Repair Washout under a slab

[guycox]

I've got a terraced area in rear of the house. Some of the older landscaping timbers have rotted and collapsed a bit. The more immediate problem is the washout under corner of slab outside the basement door. The slab runs the width of the house under deck. One corner of the slab is hanging out in mid-air because the 1st terraced area has subsided about 6" -- This is my father-in-law's house and I have no idea how long the area has been washed out..

I thinking that I'll shove some crushed rock into the wasout as far as I can get it. Build form around the washed out area and drill a hole in the slab so I can pour in enough cement to fill up the void. I think it'll take about two sacks to fill the void.

Then I'll have to put up another wall (4x4 pressure treated) to support the existing terraces - I'm afraid the pulling the existing timbers will cause the existing terrace to collapse.

Does this sound like a sound plan? Other suggestions?

[chopnhack]

If its a large enough area, you may want to get a pro that does this type of repair involved. They have good insurance for this kind of thing whereas you probably do not. If you do the repair and it still fails, the homeowners may not pay out the claim citing undocumented work. Look for structural repair pros or if your area uses them folks that do pile drivings and earth anchors. HTH

[jking]

Is this just a patio slab, or, is it the basement slab?

[iceman61]

If you get crushed aggregate under it, then you will have trouble getting the cement to fill the voids between the aggregate. If you build a form flush with the outside edge, put alot of water in your cement to make it flow to fill out the void. you might be better off. I would use straight Portland cement also. (smaller aggregate size, or gradation) Just an opinion.

I once saw a home improvement show on the DIY Network where they raised sagging slabs by core drilling the slab & injected "flowable fill" concrete into the hole in the slab until the slab came back up to grade. Not what you're looking for but that's the type of flow you need to completely fill out the cavity. Flowable fill concrete is what you see coming from the concrete truck that have a long hose pumping the concrete.

Here is some good reading on the subject: http://www.nachi.org/forum/f23/erosion-under-slab-1982/

[chopnhack]

Watering down the cement weakens it greatly. Instead of doing that you can use grout, even when mixed to a flowable consistency you would still get 8kpsi ay 28 days of curing.

http://www.sakrete.com/products/deta...truction-Grout

[Seedy]

I wouldn't use gravel. You will never get it packed tight. Drilling a hole to pour in cement will weaken the slab and it will want to crack where the hole is.

A better idea would be to dry-pack underneath. Clean out all loose material. In a wheel barrow mix equal parts mason sand and portland cement. Mix well. Mist with water while mixing. Only add water to the point where the sand is dark and will stick together when squeezed in your hand. Shovel some under the slab and pack itrepeatedly with a piece of 2x4 or 2x2. Do it a little at a time so that it all gets well packed.

You can fill to outside the edges of the slab, then cut the edge off square with the slab. You can leave like that or brush lightly with a wet paint brush.

This method will give constant support to the slab. It won't wash out and shouldn't settle.

-craig

[guycox]

It's a patio slab outside a walkout basement.