SS9901-CS040505From the desk of...
Ken Griffin, P.E.
Director of Engineering &
Public Works
Date:
_~.S, ~
IInitiated by:
ATM
"Gig 'em Aggies "
City of Coppell
P.O. Box 9478
Coppell, Texas 75019
(9 72)304-3686 / Metro (972)4 71-2251
TRENCHLESS TECHNOLOGY
Overc°minbgy Diving
Beneath Them
H'orizontal directional drilling offers
.another way to navigate tough terrain.
By Bruce A. Neu, P.E.
What happens when the proposed
route of a gas pipeline or water main
intersects wetlmtds, an archeological
site, or a'congested urban area?
Traditionally, engineers would de-
sign the pipeline to either go through
the obstruction or around it--not nec-
essarily appealing options given that
both would likely require additional
permits, extend project schedules, and
increase costs. However, horizontal di-
rectional drilling (HDD) offers an al-
ternative. An evoh4ng trenchless con-
snr~cdon technology, it enables engineers
and pipeline conu'actors to avoid sen-
sitive or other potentially n'oublesome
sites by drilling beneath them.
HDD does just what its name im-
plies--it drills holes horizontally. The
method has evolved from traditional
technologies used to extract water,
oil, or natural gas supplies fl-0m deep
underground. In essence, it is noth-
ing more than a traditional drill rig
turned on its side. Rather than work-
ing vertically, HDD allows contractm*
to drill across the horizontal plane.
As pressures for the development
and conse~wation of land continue
to increase, municipalities and pub-
lic m~d private utilities are often faced
with econonfic, regulatory, and lo-
gistical challenges when expanding,
upgrading, or repairing infrastrnc-
tttre systems. For example, coastal
utilities that are required to install
new pipelines or conduits to serve a
growing island commanity might
have to cross navigable or environ-
mentally sensitive waterways. Simi-
larly, water authorities serving densely
populated areas that are required to
'replace or repair aging water lines
might have to tear up roadways and
sideoalks in heavily trafficked urban
areas. Although
such jobs have been
done in the past
with open-cut ex-
ca~,ation techniques,
HDD technology
cau save time and
money, while min-
h~fing dismrlxances
of both the man-
made and natural
environments.
Horizontal di-
rectional drilIing
has many fro=reach-
tor, for example, HDD was initially
popular among petroleum and nat-
ural gas companies. Today, media and
cotnmunication firms have joined
them to install telephone, fiber optic,
and cable conduits and lines.
In the public sector, HDD's use-
fulness includes the repair and re-
placement of potable water mains,
reclaimed water mains, storm water
piping, sewage gravity piping, and
force mains.
BENEFITS OF HnD
HDD technology can provide many
benefits to resource- and cash-swapped
municipalities, especially when com-
pared to traditional cut-and-cover
ing applications,
and new uses con- The Pensacola Beach side of the Santa Rosa Sound project
tinue to develop, shows the HDD entry pit staging area with the HDD rig and
In tire private sec- rig control booth in front.
42 P.blicWorks Apd~ 2004 · www. pwmag.com
Installation of 5300 feet of 30-inch steel casing pipe placed
during the spring and summer of 2003, The pipe crossed
the Santa Rosa Sound in Escambia County, Florida,
methods of excavation. Some of the
specific benefits that HDD can de-
liver include:
Expedited projects. Since HDD
allows fl~e installation of piping and
conduit~ beneath sensitive areas--
wetlands, stream crossings, or even
hazardous waste sites--disruptions
to surface or ground conditions in
the prc!ject area are minimal or non-
existem. As a result, using HDD tech-
nolog7 could require fewer permits
and precoustruction studies from
local, state, or federal agencies. Arid
with fewer permitting hurdles, pr0j-
ects can be completed more quickly.
"Because HDD technology can alle-
viate permit requirements, inc]nd-
lng impact studies and other assess-
nlents, we have found that H1)D
projects can be completed more
quickly than those using traditional
excavation methods," says Bill ,John-
son, P.E., director of engineering for
the Pensacola, Fla.-based Escambia
Count), Utilities Au-
thority (ECUA).
