Base for an extension query?

[handyman77]

Hi, i have taken over a job from a firm that had fallen out with the customer over the make up of the base of his new extension. It was was supposed to be block and beam, but the builder put down a 100mm reinforced slab over a well compacted hardcore base (so im told!). With a view to insulating and screeding. Hence the fall out. Is there a way to now bring this upto spec with out having to break up this slab! BC is adamant is has to be redone. There was a tree (think eucalyptus with small root bulb) approx 3 metres away which was removed in July before the new foundations which are 1.6 deep where poured. I am being warned of heave although the SE says it unlikely,but still possible. Just trying to save client money and me some time! Any thoughts......

 

[Static]

Slab should come up and be done with a void or with something like cellcore under to allow of heave..

 

[daveyboyd]

Heave from what exactly?

If the tree was three metres away, now removed (7 months ago) with root ball grubbed up and the foundations are 1.6m deep, as long as the tree wasn't massive you should be ok. A rough rule of thumb is that the affected area of desiccation (area that will cause heave once re-hydrated) will be the area underneath the trees canopy.

If it was designed as a raft foundation due to the ground conditions it may be that the BCO is right but unless the slab is formed on clay then I very much doubt that any heave will occur.