Risks of differential settlement; rock versus stone

[vladhed]

Hi,

We've started to build a two story house. The foundation around the perimeter is a 4 foot ICF wall on 24" wide footings poured directly on rock. This will be filled in with clear stone, over which a concrete slab will be poured for the ground floor.

There are two internal load bearing walls, that the architect wants supported by concrete walls on footing also sitting on the rock.

This stikes me as overkill - isn't it sufficient fill the whole area with clear stone, then dig out 6" of it and pour footings on top of the roughly 3 1/2 feet of stone, and put the load bearing walls on these "surface" footings?

Is there a risk that these footings will settle, being on several feet of clear stone, whereas the perimeter walls will not - creating cracks in walls etc...?

cheers!

Dominic in Ottawa

 

[BigH]

Seems to me that it would be appropriate to do what you have suggested. I would probably use something like MTC (MTO) Granular B rather than clear stone. With nominal compaction, they would easily support your internal footings with very little settlement (likely less than 10mm) - again if the granular B was nominally compacted. Ask the architect to give you sound technical reasons for putting the footings on the bedrock.

 

[exploengineer]

If part of the footing our on solid rock and the otheres are not, your house will settle and crack rather quicky.

Everything should be on the material that is the most solid.

I have seen this type of cracking a lot!

Frank Lucca M.I.Exp.E.

http://www.terradinamica.com

 

[BigH]

Exploengineer is correct to a degree. If you dig down to the bedrock and backfill in an engineered fashion with a well graded crushed stone (such as Ontario's Granular B Type 1), it is quite doubtful that you would have more than 5mm of settlement after the footings go down and I don't think that this will create the cracks. If you were building on rock in part and on a firm clayey soil or loose sand, then you might get sufficient settlement to be concerned. Another option would be to dig down to rock and backfill the trench with a very lean mix concrete - such as 1000 psi (9 MPa). Might be cheaper - you don't need to worry about compaction and you will have 'positive' contact with the rock with an excellent material.