Page 203 - Trenchless Technology Piping Installation and Inspection

Pipe and Pipe Installation Considerations 167

bead has a negligible effect on flow. In nonpressure gravity design,
some engineers believe that the turbulent flow created by the bead is
helpful in keeping solids suspended in the flow; others feel it is detri-
mental to the flow.
Other types of joints in HDPE pipe include electrofusion and
bell-and-spigot gasket joints.
Advantages and Limitations
The continuous jointless conduit that results from the butt-fusion of
HDPE pipe makes it an ideal piping material for pull-in installations
such as horizontal directional drilling and pipe bursting. HDPE pipe’s
ability to withstand both the internal and external corrosion is advan-
tageous for both water and sewer systems. Its high flexibility is a
favorable characteristic during trenchless installations; there is no
need for very long entry pits other than for larger diameters and deep
installations. A very low internal resistance to flow makes it a good
material for both pressure and nonpressure gravity systems. In pres-
sure water systems, the expansive forces of freezing water do not
cause the pipe to crack. In very cold temperatures, HDPE has a high
resistance to failure by impact. The pipe also resists shatter-type or
rapid crack-propagation failure.
In the early years of manufacture, environmental stress cracking
was a concern for HDPE pipe. In the past 25 years, higher-quality
resins have successfully overcome this problem in HDPE pressure
pipes. The butt-fusion of HDPE necessitates the use of a skilled labor
force. The fused joint is allowed to cool for an additional 30 minutes
after removal from the fusion machine prior to installation. In the
case of thick wall pipes (> 2 in.) extra cooling may be required. The
high sensitivity of HDPE to temperature differentials requires special
design considerations for above grade pipelines. (Transitions from
HDPE pressure pipe to gasket joint pressure pipe requires a thrust
anchor.) However, buried forcemain lines require no special installa-
tion for thermal affects. Buried or sliplined gravity flow pipes gen-
erally require a wall anchor where they terminate in a manhole.
All pipes must be designed for the proper pressure and be derated
for surge and fatigue loads, temperature, corrosion, earthquake loads,
C factor, installation, and others. Table 4.14 presents the advantages
and limitations of HDPE.

4.7.4 Glass-Reinforced Pipe (Fiberglass Pipe)
The third type of plastic pipe is glass-reinforced pipe (GRP), also
commonly referred to as fiberglass pipe. Unlike PVC and HDPE, GRP
is made of a thermoset material. GRP was first manufactured in
the United States in the 1950s, as an alternative to corrosion-prone
concrete and steel materials. Using a patented centrifugally cast man-
ufacturing process, Perrault Fibercast Corporation of Oklahoma