Fencing on the Rocks
- Apr 1, 2008
Building fence can be a challenge in rocky terrain. Some parts of the country have minor problems with putting in fencing due to rocks, and other parts of the country are nearly impossible to fence with standard posts. Here are some solutions for when you are forced to "fence on the rocks."
Chuck Druin of Penrod Lumber and Fence Company in Simpsonville, Ky., says there are only a few regions in Kentucky where rocks are an issue. South of Lexington, for instance, rocky areas are occasionally encountered and he might try to move the fence a little. But sometimes this is not possible--as when building boundary fence or constructing horse pens. "Most horsemen in this part of the country want board fence or diamond mesh, which means posts must be a certain distance apart; you don't have options for where you put the posts," says Druin.
Most fence contractors use post hole diggers or post pounders if there are only a few rocks. There are several types of diggers and pounders that can be mounted on a tractor, front-end loader, or Bobcat. The digger uses an auger or drill to go down through the soil and create the proper size hole for a post. The pounder drops a heavy weight every few seconds on top of a wood post--the bottom of which is sharpened to easily penetrate the ground.
In dirt or sandy soil, an auger or post hole digger works fine, but for setting wood posts many contractors use a mechanical post pounder mounted on a tractor. It can pound posts into fairly rocky ground if rocks are small, and it eliminates the task of filling and tamping a post hole. In rocky ground, however, a wood post might be forced out of line by the rocks, or it might be shattered by the post pounder.
Michael Thomas, a rancher near Salmon, Idaho, who has built fence in rocky country all his life and also does contract fencing, uses a post pounder wherever terrain is not too steep for a tractor. An innovative tool he uses in rocky ground is a metal pilot post to create a hole for the wood post. "The pilot post is only 3 to 4 inches in diameter. The bottom 3 feet is solid steel with a sharp tip; the top part is hollow well casing so it's light enough to carry around, and the top has a solid cap for the driver to slam down on," says Thomas. "You can drive it into anything but solid rock, since it pushes aside most rocks. You drive it in as far as you can, then pull it out, insert the larger wood post and drive it into the hole. The wood post will be very tight and secure."
Chris Cozby of 440 Fence Company in Aubrey, Texas, says that if a pounder can't get a regular wood post in, or if a digger gets the hole partway down and hits a rock shelf or fairly solid rock, he just tries to chip out the rest of the hole with a chip bar.
Digging Them Out by Hand
Mike McCarthy of Aiken, S.C., builds many fences for horse farms. As a horseman and polo player, he understands the fencing needs of horse owners. He grew up in Vermont, where rocks are commonplace, and he prefers to dig holes by hand.
"In Vermont, where I lived (Lake Champlain area) the rock was slate--since it used to be ocean floor," he explains. "We'd chip down through it, because this type of rock will break off. A hand bar works as well as anything. Then I built fences in Connecticut, where the rock was glacier deposits. You might run into rocks 4 inches to 4 feet in diameter and never knew what you were dealing with until you got into it. Most of it was granite, and you can't chip through granite.
"In those cases, we tried to get the hole close to where we wanted it, then used pry bars and poles to pull rocks out of the way by hand," he adds. "You might have to make a large hole to get the rocks out. A post hole digger mounted on a tractor is helpless in this terrain. It might get through sand and gravel, but not rocks. A digger mounted on a skidder or front-end loader has a better chance, with more mobility and added hydraulic strength--if it hooks a rock it may be able to pull it up and you can yank it out."
"You may get by with a rock bit on a post hole digger, if you're digging through sandstone that shatters and breaks up," says McCarthy. "But if the rocks are granite, it's nearly impossible to break them, or takes too long."
Druin uses two methods to deal with major rocks. "For posts that must go into solid rock, we have a truck with a rock drill (with tungsten carbide bit) mounted on it, just like the power company uses for setting power poles," he says. "We also use a jackhammer for making a smaller hole. If we have a major job with lots of rocks, we bring the truck because it can dig holes fairly fast. But if we're setting 300 posts and only have a few in solid rock, we just use the jackhammer."
He has two kinds of jackhammers; one is mounted on a tractor and the other is a hand-held air hammer.
