A very striking comparison can be drawn between our extensive underground utility system and the internal system of the human body. Both cannot be seen with the naked eye, both are extensive, and damage to either system can be catastrophic. Thanks to latest technology, we have instruments that can be used to detect the exact location of organs or problems within our body.
Moreover, we would want a skilled doctor to operate on our body so that other organs are not damaged.
Similarly for pipelines, technology is available to detect the exact location of all kinds of utility pipelines that crisscross beneath our feet, and it is equally important that skilled technicians and good utility detection methods are employed to detect and mark out utilities.
Science and technology have played an important role in making utility detection precise, efficient and easy to use, and contractors should ensure that they use available technology to undertake cost-effective and environmentally friendly projects. The importance of correct identification of utilities before work is commenced cannot be overstated. (To learn more, see The Science of Getting it Right: Locating Underground Utilities.)
Detecting Utility Lines
Survey of underground services are performed on different levels such as studying drawings from utility owners, site investigation using those drawings, and physical verification and identification using trial pits or tracing devices inserted in pipelines. The extent of the survey depends on the work site congestion.
Urban areas will require a much more detailed survey compared to rural field areas. Once the survey results are out, the details should be shared with the designer and the contractor.
At no point during the project should there be a time where information is not known to all involved parties. Personnel conducting the survey and marking the utilities should have good working knowledge in the use of survey instruments and should be well experienced. This ensures that mistakes or oversights are minimized or eliminated.
Types of Detecting Devices and Locators
Commonly used detecting devices and locators include:
Ground Penetrating Radar (GPR)
GPR works by transmitting a radio signal at high frequency into the ground. It detects anomalies in the ground such as pipes and other subsurface structures. A receiver picks up the reflected signal from subsurface utilities and stores it on a computer. The time taken by the pulse to travel to the target and back to the receiver indicates the depth and location of the utility.
It gives the user the path and approximate depth of buried utilities such as gas lines, sewer lines, communication lines, septic tanks and other metallic and nonmetallic targets beneath roads and open spaces.
Radio Frequency Identification (RFID)
RFID is a technology that, if properly incorporated into the utility system, will help future generations to easily detect utilities. RFID chips are used to tag new services and are programmed with information about the service, its depth and exact location. The tag should be properly attached to the utility line to ensure that the information recovered is accurate. Specialized RFID detecting tools may be essential to detect these utilities.
Pipe and Cable Locators
This locator has internal antennas that pick up the magnetic field in its receiver. The magnetic field may be artificially generated using a transmitter or picked up from magnetic waves generated by cables carrying current. They are classified as either cable avoidance tools, which are used to locate basic services and the general area of utility location; or professional locators, which are used for congested areas, deep service locations, large pipeline locations and long-distance location. (For more, check out An Intro to Underground Pipe Locators.)
These detectors are used to detect electricity cables that radiate a magnetic field due to the current flowing through them. The hum detectors receive these signals from live lines and do not respond to direct current cables, cables where current flow is very low or street light cables that are turned off during the day.
2-Man Electromagnetic Scan
This is an inductive electromagnetic scanning method that locates unknown buried utilities. The scan involves two people, one holding a transmitter and the other a receiver. Provided the two people walk across a metallic utility together, they can recognize, mark and follow it. Four different sweeps at different sweep angles will have to be conducted to maximize its effectiveness.
Vacuum excavation is a potholing technique that is carried out by simultaneously shooting compressed air jets and removing excavated material by vacuum action. An approximate area of 1’ x 1’ is dug at the point indicated by the locator. Since no water, mechanical devices or blades are used, the process is safe for the utility as well as the operator. The service uncovered should be clearly identified because sometimes the service excavated may not be the service you traced or are looking for.
Effectively Using Detecting Devices
To effectively use a detecting device or locator, the following pointers should be kept in mind:
- The operator should be well trained to understand the function of the device through training and practical experience.
- The device should be used as per manufacturer’s instructions and should be regularly checked and calibrated.
- Terminating cables should be treated as live until proven otherwise because they are difficult to detect even when they are live.
- Pipes and cables running close together may be detected as a single signal. Signal generators can be used to accurately detect and trace utility lines.
- Locators other than GPR may not be able to detect nonmetallic pipes unless metallic tracer wires or RFID chips are installed in them. Since many old utility lines are not fitted with these, one or more devices may have to be used.
- Services that are located on the plan but not detectable on site should be brought to the notice of the service owner.
- The entire work site should be surveyed because pipelines do not necessarily always run in straight lines.
- All exposed and identified utility lines should be properly marked using waterproof crayon, chalk or biodegradable paint.
Locating an underground utility depends on many factors such as the type of utility service and the depth of its location, the effects of nearby metallic structures/services on the device, ground conditions, the reliability, characteristics and calibration of the device, and most importantly the skill and experience of the device operator.
No project should be undertaken before preliminary survey, location, identification and marking of all utility lines has been completed. This can prevent the need for costly repair work to existing lines and danger to workers operating excavating equipment.
Trenchless construction methods such as horizontal directional drilling (HDD) and microtunneling excavate through subsurface soil, and hence greatly depend on geotechnical and utility investigation results.