Page 99 - Buried Pipe Design

Design of Gravity Flow Pipes 75

P y

P X s y

P X
X y
s Ks
K

j
Figure 3.3 Infinitesimal soil cube B. Conditions for soil slip are
P x K
y .

soil. Both principles of stability and experience show a wide trench is
seldom justified.
For small deflection (5 percent or less), theoretically, the embedment
needs little horizontal strength. Good sidefill soils add a margin of
safety. See Fig. 3.3, where the infinitesimal soil cube is in equilibrium
as long as the pipe pressure does not exceed sidefill soil strength
x .
For stability, P

x K
y .
x
As long as the pipe is nearly circular, in poor native soils, the pipe
depends little on the side support from the trench wall and the trench
does not need to be wider than half a diameter on each side for both
rigid and flexible pipes. If ring deflection of a flexible pipe is no more
than 5 percent, the effect of ring deflection can be neglected. On a rigid
pipe, P y is the Marston load. On a flexible pipe, P y is more nearly the
26
prism load, H. 45,46
The height of soil cover H is not a pertinent variable in the analysis of
trench width. As soil load is increased, the pressure on the pipe increases;
but the strength of the sidefill soil increases in direct proportion.

Rules of thumb for required trench width for flexible pipes
1. Trench must be wide enough for proper soil placement.
2. In poor soil, specify a minimum width of sidefill of half a diameter
D/2 from the pipe to the walls of the trench, or from the pipe to the
windrow slopes of the embedment in an embankment.
3. In good soil, the width of sidefill can be less, provided that the
embedment is placed at adequate density.