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House foundation anchored to large rock, no footing

DoubleStud

Structural
Jul 6, 2022
464
The soil report recommends the house foundation shall be on spread footings at frost depth. What happens if they have a hard time digging to frost depth due to a very large rock at some areas? I am talking about a solid rock roughly about 16 ft long, 8 ft wide. Not sure how deep it going to be. Can you just form the wall on top of the rock and epoxy dowel into it as opposed to keep digging down (breaking rock) another 2 ft and form 16" wide footing? I should probably just call the soil engineer that wrote the report, but I want to hear what you guys think first.
 
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If it is bedrock or a rock layer it may be ok, this however sounds more like a boulder which I don't think is ideal. You can do pull tests with the adhesive to get some data on strength, but overall stability is a much harder ask to quantify with boulders. The other concern I would have is that if part of the structure is on rocks and the other part is on soil you are going to have some amount of differential settlement that could become problematic.

I'd lean on the geotech to weigh in.
 
If it is bedrock or a rock layer it may be ok, this however sounds more like a boulder which I don't think is ideal. You can do pull tests with the adhesive to get some data on strength, but overall stability is a much harder ask to quantify with boulders. The other concern I would have is that if part of the structure is on rocks and the other part is on soil you are going to have some amount of differential settlement that could become problematic.

I'd lean on the geotech to weigh in.
Wouldn't you have the same problem if lets say you keep breaking the boulder and part of the house footing sits on large boulder and other part sits on soil?
 
Your soil report should have caught the rock and told you how to deal with it. We don’t know if you have clay under the rock or what.

Houses are lightly loaded enough that soil strength is rarely a problem beyond muck and peat and such. Strength is not the only factor, though.
 
This is done somewhat often in my neck of the woods with large boulders (with OK from the soils engineer of course)
 
I usually don't have a problem with this but warn the customer that minor cracking may occur due to differential settlement. Have never had any feedback whether this has ever occurred though.
 
Best practice here now is to take all foundations down to rock if part of the house is on rock to mitigate differential settlement
No, it wasn't my idea
Yes, it's as expensive as it sounds

However, I think that would apply here if part of the house is on soil and the centre is on a big boulder
Differential settlement seems a legitimate concern here
What is the depth to rock/boulders elsewhere under the footprint
 

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