Underground Utility Location For Safer Construction
Underground construction can be dangerous and expensive if existing
underground utilities are damaged. Gas and electric services are
particularly dangerous. Damaging fiber optic cables can disrupt
communications for large areas and be very expensive to the careless
contractor. It’s not just digging – the fiber optic industry refers
often to “backhoe fade” when a cable is cut – but horizontal directional
drilling (HDD) is especially dangerous. HDD can easily puncture high
pressure gas lines causing fires or explosions or puncture sewer lines
which may go undetected for years.
One of the challenges for construction is the unpredictable nature of
underground infrastructure. Many utilities were installed decades ago
and may not appear on modern maps. Even recent installations may not be
accurately recorded. Relying solely on utility records may be a problem
due to incomplete or outdated information. Inaccurate line markings can
be misleading, especially in high density utility areas.
Every underground construction project requires the contractor use “Call
Before You Dig” services to have the local authorities and owners of
buried utilities identify the current utilities in the area of the
construction. The owners of the utilities involved will locate the
utilities in the area and mark their locations.
The locator (the person doing the location and marking) will use a
standardized color code of spray paint, stakes or colored flags to
identify buried utilities:
Natural Gas - Yellow - which also represents oil, steam, and other gases
Electric Lines - Red - which can also be other power, cables, and conduit
Telecommunication Lines - Orange - which can also be other cables, alarms, broadband, and signal wires
Potable Water - Blue - which may link to fresh drinking water faucets, fire hydrants, and more
Sewer Mains - Green - which also include storm drains and waste water lines
Reclaimed Water - Purple - often associated with irrigation or non-drinking water
These markings are not exact information. There is a tolerance zone
associated with each set of marks. The tolerance zone is the area where
the excavator must be careful, because they may still strike a line or
have a near miss when using large machinery.
Tolerance zones vary by state, but range from 18” to 36”. The tolerance
zone distance spans each side of the outer edge of the pipeline or
cable, where digging with mechanized equipment is not allowed. If the
tolerance zone is 24” and the pipeline diameter is 2” that translates
into a 50” area where no mechanized equipment may be used.
Inside of the tolerance zone, which you may think of as a simple buffer
for safety sake, or an allowance for lack of precision of the tools and
methods, the excavator must hand dig for exact location and/or use
alternate excavation techniques. Pipeline and electric-transmission
owner standards typically demand daylighting (hand digging to expose the
utilities) 15 feet on either side of the crossing and at each tie-in.
There are two other limitations. Even with the tolerance zone, depth
information is not displayed and not all utility lines are grouped
together in a conduit or layered tightly in a utility corridor. Many
utilities could be at different depths and/or side by side.
Underground Location Techniques
Today two technologies dominate the industry: Electromagnetic (EM) and Ground Penetrating Radar (GPR)
Electromagnetic (EM) Locators
EM pipeline and cable locators are essential tools for locating
underground utilities like pipelines, power lines, and communication
cables. They operate by identifying electromagnetic fields associated
with these utilities. In most cases, this involves sending a
radiofrequency signal down the pipeline or cable. If the cable or
pipeline is not metal, there needs to be a metal wire, known as tracer
wire, buried above or beside the pipeline or cable in order for an EM
locator to find it. Some communications cables will have a metal sheath
to protect the cable or a metal strength member which can also transmit
the radiofrequency. Details on tracer wire are covered after the EM
locator section.
1. Passive EM Locators: These devices detect natural electromagnetic
fields emitted by live utilities, such as power lines. They are useful
for quick scans, but may not detect inactive or non-conductive
utilities.
2. Active EM Locators: Active locators apply a specific frequency signal
to a utility line using a transmitter. The receiver then detects this
signal, allowing for precise tracing of the utility's path.
Tracer wire is simply an insulated wire (conductor) that is buried in
the same trench as a non-metallic pipe, conduit, or cable. When the
utility needs to be found, a locator transmitter is clipped to the wire
and to a remote ground stake; the alternating current that flows along
the wire sets up an electromagnetic field that an EM locator can trace
from the surface. Tracer wire became a federal requirement for new
plastic natural gas mains and service lines in 1996. Beginning in 2014,
some states began requiring that all underground facilities be
“electronically locatable,” and other states required tracer wire for
non-metallic water and/or sewer lines.
