State of Technology Report for Force Main Rehabilitation, Final ...

3.5
Replacement
Figure 3-20. Aqualiner Installation Process
Replacement is a form of renewal. Replacement involves the installation of a new fully structural pipe to
take over the functions of the deteriorated main. Several technologies are available for online and offline
replacement as summarized in Figure 3-21. Historically, the most common method of rehabilitation has
been offline replacement of an underground main by open cut construction. While it is expensive, offline
replacement has several benefits for the operating utility as follows:
• It results in a new pipeline with known condition and designed to current standards.
• Offline replacement can be undertaken with the existing force main in operation so there is no
disruption to service.
• Offline replacement offers the opportunity to build parallel to the existing line in order to
create some redundancy, allowing future inspection and maintenance works to be undertaken,
while maintaining service.
• Cost may not be much higher than rehabilitation when the cost of by-pass pumping during
rehabilitation is taken into account.
Fortunately, new trenchless methods of construction have yielded a wide variety of replacement methods
that now limit the amount of excavation required to a minimum. These trenchless methods fall into both
online and offline methods. Online is where the new pipe is laid to the same line and grade as the
deteriorated pipe being replaced. With online replacement, by-pass of the existing pipe is needed during
the replacement construction. Sliplining, pipe bursting, and pipe splitting are examples of online
replacement. Offline replacement consists of installing a new pipe using a different line and possibly
grade as the existing pipe. Normally, a by-pass of the existing pipe is not necessary with offline
replacement. Once the new line is in place and has been leak tightness tested, flow is then diverted to the
new line and the deteriorated main is retired from service. Directional drilling, microtunneling/pipe
jacking, pilot tube boring, and auger boring are examples of trenchless offline replacement methods.
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