Foundation on bedrock

[AlpineEngineer]

I've got a single family home foundation plan that I am designing. The site is SOLID exposed granite bedrock, no fractures, it will need to be blasted. The slope is about 30% and the house will be 1 story on top of a walkout basement, so slab on grade. For now I am assuming that after blasting and excavation we will be on undisturbed solid bedrock. It appears to me spread footers are not necessary due to the fact that there will be no frost heave potential and the bearing capacity is probably around 12,000 psf (plenty).

Couldn't you just pour the slab directly on the bedrock and build up from there? I'm thinking of using an 8" thickened edge/slab w/ horizontal #4's at all load bearing areas, for concrete integrity. I am also thinking I'd feel better if there were some dowels (#5 bars) epoxyed into the bedrock and then bent into the slab, is this needed? If so, how often would you put the dowels, how deep?
Thanks in advance.

 

[oldestguy]

Sounds like a good plan, except why any dowells? Doesn't sound needed.

In my experience for non-frost heaving rock,no footings are needed and it all has worked fine for light structures of masonry.

If you have not had experience with blasting, better check out a few things. Blasters like to over-blast (deeper) to be sure they have not left any hard zones. On occasion, with differing footing elevations they have gotten overboard on how deep to go and "lightly-loosened" rock is left. If you see blasting wires or other evidence of this over blasting, better investigate. They usually don't throw any rock in the air with carefully designed blasts. This can be the problem.

In these instances, I call for hand excavation of all loosened rock. A machine may miss this.

I have seen some excessive settlements of residential structures when loosened rock is built on.

 

[jdonville]

AlpineEngineer,

Not an expert, but I can only imagine that after blasting the rock will not be "undisturbed". Also, I expect that the surface will not be smooth.

Just my $0.02...

Jeff

 

[Grouser]

AlpineEngineer

I'd put the dowels in. They can't cost very much and will provide resistance against sliding or uplift if water gets under the slab and freezes. As a guess how about one every 10' along the perimeter and a few in the center (16 + 4 for a 40' x 40' slab = 20). I guess uplift calcs would be necessary for embedment/developement lengths.

Good luck.