Soil Stabilization Using Fly Ash

[MRM]

I'm looking to get general information on the use of fly ash for soil stabilization purposes for sands, silts, and clays. Has anyone used fly ash in connection with road and/or concrete slab subgrade stabilization? Erosion mitigation? Increasing the damping of dynamic machinery foundation subsoils, like punch presses for example? I'm still looking into the reactions of fly ash (and Portland Cement too for that matter) with sulfates, organics, etc. I have no particular project related to this issue yet, but just doing some preliminary checking for potential projects down the road. Thanks.

 

[BigH]

We are using fly ash extensively in embankent construction in West Bengal, India. The original idea was to mix fly ash with clayey "earth" borrow - but this was a terrible idea - how to you take a clay that comes in lumps and then mix thoroughly fly ash into it (about 40-60 ratio). You'd have to dry the clay and pulverize it and blend it - too time consuming and costly. Instead, we are using fine sand from river deposits and mixing 15% sand with 85% fly ash for the embankment constrution. Our other contract is also using it for subgrade (at 25% sand and 75% fly ash) but with a caveat. They use a full 35kg weight for surcharge loading on the soaking period - much higher than the normal 5kg weights or so with the standard lab testing. They had to jerry-rig up a frame to get all the weight on.

There has been a few papers I have seen on the subject here in India but the one - I find a bit far fetched - like if you are trying to use a lot of fly ash, why are you bothering to increase the "compressive strength" if you only use 8% fly ash!

My general impression is that the fly ash mixed with sand provides decent compaction and a reasonable embankment - don't get hung up on 100% MDD at 1.29 to 1.33 g/cc. The big concern, though, is the toxicity (or perceived values) in its use as fill. In our case, we are capping it above and we have clayey caps on the outside slopes - clay bottom. So, we have a situation where the flyash is embalmed, so to speak - unlike the normal conditions around here where everyone is using directly on the rice paddies in order to provide driveways, building lots, etc.

Best regards and

 

[BigH]

Addendum: Forgot to say that the mixing of the sand and fly ash is quite straightforward. The hardest thing is how to control you are getting 15% sand in the mix (contractual problem). In fact, based on my data, they are probably putting in something like 25%. The fly ash goes on first, sand is spread over it and then a farming disk harrow is used to mix it all together. Using a Ingersol Rand vibratory compactor to compact it. Getting 97% MDD (heavy - or, for the Yanks/Canucks - modified) without too much trouble - unless the mix gets saturated. It is highly affected by water - needs to drain (but isn't that slow at drainage) and is also quite erodable.

Hope that this all helps.

 

[Focht3]

I echo [blue]BigH[/blue]'s comments and have a few of my own.

Fly ash is quite good at stabilizing relatively thin (~3 ft.) zones of saturated clayey sands. This is a common use in the Woodlands/Kingwood areas north of Houston. Blow it on - disc it in - compact it after it sets up somewhat. This is a lot faster than trenching and sumping the site, but with added cost.

You can inject a fly ash slurry (with or without lime) into highly fissured expansive clays. The fly ash slurry not only interacts with the clays, it fills the cracks too. Woodbine Corp. in the Dallas/Ft. Worth area pioneered this approach. They were purchased by Hayward Baker - Art Pengelly in the DFW office came from Woodbine - he's their in-house expert on this approach. This is quite successful in treating slopes - but I can't vouch for its' use on building pads.

And the issue of toxicity can be an issue - the source of the fly ash is key. Cement plant fly ash is usually okay, but you have to look at the power plant fly ash on a plant-by-plant (and, occasionally, month-by-month) basis. For most construction sites this isn't an issue; but it can be a concern around wetlands, riparian areas and aquifer recharge zones.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[genomty]

Hi MRM:

I have read a brief pamflet by the Federal Highway Administration of the U.S. DOT. I can not remmenber the name of this publication, sorry, I need to look around my house and give you the exact name. This publication includes a summary of the use of fly ash around U.S. to improve even bases.

Because of fly ash is a puzzolanic material, it typically reacts with moisture and calcium carbonate, cement and hyadrated lime are the typical cementius materials used to generate the reaction. As curious comment, Mayan culture 1000 years ago, build up some structures at "El Tajin" using some kind of volcanic ash and hydrated lime, remmeber, that they used the lime to prepare "nixtamal" the base for making "tortillas", so they observed that when the spill away the water containing lime over the ground it becames hard over the time. Then some one had the idea and started to use it as construction material. there is an interesting article in some ACI journal by Mr. Raymundo Rivera, rest in peace, and a colaborator. Well sorry for the mental lapsus.

Back to the document, it also includes some contruction procedures. try to put your hands in a copy of it. I will look for it and send you the entire information.

Related to the mixing I'm not pretty sure but i think that a soil stabilizer equipment Cat-RM350B or similar piece of construction equipment can be quite helpful to treat clayey soils. This equpiment is pretty used throughout the Rio Grande Valley, southern Texas, to improve "caliche" for its usage as base material. I have seen this guys (contractors working for the TxDOT) extending the "caliche" then with a water tank truck spread a hydrated lime lechate over the layer the mix both using the soil stabilizer. This method is also used to treat the native clayey soil and gives them good results. Probably this "wet" method could be a good solution to mix soil-fly ash- and reactive material.

I hope the comments above gives you a clue to redirect your searhing efforts.

I'll try to give the data soon.

 

[Focht3]

Hmmm,

I've traveled a good bit in south Texas - and I don't think much of the use of "caliche" as road base. It's highly variable - and subject to significant softening when wetted. And the Port of Corpus Christi had a spectacular failure of a pavement built using "caliche" - it only lasted about 2 or 3 months. The failure was very unfortunate - the wearing surface was high strength cement pavers. It wasn't my pavement design; but I was paid to evaluate the failure. Even though I clearly blamed the "caliche" (no pavers were damaged at all), the pavers got the blame. A real shame, really -

And while a "Pulvermixer" might get the materials blended, remember that a significant part of the effectiveness of fly ash has to do with physical contact. Fly ash won't be as effective as lime -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[Ashayeri]

Hi,
take a look at journal of geotechnical and geoenvironmental engineering, ASCE, Vol. 127, No. 7, pp. 568-573.

goodluck.

 

[genomty]

The pamphlet previous mentioned is FHWA-SA-94-081.

 

[MRM]

Thanks again, I appreciate all of your comments. They are very helpful, indeed.