Overconsolidated or underconsolidated clay determination

[Shakta]

I have a consolidation result (Sample depth 26’, groundwater at 3’from existing grade) that shows the soil is overconsolidated with Pc=7.5 ksf. The test was performed as loading 0.1, .25, .5, 1, 2, 4, 1, .25, .5, 1, 2, 4, 8, 16, 4, 1, .25 ksf. Lab test shows soil type is CL, natural moisture 81%, PL=32, LL=91. It’s in coastal area. Seeking advise if the soil is really overconsolidated? The reason of doubt is 1) MC is very close to LL than PL, 2) in consolidation test the rebound pressure (4 ksf) already exceeded the existing overburden pressure say about 2 ksf (not sure if it has any effect on determining the preconsolidation pressure), 3) chance of disturbance of test sample

Thanks in

 

[BigH]

Would best to present the plot so we can look at it rather than giving the loading sequence.

Do you have the undrained shear strength of the sample (i.e., same tube). Could the clay be somewhat cemented? Also, can you give us a general location . . . i.e, coastal in Indonesia, West Bengal, Sind, Pakisan, Africa? Do you have any experience of the area from previous investigations?

 

[Shakta]

BigH
I don't have any previous experience at the location. The area is east coast of USA
Here is Triax and consol. These are from same tube. Thanks

 

[GeoEnvGuy]

I looked at the test results and I find it odd the dry density is 100 in your triaxial then 60 in the consolidation test, your void ratios are also way off. Given your atterberg results you are in a CH high plastic clay with 81% moisture content, you can figure out what your density should be to see which test result is not representative.

 

[oldestguy]

Without further info, these results look like the site is a sand and relatively firm.

 

[geomane]

How did you get an overburden pressure of 4ksf? If sample depth is at 26 feet, that means the unit weight of soil would be about 150pcf, which is not realistic.

The curve doesn’t look like the sample was disturbed. If your overburden pressure and preconsolidation pressure were correct, you would have an OCR of less than 2, implying only a slightly over consolidated soil.

If the triax was performed on a sample from the same tube, why are the confining pressures so low (low assuming you’re overburden pressure of 4ksf was correct)? And the initial water content of the triax specimens states 14%. Looking at the triax data, that is not the same material as was tested in the consolidation test (dry density, void ratio, initial moisture content are all different).

 

[Shakta]

Thank you everyone for your inputs.
geomane, GeoEnvGuy: the tests were done on same ud however the matarial is not same. Triax was performed on the sandy side of UD and consol on the clay side.

Geomane, in case of using overburden/rebound pressure say 2 ksf instead of 4 ksf on consol, what difference the consol result may show? Would the curve change significantly?

 

[SoilRocks]

The material appears to obviously be a soft clay - very soft (or some other collapsible soil). I drew a quick field curve, and calculated (roughly) a modified consolidation ratio (strain based) of 0.70! This could be mud in a marsh or soft marine deposit? Organics? The consolidation curve itself looks good, showing very little disturbance (i.e. good sampling techniques). Why run your rebound almost right at Pp? That's not good - it can make interpretation very difficult. Why does the lab data say 4 ksf overburden? That seems like a mistake with the high groundwater. Based on all the material to depth being mud, the high groundwater, and the Pp of the curve, it does suggest a pretty significant OCR for the material. However, it is so soft, that doesn't seem to jive. If you had run triaxial testing on the mud, then we could see if there is some cementation (sensitive soil - collapsible) that would show the soil fabric break down (collapse) at high stress. Can you tell us the depositional environment?