drained and confined test 1

[ketanco]

in a question i am solving about triaxial test, it says:

"In a drained triaxial test, a specimen is confined under pressure and axially loaded until failure."

but drained means, we let the specimen to drain, which means it must be unconfined isnt it? why do they say confined? what am i missing here?

 

[fattdad]

Let's say you have a soil at the depth of 40 ft and the water table is at 10 ft. Let's say you recover a sample at this 40 ft depth and you want to know how it'll respond to newly imposed stresses. If the unit weight of the soil is 122.4 pounds per cubic the vertical stress at the depth of 40 ft is 3,024 psf (21 psi). The confinemenet stress may be half that value. It's often the case that the newly imposed stresses could increase the confinement as well as the shearing force. Let's say you are building a sloped embankment, so the confinement stress may increase by 5 psi.

To run a CD test you'd then take the sample, put it into the triaxial cell, saturate the sample and impose a confinement stress - 10 psi, 15 psi, 20 psi? You get to pick. After the confinment stress is imposed now you impose shearing. Going back to the principal of critical void ratio, the sample will either expand or contract. With such volumetric change there will be an effect on the pore-water pressure, which will alter the effective stress in the sample. In a drained test, you have to impose sharing slowly enough that the excess pore pressure is attenuated at the exact same rate as the shearing. Well, I guess it could run slower. . .

Most labs do consolidated UNDRAINED triaxial compression tests where the excess pore pressure is measured and added or subtracted from the confinment and axial stress to get the effective stress. CD tests take some time to run!

f-d

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