Effect of Acid on Soil

[Geo69]

I am working on a project where a structure has settled, likely as a result of concentrated sulfuric acid in the soil. pH values are as low as 1.6. Some of the bedrock in the area is limestone and has likely been affected by the acid. Does anyone know of any publications that discuss the effect of acid on soil?

Thank you!

 

[fattdad]

This is an interesting inquiry! Do you know the source of the concentrated sulfuric acid? What type of soil had the 1.6 pH value? Is the water table in the limestone bedrock?

I'd think that a strong acid would certainly be neutralized by the limestone and that in turn could result in loss of volume or settlement. That said, if you have a pH of 1.6 there is obviously nothing at that location that's acting as a buffer so that would not foretell of loss of volume.

Consider a layer of soil that is nothing but quartz sand. If you were to leak concentrated sulfuric acid, you'd not expect any chemical reaction or loss of volume. So, in that environment, you'd not expect a problem (and you'd have high pH values in the quartz sand layer). At this point (i.e., by your original post), it's not clear how the limestone is playing into this concern, other than by implication.

You got my interest. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[Geo69]

f-d,
Thanks for the response. It is a unique project.

The subsurface conditions consist of about 10 feet of fill overlying clay and gravel layers. We may have encountered bedrock at 80 feet. Some of the bedrock in the area is mapped as limstone and would definitely react with the acid. I'm trying to find out whether the acid would affect other types of soil. I haven't been able to do any lab testing to determine mineral content of the soil.

Some of the upper (2 to 3 feet) of soil appears to have been affected by the acid, however much of the lower soil still has very high SPT values and does not appear to have been affected. The pH of 1.6 is at a depth of about 25 feet.

 

[Ron]

Test the pH again. Having a pH of 1.6 at 25 feet below the source is hard to believe.

Secondly, do you have any idea of the pH at or near the bedrock?

You might want to look at concrete deterioration in the footings and foundations before you worry about the bedrock.

 

[Geo69]

Ron,

I'm more worried about the surficial soil and its support capability than the bedrock.

We performed more than 30 pH tests in the field during drilling and found the shallow soil had pH values in the 1.6 range. The pH values at depth were in the range of 7 to 9.

Deterioration of the existing concrete and whatever will be used to fix this is definitely a concern.

 

[fattdad]

I thought you just said, the pH at 25 ft (i.e., not shallow depth) was the 1.6?

What method did you use to measure pH? Did you do any other chemical characterization?

Any further insight on the source of the sulfuric acid?

Any data on the strength/compressibility of the fill? Any soil classification data?

f-d

¡papá gordo ain’t no madre flaca!

 

[Geo69]

The pH is low from the surface to a depth of about 30 feet where a clay layer is encountered, then rises to about 7 to 9 down to the bottom of the borings at about 80 feet.

The gravel layers are very dense. The clay is medium stiff to stiff. The fill is predominantly gravel which is loose to medium dense.

The source of the acid is the facility. The acid has seeped into the ground over time from spills.

 

[concretemasonry]

It sounds like the clay layer is preventing the acidic moisture from penetrating it.

You may want to look at the composition of the clay layer to determine the expected long term reaction with the acid from the fill material in the upper layers. - Don't poke any more holes in the clay than necessary.

If this is the case, the limestone is effectively protected by the clay layer.

 

[cvg]

if you are worried about the surficial soil, than you need to test the chemical / mineral composition of the soil. Until you do that, you cannot make any assumptions on how reactive it is with sulfuric acid. Your other option is to neutralize the acid or remove and replace the soil.

 

[Ron]

As fattdad noted, siliceous materials are not greatly affected by acids. The low pH near the surface concerns me only in the sense of the foundation materials (concrete). The concrete will be significantly affected by the acid in the soil. Settlement will not be affected by the acidic nature of the soils, unless there is decomposition, which is not likely with silica sands.

Acids can cause deflocculation in clays, but at the level of pH you noted (7 to 9) I would not expect any effect on the clays.

You will likely need to treat the soil in the area of the foundations (lime treatment to raise the pH) so that concrete deterioration is reduced. Make sure you specify extra cover on the rebar and specify and obnoxiously require high density in the concrete.

 

[fattdad]

So, with the source controlled (I'm guessing you've stopped the leak) and knowing there's gravel (i.e., quartz) I worry about the foundation concrete as well. I'm thinking the potable water supply is protected by the clay (i.e., there's no effect to human health of the environment), you may be able to address low pH using hydrogen peroxide, which should yield water and sulfur (I'm not really a chemist, but think it'd work like this).

Regarding settlement if the facility has been there for some measure of time, the fill's not likely a bad actor. If this is a newish facility that sprung a leak, I'd consider poorly-compacted fill leading to settlement and see if that could result in a leak.

I like guess work from the comfort of cyberland!

Good luck.

f-d

¡papá gordo ain’t no madre flaca!

 

[Geo69]

Thanks to all for your suggestions. I'd considered a lot of what has been said, but its good to know I'm not out in left field.