Elastic vs Immediate

[NBRY1]

Regarding Elastic Settlements:
A lot of textbooks refer to 'elastic' settlement as 'immediate' settlement, but then throw in the caveats that it is time-independent and that no moisture change (i.e no volume change) can occur.
To me, this means it can only be an undrained condition and Eu only applies.
It appears to me that a lot of engineers assume all elastic settlements (Eu and E') are immediate; however, I disagree and would lump E' in with consolidation since there would be
volume change even in dry sand.

Am I correct? If so, seems reasonable to estimate E' from a consolidation test which I think is routinely done.

 

[r13]

IMO, it is less arguable that "immediate" settlement behaves elastically, that stress strain is linear, and deformation is recoverable. The duration of the immediate settlement is depending on soil structures and rate of compaction.

 

[Ron]

Immediate settlement can be either elastic or inelastic. It depends on loading and soil type. It can be recoverable or not.

Elastic settlement is one form of immediate settlement and can be recoverable if load is removed, but is usually not because the loads are rarely removed, particularly for a foundation. Elastic and inelastic immediate settlement can occur simultaneously.

 

[r13]

Interesting. Here are two schools of thinking. The second quote is close to the thinking of the OP and Ron. However, it is up to debate that, is unprepared soft/loose soil fitting the bill for use in general construction with defined loads?

Immediate settlement takes place in a short time (about a week) after application of load and is due to elastic distortion of the soil. As the settlement is experienced in a short time, there will not be enough time for soil mass for change in its water content. Hence, Settlement takes place under constant Volume or under un-drained conditions. Immediate settlement is computed by elastic theory. Compared to consolidation settlement, particularly in saturated clay, immediate settlement is very small and often neglected unless the structure is very important.

In the current state of practice, any soil settlement other than consolidation settlement is often called elastic settlement. It is also referred to as immediate settlement in some geotechnical reports. The question is: which one of these terminologies is more appropriate and better represents the nature of soil deformations.

Unlike steel or aluminium, soil exhibits nonlinear plastic behavior immediately after application of external loads. If you load and unload a soft/loose soil specimen and plot the load-deflection curve, the unloading curve will not intersect the deflection axis at origin , i.e., (load=0.0, deformation=0.0). To this end, use of the term “elastic settlement” seems to be inappropriate. It is recommended to use the term “immediate settlement” instead. Immediate settlement better represents elastoplastic deformation of soils which is time independent and occurs immediately after loading.

 

[NBRY1]

Seems to me that elastic settlement should be split between immediate (undrained, Eu) and primary consolidation (drained, E').
Granted, a DMT or PMT won't know the difference, just want to reconcile how theories define the settlement components; i.e. can you have consolidation settlement in dry sand without excess pwp or is this
elastic/immediate/distortion since no excess pwp?

 

[r13]

I believe that, the calculation of time dependent settlement is use total expected settlement less the immediate settlement, for both are known (can be calculated), but the time dependent settlement.