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# Long Term settlement or "creep" in Sand

## Long Term settlement or "creep" in Sand

(OP)
Hi,

Does anyone have any references for long term settlement in sand. Specifically I am looking to see if there are any arguments that say it can be limited to 5 or 10 years in sand as opposed to the lifetime of the structure.

Schmertmann includes a C1 factor to account for long term settlement. Burland and Burbridge (1985) also include a long term settlement factor. Both of those methods do not include any reference to limiting the time in years to 5 or 10 years for example?

I am hoping this reference doesnt exist.

### RE: Long Term settlement or "creep" in Sand

Duncan and Buchignani published, "An Engineering Manual for Settlement Studies" (Virginia Tech, CGPR Publication No. 2) that provides an equation for the time rate of settlement in sand. In that equation, they provide values of Ct. Ct is the value to scale the calculation of immediate settlement for time. So, if you calculate 5 inches of immediate settlement, you'd then consider how much could develop after time. In 10 years, they'd have the value of 1.4 times that 5 inches which would then suggest a new total of 7 inches or 2 additional inches after project delivery.

1 month, Ct=1.0
4 months, Ct=1.1
1 year, Ct=1.2
3 years, Ct=1.3
10 years, Ct=1.4
30 years, Ct=1.5

Sorry, it exists!

f-d

ípapß gordo ainÆt no madre flaca!

### RE: Long Term settlement or "creep" in Sand

(OP)
Cheers f-d.

I probably didnt explain well enough what I meant. I am aware of the time factor, however I am hoping that a reference does not exist that says " in sand, you must limit the time factor to 5/10 years ". Which would imply that long term settlement occurs within the 5-10 years after construction.

### RE: Long Term settlement or "creep" in Sand

Given the nominal settlement that the scale relates to the 10 to 30 year period...would it make a difference?

Half the problem in limiting the time factor with sands, lies in the definition of "sands". If I had some silt and clay in my sands, I would gladly apply this time scale to the full 30 year period. If it's undeniably a clean, well graded sand, I'd be more inclined to use the scale up to a 3 year period post-construction.

Not saying I'm right, it just seems to make sense. I doubt there are many cases where it will make or break your project.

All the best,
Mike

### RE: Long Term settlement or "creep" in Sand

(OP)
Mike,

True, i agree with you. Its a slight difference. However, for foundations that are marginal (i.e. 20mm settlement) it has an impact.

I accept that settlement calculations are an educated guess at times however we have to tell our client if the calculations are within the settlement tolerance or not. Adopting the rule of half to double the calculated settlement will not fly.

I also agree that the gut feel is that long term settlement in sands is less in clean sands etc but I am not sure if limiting it to 3yrs is the right thing to do? Schmertman, Burland and Burbridge, Duncan etc. have never stated something to this effect (that i have read) hence why I am asking to know if there are any references that state long term effects should be limited to 3 years (or 5 years).

Thanks for the replies.

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