Widening a concrete foundation

[nordita]

I'm making a blockwork building at the back of the garden (12.5m2). I have a rectangular trench fill foundation (c20). It will hold a cavity wall of dense and light blocks.

Unfortunately, I wasn't accurate and a part of my foundation narrows.

I figured it would be simple to dig out this restriction and fill the hole (0.07m3) with bagged concrete so the trench becomes more wide and even.

Is this sensible?

My alternative would be to build the cavity wall within the constraints, but as I want to keep all corners at right angles this means:
- reducing the cavity size
- and having a block corner too close to the foundation edge

 

[nordita]

Some pictures


Corner 1

Corner 2

Corner 3

 

[John D v2.0]

You can't patch in at the side without somehow fixing them together with rebar. How close is too close to the edge? You could build right on the edge, it's called an eccentric foundation and isn't likely to be a problem for something lightweight.

 

[nordita]

Thanks for your reply John. Yes I figured rebar would have been a must. I've asked around and most other people don't seem to be too concerned as long as its all on it. The closest to the edge is 33mm on corner 3 and on the interior of corner 1 about 20mm. This is my first major construction project so I don't really know what constitutes as light weight.

 

[John D v2.0]

The important thing is that your hole for the concrete had a flat bottom and vertical sides, so that the foundation won't move progressively as the soil changes through the seasons.
If it's a pad for a post holding up a corner of your house, it becomes important to get it in the middle, but for a small section of single storey wall I can't imagine anything will happen.

 

[Dereekoo]

This is my first major construction project so I don't really know what constitutes as light weight.

What is considered light weight depends what your bearing pressure your soil can take, imagine totally high water table and siht ground that a log cabin would sink into or something built on bed rock that can support a sky scraper. Building regs give you an idea of what bearing pressure different grounds can take. Unless your founding ground was really carp I shouldn't think you will have an issue with your structure.