Sub base for replacement slab

[gh483]

As summarised in my other thread about a separate issue I have broken up and removed a concrete ground floor slab in a house, which was sitting on top of about 200 mm of loose stones (with the DPM separating the two layers). These stones are just that, with not much in the way of graded finer material in the sub base. There wasn't even a sand blinding layer to protect the DPM but this was probably due to the fact that the stones are fairly rounded with few sharp edges (like beach shingle).

I have not dug out all of these stones, only in a small corner to see how far down they go and what was underneath which is as far as I can tell, well compacted stony soil.

The new slab will be roughly the same thickness (100-150 mm) and elevation as the old, but I also want to have 150-200 mm of insulation beneath it.

My question is how much of the stony layer would I need to have under the DPM and insulation? If I have the same thickness as before I will need to excavate a corresponding amount of soil, which I really don't want to do due to the amount of work and because it would be below the top of the footings.

My limited understanding is that the purpose of the sub base prevents settlement and penetration of moisture. Seeing as the soil has been holding up the existing slab for several decades, can it be said that it should already be well compacted and settled and a thin layer of stones say 50-100 mm (just to keep the DPM from resting directly on the soil) should be adequate?

Could I also get away with simply removing as much thickness of stones as necessary to install the insulation and rely on the remaining stones being compacted, or would they need re-compacting mechanically? Having said that they seemed pretty loose and easy to excavate when I first lifted off the DPM and due to their rounded shapes and lack of fine material they probably have limited compactability anyway.

 

[^woody^]

Presumably a floor slab?

Normally you remove the layer of soft soil and then whatever is left after you've deducted the thickness of the screed, slab, insulation and blinding is your sub-base.

 

[gh483]

Yes this is a slab resting on insulation and the ground (not suspended).

Conventional wisdom seems to be you need a decent layer of sub base whether thats graded MOT, stones or just rubble to spread the load and prevent settlement, although my uneducated self never fully understood this (the load isn't being spread over a larger area than the slab itself, and if the underlying ground is too soft then the sub base would just sink taking the slab and everything above with it). However in past projects whether that's a path or shed I have always followed the usual advice and laid the concrete on at least half a foot of rubble. In these cases, the ground was green field. But in my present situation the ground I am building off beneath the excavated stones should in theory be as sound as when the original slab was first poured. Why then do I need the sub base for the new slab?

 

[^woody^]

You don't need a sub base if the existing base is suitable. You wouldn't dig up suitable ground to then import new material that preforms no differently.

 

[gh483]

Appreciated, that makes logical sense to me. Will wait and see if anyone disagrees.