Before beginning excavation for any construction, infrastructure or other building project, a number of factors must be taken into consideration in order to ensure an efficient, correct and safe digging process. One factor that deserves special attention during any excavation operation is the tolerance zone for any underground facilities.

What is the Tolerance Zone?

Simply put, the tolerance zone is a pre-defined horizontal distance extending from the outer edge or wall of a line or pipe that runs underground. The exact distance that defines this tolerance zone varies from state to state, ranging from 18 inches to 30 inches on either side of the line or pipe. Almost all states are evenly split between using 18 inches or 24 inches in their tolerance zone definition; North Carolina is alone in using 30 inches.

To determine the total size of the tolerance zone area, you must know the state’s guidelines and the size of the line or pipe. For example, the total size of the tolerance zone area for a two-inch pipe in a state with a defined tolerance zone size of 18 inches would be 38 inches: 18 inches on either side of the pipe plus the two-inch diameter of the pipe itself.

Why is Understanding the Tolerance Zone Important?

The tolerance zone acts as a warning to an excavator that they need to proceed with care and caution and employ best practices while working in the area. In particular, the tolerance zone is necessary due to the inexact science of locating underground facilities.

Keep in mind that underground facilities can be located only indirectly and approximately through the use of locating equipment. All locating equipment is indirect by definition; a cable locator finds the electromagnetic fields created by the cables, not the cables themselves. While the results of this method are often practically the same thing as locating the position of the cable itself, certain factors can result in less precise positioning. For example, a change in the density, composition or moisture content of the earth above the underground facility can distort the electromagnetic field. In these cases, the strongest electromagnetic field above ground may not correspond exactly to the location of the facility underground.

It is for this reason that tolerance zones are so important. While locating equipment can only return an approximate location of the underground facility, the tolerance zone is designed to take any variances caused by distortion into account and define the area in which the utility can be presumed to be located.

Best Practices for Working Within the Tolerance Zone

While all states allow for digging within their defined tolerance zones, guidelines do exist to help protect underground facilities; anyone looking to excavate within a tolerance zone should familiarize themselves with their state’s particular rules and regulations. In general, it should always be assumed that the exact location of the underground facility is anywhere inside of the tolerance zone, and following certain best practices can help you steer clear of an avoidable disaster.

Dig Prudently

First and foremost, always employ prudent excavation techniques such as hand digging, soft digging, pot holing or vacuum excavation. Note that some states allow only hand digging within the tolerance zone. Instruments such as pick axes, digging bars and pointed spades should never be used.

When beginning to dig, loosen dirt with a blunted shovel, and then remove the loosened dirt using a regular shovel. Similarly, do not stab at the ground with your shovel; a gentle prying motion should be used instead. Digging should be done at an angle and parallel to the direction of the underground utility line; following this practice ensures that the shovel will harmlessly slide off of any pipe that you may hit.

Treat Every Line Like It’s Live

Once you do expose any underground utilities, remember to always treat any line as though it were live. Furthermore, don’t assume that a facility is correctly marked and color-coded. If you come across a line that is outside of the prescribed tolerance zone, don’t jump to the conclusion that the line is incorrectly marked; there may be more than one utility line in your excavation area.

Use Test Holes to Confirm Location and Depth

If you are employing directional or horizontal drilling or boring, you should confirm the location and depth of any facilities that are along the boring path using hand dug or vacuum excavated test holes at all points—including for any utilities that traverse the boring path.

Keep in mind that an underground utility line may not be at the expected depth. While the line should have been installed at the correctly prescribed depth, over time grade changes or erosion can result in a line that is at a depth different than at installation. Similarly, the size of the line may not be as expected. While the locator should have been able to accurately gauge the facility size when marking, these estimates are not always correct.

Never, Ever Dig Directly Up to the Tolerance Zone

Lastly, remember to never dig directly up to the tolerance zone. First expose the line, then dig to the exposure.

When planning a dig within a tolerance zone, keep in mind that these best practices can help protect you, your co-workers and your company. Your company may be held liable for any damage done to underground utilities, either directly or indirectly. More seriously, you or those that you work with could be injured or killed by not taking the proper care around buried utility lines. Before you begin to excavate, always make sure that you understand the tolerance zone in your excavation area and that you are following proper digging guidelines.

At Underground Services, Inc., we are pleased to provide a complete range of professional utility locating services across the United States. Our trademarked SoftDig system employs a non-destructive vacuum excavation process and has been in use throughout the United States, Europe and Australia for over 50 years, revolutionizing the practice of underground utility locating.

For more information about the standards that regulate our work, a portfolio of our previous clients or any other question, you may have, get in touch with us at (610) 738-8762 or use our online estimate request form.

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