Deflection of flexible pipes is not a design criterion in most specifications, because if pipes are properly installed with approved soil and compaction level, deflections will be within normal limits. However, the deflection for given loading and backfill conditions
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can be approximated for a round pipe. The traditional method of predicting deflection is the Iowa formula introduced by M. G. Spangler and modified by R. K. Watkins:
DTKWr
AX = ______ _ c______________
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EI + 0.061Er*
Values for the bedding constant may be found in Table 5.26. Because the bedding constant does not vary greatly and the bedding angle is generally not well known, it is often taken as 0.10. Values of the modulus of soil reaction are given in Table 5.27. As used in this table, the bedding material refers to the soil surrounding the pipe, not just the bedding layer on which the pipe rests. The deflection lag factor accounts for the tendency for deflections to increase over time, particularly if the soil is not well compacted or if the soil has a significant plastic content. The value of DL used ranges from 1.0 to 1.5. Generally, reverse curvature of a round flexible pipe occurs when the deflection reaches approximately 20 percent. Traditionally, a factor of safety of 4 is used, so deflections are limited to 5 percent.
