A while back we published an article titled What is an Unknown Utility? The publication covered the topic of unknown utilities, and what they mean. In this article, however, we want to discuss a different type of obstacle both excavators and utility locating companies must face: unlocatable utilities. On the face, the definition of an unlocatable utility seems pretty obvious, but what are the underlying situations or circumstances that make utilities unlocatable? Why are some utilities unlocatable in one area and locatable in another?
Any utility or structure that exists underground whose location is not known using any available sources and cannot be detected using surface geophysical equipment.
In our definition above, we state “location is not known using any available sources.” We believe that if the utility is known by an individual, utility record, or visible surface feature, then it is not unlocatable. When a utility is suspected, or there is significant evidence that it exists, even if it cannot be designated using surface equipment, then subsurface technologies exist to safely expose and locate the utility or structure using vacuum excavation before declaring the utility or structure unlocatable.
What types of utilities are unlocatable?
Ideally, we could answer this question with a black and white answer. Like most topics in the utility locating industry however, surface geophysical equipment, soil conditions, utility compositions, and other factors can really muddy the waters. There is no clear cut answer to define what utilities are unlocatable, because there are too many factors that can affect a technician’s ability to detect the location of an underground utility or structure.
The most common underground utilities that are not locatable on the surface with standard locating equipment can vary depending on the detection equipment that is being used.
In terms of electromagnetic equipment, the types of underground utilities that are unlocatable tend to be pressurized non-metallic pipes or occupied conduits that contain a non-metallic or ungrounded cable. For electromagnetic geophysical equipment to work as designed, an electromagnetic frequency must be applied to a conductor. If there is no conductor, the signal cannot travel through the utility, and thus it cannot be detected. This is why the installation of tracer wire along side the non-conductive utility is so critical, even on water and forced / pressurized sewer pipes.
Just because a utility is not locatable with electromagnetic geophysical equipment does not mean that it is unlocatable entirely, there is alternative equipment that may detect it’s location from the surface.
Ground Penetrating Radar
Ground penetrating radar works by sending pulses of radio frequency into the ground. When those waves are reflected off of objects in the ground, including the ground itself, the waves will travel back to the GPR unit at different velocities. Objects that are conductive will reflect at higher velocities, while objects that are not conductive will reflect at lower velocities. In most cases, the data is displayed and analysed in real time by the GPR survey technician.
The most common types of utilities that are unlocatable using Ground Penetrating Radar tend to be utilities whose depth exceeds the penetration limit of the transmitted radio waves, and utilities whose material composition has the same conductive properties as the surrounding earth material. When a utility has the same conductive properties as the soil, the utility will not reflect the GPR waves at a different velocity, and will essentially be invisible to the GPR technician. This happens a lot for example when attempting to find terracotta pipes in dense, moist, or clay earth materials.
Earlier we talked about our definition of unlocatable utilities. If any evidence exists to suggest the presence of an underground utility, we do not believe we can classify it as unlocatable without first attempting to safely excavate to find the location of the utility or structure. This of course can be a budget buster for most projects. You’ll have to weigh the risks to determine if the investment in exploratory excavating is really worth it.
If the utility or structure cannot be found using a reasonable amount of exploratory excavation, we can then classify the utility as unlocatable.
What can be done about unlocatable utilities?
The important thing to keep in mind is that not every utility is locatable. In the construction industry, you have to expect the unexpected and plan for everything. Hiring a reputable private locating firm is the first step. Even if a utility is not locatable, chances are that the qualified technician will have the experience and expertise to know when something doesn’t look right and will have the morals to speak up and communicate that with you.