Hi all! When evaluating the earth

[lpz31415]

Hi all!
When evaluating the earth pressure forces in a wall wich is supporting a cohesive soil, we use to consider the effective parameters (effective friction angle and effective cohesion). I've always done this in this way by inertia because I was taught to to that. (consider water table at top of the wall).
Link
Thinking about that , would not be logical to consider undraided parameters to calculate the presure forces? In other words, I dont know why to use effective parameters to calculate earth pressure in a saturated fine soils.

Thank you in advance!

 

[jdonville]

Most retention systems incorporate some sort of drainage mechanism in order to reduce the magnitude of lateral force to be resisted - i.e., remove hydrostatic loads. Since the systems are largely assumed to be drained and drained strengths are usually lower than undrained strength (a little cohesion goes a long way), for retention systems designed for long-term service (usually years rather than months), drained strengths are used. Normally, I assume friction angles based on correlations with the plasticity index and zero cohesion.

This is for cut walls, obviously, since nobody in north america (where I work) should be specifying/allowing cohesive backfill - at least immediately behind the wall - for fill-type walls.

 

[PEinc]

For most walls, but not all, that I design, drained conditions control. Check it both ways and use the more critical loading.

http://www.PeirceEngineering.com

 

[fattdad]

slope stability should consider the lowest safety factor for the following:

drained
undrained
circular
non-circular

f-d

ípapß gordo ainÆt no madre flaca!

 

[lpz31415]

The problem I've to solve is a diaphragm wall, is not the same as a cantilever earth retention wall wich have drains.
In a diaphragm wall water table is always going to be at the same height, and only 1 alternative (c,phi or c',phi') should be considered.
Please I would appreciate any aclaration about this issue.

 

[MTNClimber]

Give us all the detail you possibly can. Your first post is too vague and your last post is just starting to give us some detail of your problem.

Do you have any subsurface data to share?
Is the wall permanent or temporary?
If temporary, What’s the duration that the wall will be exposed?
Will there be dewatering inside the excavation?
Ect.

 

[Okiryu]

Undrained conditions are related to no volumetric changes.

Drained conditions are related to no excess of pore water pressures.

As jdonville mentioned, since walls are always designed to have good drainage, soil drained parameters are used.

As PEinc and fattdad mentioned, you may need to check both undrained and drained conditions and use the most critical. But similar to slope stability analysis, once you start considering cohesion for undrained conditions, I would expect that your FOS may increase. So drained conditions normally controls.

For your wall, you should also add hydrostatic pressures for the entire height of the wall.

 

[lpz31415]

Hi all!
1- Subsurface data:
-- a. 1m fill
-- b. 15m low plasticity clayley soil
-- c. 5m gravels
2- The wall is permanent (is a diaphragm wall made with a clambshell and poured with a tremie pipe). It's and cut and cover structure, maximum excavation depth is -8.00m.
3- Water table is at -3.00m.

My question is because the design software ask me only for one set of parameters, (c/phi, c'/phi' effective), and most people in the office use to calculate with effective parameters, with no arguments. My question is: ¿Why not calculate with undrained parameters too? It could be determinant...

Thank you all!

 

[lpz31415]

I hope this picture let you know better my question.
For undrained condicions, most times people use to take c=qu/2 and phi=0

The reason of this post is that I would like to know why dont use undrained parameters.

Thank you all!

 

[MTNClimber]

I would agree with PEinc, fattdad, and Okiryu, you should check both drained and undrained but as PEinc drained conditions typically control. I actually don't recall undrained controlling once but it's just prudent to check, especially if you're using software that can spit out the result in seconds.

 

[Okiryu]

When soils are excavated, the retained soils will experience an increase in shear stress. This will result in excess pore water pressures and soils will be undrained temporarily. For clays, it will take some time for excess pore water pressure to dissipate but eventually they will dissipate and soils will be in drained conditions. If you are not sure if excess pore water pressures will be generated, you will need to check both undrained and drained conditions. As MTNClimber mentioned, you will be able to do this very quickly since you are using software. By the way, drainage does not mean drained conditions. However, good drainage helps to minimize excess pore water pressures so drained conditions can be used.