The Science of Getting it Right: Locating Underground Utilities
Technology has made great strides in developing the means to locate underground cable networks and utilities including ground penetrating radar and remote sensing among others. These advancements make all the difference in allowing for a safe workplace and an efficient trenchless construction project.
Imagine a surgeon performing surgery with the lights off: that's what it would be like to dig into the ground without first locating and marking underground utilities in the area.
It is as good as digging blind and it can prove to be a costly gamble. The risk of hitting electrical, telecommunication, water, sewer, and gas lines rises exponentially, and that can cause severe injury or even death of individuals nearby.
The best way to avoid these problems is to hire reliable utility locating services that can clearly locate and mark out electrical, gas, and water and drainage lines. Also, methods of utility detection such as ground-penetrating radar (GPR) and highly specific pipe penetrating radar (PPR) provide details of subsurface utilities and underground conditions that allow for efficient and safe trenchless construction.
However, all technologies including GPR have limitations, and digging should proceed with caution. (Read also: Effectively Using Utility Detecting Devices.)
Proceeding with Caution
The following points must all be considered when planning to survey the location of underground utilities:
- While contacting the local One Call service is the first step, it should be kept in mind that One Call location crews usually mark only the public portions of underground utilities.
- Private utility locators may try to mark out all relevant utilities, but there is no guarantee that all utilities will be located.
- In some cases, the utility map may not show any underground utilities but it is necessary to use utility locating technology to confirm it.
- Conduct visual inspection including inside utility vaults and building for evidence.
- All technologies have limitations and instrument readings are subject to the operators’ interpretation.
- Non-metallic pipes can also be traced using metallic tracer wire inserted into the hollow pipe.
- Limitations of GPR include limited penetration depth and data interpretation difficulties in some sub-surface conditions.
- Utility mark out based on records can be inaccurate and incomplete. (Read also: Geomapping: Locating Aging Pipelines.)
Methods of Detecting Underground Utilities
1. Geographical Information System (GIS)
Utility Contractor Online describes the geographical information system as a computer application capable of capturing, storing, manipulating, and displaying information that has been geographically referenced. This information includes surface features as well as their attribute information.
Thie information is stored in the system and can perform complex analyses like information retrieval, overlay, and data output. Using simple commands, specific details can be retrieved from the database.
GIS has the capacity to determine the relationship between different layers like road, rail, administrative boundaries, buildings, etc., and correlate them for a comprehensive picture. The data output system has the ability to project the resulting maps on to the screen to enable planners to make the right decisions regarding a particular locality.
2. Remote Sensing
Remote sensing uses data from aerial and satellite imagery to detect objects on (and under) earth. Data received from aerial surveys using different satellites in different resolutions is widely used by planners. This data can be availed from the national agencies overseeing remote sensing technology.
High-resolution data from the satellites can give highly accurate mapping information including small features like ponds, road dividers, parks, buildings, as well as soil density, underground water rocky terrain. This technology helps gather information regarding the geological features including natural and artificial landmarks.
3. Induction Utility Locators
An induction locator operates by locating a signal that has been introduced by a transmitter into the utility pipeline. The utility line acts like a radio antenna and transmits electromagnetic signals that are picked up by a receiver. A signal can also be introduced into a utility line indirectly when a transmitter is placed above the line. An induction clamp that induces signals on specific utilities can be used to generate a direct induced signal.
Since it forms a closed loop, it minimizes interference and is preferred for non-metallic conduits. The receiver is moved horizontally across the approximate location of the utility pipeline until the strongest signal is received which indicates the approximate horizontal location. For determining the approximate orientation, the receiver is rotated until the weakest signal is detected. The depth of the utility however cannot be accurately ascertained by this method.
4. Electromagnetic and Magnetic Locators
To locate sub-surface metallic utilities such as metal pipes, conduits, buried structural steel, etc. electromagnetic survey instruments are used. This method involves sending electromagnetic pulses through the ground and recording the transmission returned from contact with metal utilities.
Magnetic locators detect objects with a high content of ferrous metal content like manhole covers, storage tanks, etc. (Read also: Why a Detailed Geotechnical Report Means Success for Your Trenchless Project.)
5. Ground Penetrating Radar (GPR)
GPR is one of the most favored methods of locating underground utilities and is capable of distinguishing the interface between two materials with different dielectric constants.
The system consists of an antenna with a receiver, and transmitter. Short duration electromagnetic pulses are radiated into the earth from the antenna moving along the ground surface. Depending on the received signal, the profile recorder converts the data into a graphic display.
6. Pipe Penetrating Radar (PPR)
Unlike ground-penetrating radar, PPR can pass through non-ferrous pipe walls, the bedding of the pipe, through conductive soil, and to a certain extent can map the outside of the pipe that is filled with air or water.
The robot carrying the PPR is allowed to traverse through the pipe under inspection. The pulse emanating from the PPR travels through the material of the pipe. Depending on any sharp change in the property of the material like at the interface of pipe and water, this pulse is reflected or refracted. The greater the difference in the properties of the materials, the greater is the amount of energy that is reflected back.
7. Radio Frequency Identification (RFID)
Radio frequency Identification is being widely used now for the purpose of tagging or marking new services. The tag will contain information such as the kind of service the pipeline is being used for, and its depth, which can be easily read by detecting devices. This system is very new and needs compatible devices programmed to read them but will be a great benefit for a growing trenchless technology.
The availability of technology to locate underground utilities should encourage every contractor and client to utilize them before undertaking any excavation project. The potential of going wrong is very high when digging blind and the economic consequences can be immense. The greatest tragedy is the loss of human life when utilities like electric lines are struck. Locating utilities should be the top priority before the drill is put to the soil.
Written by Tabitha Mishra | Civil Engineer, Technical Content Writer
Tabitha has a Bachelors Degree in Civil Engineering from Mumbai University, India, and is currently freelancing as a technical content writer. Prior to writing, she has worked as a site engineer and site manager for various building construction, building rehabilitation, and real estate evaluation projects.
Tabitha is also certified as a Primavera project management professional and is well versed with Auto CAD. In her spare time, she does private consultation for small-sized home builders and assists with plans and permissions.