Compaction of Fly-Ash?

[Riggly]

Has anyone seen the placement of fly ash where the fly ash is compacted? I thought the fly ash comes in the form of flowable fill which does not need compaction. Please comment.

 

[GeoPaveTraffic]

The fly ash that I've seen brought to job sites is a powder, much like cement. Never seen anyone try to compact just fly ash. It has always been mixed with soil then compacted.

When mixed with soil, I always recommend that the resulting mixture be field tested based on strength not density; since the fly ash tends to result in a somewhat lower density than soil alone. But very high stiffness and strength.

 

[SlideRuleEra]

I have had first-hand experience with fly ash as it is produced by burning pulverized coal at electric utilities. Fly ash is composed of microscopic spheres and, by itself, is unstable. It can act just like a liquid - I have seen it sloshing around in the back of a dump truck, looking like a very dirty water, even though it was absolutely dry. It can be mixed with water and pumped relatively easily as a slurry. When dried, it looks and feels solid. However, it is thixotropic - when vibrated it again acts like a liquid - vibrating objects, like bulldozers, can sink out of sight into it.

To make a useful construction material out of fly ash, agents such as lime or cement are added to stabilize it (such as flowable fill).

Fly ash contains the concentrated heavy metals from coal which can contaminate groundwater. Lime or cement "lock" the these contaminants in place and make the use of flowable fill environmentally acceptable.

http://www.SlideRuleEra.net

 

[BigH]

Fly ash - yes, I have seen fly ash compacted both as the only ingredient and also as FASM (Fly Ash Sand Mix) where the sand was mixed at 15% by weight. Interesting "animal". A lot depends on the gradation of the fly ash - is the material coming out from the "head" of the pond near the spigot or in the slimier end - the "tail". Actually we didn't have too much problems with compaction (10% sand sized particles in the fly ash) - we laid the fly ash down - put some piles of sand on top and used a disc to mix the materials together. Built some 90 km of highway embankment this way - a few problems but not much different, in our case, than using a sandy silt. The job next door used 100% fly ash. The biggest problem we had was that the FASM seemed to have two optimum moisture contents - one near 15% and one near 28%. The latter had a slightly higher MDD but usually the lower optimum was used - which is likely one of the reasons we had less problems with compaction. It worked out okay although I have my reservations on using it. However, with the fact that we were building on 6 m of very soft to soft clay, some of the inherent problems with fly ash (settlements, washouts through the side containments, etc. was offset by the variable consolidation settlements of the compressible clays - India is requiring that all road embankments built within 100 km of a coal-fired power plant use fly ash as embankment material. Pushed by a guy who wants to grow vegetables and fruits on it - wouldn't hear of any "environmental problems".

 

[fattdad]

I will now share with you a true story. There was a time in Richmond, Virginia when a company was marketing a product that was locally referred to as "fly-ash" fill. It was actually a mix of flyash, bottom ash, and lime kiln dust. It worked pretty well, especially for wet subgrades and for building pads/earthwork in the winter (it was very forgiving in wet weather). All of these industrial by-products were stockpiling and this "seemed" to be a good way to deliver to market the product that helped local land developers.

Several new power plants came on line that included stack scrubbers, which produce their own industrial by product - scrubber residue. The company began to mix scrubber residue with the fly-ash, bottom-ash, LKD blend and continued to deliver this to market. Seemingly all continued to go well.

Several months later (after construction completed) a poltergiest appeared at these projects. Buildings began to crack, doors failed to close and walls/slabs began to crack. Needless to say, there was a huge problem!

A cottage industry was born! What was happening. . . .

Here is the summary: Flyash mixtures when blended with lime can be very effective. If, however, there is a source of sulfur (i.e., in the case of the scrubber residue), a damaging reaction occurs in the presence of moisture; the development of ettringite. Any of you concrete guys know about ettringite; however, in a fill pad there is a different dynamic. The ettringite crystals in the pore spaces of the fill create a jacking force that is quite impressive! We measured heave of 6 to 9 inches in some slabs.

My advice: If you are using coal combusion by-products confirm for yourself that the product does not contain appreciable quantities of sulfur. I'd limit the total concentration to under 0.5 percent of the total dry mass. However, to be safe take your odometer and do a swell test and run it for 30 days (i.e., if there is any sulfur in the overall product).

These were HUGE lawsuits!

f-d

¡papá gordo ain’t no madre flaca!

