Pier foundation on boulders

redlodge · September 12, 2019, 11:09:20 AM

Hey All!
I'm new here - and have a question about a foundation for a log cabin 24x28 that I hope to put in on land I have in the Beartooth Mountains in Montana. I am not a builder - more like a homeowner who hacks away at small projects - so never built a house or cabin or anything at that level so my knowledge is limited. I have some land that is basically a boulder field on a slight grade. I have been working with a local excavator who put a road into our place and dug well lines etc. He says that a pier foundation would be the best way to go as digging out a flat spot to pour a slab would be really difficult and likely cost-prohibitive. I am fine with that if it will work. I will attach a couple of pics of rough cabin location that probably won't show much, all the rocks in the pics are sitting on top of more rocks, many of the boulders there are huge with just the tips above ground, and underground they are the size of Volkswagons. (We had to dig out two for a septic system and they were huge)

My guy has dug many foundations in this valley and seems to know his stuff, but he is not necessarily a foundation specialist, but rather more of an excavator specialist. So - he says that if you dig a pier hole and it goes down 2 or 3 feet and hits one of those boulders, it is fine to set the pier (using sonotubes kind of method) on the boulder - that it will be solid there. I've been looking for wisdom on that online and found this forum with a few similar topics so wanted to ask if any of you have had experience with this and get thoughts about when appropriate to plant a pier on a rock vs a deep poured base. I appreciate your help!

Ljohnsaw · September 12, 2019, 11:53:34 AM

That would probably work if you drilled some holes to epoxy in some pins (rebar) to keep the pier from sliding off the boulders. Will this require permits? If so, check with the building department on their requirements.

JJ · September 12, 2019, 12:03:31 PM

Hi redlodge,

I had a porch add-on to my house and used 4 sono-tubes under beam. 3 holes no problem, down 4ft, but one I encounter a bolder 2ft down. After digging around, found this boulder to be about the size of a quad (4 wheeler) so did like you mention and poured a small slab on top of the boulder and set the sono tube on that. Big mistake.. For several years, no problem I tiled the floor of the room and finished it off with drywall. Anyway one very cold year the boulder moved, and picked up the entire porch by ~3 inches. It was scary, on coldest nights the room would bang!, and I had cracks in drywall and tile floor. I had to cut the post, and shim throughout the winter and into spring to keep the porch level. In spring, the boulder dropped back (mostly) but the sono-tube was broken.

So I get a small backhoe, and dug out the broken tube an boulder, too heavy to pick up, so I dig big hole in front of boulder, chained the backhoe to a tree, and dragged the boulder into the hole and reburied it. Then put the sono-tube in correct with 4' and big foot at bottom.

over the years, that boulder I reburied started to poke out of the lawn, and each year came up few cm, by time I moved from that house, I had to mow around it.

The frost does push these big rocks around.

JJ

Don P · September 12, 2019, 02:22:52 PM

What you will find in an online search is many people, rarely builders, claiming that pier foundations are fine and they know of examples a hundred or more years old. That is true. What they don't see is all the failures. Because of the trouble these foundations have caused the building codes have tightened incrementally over the years. Currently it is an engineer required foundation in the building code. Even if there is no enforcement take that to heart.

A full perimeter footing buried below frost depth with perimeter walls is the minimum prescriptive (without engineering) foundation. The walls can be made with the local rock and if uncoursed, a rubblestone foundation, the walls would need to be a minimum of 16" thick, which in stone is usually easy to do. It can of course be concrete or block. A permanent wood foundation is another option but it sounds like that might be more difficult on that site.

barbender · September 12, 2019, 06:27:47 PM

I'd say, it depends on how rustic it will be. If it's logs and heated and used only occasionally, and you don't care if it moves around a bit- maybe. It's o e thing if it's bedrock, but if it's just boulders they will move.

flyingparks · September 12, 2019, 08:56:39 PM

I would advise against a pier foundation based on what I have seen with movement. Depending on how much you are planning on investing, I highly recommend consulting an engineer. I lived in a house that was built directly on a slab of granite. No problems. It was essentially a stepped stem wall with rebar drilled into the granite.

