Relationship between dry, drained and undrained young's modulus 1

[awa5114]

I'm a structural engineer recently converted to the dark side (geotechnical engineering). I understand there are drained, undrained and dry conditions for soil samples. Dry conditions mean no pore water exists and all the voids are filled with air. Drained conditions mean that pore water exists but the boundary conditions allow it to dissipate freely and thus no excess pore pressure exists. Undrained conditions mean that pore water exists and the boundary does not allow it to dissipate. This causes excess pore pressure to build up and modifies the soil's mechanical properties due to the presence of water.

Most numerical software allows for these three types of analysis, often providing several subtypes for the undrained case. I would like to understand better the material parameters to input in these programs. For instance:

- what relationships exists between undrained poisson's ratio and effective poisson's ratio
- what relationships exist between undrained young's modulus and effective young's modulus
- if the above relationships exist, are they dependent on the type of soil (sand, clay) and are empirical? OR can a general relationship be derived which is agnostic to soil type? If this general relationship exists what parameters are common to all soil types that it can use?

 

[oldestguy]

Using Young's modulus may be OK for some, but using the classical methods for bearing capacity and settlement I don't use it. Also it is pretty rare that shear strength calculations govern design. Usually for footing settlement governs and then the material likely is saturated.

 

[Okiryu]

awa5114, welcome to the dark side ! ... and it is an interesting question. I am not an expert in soil mechanics but found the attached extract from Budhu's book as the answer to your question #2.

I would also like to add that undrained and drained conditions also consider how the load is applied. Rapid loading is related to undrained conditions (short term loading), "slow" loading is related to drained conditions (long term loading).

For your question #1, you will see in Budhu's book extract, that for undrained conditions v=0.5, but this is because the classic theory of soil mechanics is based on saturated soils. So, if soil is undrained, then S=100% and therefore v=0.5. For drained conditions, typically 0.3~0.35 is considered.

 

[LRJ]

The ratio of undrained/drained will depend on the strain level; at small strain they will be the same.

All of the discussed relationships will be empirical and therefore will be dependent on soil type.

 

[fattdad]

drained and undrained behavior is (typically) related to shear strength. I know of no typical geotechnical analysis that's based on drained and/or undrained soil modulus. (There is one exception that's emerging right now, and that's related to the mechanistic-empirical pavement design, where resilient modulus varies by the soil-moisture characteristic curve.

To me, soil modulus is the spring constant that is unrelated to dissipation of pore pressure. Consolidation characteristics are related to dissipation of pore pressure. So, in performing settlement calculations, if I'm below the preconsolidation pressure, I, "May" use soil modulus or I may use Cr.

Soil modulus is dependent on poisson's ratio.

Drained and undrained strength is a BIG factor in fine-grained soil strength. In the program I manage, I'd require both analyses to confirm that safety factors in slope or wall design considers both failure modes.

I'm no scholar. I'm just speaking from experience.

Good luck in your retooling from structural engineering. Geotechnical engineering seems more fun to me!

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

f-d, have you ever heard about modulus for fully softened and residual conditions? Just wondering if they have any application in geotechnical engineering...

 

[fattdad]

I am familiar with shear strength determinations for fully-softened and residual conditions. I've never heard of these same terms applied to modulus.

I'd be interested though! Doesn't yet make sense to me; however. . .

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

f-d, I asked the same question to Prof. Tim Stark and he said that research has not been done for checking modulus of FSS and residual conditions. So perhaps you are right: there may not be applications for them in geotechnical engineering.

 

[fattdad]

Say, "Hi" to Tim!

f-d

ípapß gordo ainÆt no madre flaca!