Page 321 - Buried Pipe Design

292 Chapter Six

and ring deflection were constrained to zero. Also, for a given gage of
steel, the cross-sectional area increases as the corrugations height is
increased.
In Utah State University tests, it was shown that ring flexibility
2
(D/r) influences crushing strength in loose soil. This is seen in Fig. 6.4
where the curves drop off to the right as the ring flexibility increases.

Reversal of curvature. As the load increases, a section of the ring may
tend to flatten and then reverse curvature (Fig. 6.6). There are two gen-
eral types of reversal of curvature. In the case of very loose soil (density
less than the critical void ratio), as the soil is compressed downward the
pipe tends to form an ellipse, but in so doing high flexural stresses
develop at the sides. These stresses combined with some ring compres-
sion cause plastic hinges. If this deformation is carried to the extreme,
the top of the pipe comes down in a reversal of curvature, and ultimately
a third plastic hinge forms in the top center. The other type of reversal
occurs in dense soil and may be referred to as localized buckling. This is
not confined to top center. It usually forms between 10 and 2 o’clock, but
not necessarily so. Occasionally the reversal occurs in the bottom
between 5 and 7 o’clock. None has been seen in the sides between about
2 and 5 o’clock or between almost 7 and 10 o’clock. The performance

Loose Soil Dense Soil

Initial Initial
Pipe Pipe

Ogree curve
Flattening of top precedes
starts reversal
reversal of
curvature of curvature

General Reversal of Curvature Localized Reversal of Curvature
(Extreme Deformation) (Extreme Deformation)
Cusp
Hinge Hinge Hinge

Figure 6.6 Comparison of the types of reversal of curvature
observed in dense and loose soil.