Slab on Grade - Partial Settlement Below

[ConnorM]

Greetings,

Long time reader, first time poster.

I have a situation with a single family dwelling with a slab-on-grade. Some settlement in the soils beneath the slab occurred, which affected the under-slab ducts, and the sanitary line. Ducts were abandoned in favor of ducts in headers, and the slab was cut to excavate and repair sanitary. One problem, when they excavated under the slab someone 'noticed' that under slab settlement had occurred "up to 4 inches". Please note there is no visible damage to the slab from above. Needless to say, this in now a Charlie Foxtrot.

Some details:
-Slab unreinforced typically (fibermesh concrete was used, 3500 PSI)
-Slab 4" thick
-Slab is tied in around edge, and also the slab is poured overtop some intermediate supports for point-loads and whatnot (basically assume supports at 10 ft on center each way in addition to whatever soil is left in contact).
-Wall and intermediate supports are all deeper than slab ~4ft.
-Slab does not directly support any structure for the remainder of the house (no slab thickening for bearing walls, etc.).

Not sure how much is unsupported, its undocumented as far as I know and everything is currently sealed again. There is a fellow involved who seems to take pride in escalating problems and I'm not sure how to deal with that. I'm more of the opinion that if one day the slab breaks and I'm 4 inches shorter than I was a second ago, I should probably go fix that, rather than the drop several thousand dollars on something that might not actually be a problem ever. The term "Life Safety Issue" is being thrown around, but I haven't found any real support for that argument (or anything that shuts it down).

I'm looking for some advise in a "if it was your house" vein, and also some legitimate sources of information on the subject. I haven't been able to find much in the way of technical documents of any kind on this type of construction. Most books I have seem to deal with much larger scope projects.

 

[GeoPaveTraffic]

Options as I see it are to either mud jack the slab or do nothing. Both have advantages and disadvantages. However, based on what you posted, I don't see any way for this to be a life safety issue.

Mud jacking will help keep the slab from breaking. However, if there are any gaps in any sub grade utilities, the mud jacking will find it and fill them up.

Doing nothing could lead to a partial collaspe of the slab at some point in the future. Generally, if the slab does fail, you should be able to mud jack it at that point.

Good Luck.

Mike Lambert

 

[Ron]

Agree with GPT. Slabs are rarely related to life safety unless there is loss of support for primary structure. Grout injection (mud jacking) would be a simple restorative process.

 

[MiketheEngineer]

Agreed - mud jacking is the way to go. Just make sure they don't over-jack - you can lift the whole house this way

 

[tsaldana]

Connor M,
I believe a good reference would be “Foundation behavior and repair residential and light commercial” by Robert Wade Brown. I believe it's in its second edition still. Another good reference for the practicing engineer would be the Texas ASCE “guidelines for the evaluation and repair of residential foundations”. You can get a free copy of the repair guidelines from Texas ASCE’s website at

http://www.texasce.org/associations/10803/files/RepairGuidelines.pdf

Now Mud jacking may be the answer that you need, but may also want to look into using Uretek for concrete lifting and soil stabilization. They use a polyurethane foam to inject below the slab for lifting and soil stabilization. You can look at their website at

http://uretekicr.com/.

As far as life safety issues this probably isn't the case unless of course your house is next to retaining wall which also is failing. But more than likely 4 inches of differential elevation in the home would probably be felt and be picked up by a home inspector or potential buyer of your home in the future. So if you were going to sell the home, you may have to have the problem taken care of before the home is shown or just made up the difference in the price of the home. Well I hope this helps good luck.