Settlement Equations 2

[haynewp]

I have a book by Das that shows the primary settlement for overconsolidated soils equation being based on Cs (swell index).

I have a book by Coduto that shows the exact same equation except it uses Cr(recompression index) instead of Cs.

Which one is correct?

 

[haynewp]

I should have shown the equations which I am referring to:

DAS: S=Cs H/(1+eo)log(pf'/po')

Coduto: S=Cr H/(1+eo)log(pf'/po')

 

[stochas]

I happen to have the same two texts, and the only differencee in the equations is in the definition of terms. What Das defines as swelling index on p40 is the same as what Cudoto defines as recompression index on p71. Both terms refer to the slope of the unloading portion of the consolidation test. In most cases, the test is loaded, unloaded, and reloaded. The slope of the unloading (swell) is the same as the slope of the reloading (recompression). Recompression index is probably the more common term. This slope is used to calculate the settlement of a soil which has been previously subjected to a greater load than the final loading of interest.

 

[haynewp]

OK

I see in the Coduto book I have, it says "that the recompression and rebound curves have nearly equal slopes, but the rebound curve is more reliable because it is less sensitive to to sample disturbances". Coduto goes on to use the recompression index specifically where Das uses the swell index.

So it doesn't matter which one you use, since they are supposed to be the same slope?

 

[BigH]

The recompression curve is sensitive to sampling/handling disturbance - on initial loading in the oedometer. But, most of us, when we want to know the recompression index, will unload just after reaching the p'c value - then on reloading the sensitivity due to sampling is negated.
I happen to like to report the recompression index and compression index as the recompression and compression ratios - in these, you can obtain from a strain-log p curve or conversely, divide the Cr, Cc by 1+e0.

 

[wez1949]

I was wondering if this could cause crackes in the sheet rock in a existing home?

 

[gandersen]

wez1949:

What is being described is a method for estimating the amount of settlement that will occur if a load is applied to an overconsolidated soil mass and the magnitude of the load does not exceed the "maximum load" that the soil had experienced in the past. It is a little more complicated than that in reality but for the purposes of discussion, the above statement will suffice.

Cracks in sheet rock are generally caused by differential movements of the foundation. These can be due to settlements or do to heaving or some combination of the two.

Settlements of overconsolidated soils are generally quite small and hence a probably not responsible for your cracking.

 

[Focht3]

[blue]BigH[/blue] gave an excellent description of the C_R issue described by Don Coduto. I would add that I don't recommend simply using the results straight from the semi-log strain-log(stress) graph because "the" C_R doesn't exist. The unload cycle is concave "upward" while the reload cycle is concave "downward." I always make an assessment of what I believe the "representative" C_R slope is, taking into account the sample, site, stress history - and engineering judgment.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[haynewp]

So you can use either one for a starting point(swell or recompression index), it's just that the recompression may be off due to disturbances?

 

[Focht3]

Well, no.

If the test is run "properly" by incorporating an unload/reload cycle as a part of the consolidation test, you don't have to worry about sample disturbance.

Choice of C_R or C_S will affect your answer, unless you choose a "representative" value that essentially represents both -

What are you analyzing?



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[haynewp]

Nothing yet, I was trying to figure out why the 2 books have different indexes listed for the same equation.

 

[BigH]

Sometimes, it is just semantics - like why do the British pronounce solder as sawlder and Yanks sawder?
Focht3 is right that there is some curvature on the rebound/reload - but sometimes, too many take it to far back in this area (viz., too much rebound). For estimating the appropriate level of rebound (then reloading), I'd look at the approximate difference in the existing effective pressure and the preconsolidation pressue - then control the rebound value to this basic difference. It might give a better (straighter) line on both rebound and reload.