Thread: Gap under car port slab with house walls above

House built in early 70's and from crawl space there is a void under the car port and outdoor storage room slab (one pour). Basically, where the perimeter of the slab meets the earth, there is a void. The rear car port wall and storage room wall rest on top of this area but roof is truss so not much load on walls/concrete. Naturally, concrete will span quite a distance without support but I can't see how far the void extends under the slab. No signs of movement/cracking on slab or interior walls in this area. Curious as to what direction you may give client. Describe and monitor? Foundation contractor? Nothing?



063 (Medium).JPG064 (Medium).JPG.

Similar Threads:

• Pex water line entry to slab house.
• new house wiring best practice - through walls or trusses
• House Concrete slab crack and a tree
• House on Concrete Slab; tree issue
• bowed stud walls along exterior of house

Pressure grouting.

They can inject grout under pressure into the void below the slab. Typically, a 2 inch diameter or so hole is bored through the slab, the injection pipe inserted through the hole, and the grout is pumped into the void.

Typically, they pump is some first (grout is very watery, 11 inch slump or greater) which gets into the soil and solidifies into a solid mass, then a few days later they some in and fill the rest of the void - they can actually raise the slab back up if it has settled some.

Once pressure grouted, that would be a permanent fix, unless there continues to be soil subsidence ... in which case, in another 40 years they may need to repeat the process.

Your safe bet is to describe pressure grouting to your client (provided they do that in your area), then recommend a foundation contractor and let your client know that they may or may not pressure grout, they may use another method.

You are basically educating your client on the options used in your area (down here pressure grouting is common), then recommend the contractor - who will then recommend the correction they will become responsible for. "You" are not "recommending" a repair, just educating your client to "typical" local options used.

FYI:
When you're photographing a void or other hard to see dark hole, use a yard stick or some item to provide perspective for the viewer. I can't really tell how tall or how deep any of that is, or was.

Dom.

It is not clear from the pictures how big the void is. The fix depends a bit on how much fill is below the slab. If there is a significant depth of fill pressure grouting is a good fix. In my area there is only one company that will do pressure grouting for residential size projects. They charge about $5000/day. They would probably quote one day, unless the depth of fill is significant.

Slab jacking is a cheaper version that only fills the void and does not densify the soil. When there is a significant depth of fill the weight of the concrete pumped into the void can compact the soil and can cause another void.

Polyurethane injection is similar, but polyurethane is light so there is less concern. This can be a good choice if the depth of fill is no significant.

When I evaluate these I dill holes through the slab and probe the soil. That gives me a reasonable idea as to how deep and soft the fill is.

I would not tell my client how to repair it, even though I agree that foam injection would be a good repair option. I would report what I see and then tell my client to contact a qualified foundation repair contractor for recommendations on what can be done or if anything needs to be done with the void.