 

[DarthSoilsGuy]

my old company worked on a project where an energy plant in NC donated a couple of baseball fields to the town. i was marginally involved on this project (a little scheduling and office chit-chat). from what i remember, the stuff came in super-wet and they had to spread it out and let it perc-out before working it. i want to say the target density was around 65 to 75 dry pcf. it is done, but i haven't heard of putting anything on it except lawn. the fly ash we working with was the reject stuff that wasn't good enough to process & sell to the concrete company (or so they said). i would call it a poor sand. i'm sure you would have to mix it if you want to do something useful.

 

[Riggly]

Thanks for these post guys. I think these are all very informative info - more than what I expected. The reason behind me asking about compaction of fly-ash is because there is a site that I am looking at where fly-ash was used as a backfill, and where a sinkhole was later developed. One hypothesis was that the fly-ash was inadequately compacted, and the other hypothesis was that the surface water infiltrated the fly-ash fill through a porous opening and saturated the fly-ash which was placed on a clay layer. The satuarated fly-ash may have lose a lot of shear strength and eventually failed - no more than 15 ft of fly-ash was used. There are also some pipes running through this site, and another hypothesis is that leaking pipes may be responsible for eroding the fill.

 

[DarthSoilsGuy]

Hey Rig,
It wouldn't be a baseball field in NC, HAH!

i hear once you get to 1st the hard part is over and it's all downhill from there!

 

[fattdad]

Riggly, do you have any soil classification data on the "flyash" materials (i.e., grain size distribution)?

f-d

¡papá gordo ain’t no madre flaca!

 

[Riggly]

Fattdad,
One sieve analysis shows that over 80 percent is finer than #200 sieve. This was obtain from a field sample. I do not have any data from the supplier.

 

[fattdad]

Just wondering about the character of the "sinkhole". Is it something that you could stick your leg into up to the knee? Is it is something you could loose a car into? Is it a shallow (but large) birdbath? If either of the two former, I'd go with hypothesis "b". If it's the latter, then maybe hypotyhesis "a".

Good luck.

f-d

¡papá gordo ain’t no madre flaca!

 

[Riggly]

Subsided maybe up to the knee height or a little more, in a few locations, and has birthbath shape. Overall area maybe about 15'x15'

 

[BigH]

The fly ash I found was quite erodable (like loess) - so if you had/have a source of water perculation and a means to see escape, then having "sinkholes" form would be a given. If properly sealed to prevent migration of the fly ash, it should be okay. I don't see the compaction of fly ash leaving "metastable" structure (as say in loess). One other problem with using fly ash (and other materials as well) is dust if it is allowed to dry out.

 

[MSEMan]

I've seen PFA (pulverized fly ash) used for a number of reinforced embankment structures in the Docklands area of London. These grading of the emabkment fills were limited to 20% passing the 63 micron sieve. Even in this siutation the moisture condition value was a critical feature to get placement to 95% MDD. That being said, if moisture was right, the stuff goes down like concrete. As Big H stated sealing, and protection from water ingress is usually a given, if this wasn't done then water ingress may turn it back into a slurry.

 

[Riggly]

Very interesting MSEMan-the flyash bein turned back to a slurry could explain the situation that I am lookin at. Thanks for the info.

 

[BigH]

Riggly - hati-hati with the notion of a "slurry". What MSEMan is saying, I believe, is that if the fly ash isn't properly sealed (to prevent its migration and to minimize groundwater from ingressing into the fly ash embankment), the added water will erode the fly ash - if the embankment has confinement above, it can't become a "free slurry" like you may be thinking - i.e., muck. It was placed to, say 95% mdd at or near optimum. This would be a saturation level of say 90%. So, you only have 10% voids that can be filled with water - it might become saturated and therefore pump under traffic loading, but it is not a "free slurry".
In my previous assignment, given that the fly ash was being "confined" on all sides by an outer clay layer (and the base was clay as well), then the grounwater level, eventually, even with good measures installed, would eventually rise up and may, then, create a situation of saturated subgrade under the roadway (bathtub effect). This would not be a good thing. I had thought that, say, 1/2 way up to 1/3 way up the embankment, one could lay horizontal pvd wick drains within the embankment which would daylight out of the clayey confining layer and would therefore maintain the maximum groundwater "mounding" at a prescribed and suitable level.