Page 172 - Trenchless Technology Piping Installation and Inspection

138 Cha pte r F o u r

FIGURE 4.16 Wall buckling of fl exible pipes. (Moser and Folkman, 2008.)
on the type of material and the pipe geometry (diameter, wall thick-
ness, and so on). Resistance against buckling may be increased by
stiffening the pipe walls by providing corrugations or by improving
the soil stiffness. The soil stiffness may be improved by using a better
embedment material, by providing better compaction, or both.

Deflection
Flexible pipes have relatively low bending stiffness when compared
with rigid pipes. Owing to this fact, deflection is an important factor
in design and installation of a flexible pipeline. Although flexible
pipes are designed to deflect in order to initiate the passive soil resis-
tance, deflection control is necessary to avoid the pipe from flattening
or reversing its curvature. Deflection control plays an important role
in the determination of quality of pipeline installation and can be a
good indication of its long-term performance (Reddy, 2002). Deflec-
tion limits for flexible pipes usually range from 2 to 7.5 percent
depending upon the pipe material (Howard, 1996). Typical deflection
scenarios are illustrated in Figs. 4.17 and 4.18. The factors that affect
pipe deflection include:

• Pipe stiffness
• Soil resistance (type, density, modulus of elasticity, and mois-
ture content)
• Applied loads
• Trench configuration (geometry and embedment)
• Haunch support
• Construction stages
• Time
• Temperature
• Variability in construction procedures and in soil characteristics