Construction Control of Ash Fills of Variable Density

[Mad Mike]

I've recently had a good query from a Contractor.

We're working on a site with large volumes of coal ash, generally coarse (gravel-sand) bottom ash, which from all available laboratory results is an excellent engineering material.

Problem is, for a number of samples tested, the maximum density and optimum moisture content show an incredible range. I can find no set standard in my local legislature for controlling the compaction of these materials...but the nuclear gauge results we're obtaining are all over the place, indicating compaction ranges of 85 to 110% despite the material being uniformly engineered.

Any tips for the field control of variable density ash? I'm happy with visual confirmation, but I expect the authority will be requesting compaction records fairly soon...

Cheers,
Mike

 

[maxim22]

Just a thought, but could the lab results be on partly crushed particles due to lab effort of a hammer in a mould and in the field then differently crushed by the plant leading to messed up correlations.

A plate test would give a stress/strain response and a maximum deflection could be obtained from a trial fill rise.

 

[Ron]

MM...be careful of these materials. They are often loaded with unreacted or partially reacted calcium and magnesium....both of which result in expansion during reactions with moisture.

Nuke gage results are not particularly reliable for these materials. Use a positive volumetric method such as a sand cone, drive sleeve or rubber balloon.

Edward Anthony has written extensively on this subject. Research it a bit.

 

[fattdad]

I think it a reach to claim, "Uniformly engineered." I'd think there'd be potential variation in the coarse-grained fraction. If this is a big project, I'd have a large handful of Proctors, I'd make a project-specific family of curves and I'd pound one-points in the field to confirm my reference density.

Breakdown is also a concern.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Mad Mike]

I really appreciate the feedback guys,

Maxim- I always assumed the variable MDD was a result of various proportions of different ash types; viz, fly ash/coal ash/bottom slag etc. I've not found anything in the literature directly addressing these density variations.

Ron- these potentially expansive components, I assume they would expand rapidly during water application and therefore show up on the laboratory CBR swell tests and during field compaction? My lab results show 0% CBR swell but this expansion is something I was completely unaware of- I will remember for next time!

Fattdad- agreed on the sort of control required here for sensitive applications, which is far more than our standard testing of layers for approval.

The ash fill is being engineered as a subgrade improvement for a rail line rehabilitation- boxing out to 5' depth and replacing the same material in engineered layers. We're doing this replacement exercise since the site investigation showed the original ash subgrade to be consistently very loose. Importantly though, the original rail lines performed well for decades despite being supported on this very loose ash subgrade and a nominal layerworks profile. The new lines will be supported on a considerable layerworks profile to conform with modern standards.

Given my perceived non-sensitivity of the application, I've been happy to confirm a general "medium dense" or better consistency using light DCP probes, in conjunction with supervised layer thickness, watering and roller passes.

I shall be taking far greater care with ash materials in the future...

All the best,
Mike

 

[fattdad]

when it's all done you can always jam a cone through it to learn more!

f-d

ípapß gordo ainÆt no madre flaca!

 

[Ron]

MM....the expansion is highly variable and often would not show in a typical short-term swell test. This is an asphalt apron at a small regional airpark. The material was used to stabilize the subgrade. It pushed the base and surface up in many locations, creating a propeller strike hazard. It was also used beneath the building as fill and pushed the two hangars floor up to the point that the doors would not close properly. We did the investigation for the airpark authority and testified in the trial, which my client won. This is just one of several I've done over the years. The expansion rate and magnitude are not predictable.

 

[fattdad]

p.s., in CCB (coal combustion byproducts) avoid at all costs any additives that contain Sulphur. Adding lime can work, some of the CCBs are pozzolonic, but any Sulphur can lead to ruin - or at least be understood.

CCBs are typically unmineralized. They are often basic and the presence of CaO (free lime) can allow for remineralization and added strength.

When Sulphur is present; however, the Sulphur will join the party, find moisture and form ettringite or other minerals of that ilk. That's a huge and expansive problem.

Then again, you may not have such issues to worry about. Here in Central Virginia (USA), we had a big flock of lawsuits over this matter, about 20 years ago.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Ron]

f-d....we did testing on the stabilized subgrade material and found ettringite. That was my first tip-off on this one. Then looked at free calcium and magnesium.

 

[Mad Mike]

Thanks for the practical demonstration Ron- pictures speak a thousand words.

I shall be very careful indeed on future projects involving ash subgrade.

All the best,
Mike