Page 320 - Buried Pipe Design

Steel and Ductile Iron Flexible Pipe Products 291

stresses. A longitudinal seam in one panel causes a stress concentration
in the wall of the adjacent panel and triggers wall crushing. Of course,
as wall crushing develops, wall buckling is initiated and buckling near
seams causes seam failure—truly an interaction phenomenon.
In every case, the performance limit is a ring deformation observable
inside the pipe. The probable deviation in observing performance lim-
its may be as much as 10 percent of vertical soil pressure, especially
near the critical void ratio. The following are some deformations iden-
tified as performance limits in these tests.

Wall crushing. When the pipe is buried in densely compacted soil
(denser than the critical void ratio), wall crushing is often the first
indication that performance limit has been reached. Slight dimpling of
the corrugations is the first visual indication of distress. Dimpling is
not a performance limit, but dimpling portends the location of general
wall crushing. This crushing usually occurs between 10 and 2 o’clock
in the ring. Deep corrugations dimple as soon as or sooner than shal-
low corrugations, but general wall crushing shows up at equal or
slightly higher pressures. In general, wall crushing develops as shown
in Fig. 6.5. It starts with a dimpling of the corrugations and progresses
into an accordion effect.
The crushing strength of the wall is the yield point stress times the
wall cross-sectional area per unit length of pipe. The yield point stress of
2
the steel involved in this experiment varied between 35 and 45 kips/in .
This variation was not significant because the influence is partially
masked out by other variables, such as seam strength and ring deflec-
tion. There is little doubt that crushing strength would be directly pro-
portional to the yield point stress if seams were 100 percent efficient

Initial ring Beginning of
wall crushing

Figure 6.5 Diagrammatic sketch
Continued wall Advanced stages
crushing of wall crushing of the mechanism of wall crushing.
(accordion effect)