Bottom Ash 1

[morgwreck243]

I hope this is in the correct forum.

I am currently serving as the field engineer for a project where we are consolidating the bottom ash in the area onto the existing stockpile of bottom ash. Then we will be capping all of the waste material. Per the contract specifications, there is no density testing requirement, but only verification of compaction with a vibratory roller and lifts no thicker than 12 inches (loose). Then a proof roll inspection every 4 ft of compacted fill placed.

We've been having some problems with moisture levels so I was wondering if anyone has had any previous experience and could give some recommendations.

Our current plan forward is to start placing 6 inch lifts instead of 12 inch lifts. The thought being that this would allow for the material to dry out longer than the time it currently sits out.

Would simply allowing for the material to site & dry out longer be more helpful?

We have currently been placing the material at the top of the existing pile. Then spread out the material down the slope. Then the next day we would push the ash material where it needs to go.

--morgwreck243

 

[oldestguy]

Define what will constitute a failure of the "test" with a proof roller. Looks like a pretty loose a spec, but working on a slope may add complications.

 

[morgwreck243]

The material fails if the compacted fill starts pumping under the load of a truck (12 tons).
I agree about the spec not stating exactly what fails / passes. It just states that it will pass a Proof Roll inspection. The inspection is performed under the observation of the client's Engineer and a soil technician (our subcontractor).

--morgwreck243

 

[oldestguy]

I don't get it. Why compact this stuff anyhow? Will it support a building someday, a road?? Is there a fear of a landslide of this "migrating" off site? While that is not your job to decide, it would seem that your superiors should be more clear as to why all the fuss and maybe better direction. Sometimes getting back to them with the questions may get a change from expecting a near impossibility or high expense for no good reason. The bottom ash I am familiar with is pretty inert stuff.

 

[Ron]

This stuff can be bad news! It can have lots of unreacted materials, likely lime or magnesium based (or both), prone to long term expansion in presence of moisture. If you put a structure on top of this material now or in the future, it is asking for trouble!

 

[morgwreck243]

There is no structure to be placed on this material. It is simply being consolidated (placed in a single location) and covered with a RCRA-type cap (GCL, GDL, & topsoil). Based on our discussions with the client about the requirement for the proof roll the client wants the proof roll to ensure there are no soft spots so that the cap does not develop a low spot or other such failure.

After the landfill is completed & the sodding is established the only equipment expected to be used on top of the cap are the lawn mowers and other maintenance trucks for the sod cover.

--morgwreck243

 

[oldestguy]

Hopefully you will have some say on "what is pumping?". For earth materials pumping usually takes more than one pass of the vehicle. Perhaps the spec can be interpreted "one pass of the truck leaving no more than ____ inches of rut." I'd fill that depth in at 8". I'd go only down hill for the test. When wet there is an unlikely chance of any progress due to that criteria of no pumping.

 

[Ron]

morgwreck243...that's fine...but what about the used of the site in 10 or 15 years? Sometimes these materials have very slow ettringite formation which can cause expansion at much later times.

Warn them about it.

 

[SlideRuleEra]

morgwreck243 - The disposal technique you describe is consistent with handling of bottom ash produced by efficiently burning pulverized coal in a high temperature electric utility boiler. If so, it should be a fairly uniform more or less granular material. Not surprising that it is wet, it is typically transported by sluicing with water. This material makes a fairly good, reasonably inert fill without much compaction. Since it is a waste product, there is no specification for it... what you get depends on the coal burned. The problem with bottom ash is that it contains high concentrations of the heavy metals found in coal - that's the reason it is being encapsulated in a permanent (and hopefully lined) landfill. If you let the bottom ash dry in a pile (as you are doing), spread it, and proof roll as specified that should be adequate.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[morgwreck243]

Ron - To my knowledge, the capped landfills will not be used for anything else. They will just remain grassy mounds.

SlideRuleEra - We have figured that we needed more time to let the ash material dry out; especially since it has been pulled out of the adjacent infiltration basin. Plus, we have been hit with a lot of rain recently.

As for the landfill,to the best of my knowledge the landfill is unlined underneath. Although, during our grading operations along the edge of the existing landfill we have run across a heavy cloth which seems to be put in, like a liner, but I am unsure if that was to just seperate the clean fill berms around the sides or if it extends all the way underneath the existing ash stockpile. So there maybe a liner, but nothing I would consider stopping the transport of heavy metals contaminants.

--morgwreck243

 

[SlideRuleEra]

morgwreck243 - We need you to answer a basic question, bottom ash can be VERY different depending on how it was "made". If it from burning coal "lumps", in the traditional manner, comments from Ron and oldestguy are most appropriate. If the bottom ash is from burning pulverized coal (ground seconds before combustion to the microscopic consistency of face powder) its properties are VERY different. Which is it?

Continuing to assume this ash comes from pulverized coal, as long as the bottom of the landfill is above groundwater and the top / sides are capped that would be ok... thought a liner might be required by regulators these days in light of several recent coal ash environmental incidents.

The cloth you encountered is probably a geotextile fabric - not a liner, and was used during the original construction of the "bottom ash pond".

The objective with closing an ash pond to cost effectively relocate the bottom ash to a landfill. If taking more time for draining the ash will solve the problem, that will likely be the way to go - drying time for a waste product is cheap. This is especially true if you are working with volumes of bottom ash measured in hundreds (or even thousands) of acre-feet.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[morgwreck243]

SlideRuleEra - We have worked in additional drying time and that seems to have worked. I had a preliminary proof roll yesterday, & everything looked good (no pumping), in prep for today's official proof roll.

I'll have to look into which method the coal ash was produced from, but based on the design it would seem like it is from pulverized coal. And yes the landfill is above the water table.

We are installing a cap (GCL, GDL liner) with a drainage system to prevent any runoff from entering the waste pile.

To everyone else thanks for the information.

--morgwreck243