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Clay stockpile design

Clay stockpile design

(OP)
I was assigned to design a clay stockpile.

In natural bank clay material, according lab tests, is classified as soft clay (CL), c value of around 200 psf. SPt-N values of around 5 blows per foot according to some borings.

We plan to stockpile this clay material for further works on a earth dam as will be used as the core material or low permeability filter.

For this new clay stockpile from the geotechnical point of view, I am supposed to provide slope and bench dimensions. The soft clay will be compacted appropiately and will be placed on 2 ft lifts.

My question is:

By having compacted this soft clay (using modified proctor and moisture content) in the new stockpile, for sure SPT-N values (if i drill some boreholes) will increase and if I get new samples (remolded samples) I might get a higher cohesion value.

Is it ok if it base my slope design having these new properties (remolded properties) on this clay material?

I am not sure aboutb this....although, those new properties (remolded properties) are the actual ones on site.


Please let me know.

RE: Clay stockpile design

The undrained strength of the stockpile will depend on the amount and type of compaction. Two foot lifts are quite thick and will result in relatively low undrained strengths.

You need to determine, in the lab or using a test pad, what the undrained strength of the material will be based on the compaction/placment specification that will be used.

You can then use this strength to start the design of the stockpile.

Depending on how long the stockpile will be around, you may also need to determine the drained strength of the clay. Note that the drained strength of clays is not very dependent on the compaction.

If this material is being used in a dam, I expect there is a lot of strength data on recompacted samples.

Mike Lambert

RE: Clay stockpile design

draw a horizontal line. There's some stockpile above that line and there is some soft clay below that line.

If we just take cohesion as undrained shear strength (Su=200 psf) your ultimate bearing capacity would only be 1,100 psf. So, after piling the stockpile to a height of 7 or 8 ft, there'd be a subgrade failure. In other words, that's the first (and big) concern. Then you need to consider the slope angle of the stockpile itself. You'd be lucky to do better than 4:1 considering 2 ft lifts, high moisture content (i.e., soft) and insufficient time for moisture conditioning (i.e., to allow for design compaction).

Someone will benefit by better defining the subgrade strength and modulus profile of the foundation soil. Also a couple of tubes (consolidation and CU-bar) and better classification testing would be nice. From a bulk sample you could do a Proctor to assess natural moisture content against optimum. A CU-bar of a compacted sample would also be good, but if you want to allow for lower compaction, run the CU-bar at 88 percent relative compaction (or 90% for example). Nobody is going to stockpile at 95 percent compaction!

Just some thoughts. . .

f-d

¡papá gordo ain’t no madre flaca!

RE: Clay stockpile design

N value of 5, to me, seems a bit high for an undrained shear strength of the base material of 200 psf (10 kPa). But basically, the stockpile would support a "load" of about 1200 psf (60 kPa) or about 7 to 9 ft (2.5 to 3 m) before you would have loss of basal support. You didn't indicate how high the stockpile was to be. As pointed out, you don't have a handle on the undrained shear strength of the compacted clay - which is needed.

If one is going to all the trouble in stockpiling and compacting a clay that will later be removed and reused, then several options should/could be considered. The use of wick drains to more quickly consolidate the clay under the loads - especially at the toe and inwards to some metres behind the crest at the highest level would be advantageous. In India, building a reinforced earth approach to a bridge over a soft clay (6 m or about 20ft and Su of 400 psf (20 kPa)) we were only able to build with the use of wick drains an 11 m (35ft) high retaining wall in stages (about 55 days per stage) without problem. You could thus build, say to 2.5 m, stage it for a few months, then continue with next stage. If your area is large enough, you can do in a perimeter fashion and perhaps have very little waiting time between stages on completion of one perimeter.

On the soft clay you have, you might wish to google the New Liskeard Test Fill carried out in Canada on very soft clay. There are a number of papers in the Canadian Geotechnical Journal related to this test fill.
See, for instance:
http://www.civil.utm.my/file/file/MJCE/2005/Stabil...

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