OCR less than 1 2

[AK92]

I have seen quite a bit of samples with overconsolidation ratio (OCR) less than 1 (from oedometer tests). Some have told me that it is a testing artefact due to disturbance of the soil, they say technically OCR cannot be less than 1. Some others say that this is due to the fact that the samples are still undergoing consolidation, so the P'c hasn't reached the final P'c at 100% consolidation. What do you think?

 

[DougHole]

If it is close to 1, it is probably "normally consolidated".it doesn;t have to be bang on 1 to be "normally consolidated"

 

[jgailla]

It is common in the coastal southeast for soils to be underconsolidated due to groundwater lowering in newly developed areas.

 

[BigH]

Underconsolidation is real. Perhaps a large areal fill had been placed years before over a thick soft clay layer and the deposit hadn't yet fully consolidated.

 

[AK92]

@BigH and @jgailla

Two of the sites where I am working on with OCR < 1, one was in an area where there was regional groundwater dewatering, one had large areal fills placed recently on top of soft clays. So in my opinion these samples were under onsolidated, but there were quite many seniors who disagree, saying that it is impossible that OCR can be less than 1. I think many textbooks don't deal with OCR < 1 too.

 

[BigH]

They would deal with it had they added the fill themselves and then watched the consolidation process happen . . .

 

[Ron]

As both jgailla and BigH noted, underconsolidation is a fact. The degree of it depends on the soil type and conditions leading to the underconsolidation. Common in alluvial/coastal plains soils and notorious in "hardpan" and some other "cementing" or bridging soils.

They would deal with it had they added the fill themselves and then watched the consolidation process happen . . .

...absolutely

 

[fattdad]

Install a piezometer and see for yourself.

f-d

¡papá gordo ain’t no madre flaca!

 

[AK92]

Thanks all for the help, I guess I'm sticking to my viewpoint then, that OCR < 1 is not impossible and it shows underconsolidation.

Just to take this a bit further:

Suppose a soil element had an original effective stress of 100kPa, and a fill of 60kPa was added to it. The current P'c is 130kPa. Can I reasonably assume that the degree of consolidation = 50% based on (130-100)/(160-100)?

 

[BigH]

You are missing the point - the degree of consolidation has to do the the percent of consolidation not a ration of pressures - i.e., (settlement to date after load application)/(total expected consolidation settlement). This would take so many months/years to occur. Say that the total expected settlement is 100 mm. 50% consolidation would mean that 50 mm has already happened. Now there are those who say, and it is reasonably true, that certain portions of the materials - due to the shape of the pore pressure dissipation have fully consolidated in a very short period of time and would then be undergoing secondary consolidation while the bulk of the layer would still be going through primary consolidation.

 

[AK92]

@BigH

I agree that the degree of consolidation, U(%) is defined based on settlement. The reason I'm asking this is that I may have to backcalculate remaining consolidation settlements from underconsolidated soft clays, and I believe the excess pore pressures can give me quite a good idea of how much remaining settlement there is.