Residential Raft foundation with new slabs abutting for extension

[geopat69]

Hi all

I have an issue regarding soil movement and the treatment of new raft slabs that will abutt existing waffle pod (essentially another raft). I have attached a plan.

The waffle has an edge detail with only 150mm high nib. The main question is how treat the differential movements. i.e If the new slab are dowelled at edges A, B C D and E, the shear loads dumped in the dowels are high! (potentially the nib is not thick enough – dowel has edge distance of only 75mm either way).

If I pier the new slabs…that will create an issue clearly as the existing house wants to move. Alternatively only pier part of the new slabs…but again that seems odd. Regardless of this, any piering will require using voidformer to force the new slabs to be suspended - and againg this loads up the dowels in that nib.

Any ideas on how to deal with this. FYI the most extreme is to demolish the laundary up to the edge of the house, and simply dowel in a completely new slab along that one edge (ie a nice single edge articulation joint!). But I don’t want to do this as it is extremely costly.

Love to hear any solutions! !!

 

[geopat69]

.

 

[geopat69]

.

 

[oldestguy]

If the bricks have to stay, what you show looks good. If bricks can go, set your new edge on old footing and also dowel into old edge of slab.

 

[geopat69]

Thanks oldest guy. Appreciate your guidance.

But the bricks have to stay. My real concerns are the edge distance in 150mm nib. Secondly, the movement between each structure. Is it conceivably possible the extension can rise 40mm at edge and have a sloping floor!

 

[oldestguy]

Presumably this slab also will rest on compacted fill. Thus the edge support hardly is needed, but makes a good place to provide even support.