Major Shrinkage?

[gtd]

Has anyone out there seen shrinkage in compacted soil in excess of 40%? We just built a consolidation pile by excavating approximately 50,000 bank cys of soil from a dry river basin and when we completed the compaction process and measured the pile it was only approximately 27,000 cys compacted in place (about 12 feet of fill). Could it be major settlement due to a retreating water table because of the drought? Or since we excavated soft, dry river deposits, entrained them with water and mechanically compacted them to an ave. rel. density of 98%, it cause significant shrinkage? Any thoughts or experience with this, please advise. We are all very puzzled.

 

[Focht3]

Where is your borrow source? Is it an aolian deposit?

I experienced the same thing at Bee Canyon in southern California. Good thing it was during the investigation, and not after the 300+ ft high toll road embankments were completed...



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[gtd]

No, fluvial. Hassyampa River in central AZ.

 

[Focht3]

Dig a test pit near your borrow source, then backfill it. If you have a large loss, it's due to the low density of the borrow material.

We excavated a few test pits as a part of the study for the toll road roadway embankment. Pit was 15 feet deep; when all the soil was placed back in the hole the hole was still six feet deep! The dry unit weights were low - less than 80 pcf.

We didn't pick this up in standard sampling - all the samples had "normal" densities (due to sample disturbance); but the logger noticed the borings in this area didn't "make" any cuttings -



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[kmerl]

If the test pit method refered by Focht3 is not an option, shelby tubes can be used for collecting relatively undisturbed samples for relative Density (Dr)determination. This is also good where the material is being excavated at depth and test pits are not economic.

Keithe J. Merl

 

[Riggly]

gtd
First of all I think your terminology or wording is confusing. Shrinkage is a decrease in volume of the soil due to lost of moisture and no externally applied load. You are referring to a case of compaction. I have not seen such a case, but from there theory, I think it is very possible. If the void ratio of the soil is high there can be a significant decrease in volume. loose sand could have void ratio of up to 0.8. That type of silty sand may be common on river bed since there is not much overburden on the soil and they are generally loose.
Shelby tubes are not very suitable in sandy soils, but to prove you can try to obtain an undisturbed sample of the soil and determine its void ratio. Hope that share some light on the subject.

 

[kmerl]

I apologize, you are correct shelby tubes do not work in sands, I read soft and automatically assumed fine-grained. However in sands it may be possible to use CPT testing to determine a consolidation parameters.

Here is a link that describes a SCPT:

http://www.cait.rutgers.edu/techbriefs/FHWA-NJ-2001-032-TB.pdf

Keithe J. Merl

 

[mrwgeo]

Riggly - I agree with your definition of shrinkage, but in my part of the world (Calif.) "shrinkage and bulking" are the colloquial terms for the change in volume of material from cut to fill.

 

[Focht3]

With clay soils, it's pretty easy to test for this phenomenon. (I was thinking about silts and sands, too.)

Carefully trim a few samples by hand - check their unit wet weight, estimate their moisture content. If the densities look low, trim a sample into an oedometer (consolidometer.) Apply a nominal pressure - say, 500 psf. THEN add water. If you get a big drop within the first 4 or 5 hours, you've found the problem -



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[VAD]

Hello gtd:

I am going to move away from the answers that others have provided to ask the following questions since very often some other things are involved which can be overlooked.

1.How did you obtain the quantity from the dry river basin. Was it by cross-sectioning before and after or by load counts.

2. Was there any material wastage or was material used in building any haul roads etc. If so was this quantity accounted for.

3. How was the volume of the consolidation stockpile measured. Was it by cross-sectioning the original ground and later the completed stock pile.

4. How was the computation done.

These questions are asked since very often these aspects may result in disparities. If you are satisfied that your measurements etc are defensible then shrinkage would be the other factor. Ground settlement under the stockpile would be a concern and could add to the magnitude since this cannot be readily accounted for.

Other responders have provided comments which are reasonable.