KNOWING YOUR ASS FROM YOUR ELBOW.
In my last blog entry I mentioned Acid Sulphate Soils or ASS. I have been put in charge of the safe disposal of the ASS. Not only does this help me cover the sustainable development piece for CPD but, more importantly allows me the opportunity to talk ASS whenever I want.
A heap of ASS. Note at the time these stairs were the only egress and the black smudges/ooze.
So what are acid sulfate soils? Here comes the Science…
Acid sulfate soil is the common name for soils that contain metal sulfides. In an undisturbed and waterlogged state, these soils may pose no or low risk. However, when disturbed or exposed to oxygen, acid sulfate soils undergo a chemical reaction known as oxidation. Oxidation produces sulfuric acid which has led to these soils being called acid sulfate soils.
How and where are acid sulfate soils formed?
Acid sulfate soils are formed by bacterial activity in waterlogged conditions when there is no or little available oxygen.
Naturally occurring bacteria convert sulfate (dissolved salt) from seawater, groundwater or surface water into sulfide (another type of compound that contains sulfur). This sulfide reacts with metals especially iron in the soil sediments or water column, to produce metal sulfides (the main components of acid sulfate soils). In order to convert the sulfate into sulfide, the bacteria also need a source of energy provided by organic material such as decaying vegetation.
So what I hear you say!
Well we have just dug up a lot of this soil, exposed it to air then added a good downpour of monsoon rain onto it and left it there to get really nice and acidic. Essentially any water we pump out has to be treated like sulfuric acid and needs to be neutralised. So you can imagine my joy when the bloke came to pump it out this morning then proceeded to let it leak out of his truck on the way to the disposal site.
For Damo!
This will be where the lime is added once the water tank has been moved.
The Water holding tank is in the top right hand corner of the picture and needs to be moved to the corner of the excavation to avoid stressing the wall.
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Doug,
What time scales are you looking at once a load of soil is dug-up before you have to consider it HAZMAT and therefore dispose of accordingly?
And, although the addition of water creates the oxidisation chemical reaction, is there an amount that starts to dilute it enough for it to no-longer be deemed HAZMAT? Thereby, if you water it down enough can you safely remove it from site and not worry about possible disposal truck leaks?
Fran.
Fran to answer your question is it is considered ASS bust. If you were to dig it up and move to a dump site. You would have disturbed it in its natural state.
In theory you could dilute it enough for it not to matter but this would require a lot of water to mix with it and we don’t have the space or cost for that. The plan is for it to get balanced out by adding a base material but the plant was put in the wrong place and would exceed the surcharge on the wall. I asked them to move it. So they sucked the water out to move the container and hence the issue.
I had asked for the dewatering plan and ASS review on the previous weds.
Doug,
Classic Monday morning problem!
Did the ground investigation identify lower than normal pH levels or did it only become highly acidic once exposed/water added? A quick Google makes me think this is not an uncommon problem in Australia – was it on a risk register, is it included in foreseen ground conditions.
The BRE SD1 suite looks at geo-chemical testing and would give it a high exposure class for concrete – how does (will) your site deal with concrete and acidic soils, presuming there will be higher levels remaining when you cast foundations (have you identified the end of the source/depth of problem)?
I had an issue on my site which involved testing of the soil. The pain was waiting over a week to find out how ‘hazardous’ it was which meant no excavation until the contamination was identified and then could be dealt with. I guess you know what it is here so perhaps less of an issue.
Can you neutralise pH level on site – just add limestone for example?
Damo,
Irritatingly we knew about this as per my previous response to grab and had a plan in place. The ASS is only a problem for a particular strata of clay and the pad foundations are on phyllite so acid exposure will not be an issue.
i will check on the exposure to the piles.
We are supposed to neutralise on site as per my response to Fran.
presumably you are not back filling over your pads with anything other than imported clean fill?
Exactly the only reinforcement in contact with the ASS is the hard piles and they have 150mm cover.