×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Drained and Undrained Condition

Drained and Undrained Condition

Drained and Undrained Condition

(OP)
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

RE: Drained and Undrained Condition

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!

RE: Drained and Undrained Condition

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.

RE: Drained and Undrained Condition

(OP)
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

RE: Drained and Undrained Condition

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!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources