Drained and Undrained Condition

[beebs]

Can some one please explain me in simple terms what the difference between drained and undrained condition is? And for sand and clay what conditions to use? Specially for sand since the parameters dont change a lot for the two conditions which one do you use when performing finite element analysis

 

[fattdad]

When soils shear, there is a potential change in the void ratio. If the void ratio increases, water must flow into the sample and if the void ratio decreases, water must flow out of the sample. For the case of sands (i.e., soils with high permeability) this is almost a moot point (unless there's an earthquake). For the case of clays, this can be a big deal. If the shearing takes place faster than the soil can drain, the buildup of water pressure in the pores of the soil can have a GREAT influence on the soil strength.

There's just too much engineering judgement to answer your second question. I have no idea what parameters to use when conducting a finite element analysis because I don't know what you are trying to model.

Good luck. Maybe if there was further insight to your problem we could help.

f-d

¡papá gordo ain’t no madre flaca!

 

[GeoPaveTraffic]

fattdad makes good points.

As for which to use, answer this: What kind of loading condition are you trying to model? If the condition happens quicker than the soils can drain, use undrained. If the condition happens slower, use drained. It is common to do analysese both way if the loading can happen both ways. Also, as fattdad pointed out, unless it is an earthquake it is unlikely that the sand will not be drained.

 

[beebs]

The loading condition I am working on is for a retaining wall. Soldier pile and lagging wall. What if the sand is on the exposed height of the wall or below the bottom of excavation.

Thanks

 

[fattdad]

I'd base the loading condtions on the effective friction angle of the sand and the effective unit weights. I'd use the Terzaghi and Peck envelope for braced excavations. For the interval of the soldier pile below the base of the excavations, I'd use Rankine passive earth pressures and increase the reaction using the factor "Cp", which usually is about a factor of 3, to allow for arching. Don't know whether you need tie-backs and walers, but if you do, don't forget to consider the vertical reaction at the base of the soldier pile if you install the tiebacks on any angle (which you would likely want to do.

Good luck.

f-d

¡papá gordo ain’t no madre flaca!