The tolerance zone is the approximate location
of a underground utility facility defined by a strip of land at least 3 feet
wide, but not wider than the underground utility facility plus 1 1/2 feet on
either side of such facility based upon the markings made by the owner or
operator of the facility.
Excavation within the tolerance zone required hand-digging using extreme care and caution. (See Section 2.7 of the Illinois Underground Utility Facilities Damage Prevention Act.)