When hiring a private underground utility locating company, you expect that they will find all of the underground utilities present within the project or location that you have requested. Often times, however, they may report that there is an “unknown utility” present. But what does that mean? Surely someone must know about it, after all it was installed by humans. The answer to this question can seem simple at first, but a deeper look into the data may prove that thesis to be a facade.
Unknown Utility: What it Means
When a private utility locating company is reporting they they have discovered an unknown utility, they are reporting that they have discovered a buried object that is likely a utility, but could not collect any data to identify what type of utility it is. For instance, a utility locating company may perform a Ground Penetrating Radar survey to scan an area for underground utilities. If they find a pipe or cable, but cannot find any utility features to identify what that utility is exactly, such as a valve, manhole, or pull box, then they will likely report that utility as “unknown.”
When a utility is reported as unknown, it certainly does not mean that the structure doesn’t exist, or that it is less dangerous than other underground utilities. In some instances, unknown utilities could prove to be more dangerous or costly if damaged.
How are unknown utilities detected?
Unknown utilities can be detected using any number of technologies and methods, including electromagnetic equipment and ground penetrating radar.
There are a few methods to locate underground utilities using electromagnetic equipment. This type of equipment is capable of applying an active signal (frequency) to a conductor through the use of a transmitter. The signal that is applied is an electromagnetic frequency. The transmitter will apply the signal, and a second instrument called a receiver will detect the magnetic field of the transmitted frequency. Because this frequency works like an A/C Current, the frequency signal will travel back to the transmitting source. Often time, the signal will bleed over onto a nearby utility to return back to the transmitter. Professional Utility Locators will utilize this signal return to search for additional utilities that may be present in the vicinity of the target utility. If they are able to detect a utility, but cannot identify the origin of the utility, then it will be labeled as unknown.
Electromagnetic equipment is can also detect passive signals. Passive Signals, typically 60Hz in the United States and 50Hz Overseas, are naturally occurring frequencies that are present on a utility system, most commonly electric. Performing a grid search to detect passive signals may result in the technician marking out unknown utilities. Many utilities will emit passive frequencies, including, but not limited to electric, copper telephone, cable television wires, and steel pipes such as natural gas, water, or steam pipes. Because of the vast array of possibilities, utilities detected using only passive methods should always be considered “unknown.”
What should I do if I have unknown utilities present?
If you recently hired an underground utility locating company and they have reported “unknown utilities” within your excavation or design area, then you can relax. Reporting on unknown utilities is normal for private utility locating companies, and it is a good thing! It means that the company has done their job and found underground structures that likely would have not been located otherwise. Now that you know about the unknown utility, you will need to determine if it is a problem or not. If the unknown utility is in direct conflict with your excavation or design, your best approach is to perform a test excavation on the unknown to determine what was detected. This can be accomplished with any number of excavation methods, but the safest method is with the use of Vacuum Excavation. You must be careful when excavating an unknown utility, because after all, the nature of what is buried below is not known.
Using vacuum excavation, the utility can be safely exposed in a small, minimally invasive test hole that is approximately 15 inches in diameter using high pressure air and a vacuum hose to remove the spoils. This will allow for identification of the utility structure including size, material composition, utility type, orientation & heading, and of course the exact elevation or depth of the utility from grade. Exposing a utility with vacuum excavation may provide a new connection point to the utility, allowing the private utility locating company to investigate the utility further with electromagnetic equipment.