Page 167 - Trenchless Technology Piping Installation and Inspection

Pipe and Pipe Installation Considerations 133

flattening. Farshad (2006) reports the following major causes of fail-
ure in flexible pipeline:
• Fracture
• Buckling
• Weathering, color, and dimensional changes
• Voids, blisters, and delaminations
• Fatigue and corrosion
• Clogging of the pipe system
Specific mechanisms for failure of a pipeline also depend upon
the type of material of the pipe. Structurally, a pipeline is said to have
failed when the performance limits of its material have been reached.
Although underground pipelines and the soil embedment around
them are designed as an integral system, structural failure of the
embedment soil leads to eventual failure of the pipe material. There-
fore, the following discussion on failure mechanisms for rigid and
flexible pipes focuses only on the failure modes of the pipe itself.

4.4.1 Failure Modes in Rigid Pipes
Rigid pipe failures occur when the performance limits for the pipe
material are reached. Performance limits for rigid pipes may be cate-
gorized into

• Ring flexure
• Longitudinal flexure
• Shear
• Radial tension
• Longitudinal tension
• Cracking
• Wall crushing

Depending on the specific pipe material, actual mechanisms of
failure may vary. For instance, radial tension due to hoop stresses
may cause rupture of reinforcement in prestressed concrete cylinder
pipe (ASCE, 2009). Ring flexure in concrete and clay pipes causes
quarter point cracking as shown in Fig 4.9. In cast-iron pipes, longitu-
dinal bending forces could cause circumferential failure of the pipe as
shown in Fig. 4.10 (Makar, 2001).
Other mechanisms of failure include longitudinal cracking caused
by pipe wall thinning and pressure surges in cast-iron pipe as shown
in Fig. 4.11 and spiral cracking of cast-iron pipe as shown in Fig. 4.12
(Makar, 2001).