Underground Obstruction at Planned Post Location: Relocate vs. Bore vs. Stop Decision Tree
Why this matters
Hitting an obstruction mid-dig is one of the fastest ways a fence job turns into a cost overrun, a safety incident, or a liability claim. The wrong response - forcing through without identifying what you hit, relocating a post without checking the permit layout, or walking off the job without documenting - creates problems that outlast the job itself. This tree gives the crew a structured decision path from the moment the auger binds or the digging bar deflects.
Step 1: Confirm utility locate status before interpreting anything else
Before diagnosing the obstruction type, answer two questions.
First: was 811 called and were the locates marked? If the locates are not on the ground, stop augering and get them. No obstruction identification is safe until you know where the public utilities run.
Second: were private utilities identified? The 811 one-call system marks utility company infrastructure - gas mains, electric distribution, water mains, telecom trunk lines. It does not mark private lines, including the gas line running from the meter to a detached garage, irrigation zone lines, invisible dog fence wire, low-voltage outdoor lighting runs, or private well lines. These are the owner's responsibility to locate and mark before dig day. If the owner did not provide private utility markings, do not proceed past hand-dig depth (typically 18 inches, verify with local jurisdiction) until the private lines are confirmed clear.
A public utility locate clearance does NOT mean the ground is clear of private lines. Gas lines to detached structures, irrigation, invisible fence wire, and low-voltage outdoor lighting are routinely buried in the exact path a fence follows along a property line or around a structure. Always ask the customer before the job starts: what private lines exist and have they been marked? Document the answer on the work order. If the customer is unsure, hand-dig to confirm or require a private locator before augering.
Step 2: Identify the obstruction type by resistance and sound
Stop augering before the bit seats fully on the obstruction. The auger's behavior as it contacts resistance tells you a lot.
Rock or concrete: the auger contacts with a grinding or ringing metallic sound and the bit deflects or spins without advancing. The digging bar produces a sharp clank rather than a dull thud. Rock is often irregular; old concrete is flat-topped if it is a footing remnant.
Tree root: the auger binds and then pops free as the bit cuts through. You will see root fiber in the spoil. Large roots (over 3 inches diameter) will bind a small auger; the spoil will have a distinctive smell.
Unknown hard object: if you cannot characterize the obstruction by sound and behavior, expose it by hand digging around the perimeter of the hole before making any decision. An unmarked metal object - pipe, conduit, cable - looks like a rock on the auger until you see it.
Step 3: Relocate the post?
A 6-inch post relocation in any direction is typically acceptable on residential privacy fences without triggering a permit revision. The rule of thumb: if the relocation keeps the post on the correct side of the property line, maintains panel-to-panel spacing within the structural tolerance of the rail system (usually plus or minus 6 inches on a standard 8-foot panel), and does not move the fence line out of the permitted setback, the relocation is a field adjustment that the foreman can authorize.
Relocations that require customer and possibly inspector sign-off: any move that pushes the post past the property line, any move that changes the setback from a structure or from the street, or any relocation on a permitted fence where the inspector has marked post locations on the approved plan.
Document every relocation with a photo showing the original hole, the obstruction in place, and the new post location relative to the original.
Step 4: Bore through it?
If relocation is not possible, assess whether the obstruction can be worked through.
Rock: hand drilling or pneumatic hammer drilling is viable for soft to medium rock (limestone, sandstone, weathered granite). Hard granite or basalt requires a specialty drill rig. Get a cost estimate before committing to this path - the per-hole cost can exceed the value of keeping the post in the original location.
Old concrete footings: breaking out a buried footing with a breaker bar or pneumatic hammer is usually possible if the footing is a remnant (previous fence, old structure) and not tied to active structure. Confirm with the GC or owner that removing the footing is authorized before breaking.
Tree root: cutting through roots under 3 inches in diameter is standard. Roots over 3 inches should be discussed with the property owner - significant root cutting can stress or kill mature trees and creates a neighbor or warranty dispute if the tree dies later.
Step 5: Stop and document?
Stop work and contact the customer or GC when any of the following apply: you cannot identify the obstruction and it could be an unmarked utility; the obstruction requires a specialty crew and the cost changes the job economics materially; boring through it requires removal of a structural element (old footing tied to a wall, large tree root from a protected tree); or the permit layout requires a post in this exact location and relocating is not authorized.
When stopping, document: photo of the hole with the obstruction visible, depth to top of obstruction, description of obstruction type and how it was identified, and the name of the person notified. This documentation is your protection if the customer disputes the delay or the added cost.
References
- OSHA 29 CFR 1926.651 - excavation requirements and utility notification.
- Common Ground Alliance Best Practices for Damage Prevention (CGA BP-2023).
- 811 national call-before-you-dig requirements by state (call811.com).
- International Residential Code (IRC) Section R105.2 - exempt and non-exempt work (post-location permit applicability varies by jurisdiction).