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answersthatwork (Structural) (OP)
20 Jul 09 15:19
Good Day to all!

We are designing a home on a site which is 95% surface Bedrock.  The Topo conditions vary from moderate to severe slope conditions.  The Code Frost depth is 3'-6".  My question may be silly but, does the surface bedrock need to be excavated (by blasting or hammering) down to the 3'-6" depth in oder to avoid any possible damage due to freezing/thawing?  If so, is there a documented reference book I could purchase for insight?  If not, what depths and or method would be proper?  

Most likely I am mistaken, but for some reason I keep thinking that a footing or foundation wall, which has been pinned properly to bedrock but ABOVE the 3'-6" frost depth, may have water infiltration between the rock and concrete over time.  If so, would'nt this cause problems?

Your help is greatly appreciated and respected!  Thank-you!
jdonville (Geotechnical)
20 Jul 09 16:57
atw,

Rock can be tricky to engineer, depending on the loads. Suggest you contact a local geotechnical engineer who will be more eperienced with local code requirements and past building experience in the area for similar strucures.

Jeff
 
GeoPaveTraffic (Geotechnical)
21 Jul 09 7:02
Code and inspector issues aside, depending on the type and compentency of the bedrock; I would generally say that footings need not be excavated into bedrock for frost protection.  I would recommend that the footings be tied to the bedrock with rebar and that the surface of the rock be clean prior to placment of the concrete.

Site grading to direct water away from the footings should also be incorporated into the design.  

Good luck with your new house.
BigH (Geotechnical)
21 Jul 09 8:58
Agree with GPT - unless there is a propensity of the rock to swell/shrink due to frost (e.g., Queenston Shale). It sounds as if the rock is so close to surface that you should be able to tell if the upper surface is affected by frost or not.  Funny how codes should be the "guiding light" but put all into a restrictive box . . .
answersthatwork (Structural) (OP)
21 Jul 09 12:25
jdonville,GeoPaveTraffic,and BigH

Thank-you very much for your suggestions and guidance.  We are looking to bring in a Geotechnical Engineer.  We have isolated the rock structure as Proterozic Rock with no sedimentary covering.  More specifically, Horne Blend Granitic Gneiss.  

Thank-you again!
 
GeoPaveTraffic (Geotechnical)
21 Jul 09 19:42
Getting a local geotech that is familure with that rock unit is well worth your while.  I know nothing about that unit and don't know what, if any, problems you can expect putting a footing directly on it.

Good luck.
oldestguy (Geotechnical)
21 Jul 09 21:24
One problem these types of jobs develop is how to get the inspector to agree that there is no problem with footings on shallow depth rock.

Generally the Geotech or structural engineer has to have a P.E. and sometime4s it takes a meeting with the inspector's boss.

Another one is where does that magic depth of frost figure come from?   
BigH (Geotechnical)
22 Jul 09 8:19
I never thought, unless the rock was an expansive shale or schist that any would think there is that much a problem for a residential home. If you think that it might be too much a problem for the local building inspector, put a couple of feet of styrofoam (2 inches or so thick) horizontally out from the top of the footing so that the "path" of frost is longer than 3'-6".  Might be cheaper in the long run . . .
darthsoilsguy2 (Geotechnical)
9 Aug 09 23:32
don't you want to put some fill down anyway for the yard and pipes.. hope you've got sewer service
swazimatt (Civil/Environmental)
19 Aug 09 6:51
I have a similar query regarding bedrock foundations
We are currently building a long (60m) double story building, with two expansion joints at aproximately 20m.
the foundations are reinforced concrete strip foundations in fairly good weathered granite gravel soils.

The problem i have is that the last 10m of one of the strip footings will end up on exposed solid granite rock.  the site is quite steep with the rock section sloping at about 30deg.

I am worried that i will end up with cracks at the transition from insitu material to rock if i continue with the strip footing onto the rock (it will be pinned into the rock by drilling and grouting in reinforcing bars)either from differential settlement or from differential expansion contraction of the rock as in winter here we have quite warm days (15-20 deg C) and cold nights (down to about -3 deg C).

should i add another expansion joint at the transition? any suggestions will be appreciated
ishvaaag (Structural)
19 Aug 09 8:19
At 20 m and with such soils you should see small problem coming from differential settlement. Even thermal issues must not be very relevant but an analysis may determine the extent of the problem; surmise fixity then look for slippage reactions to see if friction will be enough etc. This said, I have seen 12 m long houses expand and show expansion joints (hairlines) besides concrete secondary beams, and railing break the masonry it was embedded in; also a structure corner (5 floors tall) stand without any damage 4 m cantilevered on an unsupported mat foundation that had lost its ground below on slippage.

Analyze the thing and you will know where you are.
 
escrowe (Geotechnical)
19 Aug 09 14:48
It's worth considering isolation joints between the rock- and soil-suported portions of the structure.  You might also consider overexcavating the rock 6 inches (15 cm) or so and backfilling with gravel or compacted native soil to provide more uniform foundation support.   
swazimatt (Civil/Environmental)
20 Aug 09 5:15
thanks ishvaaag and escrowe.  

Overexcavating the rock would require blasting, and it is not an option. think we will go with the isolation joints
ecanuck (Geotechnical)
23 Aug 09 12:43
Hi answersthatwork.

You've got pretty massive bedrock that wouldn't soak much water, nor bump around too much from frost, I shouldn't think.  

Still, It would be prudent to drain the grade away from the foundations with impermeable soil, or synthetic and maybe add foam insulation, but thats your call since you need to consider some heat movement from inside....

It would be advisable to add some rebar in the footings to provide a transition between the rock and overburden.pipe
DRC1 (Civil/Environmental)
23 Aug 09 23:14
Any time you transition from one foundation system to another, it is prudent to isolate the two systems as much as is practcal, so I would vote for the additional joints.

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