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

3.5.1.3 Pipe Slitting. Pipe slitting is a variation of the static method incorporating cutting wheels in
advance of the bursting head. The cutting wheel slits the ductile pipe, such as DI or steel, allowing the
bursting head to then open up the slit pipe. All other aspects are similar to static bursting. Pipe slitting
has been carried out on pipes from 6 to 24 inches (150 to 600 mm) in diameter.
3.5.2 Offline Replacement. As the name implies, offline replacement simply involves the
installation of a new pipe without regard to the line and grade of the existing pipe. Normally the existing
deteriorated pipe being replaced is kept in service (at reduced operating conditions if necessary), while the
new replacement pipe is being installed. Once the new pipe is in place and has been leak tightness tested,
a switchover is made.
3.5.2.1 Open Cut. Historically, the most often used renewal method of a deteriorated sewer force
main is open cut replacement. Force mains, like water mains, are generally not buried very deep so the
cost of open cut excavation is less than for deep gravity sewer mains. However, other indirect costs such
as disruption to traffic and the general public or interference with other underground structures can raise
the cost of open cut replacement to a point where rehabilitation or replacement with trenchless means is
more cost-effective.
3.5.2.2 Directional Drilling. Directional drilling is a trenchless excavation method. First, a smalldiameter
pilot hole is drilled along the designed directional path. The drill head can be steered both
horizontally and vertically and is equipped with a head-location device (sonde) for shallow drilling
applications. The pilot hole is then enlarged and finally the replacement pipe is pulled in to the reamed
hole. Typical pipes that can be pulled into the hole are steel, HDPE, PVC, and DI. Steel pipe would
either have welded joints, or mechanically-locked joints such as Permalok. Butt-welded joints are used
with HDPE and either fusible joints on PVC or mechanically-locked joints such as Certalok from
Certainteed or Terrabrute by IPEX. The radius of curvature commonly used for designing drill paths is
1,200 times the nominal diameter of the pipe. This is based on established practice for steel pipe. HDPE
and PVC, which have greater flexibility than steel could accommodate a tighter radius if needed.
Diameters that have been installed by HDD are 2 to 60 inches (50 to 1,500 mm) and lengths up to over
10,000 feet (3,049 meters). HDD has been especially useful on river crossings and for the installation of
service connections. In 2005, the American Society of Civil Engineers (ASCE) released a manual of
practice (MOP #108) for Pipeline Design for Installation by Horizontal Directional Drilling (ASCE,
2005).
3.5.2.3 Microtunneling/Pipe Jacking. Microtunneling or pipe jacking involves the installation of a
new pipe behind a tunneling shield or tunnel boring machine (TBM). On short to medium length drives,
the pipe string and shield are driven forward by hydraulic jacks operating from a drive shaft (on long
drives, intermediate jacking stations may also be installed at intervals along the pipe string). Once the
TBM reaches the reception pit, it is removed. The jacked pipes can be the replacement pipes themselves,
or they can serve as a casing for subsequent installation of the replacement pipes by sliplining.
With the different types of tunneling machines available, including slurry and earth pressure balance
(EPB) machines, with cutting heads to handle rock and mixed ground conditions, a wide variety of
ground conditions can be handled. A Geotechnical Design Summary Report (GDSR) and a Geotechnical
Baseline Report (GBR) typically are used to define the geotechnical parameters of a tunneling project so
that there is a clear understanding of the geotechnical conditions expected on a project. Diameters from 6
to 120 inches (150 to 3,000 mm) can be microtunneled and curved alignments with joint deflections of up
to 5% can be accommodated (although curved alignments currently are uncommon in the US).
The selection of the right jacking pipe is paramount. Typically, the loads imposed on the jacking pipe
during installation are going to control the pipe design. Jacking loads of up to 1,000 tons are possible, so
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