Page 152 - Buried Pipe Design

126 Chapter Three

where W wheel load on surface and H 2.5 ft, which is the depth to section
AA 1.5 ft 1 ft. So
W 0.477 (20,000) 2
P l 0.477 1526 lb/ft
H 2 (2.5) 2
The soil pressure on section AA is

P′ P H P d P l H 712 1526 120(1.5) 2058 lb/ft 2
where HD is the load supported by the pipe walls. Vertical soil stress on
section AA is

y P′ 2058 lb/ft 2
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 col-
lapse inward from the sides.
x is related to
y by the following equation:

y

x
K
where
1 sin
K 3
1 sin

where K is Rankine’s lateral pressure ratio and
is the soil friction angle
and for this soil
30°.
Thus, the pipe must be able to support a horizontal load of
y /3 2058/3
2
2
686 lb/ft , but it will only support 180 lb/ft (vertical dead load on top of
pipe). To remedy the situation, one could
1. More than triple 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.

Example Problem 3.4 What is the vertical soil stress at section AA of
Fig. 3.26? The pipes are 72-in-diameter HDPE profile-wall pipe. There is 24
in of soil between the two at the spring line. Soil cover is 1.5 ft of soil at a
3
unit weight of 120 lb/ft . A surface wheel load of W 20 kips is anticipated.
This is similar to the last example.
Area per unit length effective thickness 0.675 in
Stiffness F/ y 18 lb/in 2

Tensile strength 1000 lb/in 2
Compression strength 3000 lb/in 2