Page 56 - Trenchless Technology Piping Installation and Inspection

New Pipeline Installations 29

clear pathway is created through poor soil conditions, allowing the
actual drilling operation to begin in more preferable soil conditions.
Areas with loose, unsupported soils are prime candidates for the con-
ductor barrel method.
During the conductor barrel process, open-face casings are
rammed into the ground at a predetermined angle until desirable soil
conditions are met. The spoil is removed from the casing with an
auger or core barrel. Drilling then proceeds within the casing, begin-
ning at a point where more desirable soil conditions are encountered.
In addition to assisting drilling operations at the start of the installa-
tion, the conductor barrel can prevent situations in unstable soils in
which drilling fluids under pressure force their way into waterways
or wetlands, acting in a similar fashion to containment cells. The con-
ductor barrel may also serve as a low friction section, facilitating
pullback.
*
Impact Moling Impact moling (also known as compaction method,
earth-piercing tools, soil-displacement hammers, impact hammers,
percussive moles, or pneumatic moles) is used to install small-
diameter pipes, ducts, and cables (less than 10 in.). In this method,
percussion or hammering action of a pneumatic piercing tool is
used to create the bore by compacting and displacing the soil rather
than removing it. The method typically is nonsteerable and should
be used with caution. Steerable systems have reached the market in
recent years.
When properly planned and executed, impact moling can be a simple
and cost-effective trenchless installation method. Utility companies
widely use this technique for installation of service connections to
gas, water, and sewer mains, usually under sidewalks, driveways,
and other short crossings under 150 ft. General advantages of impact
moling are low operational and investment costs, relative simplicity in
operating, minimal or no excavation beyond the necessary connec-
tion pits or termination points for the installed product, and minimal
public disruption. Support equipment is limited to a small air com-
pressor, and perhaps a small backhoe or trencher, to open and rein-
state the connection/termination pits.
Feasibility of the method is restricted by its generic limitations
(limited boring diameter and length) and by local ground conditions
that can greatly affect performance. Adverse ground conditions
may include cobbles, dense dry clays, and other noncompactable
soils. Such soils may drastically reduce penetration rates and con-
tribute toward surface upheaval and/or deviation from the desired

∗ For more information, visit Guidelines for Impact Moling, TTC Technical Report
#2001.03, available at: http://www.ttc.latech.edu/publications/guidelines_pb_
im_pr/moling.pdf.