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Design Specification for Compaction of Engineered Shale Fill

Design Specification for Compaction of Engineered Shale Fill

Design Specification for Compaction of Engineered Shale Fill

I've recently been tasked with providing a compaction specification for carbonaceous shale which is to be used in the engineering of large-scale industrial cut-to-fill platforms.

The shale is a grey to dark grey micaceous material recovered as platelets; sometimes hard but non-durable - the volumes being moved are a couple of million cubic metres.

The common specification for compaction of shale is to restrict the layer thickness to 8" and use very heavy compaction plant; however there is also a general caution against the over-compaction of shale fill, which may heave in response.

Question is, when I'm dealing with the construction of high fill embankments, where I'm worried that under-compaction of the shale could lead to excessive settlements, should I be worried about over-compaction and heave of the fills? My intention is to specify a layer compaction of 95% Mod. AASHTO in 8" lifts.

I'm aware in principle that there may be some heaving of the upper fill material; which I would expect to be less significant than the settlement in a high fill embankment (30 to 50 feet fill thickness). However in areas where the fill is relatively thin, say 10 to 20 feet, I'm concerned about heave if the shales are over-compacted. There is no frost in my area.

How does one design around this, short of zoning the fills with different compaction specifications for each zone?

I'd greatly appreciate any direction or case histories.


RE: Design Specification for Compaction of Engineered Shale Fill

I suspect this shale heaving may be the taking on of water effect. If that is the case, 95%compaction is too high. It would be best to do some checking on this shale as used for compacted fills and what is the long term experience there. You don't want to be the engineer developing a compacted fill project only to find the heaving gets away from you and all sorts of damage can result. I've sen this when over zealous new engineers have insisted on high compaction and the resulting expansion damage is significant. If no such experience can be located, I'd shoot for something liik 90% and note to developers of the special characteristics of the fill.

RE: Design Specification for Compaction of Engineered Shale Fill

to me the question is whether you are concerned about settlement, shear strength, or both (i.e., equally). I write to discuss shear strength.

The long-term performance of shale fill is problematic. The strength degrades over time. You may have great case histories from jobs that are 10 years old and if your design life is 15 years, you may be just fine.

In highway work, we shoot for a 75 year design life. In that regard, the strength of shale fill needs to consider such long-term performance. To evaluate the long-term strength, we'd want to see a normally-consolidated direct shear test on fully disaggregated shale fill. We'd pulverize the shale, we'd bring it to the liquid limit, we'd normally consolidate the shale in the odometer and we'd shear it at (typically) 3 normal loads. That would return some, "peak" strength and some, "residual" strength. That peak is actually the fully-softened shear strength. The residual strength would be as customary.

Good luck in your shale fill project. Initial compaction will do only so much in addressing the long-term strength.


ípapß gordo ainÆt no madre flaca!

RE: Design Specification for Compaction of Engineered Shale Fill

Better yet do some testing. Atterburg limits for sure. However, take samples to a laboratory that can do electron microscope identification of the clay minerals. If Montmorillonite, very serious. However, Illite also can be bad. These minerals with clay platelets expand when they take water into the plate combinations. Doing some detailed lab testing will help resolve any questions.

RE: Design Specification for Compaction of Engineered Shale Fill

I cant offer much in regard to compaction of shale as i have never used the material, however with the volume and size of earthworks that you are doing, a few test fill areas using different compaction lift thicknesses, number of passes, different plant etc. would be justifiable to develop an economic spec that achieves your goal.

This may not address the long term issues as f-d has recommended.

RE: Design Specification for Compaction of Engineered Shale Fill

I really appreciate all of your answers.

The shale in question is ubiquitous in my area - we have tested it ad nauseam to determine grading, compacted CBR strength, shear properties. The clays deriving from the shale are well known for being expansive, probably illite for the most part. The shale slakes rapidly during construction.

I've accounted for the poor durability of the shale as reiterated by fattdad; I've flattened the fill embankments to accommodate its shoddy performance in the long-term.

OG, thanks so much for the comments on heave and my recommended compaction- gives me the exact direction I need moving forward.

Last question which arises from it - if one compacts the shale to 90% as opposed to 95%, there must in theory be additional long-term settlement arising from this lower compaction. My only concern is that settlement then becomes a bigger problem than heave! Looking at isolated test results isn't helping me much - I know the shale swells, and I also know it settles significantly in engineered high fill embankments.

What I am going to do to try and mitigate heave beneath new surface beds, is to design the upper 5' of the platform as a selected, inert granular fill material. We've got limited amounts of dolerite and sandstone which I would incorporate here; may end up being closer to 3' but hopefully it helps.

Thank you so much for the guidance.

RE: Design Specification for Compaction of Engineered Shale Fill

Fattdad, your method of pulverizing the shale before running your direct shear test; is this a recognized method for testing the long-term shear strengths of degradable shale?

It's brilliant; I'm going to run a few tests like that to support my detailed design analyses- I'm hoping its a recognized method so I don't have to list Fattdad as a reference!


RE: Design Specification for Compaction of Engineered Shale Fill

Some good recommendations as always from OG and f-d. Can i ask you guys opinion on my Foundation Engineering thread re large mat settlements. I value and respect your opinions!!

RE: Design Specification for Compaction of Engineered Shale Fill


I think I'm citing a USACE (Corps of Engineers) procedure. I'll check for citation.


ípapß gordo ainÆt no madre flaca!

RE: Design Specification for Compaction of Engineered Shale Fill

Interesting problem.

One option would be to use a higher compaction standard in lower portion of fills and a lighter standard in the upper portion. The theory would be to worry about settlement where the over burden stress is greater that the swell pressure and to worry about swell otherwise.

The fill plan could be something like: Compact to 95% MDD when more than 15 feet of fill is remaining and to 90% MDD otherwise. You should also specify a maximum allowable compaction, not just a minimum. Just an example since we don't have the swell pressure or the compacted moist unit weight.

A couple of additional thoughts. When you think about the compacted unit weight, make sure to use something between the specified minimum and maximum since the contractor will likely not compact just to the minimum. Also you could also consider adding a layer of non-shale (non-shrink/swell) material at the top of the fill to reduce the potential for heave. If you go this route, my preference has always been to use lean clay instead of crushed rock. I always thought the crushed rock just provided a pathway for moisture to get the shrink/swell susceptible material.

Good luck.

Mike Lambert

RE: Design Specification for Compaction of Engineered Shale Fill

Mikes idea sounds good. Any settlement in the deeper fill will not result in localized settlement at the surface and thus can be tolerated if even noticed. Expansion of shallow fill still is possible, especially with 95% there. Again what has the experience been with that 95%?

RE: Design Specification for Compaction of Engineered Shale Fill

Thanks so much for all your input - I've got all the information I need to make an informed decision.

Mike Lambert; that sort of zoning of compaction is exactly what I was leaning towards. It is well documented that shale embankments perform better, stability-wise, with a higher compactive effort, which is why I'm very wary of specifying a lower compaction through the entire profile; I want the material heavily compacted, particularly in the fill shoulders.

I will get all the lab data together and probably specify a high minimum compaction (say 95% Mod AASHTO) through the lower fill, transitioning to a lower compaction range (say 90% min, 93% max) in the upper fill. I will then seal the shale fill off with a good capping layer and incorporate subsoil drains around the platform perimeter.

All the best, and thanks again.

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