Page 188 - Trenchless Technology Piping Installation and Inspection

Pipe and Pipe Installation Considerations 153

buried in the ground. In recent years, the ability of clay pipes to with-
stand external abrasion and relatively high compressive forces has
led to its wide usage in trenchless applications such as microtunnel-
ing and pipe jacking.
Although the inert characteristics of the material made it ideal
for use in aggressive environments such as sanitary sewers, the
inability of clay pipes until recently to manufacture effective joints
has resulted in a drastic reduction of its use in sewer systems.
Though innovation within the industry has led to better performing
joints in recent years, the millions of feet of clay pipe installed in the
ground decades ago are now responsible for I/I problems in sewer
systems throughout the United States and Canada. Its susceptibility
to shear and beam breakage owing to poor bedding conditions and
ground movement has caused numerous leaks and cracks in a
number of sewer systems where it was used. The short lengths in
which clay pipes are manufactured (usually 8 ft or less) increases
the number of joints in a sewer main line, thus raising the chances
of leakage through joints. Root intrusion through clay pipe joints
has led municipalities to institute annual root control measures in
older systems.

4.7 Plastics Pipes
Plastic pipes were introduced in the late 1950s in North America. The
three main types of plastics pipes include PVC, high-density polyeth-
ylene or cross-linked polyethylene (HDPE, PEX), and glass-reinforced
pipe (also called fiberglass pipe). PVC and PE fall into the group of
thermoplastics, while GRP and PEX is a thermoset pipe. The properties
of thermoplastics for construction material applications are better
appreciated with an understanding of viscoelastics, discussed in the
Sec. 4.8.1. Thermoset plastics are processed by a combination of
chemicals and heat, and once formed, cannot be reshaped. Both ther-
moplastic and thermoset pipes are considered to be flexible pipes and
are designed accordingly.
In the field of municipal trenchless construction, PE has been the
dominant piping material in the past decade. By butt-fusion of suc-
cessive lengths of PE pipe, or by using coiled PE, a long jointless pipe
is created, which can be installed by trenchless methods such as hori-
zontal directional drilling (HDD). PE is also used in open-trench con-
struction. Traditionally, PVC has been the most dominant material for
open-cut installations in the North American water and sewer mar-
kets because of its bell-and-spigot gasket-joints, its light weight in
smaller diameters, and ease to work with. In recent years, several
manufacturers have also created proprietary PVC products with
modified joints for trenchless installation. GRP pipes are used in the
United States for both pressure and gravity applications, though the
latter is the more prominent use.