taking primary vs secondary and primary consolidation when calculating settlement

[ketanco]

Hello

For civil pe preparation, On 6 minute solutions for geotech book, problem 42. why does it say it is normally consolidated and take only primary consolidation into account and not secondary consolidation as well?

it is basically a question that has sand and below the sand there is clay. you excavate some of the sand and refill and apply surcharge. it asks for the clay settlement underneath.

Thanks

 

[BigH]

I don't have the book - but typically one doesn't really consider, unless the project necessitates it, secondary settlements as they are a long time coming and are quite small compared to consolidation settlements (peats and organic clays, etc, notwithstanding). One aspect, and this may be a bit more involved than the question, is that very few soils are "normally consolidated" since, if they are not recent (and still undergoing consolidation settlement by self weight), aging has an effect of providing a bit of preconsolidation to a clay so using normal consolidation only would be a bit to more conservative.

 

[ketanco]

Hello and thanks a lot for answer. I didnt understand the part you said :

"very few soils are "normally consolidated" since, if they are not recent (and still undergoing consolidation settlement by self weight), aging has an effect of providing a bit of preconsolidation to a clay so using normal consolidation only would be a bit to more conservative."

could you explain here a little more?

 

[jayrod12]

If I understand BigH's point correctly he's basically saying that "normally consolidated" is a conservative approximation as most soils, unless they are freshly placed, are pre-consolidated and therefore stronger than their normally consolidated counterparts.