Virtual Stringline: Exploring Subsurface Utility Mapping
by Harry O. Ward
January 5, 2012
In almost any project occurring in an urban or suburban area, we need to be aware of the utilities beneath the site.
We must know where the water trunk lines are, where the sanitary tap-ins are located, and where the power and gas lines are. We need this information so we can hook into them for the project and avoid them when a backhoe is digging a trench nearby. If we don’t locate the utilities beforehand, the results can be damaging and costly.
A few years ago Site Prep published an article on dowsing, the art of finding water, conduits, electrical lines and voids beneath the surface. Almost all respondents affirmatively stated that this “water witching” does indeed work, although none could explain why. Many organizations routinely use dowsing to locate utilities and other underground features and have for hundreds of years. But more modern methods for utility locating do exist, of course, and I want to discuss some of these methods in this column.
Collecting Data and Related Research
In the case of identifying subsurface utilities for low-level efforts—those where you want to be aware that a utility exists in the vicinity—contractors should begin with basic data collection and research. They should check with landowners and agencies that are responsible for the area, including the local water authority, county property records, Call 811 and other organizations that maintain records of underground features.
This level of identification may not provide highly accurate data; rather, it provides an understanding of the utility’s purpose, the density of utilities and an estimated location of where they are. But if you miss a document or perhaps if the utility was never recorded, then some utilities can be overlooked completely. For example, when I bought my house 20 years ago in a rural setting, I chose to have the septic system maintained. When the contractor came out, he asked where the tank was and where the leach lines were. Having just acquired the land, I didn’t know, and the county property records dating to 1945 for my house did not show them either.
Another example occurred when an early GIS system for a major U.S. city. We compiled all of the data we could find and entered it into the database. After the system went into production, contractors’ feedback indicated that some of the utilities were a street width away from where they were recorded. This proved that a more accurate method was required to better identify locations.
Getting a Better Look
If you need more accuracy, an above-ground survey can be conducted. Aerial mappers will fly the site, photogrammetrists then digitize the data into a CAD system, and surveyors supplement it with field verification. Above-ground utilities, such as overhead lines, manholes, pedestals and more, can be extracted from the orthophotos. Linework connections are surveyed in the field to ensure that the manholes or other features are correctly linked together. Simultaneously, manhole inverts, pipe sizes and other needed data may be captured and recorded so that the utility is actually in 3D. In fact, many surveyors are collecting GIS attribute data as part of their routine surveys now.
For precisely measuring underground, different tools are needed. These tools include:
Metal Detectors—These devices use magnetic field measurements to identify underground utilities, and they can trace lines and compute depths as well. One of the drawbacks of this device is that it can produce false signals from wet ground foliage, pavement materials or mineralized ground soils, and some will identify only ferro-magnetic metals like iron, steel, or other magnets. The size of the coil in the metal detector helps determine how deep the equipment can read. These magnetometers are highly accurate instruments that measure local magnetic fields quite precisely. They operate using proton-based fluids surrounded by an electric coil. Once a current is applied to the coil, it generates a field of magnetism that temporarily polarizes the protons. When the current is terminated, the protons realign along the line of the earth’s magnetic field. The proton movement creates a measurable electric current in the coil and reads a frequency proportional to the magnetic intensity. Metal detectors can only read a few feet beneath the surface under most ambient conditions.
Gradiometers—These devices measure magnetic field gradients instead of total field strength. Magnetic gradients (changes) may provide a better definition of shallow buried features, such as below ground wells, tanks or buried metal debris, because the depth of magnetic surveys is unaffected by elevated electrical ground conductivities. This makes them more useful on sites with other conductive materials, such as salt-infused groundwater, highly conductive clays or other contaminates.
New Technologies—A new tool that has appeared recently is Optimal Ranging’s Spar utility locator. Its FieldSens technology computes the effects of ground conductivity and field distortions in real time and combines it with GPS to map utilities in 3D. It works using the alternating current and its magnetic field to locate the feature. It assumes that a signal can be located on the item, and if one doesn’t exist, the technician can often induce a signal from an exterior source.
Acoustic Technologies—Acoustic location methods usually apply to waterlines or lines with a flow in them. A sensitive acoustic receiver reads background sounds of water flowing at joints and leak locations or sounds introduced into the water main using a transducer. This method may have good identification results, but can be inaccurate.
Ground Penetrating Radar (GPR)—GPR is a method that uses radar pulses to image the subsurface. It uses electromagnetic radiation in the microwave band of the radio spectrum, and detects reflected signals from subsurface features. It produces a geologic cross-section of the soil based on the electrical properties of the ground. How deep the signal will penetrate the soil is based on the conductivity of the soil—the more clay-based soils, the less depth you can anticipate. This equipment is fairly heavy and may be mounted on a carriage so it can be pushed anywhere on site. GPR is effective in locating underground tanks and utilities, delineating pits and trenches that contain metallic and nonmetallic debris, identifying previously excavated or backfilled areas, mapping rebar or voids in the subsurface, measuring pavement depths and more. It is good for detecting objects, changes in material, voids and cracks.
Going
Deeper
There are times when you may need to physically locate the utilities. This task
is typically accomplished with heavy-duty mobile machines that physically
remove the earth around the utility so that it can be seen, touched, measured
and recorded very accurately. Because the utility is physically exposed,
developing test pits is the most accurate and reliable method we have to ensure
that a utility actually does reside exactly where it is recorded.
In order to physically locate the utilities, nondestructive excavating
equipment is employed at critical junctures to establish the precise 2D and 3D
positions of underground utilities. Data collected includes the type of
utility, pipe sizes, buried condition, pipe material, soil contamination and
paving thicknesses. By knowing exactly where a utility is positioned, the
designer or contractor can make small adjustments in the project to avoid
relocating these existing utilities. This method can locate utilities and other
features as deep as 30 feet below ground.
For this method, contractors use machines to excavate test holes or test pits.
This can be done using air and hydro vacuum excavators. Vacuum excavators are
available in truck-mounted form, as well as in portable units designed for
areas that are less accessible due to height, width or weight concerns.
By reviewing the technologies discussed above, you can see there are several
choices available for identifying utilities below ground. For simple
situations, dowsing may work fine, but more sophisticated equipment may be
needed as other information is identified that is required.
For most applications metal detectors, gradiometers, acoustics and GPR may be
satisfactory. For an absolutely positive and flawless determination, a test pit
must be developed where the utility can be physically exposed. If you use one or
more of these technologies, chances are good that you may never hit a gas or
power line again.
Harry O. Ward
hward@harken-reidar.com
Harry
O. Ward, PE, is president of Harken-Reidar Inc. He has been a member of the
engineering faculty at George
Mason University
since 1997 and was named "Outstanding Adjunct Professor" for GMU in 2010. He
can be reached at hward@harken-reidar.com or hward@gmu.edu.
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Title: Now 3D is very valuable
By: utility mapping
Posted: May 14, 2012 2:23 AM
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