Page 66 - Trenchless Technology Piping Installation and Inspection

36 Cha pte r O n e

they can be identified and removed. Many options are available for
handling the soil conditions.
In utility tunneling, only a small jacking force sufficient to drive
only the shield or TBM has to be developed. Also, jacking pipes are
not required, so potentially pipes with less wall thickness than jack-
ing pipes can be used.

Limitations
Pipe jacking and utility tunneling are specialized operations. As with
any other trenchless technology methods, these methods require a lot
of planning and coordination. While these operations can potentially
be conducted on a radius, it is recommended that all directional
changes be made at the shafts. In the pipe-jacking method, the pipe
should be strong enough to resist jacking forces. Hence not all types
of pipes can be used for this operation.
In the utility-tunneling method, the liner is classified as a tempo-
rary structure. Therefore, a product pipe (also called carrier pipe)
must be transported and installed inside the tunnel and the annular
space between the product pipe and the tunnel liner filled with grout
to provide adequate support.

1.8.2 Horizontal Auger Boring

Advantages
The major advantage of auger boring is that the casing is installed as
the borehole excavation takes place. Hence, there is no uncased bore-
hole that substantially reduces the probability of a cave-in, which
could result in surface subsidence along the bore path. Also, with use
of proper cutterhead and construction practice, this method can be
used in a wide variety of soil types, which makes it a very versatile
method.

Limitations
Horizontal auger boring is generally unsteerable; however, some
basic steering systems are available and some contractors may use
innovative methods (walk-over tools, inertia systems, etc.) to
achieve accurate line and grade installations. This method requires
entry that must accommodate pipe section lengths (usually 20 ft
and up) and reception pits to retrieve the cutterhead. To identify
obstacles such as large boulders, running sands, or very soft
ground conditions, a thorough site investigation is required. Hori-
zontal auger boring can accommodate rocks up to one-third of the
diameter of the casing. The casing must be made of steel, to accom-
modate the steel augers turning inside the casing. With larger cas-
ing pipe installations (worker entry), it is possible to use smaller