Minimal impact
on critical habitats.
One of the great-
est benefits of HDD
is Ihat its intpact
on the environment
is substantially less
when compared Io
more traditignal
excavation meth-
ods. For example:
when the ECUA re-
cently needed a
new water line, it
discovered that the
initial route would
disturb an envi-
ronmentally sensi-
tive sea grass bed.
Using open-cut con-
struction tech-
niqnes, ECUA wmdd bare had to
cmnplete thne-consuming and costly
environmental impact and mitiga-
tion studies to gauge the effect on
the sea grass bed.
"Going under, rather than through,
the sea grass bed using HDD was a
great allernative that actually elimi-
nated our need to prepare environ-
mental impact studies," says John-
son. "This saved us time and money
aud also kept a critical habitat free
from disturbance."
Smaller footprints. Though rite
length to which some directional
drilling boreholes ca2~ extend is vast--
the ECUA's Pensacola Beach sub-
aqueous water line is 5300 feet long--
the combined impact of these projects
is limited to the size of the staging
areas at either end of the bore. Gen-
eral]): stag4ng areas for even the larger
projects may require an area of only
100 feet by 150 feet. Smaller project
footprints mean less site cleanup and
quicker restoration once the project
is complete.
Simpler projects. HDD can make
projects simpler. By boring holes be-
neath occupied, congested, or envi-
ronmentally sensitive areas, project
teams encounter fewer construction
activity restrictions. In addition, HDD
can simplify permitting. For exam-
ple, in lieu of a special-conditiou
dredge and fill permit fi'om the U.S.
Army Corps of Engineers and the
U.S. Coast Guard, a project using
HDD would require much less time
and paperwork--a general permit
would probably suffice, one that stip-
ulates that any drilling beneath a nav-
igable waterway must be done at a
level lower than the limit of the Coq?s'
designated channel depth, withont
disn~ption to marine traffic.
Greater rises. When introduced al-
most 40 years ago, HDD trenchless
technology was mostly used for short,
small-diameter utility crossings. Ad-
vances in technology' have allowed for
longer, deepei; and larger diameter
bores to be drilled through various
soil conditions. Engineering and util-
ity contracting fim~s that rise HDD
are responding to increased requests
by' a diverse number of ntilities, from
metropolitan sewage authorities to
cable television companies, to use
HDD as a means of utility installation.
HOW DOES IT WORK?
Here's a brief overview of how HDD
trenchless technologyworks and steps
to take to make a project snccessfnl:
1.Perform an initial field recon-
naissance. The first field assessment
will determine if HDD is an option
or if other construction methods
should be evaluated. Soil borings, ge-
otechnical studies, and pre-project
meetings with regulatory agencies
and experienced HDD contractors
April 2004 · www pwmag.com PublicWorks 43
TRENCHLESg TECHNOLOGY
residents for stag-
ing areas and 24-
hour work peri, fits,
if necessary. Field
utility mm'kouts also
must be done, with
test pits, potholes,
or otber means of
locating under-
ground utilities, to
prevent the teton
from boring a hole
into existing buried
cables or pipelines.
3. Perform
pipeline/conduit
design. Once field
conditions and reg-
nlatory require-
ments ~u'e kno,a~l, design of the pipeline
or conduit can begin. In consnltation
with the client, the engineering team
must consider the depth, dimneter,
route, and existing surface and sub-
surlhce conditions in its design of the
bore hole. Once the desigat is com-
plete, the team ,/,'ill present the plan
to regulatory, agencies for approval.
4. Select a contractor. When se-
lecting an HDD contractor, it is im-
portant to carefully consider qualifi-
cations. Due to the rapid growth of
the industU', there are many contrac-
tors now with HDD equipment. How-
ever; ovming the equipment docs not
'necessarily qualify a conU-acmr. De-
pending on the scope, location, depth,
length, and diameter of the direcfonal
drill, all contractors bidding on the
project should be prequa]ified. A good
HDD contractor must bave an expe-
rienced staff that understands the
drilling operation, GPS, and other
mapping and borehole tracking eqnil>
ment, and be able to respond to equip-
ment m~functions, thronging soil con-
didons, and other problems.
