McElroy Plumbing and Heating Co., Inc
Servicing MS and AL since 1930
Offers the following underground utility locating
services:
Underground Utility Locating
Overhead utility lines are becoming a thing of the
past except in rural areas. The urban underground has become a spider's
web of utility lines, including phones, electricity, gas, cable TV, fiber
optics, traffic signals, street lighting circuits, drainage and flood
control facilities, water mains and waste water pipes. In some locations,
major oil and gas pipelines, communication lines, mass
transit, and rail and road tunnels also compete for space underground. The
deregulation of utility services is adding to the problem as multiple
service providers seek to place their networks underground.
Utility lines are all susceptible to being damaged
as construction and renovation equipment excavate in their vicinity.
Records are often poor with inaccurate utility positions and/or depths.
Some live services are not even mapped out on the utility plans. This means that
the ability to physically determine on-site the location, nature and depth
of underground utility services is critical to reducing the risk and
consequences of inadvertent damage during construction.
McElroy's adjoining utility detection services coupled with our
Hydrovac Excavation technology
allows McElroy to not only detect the exiting utilities but to safely dig with high
pressure water minimizing if not altogether completely preventing any harm to existing pipes to
confirm the location of underground utilities (pipes, conduit, etc.)
_________________
There are several potential
disasters waiting to occur at each excavation site. High on the list of
things to avoid is the accidental rupturing or breaking of underground
utilities such as electrical power cables, communication wires, and
pipelines. Though accurate information concerning their location and depth
is usually available for most of these utilities, contractors must
ensure prior to breaking ground that unrecorded utilities are not present.
Though many of these unrecorded utilities might have been deactivated and
abandoned in place, there remains the serious possibility that an
excavation crew might stumble upon an undocumented, active utility—with
disastrous results.
Though it
would be nice to have x-ray vision to allow us to see through the ground
surface or pavement to determine what lies beneath, the next best thing
can be obtained from a range of subsurface detection technologies on the
market. This article will examine basic subsurface investigation devices,
their operational techniques, the results they provide, the effects of
soil characteristics on their performance, and other limitations.
Utility
Maps and Agency Information
The first
source for information concerning underground utilities is the local
utility offices (telephone, electrical power, cable television, gas, and
so on) and the county engineer’s office (storm sewers, sanitary sewers,
water lines, and so on). Plan drawings showing the location, alignment,
and depth of known utilities will be available at least in hard-copy
format. Only occasionally will a utility or engineer’s office have an
electronic file drawing in CAD format, which is unfortunate since these
files can be easily integrated into a global positioning system (GPS) for
field surveying. As more resources are made available for recordkeeping,
more hard-copy records will be translated into electronic format.
Especially in
older urban industrial environments, utility records are going to be
sparse or nonexistent. The buildings that once comprised old factory
complexes (some dating back to the 1800s) may contain myriad steam lines,
gas pipes, and other utilities. Often the only records (such as they are)
might be extracted from the archives of the industrial company—which might
or might not still be in existence. Even if a gas line has been turned off
and isolated for many years, there is no guarantee that potentially
dangerous amounts of gas are not trapped in the abandoned pipeline
segment. Old sanitary sewers could also have accumulated methane at
isolated high points. These older commercial and industrial sites might
also contain deposits of inert metallic waste and debris that can be
confused by the detector with utilities. At no time should it be assumed
that just because a utility is old and abandoned, it is no longer
dangerous.
Finding
Pipes and Other Objects: Metal Detection
Magnetic
detection can be used to find the following:
-
iron,
steel, and copper water lines
-
metal gas
lines
-
surveying
pins (property markers)
-
copper
tracer wire
-
copper and
aluminum electrical wires
-
steel
cables
-
telephone
and TV cables
-
aluminum
conduit
-
any
continuous metal pipe or line
______
Various steps involved in
data collection for existing utilities consists of various steps, starting
from quick reconnaissance to detailed investigations. Various techniques
used are:
Historical Utility Records Research
The data collection under this stage is aimed at obtaining basic
information on possible locations, congestion and orientation of
utilities. Such information is highly inadequate for use by trenchless
contractor, but immensely useful for SUE contractor to plan density and
orientation of survey lines, choose the right equipment, and plan the
survey operations.
Designation
Designation is the process where by the approximate horizontal location of
a utility is determined. Following a rough approximation of the general
location of facilities provided by Historical records research and visual
site assessment, a number of geophysical technologies can be used,
selected by applicability, for identifying the horizontal locations of
particular utilities.
Induction Utility Locators
Induction utility locators operate by locating either a background signal
or by locating a signal introduced into the utility line using a
transmitter. There are three sources of background signals that can be
located. A utility line can act like a radio antenna, transmitting
electromagnetic signals that can be picked up with a receiver. AC power
lines have a 50HZ signal associated with them. This signal occurs in all
active AC power lines regardless of voltage. Utilities in close proximity
to AC power lines or used as grounds may also have a 50HZ signal that can
be located with a receiver. A signal can be indirectly induced onto a
utility line by placing the transmitter above the line. Through a process
of trial and error, the exact above position can be determined. A direct
induced signal can be generated using an induction clamp. The inductor
clamp induces a signal on specific utilities. This is the preferred method
of tracing, where possible. By virtue of the closed loop, there is little
chance of interference with the resulting signals. When access can be
gained to a conduit, a flexible insulated trace wire can be used. The
resulting signal loop can be traced. This is very useful for non-metallic
conduits. Finally, these signals can be located horizontally on the
surface using a receiver. The receiver is moved across the estimated
location of the utility line until the highest signal strength is
achieved. This is the approximate horizontal location of the utility. The
receiver is then rotated until minimal signal strength is achieved. This
will give the approximate orientation of the utility. Vertical depth,
however, derived from this equipment is subject to gross error.
