Boring - Trenchless International

Powering New York State
by Nick H. Strater, Brian C. Dorwart, Brierley Associates, LLC and Lane Puls, Burns and McDonnell
The recent expansion of an electric substation in rural New York State required
trenchless installation of a 345 kilovolt duct bank through an existing rock slope,
below live overhead wires – an optically guided, downhole hammer was selected to
complete the job.
North america
April 2010 - Trenchless International
Representatives of Burns &
McDonnell, Brierly Associates LLC, and
Construction Drilling Inc. collaborated in
the development of a plan to drill large
diameter holes through the rock slope
using the optically guided downhole
hammers.
The existing substation rests within a
topographic “bowl”, and sits near the toe
of a 35 foot high, vertical rock slope. With
the close proximity of live electric facilities
at the crest and toe of the slope, conventional
rock excavation by blasting was not
feasible, and a rock breaker would have
been too expensive due to rock strength
and work area restrictions. Therefore, it
was determined that trenchless installation
of the cables would be required to
minimise impact to the slope and adjacent
substation facilities.
A powerful solution
Trenchless design and construction
planning was restricted by the presence of
overhead, high voltage power lines at the
crest and toe of the slope, and the requirement
that the trenchless method could
not use drilling fluid. This constraint was
imposed by the substation owner, due to
concerns about the impacts of inadvertent
drill fluid returns on the adjacent electrical
equipment. Although horizontal directional
drilling (HDD) was initially considered as a
possible means of completing the installation,
it was eventually ruled out due to site
access limitations, the need to use drill
fluid, and relative cost.
Based on these design constraints and
the subsurface conditions, the design
team recommended that the duct bank
installation be completed in holes drilled
with an optically guided downhole
hammer, using methods perfected by
Construction Drilling, Inc. (CDI), of western
Massachusetts.
Developing downhole hammers
CDI has developed a means tracking
downhole hammers during drilling.
The guidance system consists of a target-mounted
drill bit, theodolite, and
monitor, which provides the operator with
continuous, real-time information about
the deviation of the desired drilling profile.
Specially designed drill bits and drilling
techniques permit steering. Prior to this
project, CDI had used this technology to
enable precision rock anchor installations
in and around sensitive structures, including
dams and bridge abutments.
For this installation, the optimum trenchless
layout was to install ducts in three
holes; two at 28 inch diameter (electric
ducts), and one at 18 inch diameter
(grounding and communications ducts).
This layout resulted in cost savings, as
the tooling for holes larger than 28 inch
diameter becomes significantly more
expensive. Each of the holes was approximately
60 feet long, located 10 feet apart
(centre-to-centre), and inclined 30 degrees
downward from horizontal. The holes were
each drilled from the top of the rock slope
downward into the substation.
Ultimately, the staging and performance
of the trenchless work for this project were
governed by the overhead electric cables,
which remained energised throughout
construction. The situation was complicated
by the tendency of these cables
to expand and sag downward during
periods of heavy energy loads. To ensure
worker safety, a phased work plan was
developed dictating maximum equipment
height during construction. To meet the
height restrictions, the surface grade in
the vicinity of the drill entry was lowered
by about 7 feet prior to mobilisation of
the drill rig. This was done using small,
low-profile equipment, including Bobcatmounted
hoe rams and excavators.
Using an (Egtechnology) EGT MD 3000
multi-purpose anchor drill, CDI elected
to use a multiple-pass approach for the
drilling. This involved completing a pilot
hole with an eight inch diameter hammer
bit, followed by an 18 inch, and then a
28 inch hammer. The rods required for
this type of drilling need to be very stiff to
accommodate the shallow alignment, and
to withstand the stress needed to withstand
the impact force imposed by the
hammers. In this case, the rods ranged
from 12 to 18 inches in diameter, and were
equipped with a hexagonal quick-release
joint.
The 18 and 28 inch hammers were
equipped with a leading edge extension,
or “stinger”, designed to follow
the pilot hole. The 28 inch hammer was
designed specifically for this project, and
was forged from a single block of steel.
Like most downhole hammers, the tools
employed by CDI are driven by air, with
a velocity of about 3,000 feet per second.
The air exits the hammer face, where it
then serves to remove the cuttings from
the hole. In an effort to protect the adjacent
electric facilities, all cuttings and
dust generated during drilling were captured
in a stuffing box and diverted to an
adjacent containment device, before the
air was discharged into the atmosphere.
The vibrations generated by this equipment
were minimal, well below the site
threshold of 0.5 inches per second, and
the noise generated the hammers was likened
to that an large excavator-mounted
impact hammer (ie, a hoe ram).
During drilling, the passage of the 8 inch
pilot and 18 inch hammer were completed
without issue; each appearing at the toe of
the slope within 6 inches of the surveyed
High Tech in Trenchless Technology
35 years of Swiss Engineering
Underground Piercing (Moles)
from Ø 45-190 mm (1.75“-7.5“)
exit point. It was noted during drilling,
however, that there was a “soft” zone in
the rock, approximately 30 feet from the
drill entry location. The soft zone was
interpreted to represent a zone of highly
weathered rock, parallel the high-angle
foliation observed in the rock mass. To
maintain the line and grade across this
zone, the heavier, 28 inch hammer was
equipped with an 8 foot long, 27 inch
diameter steel “barrel” shroud intended to
help bridge the soft zones in the rock, and
keep the bit from dropping during drilling.
Drilling with the shroud proved effective,
although the decreased annulus diminished
the efficiency of cuttings removal,
which in turn caused occasional jamming
of the tools. Although drilling with the
shroud was somewhat more time consuming,
the 28 inch hammer ultimately exited
the toe of the rock slope with six inches of
the design alignment for each hole, which
was well within the limits required for successful
duct installation.
Following completion of the holes, and
duct bundles were assembled using
mechanically-coupled PVC, that minimised
the need for bundle lay-down room.
Following installation of the duct dandles,
a concrete bulkhead was cast at the bottom
of each hole, and the annulus around
the installed duct was tremie-grouted with
a thermal grout.
HDD Machines for directional bores
up to 400 m (1‘300 ft) and Ø 1’000 mm (40”)
TERRA AG, Hauptstrasse 92, 6260 Reiden, Switzerland
phone: +41-62-749 10 10 fax: +41-62-749 10 11 e-mail: terra.ch@bluewin.ch www.terra-eu.eu
Cable Bursters with pulling forces
up to 40 tons (88‘000 lbs)
North america
April 2010 - Trenchless International
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