Need help with foundation design on Bedrock 1

[benwr]

I am building in Northwestern Ontario (Canadian Shield) on bedrock. I had the site scrapped down to rock when i had my road built and will shoot some elevations this weekend.

The area is fairly flat with only a few impressions, i would estimate the elevation difference, highest point to lowest point be approx 2ft. I will know for sure by Monday.

I am building a one level home, with in slab heating. I was planning on a thickened edge slab as shown in the attachment. I am not interested in a crawlspace, as I see the problems they create everyday ay work (I know if they are built correctly they can be problem free, but want to avoid it).

I had a builder I know looking at my site and he was concerned that bringing in fill for a thickened edge slab is a bad idea, as the hydrostatic pressure would cause it to move down hill (it slopes slightly towards the water and a very high cliff for that matter)and will not get a good compaction.

I am sure many of you are firm believers of building on bedrock, as all contractors/engineers in this area are.

I am looking for inputs on how to scribe my footings to bedrock, while having a "slab on grade" type home. I will attach another sketch, showing a 10inch footing, scribed to bedrock, 8 inch min height, with a 6 inch slab on top I am thinking of. My concern with this is the rebar left exposed that will go into the slab will be in the way of the compactor. A guy i work with assumed they can just bend the bar down after compacting the fill and laying the ridged insulation, but his background is in mining.

I am fairly new to foundation design and all the input is appreciated.

 

[benwr]

Footing Sketch

 

[darthsoilsguy2]

benwr's detail is better. the underslab fill will go nowhere if completely surrounded by concrete ring anchored to the rock. however, i'd be hesitant of bringing a contractor on-board that is nervous about the design principals.

Don't ask about what to do with the sewage....

 

[jhoulette]

How about a turn down edge slab-on-grade just as your first image shows but adding thickened ribs at roughly 12 to 14ft apart and use a 6 1/2" to 7" slab. This is will help make a stronger slab that can span over any soft areas in the compaction.
Also, I recommend the rebar dowels if sliding is indeed an issue. But I wonder how much of an issue it is. You described a flat site and not much of a reason to worry about it. If the pad is graded flat prior to placing the foundation then how does the slope around the pad effect it from sliding?

Jim Houlette PE
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http://www.evstudio.us

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[Ron]

benwr...either of your sketches would likely work; however, I suggest that you consult a local geotechnical engineer to confirm your approach. Structurally, they are fine. jhoulette's suggestion is good for conservatism.

 

[beton1]

Agree with Ron. Why? You cannot state that Geotechnicals, just pull out a report and copy it. Worked with a world wide Geotech outfit when I started out. Have nothing but respect for those guys. It may seem that they are using (sometimes) what appears as 'off the shelf', but it is thought out and they know what works. Yes your design may work, but how do you know if it is underdesigned or perhaps even overdesigned. A geotechnical specialists input is imperative. They understand the underlying geology and fractures and other, that is beyond the scope of most of us. If it works for you, fine. You can contribute it in a appropriate manner or as a suggested method (general idea) but not as a opportunity to 'critize' an entire profession. You have to respect their knowledge. Is it not a liability issue as well?
If you have resources and specialists, why risk not using them? Sorry if I am ranting.

 

[benwr]

As far as getting a Geotech to do a design, that is not an option as I simply have no money for something like that, its not that I don’t think it is worth it at all.

Also the only design firm in town (closest city is Winnipeg, 2.5 hrs west) does not have a geotech guy on staff. I work with 2 former employees of the firm and have been picking their brains on the subject. As far as liability goes, it will be an owner design/build and the permitting in the area does not require stamped drawings, I do not think their will be any issues there.

As far as the site goes, there is no question at all it is solid bedrock. I am looking for the most practical way to build a proper foundation while maintaining the in slab heating and avoiding crawlspaces.

 

[Patgeotech]

Benwr,

Just wanted to add another point - should you go for the grouted dowel into the rock (which we use often in foundations on the mountainside in Cape Town), go for a slightly thicker bar so that allowance is made for dowel corrosion over the long term. This is minimal in terms of cost but gives one peace of mind for the long term. I assume that a seasonal perched groundwater exists at your site which would affect the grouted dowel bar.

Cheers and good luck.

 

[benwr]

Patgeotech,

Good tip, I will definitely keep that in mind when I decide on a design.

Thanks,