Magnetic Locators
Ferrous Metal or Magnetic locators operate by indicating the relative
amounts of buried ferrous metals. They have limited application to
locating and identifying utility lines but can be very useful for locating
underground storage tanks (UST's) and buried manhole covers or other
subsurface objects with a large ferrous metal content.
Electromagnetic Surveys
Electromagnetic survey equipment is used to locate metallic utilities.
This method pulses the ground and records the signal retransmitted back to
the unit from subsurface metal. Particularly useful for locating metal
pipelines and conduit, this device also can help locate other subsurface
objects such as UST’s, buried foundations (that contain structural steel),
and pilings and pile caps (that also contain steel).
Ground Penetrating Radar
Ground Penetrating Radar (GPR) is an electromagnetic method that detects
interfaces between subsurface materials with differing dielectric
constants (a term that describes an electrical parameter of a material).
The GPR system consists of an antenna, which houses the transmitter and
receiver; and a profiling recorder, which processes the received signal
and produces a graphic display of the data. The transmitter radiates
repetitive short-duration EM signals into the earth from an antenna moving
across the ground surface. Electromagnetic waves are reflected back to the
receiver by interfaces between materials with differing dielectric
constants. The intensity of the reflected signal is a function of the
contrast in the dielectric constant at the interface, the conductivity of
the material, which the wave is traveling through, and the frequency of
the signal. Subsurface features which may cause such reflections are: 1)
natural geologic conditions such as changes in sediment composition,
bedding and cementation horizons, voids, and water content; or 2)
man-introduced materials or changes to the subsurface such as soil
backfill, buried debris, tanks, pipelines, and utilities. The profiling
recorder receives the signal from the antennae and produces a continuous
cross section of the subsurface interface reflections, referred to as
reflectors.
Depth of investigation of the GPR signal is highly site specific, and is
limited by signal attenuation (absorption) of the subsurface materials.
Signal attenuation is dependent upon the electrical conductivity of the
subsurface materials. Signal attenuation is greatest in materials with
relatively high electrical conductivity such as clays and brackish
groundwater, and lowest in relatively low conductivity materials such as
unsaturated sand or rock. Maximum depth of investigation is also dependent
on antennae frequency and generally increases with decreasing frequency;
however, the ability to identify smaller features is diminished as
frequency decreases.
The various GPR antennas used are internally shielded from aboveground
interference sources. Accordingly, the GPR signal is minimally affected by
nearby aboveground conductive objects such as metal fences, overhead power
lines, and vehicles.
A GPR survey is performed by towing an antenna across the ground along
predetermined transect lines. The antennae is either pulled by a person or
towed behind a vehicle. Preliminary GPR transects are performed over
random areas of the site to calibrate the GPR equipment and characterize
overall site conditions. The optimum time range settings are selected to
provide the best combination of depth of investigation and data resolution
for the subsurface conditions at the site. Ideally, the survey is
performed along a preselected system of perpendicular or parallel transect
lines. The configuration of the transect lines is designed based on the
geometry and size of the target and the dimensions of the site. The
beginning and ending points of the transect lines and grid intersection
points, or nodes, are marked on the ground with spray paint or survey
flags. A grid system is used to increase the probability of crossing the
short axis of a target providing a more definitive signature in the data.
The location of the antenna along a transect line is electronically marked
on the cross section at each grid intersection point to allow correlation
of the data to actual ground locations. The location of the targets can be
marked on the ground surface using spray paint or survey flags.
Acoustic Location Methods
Acoustic location methods generally apply to waterlines. A highly
sensitive Acoustic Receiver listens for background sounds of water
flowing; (at joints, leaks, etc.) or to sounds introduced into the water
main using a transducer. This method may have good identification results,
but can be inaccurate. Acoustics can also being utilized to determine the
location of plastic gas lines.
Color Codes for Radio Location
National
Underground Utilities
1-800-424-5555
Yellow – Natural Gas,
Oil, steam
Red – electric
Orange – telephone or cable
Blue – water
Green – sewer
Mississippi
http://www.ms1call.org/excavation.html
GROUP
IDENTIFYING COLOR: UTILITY OR TYPE OF FACILITY
· SAFETY RED: Electric
· HIGH VISIBILITY SAFETY YELLOW: Petroleum Product /
Hazardous
Flammable/Corrosive/Toxic Materials, Product and Steam Lines,
Gas or Gaseous Material.
· SAFETY ORANGE: Telecommunications (including fiber
optic) and CATV
· SAFETY PRECAUTION BLUE: Water and Irrigation Slurry Lines
· SAFETY GREEN: Sewer and Drain Lines
· HIGH VISIBILITY PINK: Temporary Survey Markings
· WHITE: Proposed Excavation
SOURCES: Laws, 1997, Ch. 483, § 4,
Effective from and after July 1, 1997
Alabama
http://www.al1call.com/faq.html
Utility owners
follow the standard for color coding their facilities as outlined in
Alabama's law. The following colors are used:
RED
Electric Power Lines, Cables, Conduit and Lighting Cables
YELLOW
Gas, Oil, Steam, Petroleum or Gaseous Materials
ORANGE
Communication, Alarm or Signal Lines, Cables or Conduit
BLUE
Water, Irrigation and Slurry Lines
GREEN
Sewers and Drain Lines
WHITE
Proposed Excavation
