Page 57 - Trenchless Technology Piping Installation and Inspection

30 Cha pte r O n e

straight-line path. It is highly recommended to trace the bore path
while boring to detect path deviation so the bore may be aborted for
another attempt before damage to pavement or nearby facilities are
caused. Steerable moles address the path deviation shortcoming of
nonsteerable moles and are poised to expand the usability of the
method by virtue of being able to bore the curved path.
Method Description Impact moles consist of an enclosed steel tube
containing an air-powered piston (also referred as the striker) that
strikes the nose of the tool driving it forward. A bore is formed by
displacing and compacting the soil laterally. The friction between the
ground and the mole body prevents the mole from rebounding back-
ward. Repeated impacts of the piston advance the unit through the
ground. There is no rigid connection between the mole and the inser-
tion pit, and the progress of the mole relies upon the frictional resis-
tance of the ground for its overall forward movement. There are two
main types of impact moling:

1. Nonsteerable moles typically involve the excavation of two
pits: an insertion pit and a receiving pit. After the careful
alignment of the mole in the insertion pit, the tool is expected
to advance through the ground in a straight line. A single per-
son can operate the mole. Due to potential steering control
difficulties and possibilities of damaging the road structure,
some highway departments and municipalities have banned
this method.
2. Steerable moles may be launched from the surface or from a
pit. The operation requires a two-man crew. A walkover
tracking system is used, as in directional drilling industry,
where one operator walks the bore route with a walkover
locator device and monitors the progress of the tool in the
ground. The other operator is a tool operator who imple-
ments the required course corrections using the guidance
controls. A product pipe, cable, or cable duct can either be
directly towed into the bore during the boring procedure or
subsequently inserted into place after the borehole is com-
pleted (providing suitable soil conditions for unsupported
borehole). Usually the mole first creates the unsupported
bore, and on removing the unit from the receiving pit, the
product pipe is attached to the air hose and pulled into the
bore (the most popular mode of operation), or the pipe is
sometimes pushed into its place.

Impact moling can also be used for dead-end bores, in which case
the tool is reversed after the bore is completed and removed from the
ground through the insertion pit. Figure 1.23 illustrates a schematic
of impact moling operation.