Boring - Trenchless International

North america
April 2010 - Trenchless International
Dissecting the record-breaker
Trenchless industry professionals are consistently pushing the boundaries of achievement in length, timing and
diameter. Here Robbins details the latest record breaking auger boring projects in America and outlines the reason
for the continuing success.
When contractor Gonzales
Boring & Tunneling of Oregon, in the
Pacific Northwest of US, completed a
183 metre long trenchless crossing, they
knew they had a landmark project on their
hands.
“Preparation, a qualified crew, and the
right cutting head matched to the right
auger boring machine made for a successful
crossing,” said Jim Gonzales,
President of Gonzales Boring & Tunneling.
The project represented a new distance
record for auger bores using Robbins
Small Boring Units (SBU-As), a type of
cutterhead utilising disc cutters for hard
rock and mixed ground.
In addition to the Oregon project, two
other record-breaking trenchless projects
have pushed the limits of auger boring
with SBU-As to distances of 107 metres
and 166 metres within their respective
size classes. The long crossings highlight
rapid advancements in technology, particularly
for boring attachments using disc
cutters. However, achieving groundbreaking
status is anything but straightforward
– optimal machine performance from both
the auger boring machine (ABM) and boring
head is most often determined by a
complex mixture of variables.
“The truth is that record-breakers are
determined by several factors,” said Chris
Sivesind, Robbins SBU Sales Manager –
Western US.
“These include improvements in auger
boring machines and disc cutterhead
technology, as well as geology. One of
the most important aspects is often the
contractor’s confidence in equipment and
crew, which eases the heightened level of
risk taken on demanding projects.”
Factors contributing to recordbreaking
projects
Much of the ability for trenchless
projects to excavate longer lengths is due
to significant improvements in ABM technology.
The average 1.5 metre diameter
ABM marketed in the late 1970s operated
with only 2,200 kN of thrust and
160 HP, and was limited to projects less
than 150 metres in length with 1,500 mm
diameter casing in soil. Within the last 40
years, the technology has grown in both
power and diameter range, operating at
up to 300 HP and 8,000 kN of thrust using
2.4 metre steel casing.
Other major ABM improvements in the
last 15 years include larger diameter
hex drives, which allow for the larger hex
augers used on long distance bores at high
horsepower and torque. Improvements in
steering accuracy have allowed installation
of small diameter casings on line and
grade, using a pilot tube boring system
or hydraulic steering system. In addition,
specialised cutting heads have extended
the range of geology that an ABM is capable
of excavating, from soft ground and
rock less than 75 MPa UCS to hard rock
greater than 200 MPa UCS and consolidated
mixed ground.
Advent of cutterheads
using disc cutters
First developed in 1996, the Robbins
SBU is a disc cutterhead used with standard
auger boring machines. The SBU-A,
in diameters from 600 mm to 1.8 metres,
consists of a circular cutterhead mounted
with single disc cutters capable of excavating
rock from 25 to over 175 MPa UCS. In
the launch pit, the machine is welded to
the lead steel casing, while the ABM provides
both torque and forward thrust to the
cutterhead. Openings in the cutterhead
called bore scrapers collect spoil from the
face, where they are transferred to a fullface
auger for removal.
Over the last ten years, SBU cutterhead
technology has improved for specific
ground conditions. Cutterhead configurations
that feature a combination of carbide
bits, two-row tungsten carbide cutters,
and single disc cutters are being used
for a variety of types of mixed ground.
In addition, tool steel for the disc cutters
themselves has resulted in fewer cutter
changes and less downtime, particularly
for the relatively short distances of most
trenchless crossings.
Larger hex augers
Hex augers of 127 mm and larger
allow the auger to withstand the higher
torque loads experienced during long
trenchless bores. A typical 127 mm hex
auger is capable of withstanding up to
163,000 Nm of torque.
Adaptations in auger diameter
On long bores, contractors seeking
the greatest efficiency use hex augers at
least 150 mm smaller in diameter than the
A record 183 metre long crossing in
Oregon, US was completed with a
Robbins Small Boring Unit.
Contractor Gonzales Boring &
Tunneling cited quality field service and
an experienced crew as major factors
in the record-breaking project.
size of the bore. The smaller auger diameter
prevents torque building and sudden
torque unwinding. On long crossings with
higher torque requirements, larger diameter
augers can flex against the side of the
casing, building up torque and unleashing
the force suddenly – a scenario that can
destroy gearboxes and damage valuable
equipment.
Ground conditions
Whatever the ground conditions may
be, consistent ground seems to be a contributing
factor for many record-breaking
projects. All three of the record-breaking
SBU crossings were excavated in uniform
medium to hard rock, with no fractures
and little ground water present.
In addition, ground that is too hard or
too soft can hinder progress. Very soft
rock less than 20 MPa UCS can clog
the cutterhead, requiring slowed rotation
and advance, particularly if groundwater
makes the cutting face sticky. Very hard
rock of 250 MPa UCS or more requires
higher thrust loading on the disc cutters,
and can also slow progress.
