Immediate and Consolidation Settlement

[jjsim]

I was told that the 'mv' value from the oedometer 1-D consolidation includes the elastic, i.e. immediate settlement.

Hence, the total settlement is given by the change in stress x mv x height of soil layer. There is no need to add the immediate settlement, which is a function of the load applied and the soil modulus of deformation E, to the consolidation settlement calculated from mv.

Is this correct?

 

[fattdad]

The application of load changes the void ratio. In soils with higher permeability (i.e., sand), we don't often worry about consolidation. We consider the elastic response. You can obtain constrained modulus values from the odometer and relate those values to elastic modulus.

When using consolidation theory, Mv informs the determination of time. That's an important determination. However, to determine the extent of settlement, Mv is not used. Rather, it's the stabilized change in void ratio that occurs after sufficient time has been allowed for the dissipation of pore pressures. Such time can vary depending on whether you are below Pp or above Pp.

Once the extent of consolidation is known, we then use Cv (which is associated with Mv) to evaluate the time for such consolidation to occur. That depends on other boundary conditions, such as the length of the drainage path.

In sands (i.e., soils with higher permeability), there is also a time-related component - something often ignored. However, if you calculate 10 inches of elastic compression, you may also want to assign some measure of compression beyond the immediate response. Such post-immediate compression can add another 20 or 30 percent in the 20 or 30 years to come.

f-d

ípapß gordo ainÆt no madre flaca!

 

[L0k]

I think mv is not related with immediate settlement. Immediate settlement has on the way when you place the porous stone, consolidation cap and load hanger while the dial gauge is not install yet.

 

[Okiryu]

jjsim, No sure if this will answer your question, but I had some correspondence with Professor Paul Mayne about this. I asked him a question related to a paper he wrote for the settlement analysis for a dormitory building in Atlanta (Unexpected but foreseeable mat settlements on Piedmont residuum). You can google and download it.

My question was about a paragraph in page 7:

"....Calculations of total displacements due to drained primary consolidation are best handled by either the classical evaluation of elastic stress distributions beneath surface foundations coupled with e-logσv' data, or elasticity theory using displacement influence factors and appropriate moduli."

My question was: does this means that elastic settlements analysis can be also used in lieu of consolidation settlement analysis? Are any other special considerations when conducting an elastic settlement analysis to predict settlements due to consolidation? My understanding is that consolidation analysis considers volumetric changes due to dissipation of pore pressures, however the elasticity approach does not consider variation in volume. Furthermore, I have been considering elastic settlements in saturated soils to be zero.

Here is Prof. Mayne's answer:

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 moreso 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).

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In Prof. Mayne's paper, you can see that he calculated the settlement using the elastic theory with the constrained modulus. My interpretation is that he used this approach because the Piedmont soils consolidate rapidly. On the other hand, I think that if you have fat clays, using the consolidation coefficients (Cv as f-d mentioned above), it is a better approach. So, basically, this is governed by the permeability of the soils and boundary conditions (again, as f-d mentioned above: the time of consolidation to occur).

 

[fattdad]

within the realm of, "The Piedmont" soils, sure it'd be about the same to use consolidation theory or elastic theory.

within the realm of our Coastal Plain, no way! I'd bet Dr. Mayne recognizes the same.

f-d

ípapß gordo ainÆt no madre flaca!