Page 149 - Buried Pipe Design

Design of Gravity Flow Pipes 123

X width of section AA between pipes

V total vertical load per length on section AA w d w l
V′ V HD vertical load per length supported by soil at section
AA total vertical load reduced by load that is supported by
pipe walls
H height of soil cover over pipe

For design, the strength of the column at section AA must be greater
than the vertical load. Failure (a performance limit) occurs if either of
the following happens:

1. Thrust in the pipe wall exceeds the ring compression strength.
2. The vertical soil stress at Section AA exceeds the compressive
strength (vertical resistance) of the soil.

The ring compression strength of the pipe wall is usually the yield
strength. For rigid pipes,
f is crushing strength of the wall. The value
for
f can be obtained from the pipe manufacturer.
The strength of the soil is found as follows. Assume that the
embedment is granular and compacted. Soil strength is vertical
stress
y at slip. Horizontal soil stress is provided by the pipe walls.
Approximate soil strength may be found from triaxial soil tests in
which interchamber pressure is equal to the horizontal pressure P x of
the pipe against the soil. For circular, flexible pipes at soil slip, P x
P d H.
Stresses in the pipe and soil are each calculated independently. This
is because the bond between soil and pipe can be assumed to be zero.
The bond cannot be assured because of fluctuations in temperature,
moisture, loads, etc., all of which tend to break down the bond at the
soil-pipe interface.
The pipe must be adequate. Therefore, before the soil column is ana-
lyzed, design starts with the ring compression equation

PD
f

2A SF

For worst-case ring compression, the live load W is directly above
the pipe where P P l P d . The live load effect P l can be found by the
Boussinesq equation (see Chap. 2). If W is assumed to be a point load
directly over the pipe, the Boussinesq equation reduces to

0.477W
P l (3.25)
2
H