Digging Tolerance Zone and Excavation Documentation

 In Damage Prevention of Utilities

When digging in the tolerance zone, the excavator must be familiar with the state’s requirements for the tolerance zone size, excavation methods, and requirements for documentation.

What is the Tolerance Zone?

In damage prevention, excavation, or the utility locator’s world, the tolerance zone is a horizontal measurement beginning at the outside edge of an underground facility. Each state has its own measurement which currently ranges between 18-30 inches. We use this dimension to create a tolerance zone area that consists of both sides of the pipe as well as the utility line itself. The purpose of doing this is to warn the excavator to proceed with extreme caution and use sensible digging techniques when working in this area.

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Locating Equipment Approximates Location

Utility locating equipment approximates the location of utilities based on electromagnetic fields. Unfortunately, electromagnetic fields can be distorted by other subsurface utilities, changing soil density, terrain composition, and moisture content. Therefore, the underground utility may not present itself directly beneath the strongest locate mark. For this reason, the experienced excavator will always anticipate that the utility can be anywhere within the tolerance zone area.

Digging Tolerance Zone Dimensions

The actual size of the tolerance zone varies depending on the state. Excavators need to know the dimension for their state, and they also need to understand the state’s requirement for excavating within the tolerance zone area. The chart shows tolerance zones for states within our region.

TZ =18” TZ =24”
  • Colorado
  • Montana
  • Nebraska
  • New Mexico
  • South Dakota
  • Texas
  • Arizona
  • Idaho
  • Nevada
  • North Dakota
  • Utah
  • Wyoming

The extent of the affected area depends on your state and pipe size. A simple and easy formula to help estimate the tolerance zone area is:

Tolerance Zone Area = (TZ x 2) + pipe dimension

For example, if an excavator in Colorado is working around 4″ pipe, the tolerance zone area would be 40″ wide. [Tolerance zone area = (18″ x 2) + 4″ = 40″]

State Digging Requirements within the Tolerance Zone

As mentioned earlier, the excavator is responsible for checking their state requirements before digging, to understand how to proceed if digging within this zone. For Example, some states may require only soft excavation methods in this tolerance zone area (i.e. vacuum excavation or hand digging).

As of the date of this post, in Colorado, for example, the requirements are:

Special Digging Requirements within Tolerance Zone
When a person excavates within eighteen inches horizontally from the exterior sides of any underground facility, such person shall exercise such reasonable care as necessary to protect any underground facility in or near the excavation area. It shall be the responsibility of the excavator to maintain adequate and accurate documentation, including but not limited to photographs, video, or sketches, at the excavation site on the location and identification of any underground facility throughout the excavation period. Colorado Revised Statutes § 9-1.5-103 (4) (c) (I)

In other words, it is not required (though may be prudent) to hand dig or vacuum excavate within the tolerance zone, though, the excavator must proceed with reasonable care and document the dig.

Excavation Documentation

Excavation safety begins before the dig. When excavating within the tolerance zone in many states, including Colorado, it is required to provide excavation documentation. The Rhino Hit Kit was assembled to satisfy this requirement and includes all the materials needed for accurately documenting a dig near and within the tolerance zone area.


  • Excavation Safety Guide, https://www.excavationsafetyguide.com/guidePDFs/2014_ESG_page_27_28.pdf, (accessed June 26, 2014)
  • Title 9 Safety- Industrial and Commercial Article 1.5 Excavation Requirements, https://colorado811.org/c/document_library/get_file?uuid=f4aab8c7-d2d9-481c-a5ed-27e897d312da&groupId=18, (accessed June 26, 2014)
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