DWyatt · September 13, 2019, 07:14:31 AM

I am certainly not a builder or foundation expert, but I would not do what you are thinking. At work we have designed sanitary sewer to be 4' deep (below the frost line here) and the owners thought they were smarter than us and put the sewer in 2.5'-3' deep. That was fine for the first 10 years or so but now they are running into the problem that their sewers are no longer at correct grade. They actually have a bunch of "humps" in the lines from frost heaving the lines that is causing major problems during high flow events within the park. Moral of the story is that frost heave is a very real thing and has more power that you could ever restrict, no matter the size of the boulder underground. There is a reason all of the codes specify foundations below the frost line, whatever that may be for your area. It may be fine for 10 years or so, but what happens when you have a harder that normal winter like JJ and end up having the finishing touches of the cabin broken/cracked when mother nature rears her neck.

redlodge · September 14, 2019, 12:02:43 AM

Thanks all! I appreciate your thoughts. Unfortunately, if we can't do a pier foundation of some kind here don't think we would be able to build here, not sure how we could pull it off since there are boulders on top of boulders. So will let you know what happens!

curved-wood · September 14, 2019, 12:40:24 AM

My house is built on rock and it didn't move a bit. Pins in the rock at every 4 feet.
Building on piers could be done, all the highrises are done that way but it has to sit on solid stuff. For a house it need good size footing under the pier. There could be some difference in settlement if some parts sitting on rock and some other on soil. As usual footings are under the frost line and on undisturbed soil. Consulting an engineer might be a good idea. But it could be done on piers

Bruno of NH · September 15, 2019, 08:38:41 PM

You could use adjustable post bases on top of the piers.
Like they use in green log home building and in Alaska when building on perma frost.
It can be done I built a house for a client on ledge and tank size boulders over looking a lake and have had no issues.
It's all built on post and piers.
20 + years and counting.
That project has no adjustable footing plates.

Don P · September 15, 2019, 10:18:43 PM

Building braced walls doesn't do much good if it sits on an unbraced foundation.

I know it is in our better nature to be helpful but the best help in this situation should come from a local engineer who has seen and correctly dealt with this situation before.

ponderosae · September 17, 2019, 05:17:44 PM

I guess some people add more rocks...

QuoteStone Foundations Questions and Answers

Q: I am building a "cabin" 22' x 36' in Ashfork, AZ. The elevation is 5700 feet and I'm on the Coconino rock shelf. While digging the trench for the coconino rock footers, I consistently dig to solid white rock. I took out the smaller ones (400lbs or less) and left the ones I couldn't remove with the mini-backhoe. Some of these "large" rocks approach the surface. Seems like I'm safe to use them as bases to my rock foundation? Also it sounds like you do not recommend lime in type S mortar - is this correct?

A: You should be fine building your foundation over the big rocks. I feel confident that the bottom of the
boulders are below the frost line with no threat of heaving. The type s masonry cement has lime in it and is formulated for use in stone masonry that needs a structural characteristic so no additives are needed.

dirtmotor · September 17, 2019, 09:35:16 PM

This old house of mine has been sitting on a rock foundation that is stacked on top of more rocks since 1906 and still solid . I believe it stays that way because of its balloon construction and the ridged way it was built , the foundation does move and shift but house stays put .So I think if you design your structure to be able to ride a less than perfect foundation it will work .

Banjo picker · September 18, 2019, 07:22:09 AM

@Don P gave you the advice you need get a local engineer involved. I have poured thousands of yards of concrete...set up 3 complete limestone crushers. At times a footer woul have to be placed on existing rock and we would put dowels in to tie our rebar to ,but these were simply too large to remove....much bigger than your Volkswagen sized rocks....and it was approved by an engineer every time.

