Elastic Settlement as a Separate Quantity in Settlement Analysis 3

[ScarpShooter]

I have often wondered why there is a convention to include elastic settlement as a separate value in addition to consolidation (primary & secondary) settlement. It seems to me that when you run an oedometer test you have an initial stress and void ratio and a final stress and void ratio. This should be all you need to estimate settlement? Wouldn't the addition of an extra elastic settlement amount, which I imagine is calculated extraneous to the oedometer test data, change your void ratio to something other than what is measured in your test? How can this be justified if the oedometer test fundamentally represents the relationship for a change in void ratio with a change in stress?

Part of my frustration with elastic settlement is that it seems like there is never really good input data (other than loading conditions). Why not run an oedometer test on a remolded coarse-grained (sand) sample? Wouldn't this also give you a volume change/stress relationship from which a modulus value could be calculated even though the void ratio/log time plot from the test wouldn't be worth much? Maybe even better (two birds with one stone) is to use void ratio/stress data from a series of specimens being prepared for a direct shear test (maybe unlikely that the shear test normal loads are in the same range as the consolidation loads of interest, but for arguments sake). Regardless, it seems like you could get a modulus out of that data for elastic settlement calcs?

I'm thinking that the approach of using actual lab test data as opposed to taking a conservative tabular value from some reference is superior but I just haven't seen this called out as a good idea anywhere and found myself in this forum today. Thanks in advance.

 

[geotechguy1]

But when I use the correlations available to me for 'Constrained Modulus', I end up with values of 'Constrained Modulus' that are, much lower than the values given by the correlations for 'elastic modulus' for a given soil, resulting in larger settlements.

I've started putting the terms in quotes because I'm becoming convinced that every engineer in the industry is talking about something slightly different.

Every day I work in this industry I understand more and more why contractors are always yelling 'EVERY GEOTECH SAYS A DIFFERENT THING' followed by expasperation

 

[Okiryu]

What are the correlations that you are using?

 

[geotechguy1]

We normally use 300 to 600 times the undrained shear strength. There is a paper in the ASCE database called 'soil modulus correlations' that I purchased, I also use some correlations in there.

 

[Okiryu]

I am not aware of the ASCE's paper, but it looks like that is the Duncan's correlation for elastic modulus. I believe that it depends also on PI.

You can also get the constrained modulus with the elastic modulus and the Poisson's ratio.

 

[EireChch]

CIRIA - guidance for embedded retaining wall uses a correlation for Young's modulus of 750 x Undreained shear strength. Can't remember the details exact. It I can did it out of you want

 

[geotechguy1]

The paper isn't by the ASCE I just purchased it from their database. It is 2013 paper by Duncan & Bursey titled 'Soil Modulus Correlations'.