foundation settlement problems

[shaneelliss]

I have some foundation settlement problems under some smallish lightly loaded spot footings and under some very large heavily loaded mat foundations. The settlement in both cases seems to be mainly caused by acidic runoff seeping through the soil and dissolving the underlying soils (and probably some soil transport as well). I believe the causes of the settlement have been taken care of for now, but I would like to shore things up for future problems. What are the most common remediation options for stabilizing the soil and the settlement? The couple options I know of are grout injection and installing deep piles adjacent to the mat (or maybe through the mat) and tying them into the mat. Are there other options? Are the options different based on the spot footings versus the large mats?

 

[Ron]

What kind of soils and what is the source of the acidic runoff? That would be unusual.

What is the subsurface profile?

 

[shaneelliss]

It is an industrial plant where there have been acidic leaks that were previously allowed to flow into the soils. These have been mostly contained by containment slabs, etc. but it is possible that future leaks may occur. The soils are mostly layers of 10'-20' lenses of silty sands, then silty clays, alternating over a depth of 100' or so. Water table is approximately 8' below grade. Maybe it is not possible to ignore the details, but I really wasn't looking for a detailed answer to a specific problem. I mostly was just wondering what common options there are for stabilizing soils or various types of foundations on unstable soils. Is that possible, or is every solution highly dependent on the soils?

 

[Ron]

Unless your sands are calcareous, I would not anticipate any deterioration of the soil as a result of the acidic exposure. It will attack the concrete used for the foundations. If you had clayey soils, it can deflocculate the clay and cause instability, but consolidation tests using the actual groundwater or leachate should be used.

If your groundwater is at 8', the pH of the groundwater at much lower depths is not likely to be significantly lower than normal.

Do some pH profiling and sampling for future efforts.

 

[shaneelliss]

Thanks Ron. The sands are oolitic sands and very calcareous and whatever gravels are found in them are limestone. All in all, the stuff dissolves in acid. That being said, I am really looking for general options on what to do about settling foundations. Can anyone provide a general list of options?

 

[msquared48]

Sounds like a different mechanism to create a sinkhole to me. I would first try to stabilize the acidity of the soil if possible, then do grout injection, similar to slabjacking.

Do you know the acid(s) involved so that a neutralizing agent could be injected? If this is possible, it would have to be done under and away from the structure as side leaching will occur with time. This could be a periodic maintenance process until the soil stabalizes.

Mike McCann, PE, SE (WA)

 

[Ron]

Agree with Mike. Most acids can be neutralized; however, you need to make sure that the neutralization remains neutral....for example, the "salts" created from neutralization of acids by various bases can be expansive....not good.

You might consider foam injection into the soil.