Page 426 - Trenchless Technology Piping Installation and Inspection

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installation technique or a pipe material or a specific pipe joint (such
as welding and butt fusion versus restraint joint).

9.2.6 Length of Installation
The length of the pipe segment that can be installed by trenchless
technology is another detail that will be impacted by the material
and selected trenchless method. For example, when segmental
rigid pipe is chosen; the shaft/pit location can be placed wherever
it will have the least impact on the area and the costs. If new man-
holes are being constructed or the old ones must be replaced, the
shafts/pits will be located at these manhole locations. If the man-
holes are being saved, the shafts/pits could be located at service
lateral excavations and downstream manholes are used as receiv-
ing shafts/pits.
When flexible pipe is chosen, the impact of fusing, welding, or
using restraint joints for new pipe sections will dictate shaft/pit loca-
tions. The owner must recognize the limitation of tensile and jacking
strength of any selected pipe system (material, joints, etc.). In addi-
tion, possible jacking (drive) distances are dependent on the pipe
material, diameter, geometric tolerances, pipe material moisture
absorption, steering of the cutterhead, trenchless installation method,
friction of the surrounding soil on the pipe outer surface, size of
overcut, and type and amount of lubrication used during the instal-
lation. There is presently no method to accurately estimate pulling or
jacking forces and to quantify these effects. Therefore, allocation of a
factor of safety (usually 2) for a given pipe system (specified mate-
rial, diameter, wall thickness, joint, etc.) during the design phase is
necessary.

9.2.7 Alignment Considerations
As in all pipeline projects, identifying feasible trenchless technology
alignments involves evaluating available right-of-way (ROW) and
easement acquisition issues, and determining the location of the
existing utilities. Sometimes, alignments are not feasible or economi-
cal for open-cut method, but can be achieved by trenchless technol-
ogy methods. Pipes installed using trenchless technology methods
can be installed deeper by only increasing the depth of the shaft/pit.
This can be of significant advantage if a deeper alignment can avoid
potential conflicts with existing underground utilities, reduce pump-
ing stations, and utility relocations.
Straight horizontal alignments are generally preferred for trench-
less technology projects. Straight alignments provide for more accurate
control of line-and-grade and for a more uniform stress distribution on
the pipe and pipe joints, reducing the risk of concentrated loads that
could damage the pipe.