HardCore you know the score!

[twm.bwen]

In the context of laying footings and slab for a conservatories/porch what the main purpose of the layer of hardcore? (other than to fill in between sides of footing trench and internal block skin.)

Also how specific do I have to be about what I use as hardcore?

Due to existing levels (internal and external )the FFL of my porch is going to have to be only 50mm (at best) above external ground height, so considering I will have to dig out about 200 for sand, dpm, celotex and slab do I really have to dig out another 100-150 for hardcore or can I lay slab etc on earth or even some paving slabs I have spare?

(porch is not bc notifiable)
Thanks

 

[noseall]

so long as the subsoil is compact and you can guarantee that it will not give or subside, oh and the hard core (slabs), or indeed that which is below the slabs, is sulphate free then, yes go ahead.

 

[twm.bwen]

so long as the subsoil is compact and you can guarantee that it will not give or subside, oh and the hard core (slabs), is sulphate free then, yes go ahead.

What do sulphates do which makes them bad?

And if I mass fill between trench and inner skin with concrete that forms the slab that should prevent subsidence.

Thanks for this help btw

 

[noseall]

sulphate gas attack can cause a floor slab to lift creating a bow in the slab and eventually cracking.

a floor membrane should alleviate this but it is no guarantee.

 

[twm.bwen]

So basically, in the event of not needing to make up the floor level, it would be ok to lay the slab without hardcore.

The island in the trench is likely to be well compacted as it currently under a thick concrete front step/slabby thing.

I did phone my local BC but his only answer was " Its just standard practice but its on your concience" and when I asked if there was a structural reason why the hardcore had to be there he couldn't really offer one.

So I may save myself the hassle of digging 150mm just to have to find hardcore to fill it in again!

Still not too late if someone knows why I shouldn't.

 

[nwhessian]

Its a porch! Surely it cant harm just to dig out for the hardcore too, wont take much longer.
150mm might be overkill for a porch mind. Just chuck in the slabs and break em up a bit with a sledge?