5. Mobilize onsite. As the HDD
contractor submits material design
drawings and procedures for review,
he begins to mohilize. The drill rig
and accessm-/equipment are set up
on both sides of the bore, and the
pipe or conduit to be installed is de-
livered to the exit side of the bore
hole for assembly and above-ground
testing, as needed.
Drifting the pilot hole inside a 14-inch steel casing helped
to stabilize the initial bore hole near the surface, This proj-
ect links a water main from Gulf Breeze to Pensacola Beach
under the Santa Rosa Sound.
will improve the chances for success.
Although technologD' exists to drill
through most soil types, including
rock, dense sand or clay is still pre-
flrrred. Rocky soil requires special
techniques and equipment, while
bores drilled throngh soft sands or
silts run the risk of not accurately
following the proposed bore path or
even collapsing on themselves.
Proper depth is equally irnportant~
It' directional drilling of a pipeline
is too shallow, a "frackout"--a blowout
of the pressnrized drilling mud--can
occun In some cases, if the drilling
is too shallow and frackout does not
occur, the pressurized drilling rnnd
can heave the ground or pavement
above. In addition to understandiog
the geologic conditions below the
ground, there also must be adeqnate
surface land available to be used as
staging areas at both ends of the bore
to set up eqnipment, string out the
pipe or conduit that is to be installed,
and to operate the drill rig.
2. Plan the project. It is critical
thai the project team and other stoke-
holders meet to thorougbly discuss
all aspects of the work. Successfld
planning allows all interested parties
to express their concerns, and helps
the project team understand all local,
state, and federal regulatory re-
quirements. For example, since large
HDD projects often require round-
the-clock operation, they must be co-
ordinated with local agencies and
6. Start drilling. Now the most chal-
lenging aspect of the project begins
with the drilling of a pilot hole of a
pre-determined diameter..~ tl~e pilot
hole is being admnced, the drill bead's
location is constanfly monitored to
ensure that the planned route is fi)l-
lowed. Using a magnetic guidance sys-
tent, a u'ansmitter located in the drill
head is gnided by either a fixed-wire
line or "walkover" receiver above the
ground over the pilot hole. Once the
pilot bore hole is complete, its ronte
is plotted ttsing the data collected from
the magnetic gnidance system and
submitted to the engineer or owner
to ensure it complies witb the pro~
posed design route of the bore. The
pilot hole is then gradually enlm'ged
with a series of reamers until the pilot
hole is roughly 1V2 times the size of
the pipe to be pulled into place.
7. Test and install the pipe. 3.'he
pipe, depending on its intended fi~nc-
tion, can be of several different ma-
terials-steel, high-density polyeth-
ylene (HDPE), polyvinyl chloride
(PVC), or ductile iroo. The pipe is
connected to the deli1 stem with a
swiveled phil head to prevent it fi'mn
twisting during pullback, and is pulled
into place by the drill rig. To avoid
hydrolock-the sticking of the pipe in
the bore hole due to the pressure
buildup of drilling mud or soil-pull-
back of the pipe has to be a contin-
uous operation. During pullback, flue
pipe has to be monitored constantly
to ensure that the manufacturer's al-
lowable tensile, torsional, and radial
forces are not exceeded. Once the
pipe is pulled into place, it is cleaned
and pressure tested. ']?he HDD con-
tractor breaks down the operation
and demobilizes from the site, leav-
ing no trace of the massive opera-
tion that has taken place.
IIDD is an evoNing technology that
offers public works officials the op-
portnnity to save time and money.
HDD holds great pronfise not only
for its capacity to streamline prpjects,
but also because it can cortsen,e htanan,
financial, and natm'al resources. ~
--Neu is the environmental malinger with
ttatch Mote MacDonald in Pe~ewok~. Fla.
44 PublicWorks April 2004 · www.pwmag.com