Page 136 - Buried Pipe Design

110 Chapter Three

Overdeflection
Strain limit
Longitudinal stresses
Shear loadings
Fatigue
Delamination

Wall crushing. Wall crushing is the term used to describe the condition
of localized yielding for a ductile material or cracking failure for brit-
tle materials. This performance limit is reached when the in-wall
stress reaches the yield stress or the ultimate stress of the pipe mate-
rial. The ring compression stress is the primary contributor to this per-
formance limit. (See Fig. 3.21.)
P v D
Ring compression (3.11)
2A
where P v vertical soil pressure
D diameter
A cross-sectional area per unit length
However, wall crushing can also be influenced by the bending stress.

Mt/2
Bending stress (3.12)
I
where M bending moment per unit length
t wall thickness
I moment of inertia of wall cross-section per unit length

Wall crushing is the primary performance limit or design basis for
most “rigid” or brittle pipe products. This performance limit may also
be reached for stiffer flexible pipes installed in highly compacted back-
fill and subjected to very deep cover. A quick check for this performance

Figure 3.21 Wall crushing at the
3 and 9 o’clock positions.