foundation on bedrock

[dvellone]

having recently excavated for my new passive solar house project I'm now confronted with bedrock and forced to go back to the drawing board. I'd planned on a full basement.

The high point of the ledge is just about at grade and off center of the footprint by a little bit. The grade falls away in every direction from that point to a low of 56" below grade.

backfilling and compacting for a slab-on-grade isn't an option as it cuts too deeply into my budget and presents a difficult challenge of getting the compression to 95%.

So my option seems to be limited to pinning the foundation to the ledge and living with a crawl space . It doesn't seem too bad an idea as I can still have a place to route my wiring and plumbing as well as some cool storage. The foundation as well as the whole house up to the top plate will be icf construction.

My question is this: is the bedrock going to be heat sink drawing away from my house or will it benefit me by maintaining a stable year-round deep ground temperature below my floor. I'd think that my approach would be to insulate the floor very well. I'd have that 45-50 degree temp below the floor.

Am I thinking right on this? I don't think that moisture will much of an issue as the ground is well-drained gravel and sand, and the ledge and building site is a high point.

Thanks for any input

 

[Reggie Dunlap]

Not sure how much rock you have or how hard it is. You could blast if your budget can handle it, if it's just one chunk it may not be too expensive. You could also use an excavator with a hammer and break it up. Here it VT a big machine with a hammer is around $250 per hour.

If it was my house I'd be willing to pay the upfront cost to get a full basement. They call it a crawl space for a reason...

Where are you in the Adirondacks?

 

[semipro]

As far as heat gain/loss and moisture go I can't see how the bedrock would differ much from a sandy soil. The specific heat of the material is essentially the same. If the bedrock is contiguous and unfractured I guess moisture moving from the soil upwards might be less than with soil. I'll bet the bedrock is fractured though.

I think you treat it like any other crawlspace and to do that properly you need to control moisture. The Building Science Corp. website has some good publications on how best to build crawlspaces. Essentially you need to install a vapor barrier on the floor and excavated walls to keep soil from ground away from your structure.

When you build your foundation be very aware of differential settling. That is, watch out for the portion of the foundation that's built on soil settling more than that portion pinned to bedrock.

 

[dvellone]

Not sure how much rock you have or how hard it is. You could blast if your budget can handle it, if it's just one chunk it may not be too expensive. You could also use an excavator with a hammer and break it up. Here it VT a big machine with a hammer is around $250 per hour.

If it was my house I'd be willing to pay the upfront cost to get a full basement. They call it a crawl space for a reason...

Where are you in the Adirondacks?

Lots of rock... It's the granite bedrock that comprises the bulk of the Adirondacks. I've got a blaster coming by tomorrow to look at the site and give me a quote, but I'm not that willing to spend a whole lot on a basement as I've already got a large insulated utility space and will be building a garage as well. The crawl-space actually will have a portion with a ceiling height of at least 74" and it happens to be under the kitchen and 1st floor bath... so everything didn't go completely wrong.

I'm in the west-central Adks near Old Forge.

 

[peakbagger]

Berlin NH is on mostly solid granite. THe local water and swer utilities have been replacing the water and sewer lines for several years. They used to blast but switched to a Backhoe with jackhammer mounted on it (called a ram hoe). They havent blasted for for three or four years. The blasting costs a lot due to liability insurance for the inevitable claims from neighbors. If there any fracture lines in the granite, a ram hoe works well. If you do go this route get the biggest back hoe you can find, this is not the spot for a crawler loader. I uses to pay $1,200 per day for the ram hoe and operator.

One thing to be concerned about is if you build on granite , you probably are going to have radon issues. The easy way is to vent the crawl space year round, but that means you have to insulate the floors and all the utilities under the floor.

Lot to be said for backfilling around th hours to raise the grade, as removing rock is not cheap.