Page 267 - Buried Pipe Design

238 Chapter Four

Conclusions

1. Rerounding is influenced mostly by internal pressure and possibly
to some extent by time.
2. It has been shown that rerounding is generally insensitive to soil
density, burial depth, pipe stiffness, and pipe material.
3. Maximum combined strains occur when starting deflections are
highest.
4. Rerounding generally increases linearly with increasing internal
pressure and may be represented by the rerounding factor R com-
puted as follows:

P n
R 1 (4.25)
435
It was found that rerounding is primarily a function of pressure,
and a rerounding relationship is proposed which accounts for the
reduction of bending strain as pressure increases. It was also found
that the highest allowable deflection for a pipe determines the cal-
culated behavior since the residual bending strain for an initial
highly deflected pipe is more than the residual strain for a low initial
deflected pipe.
Recommendation. Combined strain should be calculated as follows:

PD t y
C RD f (4.26)
2tE D D
where C combined strain
D f shape factor, range 3 to 8 (3 for uniform compaction and a
2
pipe stiffness greater than 40 lb/in , 6 for poor haunch or
nonuniform compaction, 8 for nonuniform compaction and
2
pipe stiffness less than 15 lb/in )
R rerounding factor 1 P n /435
P internal pressure
t wall thickness
E Young’s modulus
D pipe diameter
y vertical pipe deflection
2
P n internal pressure, lb/in (0
P n
435)
Thrust restraint
Unbalanced hydrostatic and hydrodynamic forces in piping systems
are called thrust forces. In the range of pressures and fluid velocities
found in waterworks or wastewater piping, the hydrodynamic thrust