Concrete Footings

ol blueeyes

hey guys,ive got something buggin me here you might shed some light on.lol.
im building myself a new cabin this spring at my lake up in canada,sask.
now my site has some sloping elevations to it,so my foundation at the front will have the 1 foot exposed and at the back it will be 6 feet exposed.(2 feet in ground)
now i know everyone puts their footing on undisturbed soil for the most part and thats fine,but at the back of my cabin where it will only be 2 feet in ground in concerned that the soil wont be stable enough since its not that far down,im sure the first foot or so is leaves and such.
not to sure what to do here,how about twisting in 4 foot deep piles every 8 feet at the back footing to relieve my concern of it sinking?or bringing in 8 inches of base and packing the hell out of it?or am i worried for nothing?
to help combat frost,im wrapping the footing with 2 inch riged insulation and backfilling a couple of feet.

Chris Johnson

The only non engineering answer we can give is...dig down to solid virgin soil

everything else is an engineer call

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Chris

concretemasonry

Only 2' deep in Sask? - They will probably heave up from the frost.

You did not say if you had continuous footings or footings for piers, but they should all be below frost in solid, undisturbed soil unless the are abchored to soild rock.

Even if you may not need to comply with the code, it would be a good ideas to look at it to see what is right and suggested for a depth.

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Dick

Engineer, designer and consultant recently active domestically and internationally on construction and design in about 35 countries.

ol blueeyes

they are countinous footings,but i was thinking or drilling 4 foot piles under the shallow end of the footings every 8 feet,that way im below frost with the piles and they enturn help if their is any movement with that end.
its funny,we get -40 temps out their and i see everyone gigging in thoer footing only 2 feet all the way aorund for their footings!i dont beleive in that.my basement will be 75% below frost,but im worried about the back end.

Chris Johnson

Yes, but even with piles driven in and a continuous footing/grade beam on top of that and attached, if the bottom for the footing/grade beam is not below the frost table, frost can get below and heave it up.

Central Ontario was 4' when I lived there, you are much further north so I'm thinking your frost table is 5' or better. Check with the local Building Department or you frost zone in the Building Code.

You can do a shallow footing detail, you must use styrofoam extending straight out from the foundation, for every foot you are short of the frost table you need 4" of foam extended 1' from the foundation, i.e. frost table is 4', you are only 2' from grade, 4" of foam on top of the footing 2' out from the foundation wall. Again that was Ontario rules where I lived, check locally for your requirements.

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Chris

KentWhitten

That's what I was thinking. Sask. shoudl be 4'-6' deep shouldn't it?

Soils test will also tell you how good your soil is to put footings on.

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Custom Cabinetry - Portland, Cape Elizabeth, Scarborough, Kennebunkport, Yarmouth, Falmouth, Cumberland, Ogunquit, Maine

Salmon Falls Cabinetry

big steve

In NJ we have to be 3' below grade.I dont beleive we have had 3' of frost in my lifetime.Can you step your footing down on the lower potion so its still farther below grade probably closer to 4 '?
Steve

ol blueeyes

i do have in my plans rigid styrofoam protecting the footings,the frost controll im not to concerned with as much as im concerened with stable soil.if i only dig down 2 or 3 feet is that sufficent enough to reach stable virgin soil,im worried that it would be spongy and i dont want anything sinking you know?but then again once i start digging everything will be much eaiser to see i guess.
i guess if it is still spongy i can dig my basement even deeper and step my foundation and build up with wood to level it out instead of having a 12 foot basement right?

big steve

its hard to say without seeing but i often need a step on my foundations.This doesnt mean the whole area needs to be excavated out .Leave the soil where your slab will be,dig footing deeper but step the foundation in an amount that will work out to a full block so the top of your foundation will still be the same height all the way around or if you are pouring the foundation walls it doesnt realy matter.
Steve

ch0mpie

you got to keep digging until you get down to competent material. For frost heave, I'm not sure how far north you are, but this may give you an idea of what your code will recomment.

This figure is from US Navy Design Manual 7.1 "Soil Mechanics"

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