I am about to start building my garage prior to building my house next year, my question is having removed all the topsoil the spoil from digging the foundations is solid yellow clay, can I use this to build up the ground prior to MOT then concrete floor. I have been told that as it is in-organic its ok to use to build up the ground. It seems a shame to spend lots on MOT to build up half a meter. Of course it will all be compacted well.

Not really a good material for backfil. I wouldn't use it. You could always forget the slab method and go beam and block and so avoid the whole backfilling and type 1 issue. Most new garages are done this way with concrete over the top, 75mm at the door and 100 to 150mm at the back for a fall.

What's the problem with backfilling with clay, expansion when wet?

<blockquote><cite>Posted By: Ed Davies</cite>What's the problem with backfilling with clay, expansion when wet?</blockquote>

It is (I believe) too susceptible to moisture changes so can heave when wet and shrink when dry.

It depends on the clay how it responds to moisture changes.

However compacting clay is hard where backfilling, so gaps may be left that then sink.

I got the depth of hole reduced under my garage. Originally it was specced like the house to prevent movement due to trees and clay. But I pointed out that if the ground in front of the garage went up or down, then I'd really like the front of the garage to do likewise! So no deep hole under the garage, just 150 mm.

But I'd need a lot of convincing to have made up ground underneath anything but a raft.