Effects of Acid on Plasticity of Clay 1

[luke648]

I am currently working on a project in a chemical plant evaluating what appears to be heaving soils. Concrete paving is heavily cracked, tanks in the area are leaning, etc. In this particular area, there had been a problem with acidic water (sulfuric acid used in process)leaking onto the area. A couple of tanks in the area have been in place since 1966, some are a little newer. Some movement was noticed around 2000, and the worst seems to have occured between that time and 2004. A survey of foundation points was initiated in 2004; however, a welding company was hired to do the survey, and there was little control (i.e. survey points, personnel, condition of benchmark, etc.) and the numbers are, to us, inconclusive. At some point between 2000 and 2004, the acidic runoff was diverted to drainage ditches. Aggravating the issue, there whave been some significant personnel changes, and some of the people in place now can't answer some of the questions about when and if operational changes were made that changed the environment, etc.

The tanks are founded on shallow footings bearing about 4 feet below finished grade. We have have one boring in the immediate area and one 30 feet away out of the process area - and some lab analyses...foundation zone soils near immediate area have high moisture content (24 to 31), sandy clay and clayey sand, LL from 24 to 34, PI from 10 to 22. In a couple of cases the moisture is approaching the LL, but not all. Perched water encountered 3 feet below finished grade. Soil pH runs between 3.3 and 3.5, while water tested at 2.7. Soil pH in the removed area runs between 5 and 8.

One thing we were told is that they had a similar problem in a different area that had been subject to acidic water. In doing some research, I have found some references that indicate clays exposed to acidic soil conditions do tend to show increased swelling in plastic soils. The chemistry is way beyond me, though!

I guess that my question is, has anyone had any experience with clays in acidic environments? If you have, how was it handled? Is this train of thought way off base? There is some argument here that we could be looking at a settlement issue, not heaving although in the field it looks like heaving. Timeframe is now limited due to internal plant schedules, so we don't have time to set up a long-term monitoring program and are looking at undercutting all the clay in the area and backfilling with flowable fill or lean concrete - it will have to be a special mix for acid resistance.

Sorry to be so lengthy, but I felt that all this information was important! Thanks in advance.

 

[TDAA]

Well, the perched water, if it extends to below the footings as well, should decrease the likelyhood of a moisture variation causing the movements. The acid on the other hans may lead to either an increase in the activity , or a decrease in the activity of the clayey materials.

The majority of work with expansive soils deals strictly with the moisture variations (typically seasonal to effect enough depth to create movement. Cation exchange in and interaction with the dipole water molecules is what will gives the placticity and the swell potential (also influenced by other factors).

That exchange is going to be affected by not only the types of minerals in the clayes, but also the solution that enters it. If you want to see what the difference is between the acid and claen water, see if a lab will run a sewll/consolodation test on your undisturbed, nutral pH samples. This could show additional swell with the acid. (run several samples, including pH before and after).

As far as some of the other items: Yes, there could be settlement, but your eyes have likely told you the right thing. Survey it if you have a good benchmark and as-built data. I think you could handle the swell from the acid with the typical methods. Just choose one that will not be affected by the pH.


It is my belief that you should get an understanding of the potential swell for the soils you have and what they are reacting with, and the depth of thoose materials that may be affected. Until you do that, there is no way to tell the best method to address it. Your removal and replace is likely a viable option. If the time frame and condition of the oter replacement is known, it may be easiest to use that, and your engineering judgement to develope recommendations for this. Lab data to back it up would be beneficial.

 

[luke648]

TDAA,

Thanks for the comments. From what you are saying it sounds like there could be some rationale for acidic water to influence swell potential given the right soil/water conditions? I took samples from both the affected area and from a "neutral" area not affected by the runoff for that very reason - to do some testing similar to what you mentioned. Although with the time constraints they have we may go ahead with undercut/backfill or stabilization with helical anchors or something, if they choose to investigate further that will be the next on the list. You're right - having an understanding of the potential swell of the soils that influence the foundations and what factors affects the swell potential is key to coming up with the best way to address the problem. If we can make sure we don't damage our laboratory equipment....

The interesting thing about this project is that a couple of those tank foundations have been in place since 1965 with no documented problems until 5 or so yrs ago. So there has been some change - in process, runoff, overall perched water elevation - something - that precipitated the movement.