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
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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.
