Page 179 - Trenchless Technology Piping Installation and Inspection

Pipe and Pipe Installation Considerations 145

Although bell-and-spigot type joints are available for open-trench
applications, trenchless methods such as microtunneling and pipe
jacking dictate that the joint be flush with the outside wall of the pipe
barrel. Therefore, the tongue and groove joint or the modified tongue
and groove joint is ideal. Table 4.6 shows joint types that may be used
in trenchless construction. Also available are mastic or mortar joints,
which are not well-suited to prevent leakage except for transient hydro-
static heads below about 5 psi. Above that pressure threshold, a gas-
keted joint meeting ASTM C443 is probably appropriate. ASTM C990
requires an in-plant hydrostatic test of 10 psi as “proof of design.”
A proprietary concrete and PVC composite pipe, PipeForm™,
with an inner and outer PVC liner that gives it the strength of con-
crete and the corrosion resistance of PVC, has been used in microtun-
neling and pipe-jacking projects for gravity applications. In addition
to preventing corrosion, the PVC outer shell also reduces external
skin friction during jacking (as long as it is not damaged by abrasion
during the process), while the internal liner improves flow. Available
diameters range from 18 through 36 in. Distribution of jacking forces
at the end surface is maximized by steel or plastic collars and rubber
gaskets. Details of the joint are shown in Table 4.6.
Advantages and Limitations Table 4.7 presents a summary of advan-
tages and limitations of concrete gravity and pressure pipes. The wide-
spread use of large-diameter concrete pipe in water and sewer appli-
cations is an indication of the material’s acceptance at the municipal
level. The vast selection of available diameters and pipe lengths is a
convenience to the design engineer and contractor. On the pressure
side, availability of various structural and pressure strengths also
makes it convenient. The ability of manufacturers to make pipes
capable of handling very high pressures in large diameters has
resulted in its specification on some high-profile trenchless projects
in recent years. Because a specialized work crew is not needed; cost
of labor for concrete pipe installation is not high. For trenchless con-
struction, the high compressive strengths give it a definite advantage
over some other pipe materials.
When used in open-cut construction, many concrete pipes
installed in gravity applications 20 to 30 years ago have shown signs
of poor performance. Leakage through joints and cracks in the pipe
has been a constant source of inconvenience to municipalities. For
contractors, the heavy weight of concrete pipes can make it difficult to
install when compared to some alternative materials. Concrete pipe’s
sensitivity to bedding conditions in both shallow and deep installa-
tions has resulted in pipe failures by shear and beam breakage in many
instances. Compared to flexible pipe types, due to their low wall
stiffness, only about 10 percent of the system’s capacity is attribut-
able to the pipe wall thus requiring that a structural “exoskeleton” of
carefully compacted, select material must be carefully constructed
around the pipe and for an additional height above the top of the pipe.