The Tolerance Zone

By: Lareina Taing

After filing a locate request with Montana 811, make sure to wait two full business days to get your underground utilities marked before you break ground. After the standard waiting period has passed, remember to check the status of your locate ticket by going to www.montana811.org and clicking on the white button named “Search and Status of a Montana Locate Ticket.” If all affected facility operators have responded, what’s next? You should dig using hand tools, vacuum excavation, or a non-invasive excavation method within the “tolerance zone”, the 18” area on either side of an underground utility plus the width of the utility itself.
 
Because location markings are approximate, the exact position of buried lines may vary from the marks. That’s why excavators should maintain a minimum horizontal (side to side) clearance of 18 inches between an unexposed facility and the cutting edge or point of any power-operated excavating or earthmoving equipment. For example, if the markings indicate a 20” pipe is buried and the minimum clearance is 18” on both sides of the facility, the entire tolerance zone is 56” wide (18” + 20” + 18”).  See below for a visual example of the tolerance zone.
 

Remember that the depth of an underground facility may vary due to installation practices, frost, erosion, and other factors. Any depth readings given by a locator, if given at all, are only estimates, and the excavator is still responsible to safely expose the facility without damage.
 
Careful digging within the tolerance zone is an important step in the damage prevention process. Protect yourself, loved ones, and the environment by always hand digging or practicing non-invasive excavation methods in the tolerance zone. By doing so, you will have peace of mind knowing that you’re helping to protect vital underground infrastructure!