He also has a small tractor-mounted drill that uses the same bit and tips as the big one on his truck. "Some of our crews keep this on the back of the tractor so if they hit a rock they just turn the tractor around, back it up to the hole, and dig it," Druin says. "The post pounder is mounted on the side, a front-end loader on the front, and the drill on the back, so they are prepared for anything. In normal rocky ground the drill digs a hole in about five minutes, but it might take 45 minutes to go through solid rock."
McCarthy sometimes uses a hand-operated hammer drill when fencing in rocky country. "If we hit a solid chunk of granite on the side of a mountain, we chip in a pilot hole for each post (with a hand bar) then use an electric jackhammer to take the hole a foot or so into the rock. Then we'd pound some rebar down into the bedrock, drill a hole into the bottom of the post, pound the post onto the rebar, and cement the hole around it," says McCarthy. This will hold the post, even though it's not very deep. If you can't get very deep, you have to make the hole wider for a larger concrete anchor.
The problem with using concrete around a wood post is that it holds any moisture next to the post, and even a treated post will soon rot off, according to Thomas. Unless you have a really good moisture barrier around the post, concrete is not a long-term solution. He's seen concreted posts rot off in a few years. "If you use concrete, do like the power company does and wrap the posts with a nonpermeable grease/paper combination," he says. "You'd be better off to make a concrete base/anchor in the ground that you tie into with a bracket that you attach to the bottom of the post. Then the post is only sitting on the concrete rather than surrounded by it, and it won't rot so much."
Backhoe and Other Options
McCarthy says that in the worst terrain he ended up bringing in someone with a backhoe or an excavator. "When the rocks are so horrific you're wasting too many man hours digging, barring, or drilling, it's faster to use a backhoe to dig a bigger hole quickly, then just back fill it when you set the post," he says.
Druin has done some fencing in New York, where there are major rock shelves. "What they often do there is go along the fenceline and chisel out a strip 4 feet wide, then fill it all in with dirt, so we can come along and put the fence in that line of dirt," says Druin. "Some places in New York have only about 6 inches of topsoil on top of granite. You might spend two days digging one hole. So they prefer to dig a trench or blast and scoop it out, or pre-drill the holes for us before we get there. Some companies, you just give them a map/diagram of where you want the fence and they drill all the holes for us."
In some instances you might make a jack fence (buck fence) rather than dig holes in solid rock. In windy country, however, a strong wind can tip over a whole section of fence unless it's well-anchored.
"To keep it from blowing over, hang a large rock under one of the jacks every so often, or make a small rock basket under some of the jacks, with the jack secured to the basket," says Thomas. To make a jack fence safer, saw off the top of the inside jack a little more flush with the fence so a horse isn't as likely to injure himself.
Where terrain is too rocky for setting posts, with rocks on top of the ground also, any fence can be easily braced with a rock basket instead of posts. "Gather and stack the rocks and secure them with net wire, or make the wire cage/basket first and fill it with rocks," says Thomas. "Rocks are heavy enough that a cage 3 to 4 feet in diameter/width (and however tall you want it) is adequate to make a solid anchor for any wire fence."
Cozby of 440 Fence Company says that when you go to a lot of trouble to build a fence in rocks, it makes good sense to put in something permanent. He sells pipe fencing, which lasts longer than wood or vinyl. "If you build a cheaper fence and have to keep repairing it, and then a few years later have to replace it, it may cost two to three times what you paid originally--since costs of fencing keep going up. It pays to build it just one time and be done with it," says Cozby.
"If you can keep steel from rusting, it will last forever, and we've done that with the finish we put on it," he adds. "We also use stainless steel screws and pins in all the connectors."
If you set this type of fence in dirt or rocks, it won't deteriorate or corrode. If you're fencing in rocks, you don't have to dig a very big hole for the posts; the metal is smaller diameter than wood or vinyl posts, and you can use a rock drill to make a small hole. Since rock is very solid, the hole doesn't have to be as deep as what you'd need for an ordinary fence post.
When you have to put up a fence on rocky ground, there are a variety of ways to solve the problem. Depending on the type of rock, sometimes drilling or pounding through it works. Other rock layers are too thick and require fencing that sits on top of the ground. Whatever method you use, make sure the fence is sturdy and safe for horses.
About the Author
Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog, www.heathersmiththomas.blogspot.com, she writes a biweekly blog at http://insidestorey.blogspot.com that comes out on Tuesdays.