Normally Consolidated clays specimens

[Geoengineer67]

Hi All;

i want to carry out triaxial test and consolidation test on both normally consolidated clay specimens as well as over consolidated clays. When we compact the soil using reconstitution method (using a hydraulic jack) on releasing the load the stress on moulded soil specimen will be zero and specimen starts swelling a little bit.
The question is, can this soil specimen be considered as normally consolidated clay during triaxial test or consolidation test?

If not please give me some hints about how to prepare normally consolidated and overconsolidated specimens for triaxial test.
I know it might not be correct to say praparing normally consolidated specimens as it is highly dependent on applying loads during shear stage. I meant by this, preparing a soil specimen with compaction stress of maximum 50 kPa so when we start consolidation at 50, 100, 200, or more, we can make sure the specimen behaves as a normally consolidated clay.

Many Thanks for any helpful comment
Mehdi

 

[AussieGeoEng]

There are two procedures to produce specimens in the laboratory. Compaction or reconstition. You mention both so a little confusing? Reconstitution is preparing a sample at a moisture (well) content inexcess of the liquid limit. Then consolidating (note the difference between compaction - extrusion of air, and consolidation - extrusion of water from the voids).

The specimen can then either be isotropically or anisotropically (Ko, 1D) consolidated.

Both the above steps/methods will significantly affect the results. Selection of a procedure will depend on you end aim?

For the NC samples in the triaxial I would make sure that the effective stress at the beginning of shear is well inexcess of the previous to describe as NC.

To test OC in the triaxial, I would first load to a stress (same same as the NC sample), then unload to the desired OCR.

For the oedometer - start directly from a specimen with moisture greater than liquid limit.

 

[Geoengineer67]

Hi Dear AussieGeoEng;

Thanks indeed for your rich reply. Yes you are right i was meaning sample preparation using compaction not reconstitution.

1) Well, suppose that we prepare a soil specimen from a dry powdered clay using compaction method at a certain dry density and moisture content. Then we carry out a triaxial test with consolidation at 100 kPa. Is this soil act as an NC clay? (i would say before preparation it had not experienced any stress and the only stress it has experienced was the stress at compaction which was released to zeto after finishing compaction and extracting the sample from mould)

Due small height of consolidation ring (i.e. 20 mm) if is there any posibility to reconstitute the clay sample at twice of liquid limit in there? and moreover, how can we reach to a certain ensity and moisture content at a consolidation stress (eg. 50 kPa) in consolidation apparatus.

Sorry if i'm asking too much. I wonder if you give me some refrences on sample preparation by reconstitution method (i.e. book, article, technical note, etc.)

Many Thanks

 

[AussieGeoEng]

Is the compaction static or dynamic (e.g. Proctor)? If it is static then you can measure the load. With dynamic it depends on the amount of energy exerted - I would usually esitmate around 200kPa.

Therefore no, I would not call the sample dynamically compacted NC.

With the oedometer you can modify the testing system to suit.

What are you trying to investigate?

For sample preparation you may check:
BURLAND, J. B. 1990. On the compressibility and shear strength of natural clays. Géotechnique, 40, 329-278.
FEARON, R. E. & COOP, M. R. 2000. Reconstitution: what makes an appropriate reference material? Géotechnique, 50, 471-477.

 

[Geoengineer67]

Hi;

I use static compaction for preparing samples using a stress controlled loading jack. But the range of load i need to compress my sample is about 1.5-3 kN for a 38 mm sample which produce a stress of more than 2.5 Mpa! on sample. However, when the height of sample reaches 76 mm in the mould under whatever load (eg. 2 kN),i lock the loading machine and leave the sample to be equalised due to dissipation of negative pore water pressure in the sample. When the measured load decreased to half of maximum load at compression i remove the load and extract the sample from tube. i leave the sample for a day in a sealed container and then start triaxial test. I used the same method for consolidation test. so does this behave as an NC clay?
My research is about evaluating the consolidation behaviour of fibre reinforced clay. I want to investigate both reinforced NC
and OC clays.

How do you mean by "With the oedometer you can modify the testing system to suit"

Thanks very much for all your hints. I'm going to download the papers tomorrow from uni.