Building fence can be a challenge in rocky, frozen or swampy ground where it’s impossible to dig postholes efficiently or set posts with a tractor-mounted post-pounder. Options in rocks include digging holes with a backhoe or chipping away the rock, if it’s a formation that will chip and break; prying rocks out with a hand bar; or using a hammer drill. “This works for drilling small-diameter holes into solid rock to insert a steel post, or even a bigger hole for a brace post,” says Michael Thomas, Thomas and Son Custom Fencing, Baker, Idaho.
“You can use either a pneumatic hammer drill that runs off an air compressor, or an electric hammer drill and a generator. These work fairly well, if you use a heavy-duty industrial model. A small one won’t do the job in big rocks,” he says. With a good drill, you can put posts into fairly solid rock.
In some terrain, where it’s not too steep and there are surface rocks, you can create an aboveground cage of rocks as a brace to anchor the fence, in lieu of brace posts.
Gather or stack rocks and secure them with net wire, or make a cage and put rocks into it. “A cage 3 to 4 feet in diameter makes a solid anchor to secure your fence wire and stretch it from,” he says. If terrain is too rocky to set wood posts, you can usually put steel posts in deep enough to hold, using rock baskets every so often for braces.
Another strategy for rocks or frozen ground when using a post-pounder is to create a pilot hole. A metal pilot post will often go through rocky ground if it’s not solid bedrock — it will push aside the rocks and penetrate frozen ground; whereas a wood post would be forced out of line or shatter.
The pilot post is only 3 or 4 inches in diameter and creates a hole for the wood post. The pointed bottom is solid-drill steel about 3 feet long to break the rock. Drill-stem cone won’t hold up, but drill steel in the tip will break most rocks or push them away, he says.
The rest of the pilot post is hollow, like well casing. This makes it a little lighter to carry. The top has a solid cap for the post-pounder to hit.
In some situations, the rock may be too solid to break or push aside. “If it’s a pasture or range fence and you are not concerned about the exact location of a post, you might have to move it a little one way or another on the fence line." If that's not an option, you may have to hand-dig. "The pilot post won’t drive through big, solid rocks.”