Consolidation: Cc and Cr indices - which to use? 1

[CopperGalaxy]

Hi everybody,

I'm just an undergraduate student so this will probably seem like a dumb question but I can't find anything about it on the internet or my books and my professor tends to disappear at this time of year conveniently -_-

I'm currently studying for a geotechnical engineering exam and I was wondering: when calculating the void ratio change (delta e) associated with the initial and final effective stresses, how do you know whether to use the equation containing the Cc index, or the Cr index?

I understand that the Cr index is associated with the unloading/reloading curve, and the Cc index with the normal consolidation curve - do you basically just find the consolidation ratio and if it's above 1 for t infinity, then it's overconsolidated and you use Cr? Likewise if the OCR is below 1 or equal to 1 at t infinity, the soil is underconsolidated/normally consolidated and you use the Cc equation?

Sorry if this seems elementary - i'll get there in the end!

Cheers

DJ

 

[BigH]

It is pretty basic - and is in most textbooks - but yes, you use the Cr value up to the pre-consolidation pressure and Cc afterwards - but in using these you only need eo to determine the settlement. You can play about with the e-logp curves directly - and don't need Cc and Cr. I've attached the equations for you . . . you can set the right side of both equations to each other, cancel out terms and determine change of void ratio directly in relation to Cr and Cc and the appropriate pressures. (note - if the increase in pressure is less than the pre-consolidation pressure - i.e., no virgin consolidation, you will need to adjust equation and use sigmao plus deltasignma instead of sigmac in the Cr term)

Now, think about using strain-log(p)curves or you can look at log(e)-log(p) curves - some interesting things can be seen. Then, too, there is the lab curve vs the field curve of e-log(p).

Note: I have not put in the primes on the pressures!