Elastic Settlement as a Separate Quantity in Settlement Analysis
Elastic Settlement as a Separate Quantity in Settlement Analysis
(OP)
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.
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.





RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
Consolidation is a process by which soil grains are reoriented and relocated to fill voids, thus decreasing overall soil matrix volume.
Elastic settlement is a different type of process in that the soil grains are compressed and elastically deformed....not re-oriented or otherwise moved to fill voids. Elastic settlement has little to do with a void ratio change.
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
Regardless of whether the deformation from a load results in a change in void ratio or an elastic deformation, the volume change measured in the oedometer test captures both phenomena because they both have adequate time to develop in the test. True elastic deformation, the kind I believe you are referring to, in soils is likely very minimal as the individual soil grains have very high moduli values 10^6 psi vs applied stress levels often in the 10^1 psi range.
In soil mechanics practice it is common to calculate settlement in a cohesionless soil using an elastic deformation analysis (usually rigid or flexible foundation over elastic half-space). The elastic modulus used in this analysis will vary depending on the in-situ density of the soil. This is simply an approximation of the actual soil behavior where the actual deformation is due mostly to the rearrangement of soil grains. As we know, the elastic modulus of the individual soil particles will not change whether the particles are in a loose or dense configuration. I think that the important thing for me, and maybe others, to realize is that this settlement analysis using the elastic half space and a moduli value representative of the bulk soil condition with collapsible voids is not the same as elastic soil deformation due to compression of the individual soil grains (the kind you speak of).
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
In the odometer you will also obtain the coefficient of secondary compression for clay and organic materials. Elastic theory in geotechnical practice has a different term for the same thing though. We call that, "C-sub-t," which ranges from 1.1 to 1.5 depending on whether you are looking at the creep response for 10 or 50 years. Sort of a parallel to secondary compression. However, again, we can't measure it in an odometer.
I would accept elastic theory in a clay though if I knew there was no chance of exceeding the preconsolidation pressure, the soil was not saturated, and there was some basis for the assignment of modulus. So, you have a point for one aspect of our field, but it's not universal.
f-d
ípapß gordo ainÆt no madre flaca!
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
"The short answer is that when you use consolidation theory, you also use elastic theory to calculate the stress distributions with depth. The stresses are used to position where you are on the e-log sv’ curves. An alternate (used more so outside the USA) is to represent consolidation data as “constrained modulus” (See Lambe & Whitman 1979).
Otherwise, the (same) elastic theory can be used to obtain displacement influence factors – here, the stresses attenuated with depth are represented as strains
All things equivalent, you would get same answer, either using stresses or strains.
For the case study in the Piedmont geology, the coefficient of consolidation is high (sandy silt) and the time it took to build the dorm hi-rise (12 months) was essentially all drained settlements. That is, it consolidated quickly as the building was constructed (slowly)."
I just wanted to share this with the forum as I thought it is related to the OP's discussion...In addition, I also apply what you have indicated in your last paragraph from your above reply...
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
Of course consolidation can be predicted with elastic theory if you use the correct E value. Lambe and Whitman's chapter 32.6 gives example for this. You can use drained elastic modulus to calculate total settlement and subtract the undrained settlement (elastic as we called it) which we calculated with undrained elastic modulus.
At this point, correlations like E'=0.6Eu becomes meaningless. (I think Poulos, Carter & Small, 2000?)
Why?
Because elastic settlement is around 0.1.-0.3 of the consolidation settlement. So, if consolidation settlement is x, then total settlement is 1.3x. Using elastic theory we can estimate E'=0.8Eu or something close. Difference between E' and Eu should be small since it is usually like this.
Last point: I understood what SkarpShooter meant. I didn't think it before. He says that since we load our samples in oedometer, we are probably seeing elastic settlement too. We observe both elastic and consolidaton settlement during oedometer. However, SkarpShooter, you should be aware that, we use mv or cc after initial settlement. Remember the consolidation curve, initial settlement already occured between 0 and first loading. So, based on this, I might choose not to calculate cr from 0 to first loading, since it may include elastic settlement too.
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
For normally and underconsolidated clays, you may need to calculate both, but as you now, the time that takes to consolidation settlements to happen is normally more critical.
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
Modulus derived a such may be mixing intent; however. You just want total change in height? Sure, use it! You want to know if the dissipation of pore pressures has an influence on the change in height? Modulus won't help.
f-d
ípapß gordo ainÆt no madre flaca!
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
I'd agree that such use would negate the Cc, Cr approach.
I've never used constrained modulus for settlement.
For me, modulus settlements (i.e., "Immediate settlement") occurs quickly and unlikely to affect the completed works. Follow-up deformation is what I'd more likely consider - i.e., what happens in the decades after the, "Immediate settlement" is complete. This returns to the earlier post on Ct.
f-d
ípapß gordo ainÆt no madre flaca!
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
Interestingly this is what my firm has been doing, but no one could remember why.
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
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
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
You can also get the constrained modulus with the elastic modulus and the Poisson's ratio.
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis
RE: Elastic Settlement as a Separate Quantity in Settlement Analysis