This protects against a bore head that has wandered. Bore heads can change direction upon hitting a rock underground, or by gradually migrating towards loose soil. When crossing a marked, underground facility, it is vital to expose that facility and dig holes on either side of that utility to track the directional bore head. Do not assume that you are boring deep enough. When back reaming, track the path of the back reamer as you would the bore head.
Back reamers can veer off track as well.
In addition, make sure when boring that you have allowed for the additional size of the back reamer when crossing above, below, or along side of underground facilities. Back reamers will be larger in size than the bore head and that extra size needs to be accounted for when working near facilities. Lastly, it is becoming more common to try and avoid problems by burying deeper and deeper. Burying new facilities 25 to 30 feet down to avoid problems in installation will only create a major problem later on. That problem is the locating of those facilities.