thus, the pipe must be able to support a horizontal load of
y /3 1603/3
2
2
534 lb/ft , but it will only support 180 lb/ft . To remedy the situation one could
1. Increase the space between the parallel pipes.
2. Place the pipes deeper to diminish the live load, and increase the verti-
cal dead load.
3. Place a concrete slab on the soil surface to distribute the live load.

Rigid pipes
For a rigid pipe, the pipe wall will take almost the entire load because
of the great difference between the modulus of elasticity of the pipe
wall and the modulus of elasticity (compressibility) of the soil. Unlike
flexible pipes, rigid pipes do not exert pressure P x P, against the soil.
Total load Q is supported by the pipe walls in ring compression and
the soil in vertical passive resistance.