Hole in foundation / slab floor

[catdog]

We have a 1970s house with a cement slab foundation. On top of the foundation, there is one row of cinderblocks and on top of that, the stud wall. Near a corner, the foundation (below the cinderblock) is gone over a width of 5" or so, probably eroded from a downspout that has now been fixed. If I stick my arm in the hole, I can feel the bottom of the drywall on the inside, so there seems to be a gap between the cinderblock and the floor slab. We have had moisture coming in in this corner.

My plan so far is to spray in expanding foam (to fill any cracks and create a moisture barrier), and then patch up the outside with cement. I am not sure how deep I should dig -- if I dig to the full depth of the foundation, will I risk making it more unstable?

Would this work? Any advice?

[HayZee518]

mix up a batch of hydraulic cement and apply it from the outside (as you already found out how) just keep shoving it into the hole. hydrauic cement will start to set up within ten minutes so mix up small batches or what you think you can apply within ten minutes. hydraulic cement expands when it hardens. I don't think the foam would do the job even tho it cures depending on moisture, but it won't cure under water or in water.

[pushkins]

If you can stick your arm in the hole then your foundation/slab is eroded.

If that is the case then you could dig down the length of the troubled area until you find the original foundation, place a plank or length of board big enough to go from one side of the foundation to the other and tall enough to come up from the good foundation level with the top of the foundation level, brace it against the foundation (back fill with a little dirt) then into the cavity left from erosion pour in mixed cement until it matches the original remaining foundation in height. You wont be able to get it to fill completely under the cinder blocks so once its dry then as previously suggested use hydraulic cement to repair the remaining damage.

Mostly damage like your talking about is mortar erosion and simply fixed like mentioned in the previous post.