Consolidated Drained Triaxial

[learning2geotech]

Hi,

In a Consolidated Drained Triaxial test, you already let all the water out of the soil specimen during the consolidation phase. So when you let the water out again during the shearing phase, where does this water come from?

 

[fattdad]

The sample is saturated. You don't let the water out, literally. You let the sample drain as it's consolidated and you let the sample drain during shear.

f-d

ípapß gordo ainÆt no madre flaca!

 

[learning2geotech]

but isn't the point of consolidation phase is to remove all the water inside the soil? So after the consolidation phase, there should be zero pore pressure in the soil right?

 

[GeoEnvGuy]

To remove all the water from a soil you would need to compress it to a void ratio of zero which will bring you back to the specific gravity of the original rock, this does not happen during CD tests. Consolidation reduces the void ratio but the water remains at the lower void ratio. You may want to review soil mechanics textbooks to learn more.

 

[learning2geotech]

thank you!

 

[EireChch]

Further to the above good replies, consolidated drained (CD) test takes the longest time because you are shearing the sample very slowly to avoid the build up of pore pressure. Thats what consolidation is essentially , the dissipation of pore pressure. Instead of thinking about it as letting water out, think of it as letting (pore) pressure out.

 

[Ghaith MS]

During Consolidation stage, you are applying a confining pressure which will generate equal amount of excess pore water pressure in the sample. So, the consolidation process will dissipate only this amount of pore pressure. During the shearing stage, additional pore pressure will be generated; and here comes the role of the "Drain" ability to discharge this additional pressure.

 

[fattdad]

Not sure about Ghaith MS's point. CD tests do not allow the development of ANY excess pore pressure. So, during the shearing stage, there is not additional pore pressure being generated! CU-bar tests do work this way and you'd need a pore pressure monitor to know what's actually going on!

f-d

ípapß gordo ainÆt no madre flaca!