Soil under slab (sidewalk) washed away and is now causing cracks down the middle

Question

I have a 50' x 6' concrete slab in front of our basement that supports the same size wooden deck above with 6 post for the upstairs. The soil under it washed away due to poor drainage. I have fixed the drainage problem and now need to support the slab.

ISSUES:

1. slab built into steep sloping bank
2. dirt is gone from the outside edge back as far as 3' underneath
3. no footing at all
4. outer bottom edge of slab is above the ground by 12" for atleast 15' in the center.

How do I add footings to an existing slab. One thought was to form up the bottom of the slab, cut holes and pour concrete in to fill the void. Slab Jack estimate was over $5000

Pictures http://imgur.com/a/NmRqr

The other choice would be pour pier footings in front of the existing slab and transfer the load.

Answer 1

Any concrete that is wet enough to flow into that space is probably too wet which weakens its strength. "Pouring" concrete is a misnomer. When it is properly mixed it has to be placed or pushed into position.

I don't think you a footing anyway you just need to replace the soil that was washed away. Usually concrete slabs for roadways are supported by packed gravel/sand mix with some clay content called road gravel. This mix is also used for county gravel roads.

"Road gravel" from a local supplier could be packed under the slab with a make-shift ram using a 4x4 post or other such device. You could also rent a jack-hammer with a compactor attachment. It sounds like a lot of hard work but rammed earth walls are an accepted way of building your own house with simple tools and a strong back.

Good luck!