Page 381 - Trenchless Technology Piping Installation and Inspection

Inspection and QA/QC for Trenchless Installation and Replacement Methods 331

then jacked along the starting alignment track with simultaneous
excavation of the soil being accomplished by a rotating cutting head
in the leading edge of the casing pipe’s annular space. The spoil is
transported back to the entrance pit by a helically wound auger rotat-
ing inside the casing pipe. Horizontal auger-boring typically provides
limited tracking and steering as well as limited support to the excava-
tion face, unless a modified auger-boring machine is used. This
method may also be referred to as bore and jack.
Horizontal auger-boring is a well-established trenchless method
that is widely used for the installation of steel pipes and casings,
especially under railways and road embankments. It is an economical
trenchless pipe installation method that can be used under various
soil conditions in diameters 8 to 72 in. This method can be used
advantageously to reduce damage to pavements and disruptions to
traffic, hence reducing the social costs associated with pipeline instal-
lations. Conventional pipe jacking (used for diameters 42 in. and up)
and pipe ramming (used for diameters 4 to 140 in.) are other methods
of casing installations.
The basic components of a horizontal auger-boring system
include the base unit, casing pusher, power pack, auger sections,
track and track extensions. Cutting bits are available for different soil
conditions.
For successful execution of boring and jacking projects, presur-
vey of the site conditions for surface features and subsurface geotech-
nical conditions must be done, and utility data must be gathered and
incorporated in the early stages of design process. It is required that
the design engineer provide the contractor with sufficient informa-
tion about the nature of the site and the possible obstacles. This will
help in determining the suitability of utility installation by boring
and jacking method.
The predesign survey includes the investigation of the general
site condition, examination of existing underground utilities and
potential for obstructions, geotechnical investigations (including
groundwater), environmental conditions, required drive lengths,
pipe diameters, site access, depth, grade, tolerances, impact to sur-
face activities, location of existing/abandoned/proposed utilities,
and rights-of-way requirements.
Geotechnical investigation of the site must be performed to iden-
tify the general and any special subsurface conditions. The extent of
the investigation may vary depending on the knowledge of known
local geological conditions.
Prior to the design of a boring project, all utilities must be located.
This process involves the use of several tools, but is not limited to,
utility locators, vacuum excavators, and use of utility maps. Each
existing utility is then mapped or plotted, and the bore path estab-
lished to avoid the existing utilities.