footings for cement slab?
Answers:
hope this helps but your not going to like it,
would not suggest putting footings in afterward, they will never last, you have to tie the footings into the slab with rebar, footings will eventually push out
Hate to say it you are going to have to rent a cement cutter and cut off about 10 inches of the sides all the way around, then drill into slab and put rebar about every 18 inches
The footings should be a minimum of three feet deep depending on the size of the slab, I personal would not have any footing under 4 feet deep.
Hope a contractor did not pour this.
good luck
Other answers:
It would be very difficult, what i'm thinking is that you might be able to dig out around the edges of the slab to the depth required for the footing then pour it in but you'd have to pump it into your formwork to use as little concrete as possible and not have a gigantic footing the shape of the trench. You'd have to brace your forms to allow them to handle the pressure from the pumpable concrete, If it'll get through codes (if you have to comply) you'll probably want to get a high slump or self consolidating/compacting concrete to hopefully reduce voids.
It would be very difficult, what i'm thinking is that you might be able to dig out around the edges of the slab to the depth required for the footing then pour it in but you'd have to pump it into your formwork to use as little concrete as possible and not have a gigantic footing the shape of the trench. You'd have to brace your forms to allow them to handle the pressure from the pumpable concrete, If it'll get through codes (if you have to comply) you'll probably want to get a high slump or self consolidating/compacting concrete to hopefully reduce voids.
That's a good answer and if you need a footing somewhere in the middle of the slab for a lolly column or other support, a concrete saw or electric jack hammer could easily make a nice hole in the slab surface. Then dig a hole to the code specification and pour the footer. You might want to put some rebar reinforcement in the footer too, and use a higher strength 6 bag mix for good measure. You could also put the footers I described periodically around the perimeter of the slab, and pour a rebar reinforced lintel on top of the slab (that tied in to each of the footers with rebar) between each of the new perimeter footers. Of course the exterior wall partitions could sit right on top of the new perimeter lintel which could run continuously wherever there were no breaks in the partition.
In some parts of the country there are contractors that level and stabilized cracked foundations. I bet they could help you with your problem.
There are some things you really don't want to do you self and this is probably one of them.