spread footer over a granite boulder

[AlpineEngineer]

I have a situation where this builder wants to (for asthetics) pour a spread footer right on top of a large natural granite boulder, the boulder is 15' long, 6' wide, and 4' tall. He wants to stone veneer this area of the house and make the boulder look intergral with the home.

This particular section of foundation is a spread footer 28' long and it crosses the boulder in the 15' direction, so more than half of this section of footer will be on the boulder. Do you guys think there are any frost heave concerns with this boulder, and do you think there are any sliding concerns, the natural slope is 32%!! If you think its ok to pour the footer on the boulder shouldn't there be some kind of rebar dowels drilled into the boulder?
Thanks a ton,

 

[csd72]

Frost heave forces are huge, I wouldnt rely on the weight of the boulder to resist this.

Why not remove the boulder, run a footing underneath it, replace the boulder on a bed of grout and then cast the remainder of the footing across the top of the boulder as planned.

I agree that dowelling into the boulder is a good idea to keep it in place.

csd

 

[Lion06]

I would be concerned about pouring a fooitng on a 32 degree slope.
Can you cut into the boulder where your footing needs to be to provide a level bearing surface?
Although, then you still have the frost heave issues. Does the boulder come close to reaching your required depth for frost?

 

[AlpineEngineer]

Thanks for the input. I don't think it is feasible to cut into the boulder, the top of the boulder is fairly level, its the natural grade thats 32%. The frost heave definately concerns me. I like CSD72's idea about pouring a frost wall under it and then putting it back in place and then doweling into it, I sure don't think there are any bearing capacity concerns of the boulder. I just don't know if an track mount excavator can pick this thing up and move it or not. I will have to ask the excavator. At 200pcf this thing would weigh 50,000lbs+, which brings up another thought, could this much mass resist frost heave? I have never looked at frost heave forces, is there any direction on how to analyze them? I don't see anything in ASCE 7-05.

 

[jdonville]

Alpine,

Ideally, you want your foundation (any foundation) to have a uniform level of support. Don't place half the footing on the boulder and half on soil - that's non-uniform support and you're just asking to have your footer crack unless it can take the differential pressure and/or movement (unlikely).

What you might be able to do is leave the boulder in place and pour your concrete to the same depth as the bottom of the boulder. This will require excavation (and a heckuvalot of concrete), but at least you won't have to move the boulder and then your footing will have more uniform support.

You should have a local geotech give you advice in this situation.

Jeff

 

[csd72]

Alpine Engineer,

There is something in ASCE7, under Masonry(165pcf) or stone (97pcf). I would believe the larger one myself I would expect it to weigh a bit more than concrete.

 

[theCorkster]

Alpine Engineer;

While unusual, you might want to start with the stability of the boulder. What underlies the boulder? Or is it an outcrop? If it is a boulder, and you can't satisfy yourself that the boulder is stable and won't undergo movement, don't build on it!

However, if it is stable, as jdonville noted, differential settlement between foundation supported by the boulder and the adjacent soils (?) becomes the next issue to resolve. You might want to check with your structural engineer to see what his tolerance is with respect to this.

 

[garrettk]

Instead of using the boulder for actual bearing support, why not make structural columns at each end of the boulder and run a header across the top of the boulder? Use some false work and grout fill around the boulder and the header to make it look like the footing and boulder are working as one, but keep the systems isolated.

 

[AlpineEngineer]

Thanks for the thoughts, I thought of the header idea but I'm worried about frost heave under the boulder. I don't know how much the rock may heave, even if I left 12" between the header and the boulder and put stone veneer, the rock may heave, crush the veneer and then jack up the header. I ended up telling the builder he just needed to get the thing moved, not sure how they will do that.
Thanks again.

 

[oldestguy]

In my experience I have seen houses settle where a "boulder was removed". That was probably because they didn't replace the space with compacted soil.

Should you leave it in place, that form of structure should have control joints anyhow and using them, inside and out, would be a way to tolerate movements of most types.