Underground Construction - January 2019 - 30

Solving a Puzzling Project

Snelson, Brotherton Team-Up to Find Unique Solution to
Challenging HDD Job
Jeff Griffin | Senior Editor

Drill a pilot hole; enlarge the
bore hole by backreaming as
necessary; prepare product for
pull-in at the exit end; pull it back
through the pilot hole - the basic
steps of a horizontal directional
drilling (HDD) installation.
But what if there is no place to
lay out and prepare the product
for pull-in?
That was the problem faced
by Pacific Gas & Electric (PG&E)
for a portion of the company's
L-109 pipe replacement program
in an area near Redwood City,
Ca., where construction was
subject to strict environmental
restrictions, limited work space
and a short, six-month window
to complete.
The project was part of the
final phase of PG&E's Pipeline
Safety Enhancement Plan.
Open-cut construction was
used on much of the project, but
HDD was determined to be the
only way to address one 2,400foot segment.
"The bore fit into an approxi-

JANUARY 2019 | UConOnline.com

mately 11,000-foot section of replacement of 24-inch steel pipe,"
said Casey MacIntyre, project
manager for primary contractor
Snelson Companies. "Conventional open-trench excavation
would have impacted environmentally protected zones and
inflicted a greater construction
impact on the sensitive land.
Horizontal directional drilling
provided the best means to reduce impact and avoid digging
along a steep grade."

Restrictions
Restrictions preventing the pullback were both geographical and
environmental.
"The project paralleled Crystal
Springs Reservoir, a highly-protected lake which is San Francisco's main source of drinking
water," MacIntyre explained.
"Due to the land and proximity to
the lake, we had close oversight
on all operations and took extreme preventative measures to
prevent any spills and maintain

protection to the area. A team of
biologists and archeologists carefully monitored work throughout
the project.
"Because the bore exit was on
the side of a hill and there were
existing gas pipelines, it would
have been extremely difficult to
set the rig in this location. And
the length of pullback would not
have been possible due to the
restricted areas of protection and
a creek that would have limited
any construction. The degree of
entry against a slope presented
crane limitations and potential
compromise of the pipe during
installation."
Snelson and PG&E brought
in Jim Brotherton, president of
Brotherton Pipeline, to assist
in developing a plan that would
allow the 2,400-foot HDD installation to be made.
Brotherton collaborated with
PG&E's engineering team to
design the bore with the use
of a pipe thrusting machine to
push the pipe string through the

bore hole from the launch end.
Pipe thrusters typically are used
with tunnel boring machines or
to assist with HDD pullback in
difficult conditions, but not as a
single-acting force.
"Manufacturer of the thrust
machine used, Herrenknecht,
confirmed that this is the first
time a thrust machine has been
used in this way," MacIntyre said.
Brotherton designed a 36-inch
bore to be drilled and then the
24-inch pipe to be pushed into
the open void without pull assistance for the full length of the
bore. The bore was designed at a
depth favorable to encountering
solid formations, and multiple
swabbing passes were used to ensure a clean hole.
As a subcontractor, Brotherton
made the pilot bore and pullback.
Snelson assisted Brotherton with
the use of cranes, sidebooms, excavators and coating inspection
during the installation.
A Cherrington 350 HDD unit
was set up in a somewhat level

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Underground Construction - January 2019

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