Do you think your excavation guy is gonna come back and help you if those boulders heave out of the ground? You might get away with that here in the Deep South but I wouldn't try it in Montana. My2 cents worth, and it didn't even cost you that. Banjo

ScottCC · October 06, 2019, 08:22:55 AM

Do yourself a favor and install a foundation by building up with good crushed stone that drains. Put footers on that are insulted inside perimeter and out. Cover with with some dirt and go. The depth of footer, thickness of insulation and horizontal distance required are an engineers world. I did one last week in a zone 5 10 minutes from Canada that has 18" of stone under footer. 2' of 2" insulation outside footer horizontally under 2' of dirt and there will be no issues. You can replace to some degree depth of footer with insulation and make house essentially float.

Raider Bill · October 06, 2019, 02:58:36 PM

I have no answer to your question but I was in Red Lodge a couple weeks ago and rode the Beartooth Pass. You live in a beautiful area!!!

Don P · October 06, 2019, 07:46:04 PM

Quote from: ScottCC on October 06, 2019, 08:22:55 AM
Do yourself a favor and install a foundation by building up with good crushed stone that drains. Put footers on that are insulted inside perimeter and out. Cover with with some dirt and go. The depth of footer, thickness of insulation and horizontal distance required are an engineers world. I did one last week in a zone 5 10 minutes from Canada that has 18” of stone under footer. 2’ of 2” insulation outside footer horizontally under 2’ of dirt and there will be no issues. You can replace to some degree depth of footer with insulation and make house essentially float.

The general rules for a "frost protected shallow foundation" are in chapter 4 of the IRC giving depths, insulation etc.

We started laying block this weekend on this little house, luckily just the one boulder which the owner in the background thinks is cool. We got around it with just a little chisel help from the Hilti.

Crusarius · October 08, 2019, 12:13:55 PM

That spells future mightmare!

reride82 · October 09, 2019, 05:58:02 PM

Welcome to the forum!

Somewhat local engineer, 170 miles west of you, and my opinion would be to find a bigger excavator. If you could squeeze a septic system in, a crawl space should be easy. Personally, I'd daylight a basement since you're on a grade. We deal with rock daily, and a decent operator on a 330 cat can move mountains. If not, there are plenty of blasting guys around to make the pieces smaller. You're not far from Stillwater mine, so there should be some blasting contractors in your area. The last pier foundation I spec'd required geotechnical bore holes(read $$$) to find the bedrock layer with average pier depths of 12'. Shop around and get a few opinions. I have family in Red Lodge, my cousin has a pharmacy there.

Levi

ponderosae · October 26, 2019, 12:30:56 PM

By the way, there are boulder houses too (something I stumbled upon looking into earthbag construction).

Kainkelly · October 26, 2019, 09:57:11 PM

Quote from: ponderosae on October 26, 2019, 12:30:56 PM
By the way, there are boulder houses too (something I stumbled upon looking into earthbag construction).

Somebody is an artist with explosives or has the patience of 1000 men....

Kainkelly · October 26, 2019, 10:04:52 PM

Quote from: reride82 on October 09, 2019, 05:58:02 PM
Welcome to the forum!

Somewhat local engineer, 170 miles west of you, and my opinion would be to find a bigger excavator. If you could squeeze a septic system in, a crawl space should be easy. Personally, I'd daylight a basement since you're on a grade. We deal with rock daily, and a decent operator on a 330 cat can move mountains. If not, there are plenty of blasting guys around to make the pieces smaller. You're not far from Stillwater mine, so there should be some blasting contractors in your area. The last pier foundation I spec'd required geotechnical bore holes(read $$$) to find the bedrock layer with average pier depths of 12'. Shop around and get a few opinions. I have family in Red Lodge, my cousin has a pharmacy there.

Levi

If all else is out English Mason's have ways of drilling and splitting rock. Would be slow going and take a long time, but will remove them in time.

ponderosae · October 27, 2019, 08:56:01 PM

I tried it the other day on some kind of granite boulder, that stuff really doesn't drill or chisel easily (I managed to drill down and get the chisel stuck for a minute though). I also tried drilling shallower holes and slamming a spiked chisel between them, but it ricochetted and flew back at me (which I suspected might happen, just not that fast). So I would have had to pulverize it with a chipping hammer for hours and hours, or get a powered version of one (but didn't need to break it up that badly).