Sunday, June 23, 2013

Underconsolidated Clay. Is it possible?

Ini salah seorang teman yg nanya tentang underconsolidated clay.

The answer is is possible..There are many reasons why soil is not under normally consolidated condition.

1.Looks to me the cause of the low pre-consolidation pressures are mainly due to Sample Disturbance : 
a. Sampling and handling - unless you took the sample using sample freezing technique the disturbance due to sampling and handling is inevitable. It is almost impossible to sample soft to very soft clay without highly disturbing it. 
 b. Stress relieve - The sample experiences stress relieve when we took it from the depth, from whatever initial pressure was to zero (atmospheric pressure). Even if you took the sample using the sample freezing technique, you are still facing the disturbance due to stress relieve when the sample is thawed, which highly affects the pc' of your sample. When you did the Constant rate of consolidation (CRS) test, what you are actually measuring was the pc' when the sample was tested. Suzanne Lacasse and also others from NGI have discussed this in many of her/their papers. For example, Lunne et al -2006 - discussed this here:

 2. I do not see any problem to calculate the settlement. I do not expect to have negative settlement (I assume you meant negative settlement=heave). On the other hand, I would think that the calculated settlement tends to be higher than what may actually observed due to lower pc' than the actual pc' on the field (due to disturbance in point 1). As long as you have higher insitu stress than pc' you still get settlement (positive settlement). 

3. The fact that some of the soils are under-consolidated does not surprise me. It is real and it only tells us that the consolidation is still in process. Please remember, in addition to cv, the rate of consolidation also depends on the thickness of the soil layer and the drainage condition on site.  In your case, the drainage is only provided by the lower sandy gravelly soil, which means only 1-way drainage. That will takes much more time to consolidate than if you have both upper and lower drainage layers.  In your case, may be it is the number 1 above (sample disturbance) that has more pronounced effect on your low pc'.  

I hope this is useful. 

Cheers, Hendra  Down Under

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