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Primary consolidation vs Secondary Consolidation
4

Primary consolidation vs Secondary Consolidation

Primary consolidation vs Secondary Consolidation

(OP)
Gents,

Is there any relationship or rule of thumb between primary consolidation vs secondary consolidation?. Such as Secondary should be ~20% of primary?. I know that the design life would affect somewhat the secondary consolidation.

I am asking this question because I am doing some consolidation analyses, but noticed that my primary consolidation values are around 0.5 to 0.9 in, whereas secondary consolidation values are around 2 and 3 in (design life 30 years). I am not sure about this. I thought they are supposed to be lower that primary consolidation.

Please let me know,

thanks.

RE: Primary consolidation vs Secondary Consolidation

2
I wouldn't think so. I'll also be pedantic and claim my preference not to use the term, "Consolidation" when speaking of secondary compression. I reserve consolidation to the behavior that's coupled to permeability. Secondary compression is settlement that occurs when the excessive pore pressures have dissipated.

f-d

ípapß gordo ainÆt no madre flaca!

RE: Primary consolidation vs Secondary Consolidation

There are relationships between the creep index Cα and the compression index Cc: typically just ratios of Cα/Cc for different soils. However, this only relates a rate of compression and the two phenomena are different as fattdad states. Moreover, you won't get a 'typical' ratio of the settlement components since the timescale and causes for each are different.

I agree that your secondary compression appears relatively high, but it is over a period of 30 years. Supposing you've undertaken a traditional approach to settlement analysis, in which the time to the end of primary consolidation is controlled by the cv parameter, perhaps adjusting that cv parameter will delay the onset of secondary compression, and you'd get more sensible answers. This might not be conservative, of course.

Generally I think all creep models are not quite right. An isotach-type approach seems more theoretically correct to me, but you can still get very high predictions of secondary compression, which doesn't seem right. It's likely because factors which counteract creep (i.e. swelling) are no included in those models, which is why they don't tend to reflect reality.

RE: Primary consolidation vs Secondary Consolidation

(OP)
Thanks!.

RE: Primary consolidation vs Secondary Consolidation

Re "Secondary compression is settlement that occurs when the excessive pore pressures have dissipated." This is only Hypothesis A, another one is "secondary compression also occurs during primary consolidation".

RE: Primary consolidation vs Secondary Consolidation

(didn't want to go there!)

f-d

ípapß gordo ainÆt no madre flaca!

RE: Primary consolidation vs Secondary Consolidation

For a given layer at a given depth, primary consolidation is approximately proportional to the increase in pressure above the preconsolidation pressure. Secondary compression is a function of time after primary consolidation ends; it does not increase with load. So if you double the load added after virgin consolidation begins, primary consolidation settlement doubles, but secondary compression settlement stays constant. Therefore there cannot be a unique ratio between the two for a given type of soil.

RE: Primary consolidation vs Secondary Consolidation

Primary consolidation also occurs prior to preconsolidation pressure; it's just the stress-strain gradient that changes after reaching the preconsolidation pressure. While a conventional text book approach would suggest there is no load magnitude effect in secondary consolidation, there clearly is. This is one of many reasons why the 'conventional' approach to calculating secondary consolidation is flawed.

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