Filling trench foundations with just concrete or hardcore and concrete

[stuart45]

I think you've been advised to do a nonsense foundation that is bound to fail.

Wish you had told me this before woody, I've just done the footings for a 2 storey extension based on this advice.

 

[^woody^]

I think you've been advised to do a nonsense foundation that is bound to fail.

Wish you had told me this before woody, I've just done the footings for a 2 storey extension based on this advice.

It's your own fault for not asking and just steaming ahead

Check out my underpinning thread.

 

[leoinlios]

I think you've been advised to do a nonsense foundation that is bound to fail.

Hi Woody, would you care to expand a little please on what part of the advice provided to my question here you think is invalid? Do you mean that it is wrong to have trench foundations of the size I dug up? I had to dig in order to reach solid and more load bearing ground. And as for the width of 350mm I was told it is what a foundation block would be like plus a little extra. Or are you saying the advice on how to fill up the trench is wrong? Thanks for clarifying. I have not moved on with this yet, i.e. with filling the trench, so any advice to save me from wasting money on concrete would be appreciated. Cheers

 

[^woody^]

You can't just make a foundation from random hardcore, blocks and concrete knocked up together just so it fills a trench.

You should either construct it out of concrete with fine aggregate, or lay bricks or blocks properly bonded and jointed so that when it moves seasonally, it does not all distort.

Either deep fill with concrete, or lay a 150mm strip and then build your foundation walls off that. Or lay a stepped brick or block footing - but 350mm is not wide enough for a this.

350mm is not a great width for a foundation either way. If it's mass-filled concrete, then it may be OK for a light timber frame, but otherwise 450mm would be more appropriate.

If you are are forming a raised concrete base for the wall, then you could mass fill the trench to ground level leave it rough but more or less level, and include some vertical re-bar to bond your shuttered top section at a latter time

 

[leoinlios]

Woody, thanks for your comment. Let me explain my design further a little. What will go on top of the raised foundation is my timber base, and on top of the base, all timber walls will be attached. The wall plates/studs will sit on the base edge, flush with the outer edge of the raised foundation. The idea is that to finish the outside of the wall, I can put sheathing lapping over the timber base completely, and going further down about an inch on the outer foundation wall i.e. lapping all the side to completely prevent water from ever entering the base or where the joists meet the dpc on the raised concrete. I am just explaining this in case it helps clarify my design. Picture below. What would be the benefit of your suggestion to only fill the trench up to ground level and fill the raised formwork part later? Assuming I go with mass filling with concrete, isn't it better to have a contiguous mas of concrete, rather than 2 parts bonded together?

 

[theprinceofdarkness]

Its not the way I would have done it, but a concrete "marley" type garage 20' X 11' I built on foundations to their spec, 12" X 12" trench round the edge and 4" oversite concrete on hardcore (6"?). Seemed to be OK for a 4 ton garage and a one ton car + deep freeze, loads of tools/metal/wood. . . So as this is a lightweight construction and pliable, it should do.
Frank

 

[tomfe]

You'll need oversite concrete if you construct like that or at the very leas a weed suppressor and good ventilation.
I would not mix anymore than 2 cubic meters as you wont feel like finishing and leveling it by the end and that the most important bit.

What's wrong with some pad foundations?