tough question, effective stress

[Ge0dude]

T= 0 years , in short term conditions, Total Stress Analysis is required, undrained parameter are used, Cu (cohesion) T = 150 years , n the long term conditions, Effective stress analysis is required, drained parameters are used, angle of shearing resistance (phi, friction)

The reason undrained parameters i.e cohesive properties of the soil is used in the short term conditions is because the effective shearing resistance is reduces with the increase of PWP (i.e decrease in effective stress). And as effective stress increases over time due to PWP dissipating this in turn increasing the angle of shearing resistance of the soil. So at the moment it is rather binary, short term use Cu assume phi=0, long term assume c=0 and phi=23 degrees (or whatever).

my question is at what percentage of PWP dissipation does effective stress analysis, drained conditions become valid. In temporary works design if the foundation system is required to stay over 2 months, or 4 months or 9 months, at what point does drained parameters come into play. is it a function of percentage of dissipation of excess pwp.

 

[EireChch]

Pick a copy of Duncan and Wright - Soil strength and slope stability and read chapter 3. It is an excellent book and a must have for all goetechs IMO.

Yes its a function of pore pressure dissipation. D&W more or less say, if you have a CLAY, your are looking at years or tens of years to reach T99 (where 99% of pore pressure has dissipated) which would be drained conditions. So for your temporary works discussion you would be looking at an undrained analysis.

If you have a silt it is much harder to determine when it becomes drained or undrained, in this instance you should do both drained and undrained and run with the worse case.

Also, you should be aware of what condition is actually your worst case drained or undrained. Using the assessment of drainage time then you can rule one case out depending on your life span.

 

[PEinc]

It is a good idea to design for drained and undrained conditions. You never know when a soil will get "tired" and decide to lose its cohesion.

http://www.PeirceEngineering.com

 

[GeoEnvGuy]

Great response from EireChch the only thing I would add is you can look at CPT pore pressure dissipation tests and run the not so popular Consolidated Drained Test of your sample and look at the rate of shearing and time increments required for the test.