A few of the technical issues a locator faces and needs to learn about include:
- Signals jumping from one cable or pipe to another in crowded rights-of-way.
- Different soil conditions.
- Non-metallic pipelines and cables without tracer wire.
- Broken tracer wire.
- Distortion and its effects on locate accuracy.
- Finding the correct signal when others exist on the site.
Ground Penetrating Radar (GPR)
GPR works like aerial radar but underground, locating
using radio waves. GPR equipment is used in a wide variety of
industries, but in the damage prevention industry, its most common use
is locating non-metallic pipelines without tracer wire and cable without
a metal sheath. As the locator pushes a cart (like a lawn mower) across
the ground, a radar antenna sends waves into the ground. When those
waves hit something different, like a pipe or any subsurface structure,
they bounce back. A computer on the GPR cart shows that bounce as a
shape or line on a screen. A trained technician can look at these images
and be fairly certain if it is showing a pipeline, cable, duct, or
anything else that should be marked as a part of the location.
These are some situations where GPR works well:
- Dry, sandy, well-drained soils like deserts (Arizona, New Mexico, etc), sandy areas, and power line corridors.
- Loose gravel or crushed stone backfill, like recently built subdivisions, utility corridors, railroad rights-of-way.
- Dry or moderately dry silts/loams.
- Pavement
GPR does not work as well in these conditions:
- Wet clay and silty clay loams
- Salt-contaminated or tidal soils like coastal marshes.
- Very wet, fine sand
- Rebar-dense or fibre-reinforced concrete
"Potholing" or Vacuum Excavation is a vital part of locating
It is common practice to use vacuum excavation or digging small test holes by hand for
verifying the location of buried cables and pipelines. This process is
called “potholing” or “daylighting.” The name potholing comes from the
fact that the process creates a round hole, six inches to a foot in
diameter, in the ground all the way down to the location of the pipeline
or cable. Daylighting comes from the fact that the hole puts daylight
on the pipeline or cable. There are many circumstances where potholing or daylighting is critical and often mandatory.
Potholing can be the best form of
excavation when only a small area needs to be excavated, like:
- Fence post installation
- Utility pole installation
- Sign post installation
Vacuum excavation can be done using either water or air to create the hole.
Certifications For Locators
There are no mandatory certifications or training required for locators, but the National Utility Locating Contractors Association
(Nulca), a national organization representing utility locating
professionals, published its first Competency Standard for training
utility locators in 1996. This guideline has become an industry guide
and is now in its fifth revision. In 2016, Nulca rolled out its
accreditation/certification program. This program is voluntary, but it
does create a standard for training. There are other training
certification programs in Canada, Australia, New Zealand, the UK, as
well as Staking University in the U.S., but the NULCA program has the
momentum of the industry’s largest locator organization behind it.
Fiber Optic Cables As Sensors
Even a single strand within a fiber optic cable can be repurposed as
distributed sensors. By sending laser pulses, then deciphering how the
light behaves inside of the fiber, asset owners and One Call centers can
detect digging, excavation, manhole openings, tapping, saw-cutting,
gunshots, leaks, or explosions in real time. This allows 24/7 live
monitoring when there is a fiber in place to utilize this technology.
Excavation activity can be sensed up to 30 feet away.
The process works hand in hand with sophisticated interrogator equipment
and algorithm-based analysis. Volumes of data that are meaningless
unless interpreted – indicating everything from noise to vibrations to
temperature or pressure changes – can be categorized to precise activity
types. That means the fiber can “hear” the difference and tell the
organization monitoring the data the specifics between a heavy truck
driving past a utility corridor and someone using a jackhammer several
feet or meters away.
To date, this fiber solution has not seen wide adoption. It is a proven
technology, but more stakeholders have to know about it for it to see
systemic implementation. From there, it will take sophisticated and
visionary leaders with chops in data analysis, coding, computer science,
engineering, and more to refine and hone it.
Adapted from
Holding Back Disaster The Men and Women Standing Between Death and
Destruction, Billions of Dollars of Losses, and Life-Altering
Infrastructure Disruptions By Scott Landes and Benjamin R. Dierker
Drawings of locating courtesy Scott Landes
More
on Outside Plant Construction and Installation
Table
of Contents: The FOA Reference Guide To Fiber
Optics
|
