Gate Post Hole Through Rock

[Obed]

I need to put in a 16' farm gate at the entrance to our property. I only have 1 foot of dirt before I hit what I'm assuming is a solid sandstone base. How do I dig a 3 foot deep hole in this stuff?

I can rent a 3PH post hole digger but they don't have rock augers available. I can rent a hydraulic jack hammer for $93/day with a 3 foot bit. I can also rent a rock drill and compresser and buy a couple rock bits for $167/day. I don't have access to electricity where I will be installing the gate.

What is my best option?

 

[GaryBDavis]

Assuming you are only going down three feet or so, I'd just dig them by hand with 20lb spud bar. Unless the rock is particularly hard, it's faster than going and renting something.

I have a 175cfm portable air compressor with a 90lb jack hammer that I use on occasion. But I have to tell you, using a 90lb jackhammer is just about as painfull as digging it by hand. There's a reason only big, burly men run jackhammers.

 

[swines]

I need to put in a 16' farm gate at the entrance to our property. I only have 1 foot of dirt before I hit what I'm assuming is a solid sandstone base. How do I dig a 3 foot deep hole in this stuff?

I can rent a 3PH post hole digger but they don't have rock augers available. I can rent a hydraulic jack hammer for $93/day with a 3 foot bit. I can also rent a rock drill and compresser and buy a couple rock bits for $167/day. I don't have access to electricity where I will be installing the gate.

What is my best option?

I don't know the geology of the area where you're putting this in, but why would you assume that you're hitting a solid sandstone base without investigating further?

Have you tried probing around at 3' and 6' radius around the post location just to see what's there?

For example, I was putting in fence posts and hit a solid object at about 18" depth. After digging around by hand, I found out the rock was really about bowling ball size and could be removed. All of the other holes just had sand and gravel to go through, and the largest gravel pieces were only about 2-inches in diameter. Begging the question, how could I manage to locate a hole in the only place with a large rock - AND manage to hit right in the middle?

Unless you know the geology includes sandstone shelfs and not just rocks or boulders - I'd do a little more investigation before I rented jack hammers, etc.

 

[therebarely]

I just put in about a mile of fence in similiar soil. You can probably put one hole in with a 3 point auger, but it is time consuming. I actually had to put a tow chain on the hook on top of the gear box and pull it down with a truck to get enough force to go through the rock, but it will do it. I had an auger with replacable teeth, and a set of teeth would do about 5 holes before needing replaced. Keep water in the hole to, it helps keep the auger cool.

 

[RedRocker]

For one hole I'd use a rock bar. I'm renting a skid steer next weekend with a rock bit to drill 30 some holes at my place, which is all rock.

 

[Zoombah]

On a related note.....you need a very solid post to support a 16ft gate. I assume you will be using a tube gate. Common tube gates come in at least three different sizes of tube diameter and metal guage. A heavy guage 2" diameter tube gate weighs quite a bit and requires a very solid post. A lighter guage gate with smaller diameter tubes will require less support. Regardless of what grade of gate you use bigger and deeper is generally better where the post is concerned.

If 'twer me I'd be looking at an H braced 8" post buried at least 42"

Also as suggested by Gary a spud bar works pretty good on sandstone. On mine one end is flattened to a dull blade and the other end is shaped into a flat disc for tamping dirt. The blade end works well on soft rock and roots, and the flat end is indispensible for tamping dirt around posts. Might want to get one and try it out before renting equipment.

 

[Obed]

Assuming you are only going down three feet or so, I'd just dig them by hand with 20lb spud bar. Unless the rock is particularly hard, it's faster than going and renting something.

Thanks for the good suggestion. I'll try using a spud bar first before trying the other more painful and expensive routes. I have 5 holes to dig: 3 for the corner posts where the gate will be hung and 2 posts at the other end of the gate. I'll be stringing barbed wire fencing for about 60 feet on each side of the gate. The fencing is to deter 4 wheelers from nosing around the place since I don't live there.

Where is a good place to purchase a spud bar? I can borrow one from my dad but I'd like to own one.

 

[Obed]

Have you tried probing around at 3' and 6' radius around the post location just to see what's there?

Yes, I have probed with a septic rod. I have consistently about 1 foot depth of dirt in the area all around the gate post. This area of TN commonly has a sandstone base about 2 feet below the surface in many areas on the plateau.

The area is the Cumberland Plateau that runs from KY through TN down into AL. It's a rather unique geological formation of a plateau at about 1800 feet in evation that is perhaps 30 to 50 miles wide by 150 (guess) miles long. The surrounding valleys on either side of the plateau are 500 to 700 feet in elevation. The eastern edge (escarpment) of the plateau is so steep that it terrified the early wagon trains. When traveling down Walden's ridge, the pioneers would cut down a tree and drag it behind the wagon as a brake. This plateau was a significant obstacle to the pioneer expansion of the nation westward.

 

[Obed]

For one hole I'd use a rock bar. I'm renting a skid steer next weekend with a rock bit to drill 30 some holes at my place, which is all rock.

RedRocker,
What's that get-up going to cost to rent for the weekend?

 

[EddieWalker]

I've dug through rock by hand and it's a miserable thing to do. Two of us spent a full day to get one hole dug by hand. We took turns swinging a sledge hammer and holding the bar. That was when we were teens, and I don't think I'd do it now if I could help it.

Another option to digging a hole through rock is to create a large footing. You said the gate is part of a corner post. Can you trench down to the rock the full legth of the corner bracing? From the post that will hold the gate to the corner post and then make the turn to the end post? Make it a full shovel width wide and go as deep as you can. A foot should be plenty, but if you want, you can always frame it up taller to get more concrete around your posts.

Weld up the entire corner before you install it with your top pipe and then weld two lengths of rebar near the bottom of each run. To really build it up and add more strength, you could weld on some angle bracing from the top of the posts down to the rebar also, but I sort of think this is way overkill.

Set the whole thing in your trench, square it up and pour concrete. You will need hundreds and hundreds of pounds of concrete in the trench to support the gate. You might even be able to rent a buggy with a yard or half yard of readi mix in a trailer. Here, I can buy one yard of readi mix and rent the trailer to haul it in for $120. Then you just back it up to your trench and pour it in.

With all that concrete to weigh it down, nothing will be able to move it.

Good luck,
Eddie