Build up under a slab: re-using material

[Dunc]

Just trying to gather information so I don't get laughed out of the office when I speak to a SE about foundation design.

 

Ground investigation reports "a safe bearing capacity of 75-100kN/m² will be available for foundations placed on the medium-dense, coarse sand found approximately 0.8mbgl." and "recommend trench fill foundations or strip foundations with a sufficient number of courses of under build to ensure that the dwelling is founded on soil with a sufficient bearing." All good.

 

The site has previously been used and has a covering of some kind of MOT/hardcore/stone surface. The layers are, top down:

Top layer is ~ 200mm deep Soft to medium - dense, grey/brown, slightly gravelly, silty, fine to medium coarse sand with crushed angular rock with an underlying white geomembrane liner. MADE GROUND. [what as a punter I think of as "hardocore"]

Second layer is ~200mm deep Soft to medium - dense, grey/brown, slightly gravelly, silty, fine to medium coarse sand with crushed angular rock with an underlying black, hexagonal geogrid. MADE GROUND.

Third layer is ~400mm native peaty topsoil which will have to be removed.

Fourth layer is the suitable load bearing material - Medium - dense, grey, damp, slightly gravelly, silty, clayey, fine to medium coarse SAND with angular cobbles. Fragments of loosely bound clayey sand embedded with gravel. Occasional angular and rounded cobbles.

 

I'm wondering if the made ground could be re-used as the base layer of an insulated raft. If I can strip that off and save it, remove the native peaty topsoil down to load bearing ground, could I stick the made ground back in, in compressed layers. An insulated raft is probably at least as deep (400mm) as the removed topsoil. Could I end up back at ground level without having to import much material other than the draining layer and fine support layer for under a raft? 

[ToughButterCup]

3 minutes ago, Dunc said:

...

I'm wondering if the made ground could be re-used as the base layer of an insulated raft. If I can strip that off and save it, remove the native peaty topsoil down to load bearing ground, could I stick the made ground back in, in compressed layers. An insulated raft is probably at least as deep (400mm) as the removed topsoil. Could I end up back at ground level without having to import much material other than the draining layer and fine support layer for under a raft? 

 

In our case, I asked the same question.

No - was the answer. But since we built on a gentle slope, we just shoved the top layer (made ground) downhill.

[Mr Punter]

It doesn't sound silly.  As long as the raft designer is OK with the material and it is not contaminated with the peat I can't see why not.  You will need space to cleanly stockpile it.

[Russell griffiths]

Don’t bother

move it to one side and use it for the drive, turning circle for deliveries 

path around house under scaffolding. 

Bring in nice and new for the slab as specified by the slab designer, by the time you have asked and they have thought about it any money saving will be long gone. 

[Dunc]

Hmm, not a strongly positive reception there.

As most of the rest of the plot is covered in this stuff, I'm already OK for the driveway and a storage/unloading area. But I'm sure I could find some use for it.

Was hoping it might save a bit of cash at this early stage.

[markc]

Save cash where it can be sorted later, kitchen, bathrooms etc. don’t cut corners with the doors (external) windows, fabric or foundations