Contractor experience
Contractor experience with multiple
successful bores, as well as willingness to
accept risk, is key to completing a recordbreaking
project. Ideally, contractors
should have experience with multiple ABM
and SBU projects at various diameters.
Setting records in Kentucky
and Ohio, US
In Louisville, Kentucky, contractor Turn-
Key Tunneling Inc. landed a distance
record of 107 metres for a 1.4 metre
diameter auger bore. The contractor utilised
a 1.4 metre Robbins SBU-A and
1.5 metre diameter ABM to excavate the
crossing below Interstate Highway 265.
The crossing formed part of the Gene
Snyder Transmission Main, an 8.7 kilometre
long, $US6.4 million water line built
by general contractor MAC Construction.
Turn-key Tunneling launched the SBU-A
on 22 January 2009. The rock, consisting
of limestone up to 138 MPa UCS,
as well as the 2.91 per cent grade,
presented challenges early on. The cutterhead
drifted while boring through several
dirt seams as well as a mud-filled cavern,
which momentarily slowed progress.
“Controlling line and grade was very
labour-intensive at the start of tunneling,
but the head responded better and better
as we progressed, and we made
up for lost time in the end,” said Roger
Lewis, Project Superintendent for Turn-
Key Tunneling, Inc. The machine broke
through on 11 March 2009, just 4.3 mm
off of line and grade and well within its
contractual ±8 cm requirements.
The contractor cited one key factor
that helped to achieve the record, “We
purchased lengths of 127 mm diameter
hex auger for this crossing, which proved
to be absolutely essential. We would not
have been able to complete the crossing
otherwise,” said Mr Lewis.
A similar landmark bore was achieved
in Clermont County, Ohio by contractor
Capitol Tunneling, Inc. in 2009.
The project required a 166 metre long
trenchless crossing below an interstate
highway in shale and limestone ranging
from 8,000 to 19,000 psi. Crews
excavated the crossing using a 900 mm
diameter SBU-A and 1.5 metre diameter
ABM. The shale was embedded with
layers of limestone throughout the bore,
making control of line and grade difficult
as the SBU tended to drift up and to the
right. Crews pulled the SBU periodically
to adjust the grade, and were able to
hole through just 75 mm off, well within
specified tolerances. The crossing was
completed in two months at rates of
12 metres per day, and required the
change of only one disc cutter.
The case for ground-breaking:
record in Oregon
The 183 metre long record crossing
in Tigard, Oregon, USA, had all the right
variables for success. Gonzales Boring
& Tunneling needed a solution to bore
a total of three gravity sewer crossings
70 metres, 183 metres, and 98 metres in
length through rock and mixed ground.
The crossings formed part of the Locust
Street Sanitary Improvements Project,
No. 6335. Approximately 1.8 kilometres
of gravity sewer were installed by general
contractor Northwest Earthmovers, Inc.,
but several areas below houses, neighbourhood
streets, a small creek, and a
service facility, needed to be excavated
with trenchless methods. Once complete
the pipeline, for owner Clean Water
Services, will increase capacity in the
area and stop overflows currently plaguing
the system.
The three crossings were initially
designed as a pilot tube microtunnelling
project using vitrified clay pipe. However,
meetings between Gonzales, other local
contractors, and the project owner eventually
resulted in the contract being opened
up to other trenchless methods, including
auger boring. “The owner has saved over
one million dollars on the trenchless section
alone over their original cost estimates
for pilot tube microtunnelling. Because the
owner listened to the construction community,
they saved both time and money,
and kept the dollars local,” said Gonzales.
After completing the initial 70 metre
crossing in clay and basalt, the SBU-A
was launched for its second 183 metre
bore on 28 October 2009. The disc cutterhead
was used with a 1.8 metre ABM
and 1 metre diameter steel casing. Rock
conditions on the second crossing consisted
of basalt at various rock strengths
from 48 to 120 MPa UCS. Crews monitored
line and grade, and were able to
maintain advance at about 12 metre per
ten hour shift.
A contractor-designed steering system
guided the SBU-A to within one hundredth
of an inch design grade. Despite the
mixed ground conditions, no disc cutters
required changing after 250 total metres
of boring.
Ultimately, quality support and contractor
willingness to attempt long crossings
may be the highest predictor of success
on the project. Gonzales and the other
contractors each had over 25 years of
experience in auger boring, and felt that
field service was invaluable. “The technology
worked very well for both crossings.
The field service support we received was
unmatched, and we hope to receive similar
support for future jobs in hard rock,”
said Gonzales.
President of Turn-Key Tunneling Inc.
Deborah Tingler also expressed support
for the technology “We are confident that,
although we have approached the limits of
the Small Boring Unit, longer lengths are
possible with the right rock and project
specifications.”
North america
April 2010 - Trenchless International
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