Cell Pressure in Triaxial 4

[Jalthi]

I am a naive in triaxial test and looking for your assistance. How do we select the cell pressure (or ?3) for a UU triaxial test? For example, for a clay sample at 10 feet deep (?sat = 125 pcf) and groundwater table at 6 feet:

?v = 10 * 125 = 1,250 psf
?v' = 1,250 – 4 * 62.4 = 1,000 psf
?h' = 1,000 * 0.4 (k0) = 400 psf
?h = 400 + 4* 62.4 = 650 psf

What would be the cell pressure in this case if I am planning a single point test? If I am running a 3-stage test, what are the appropriate cell pressures? Thank you.

 

[Ron]

Use the in situ confining stress as the confining pressure.

 

[Jalthi]

Ron, you have re-stated my question. What is the confining pressure in the above example?

 

[BigH]

I suggest that you find a copy of Bishops book on Triaxial testing. As Ron stated, you should use 400 psf (as you are using ko as 0.4. Typically it is 0.5. I would consolidate the sample under the vertical and horizontal effective stresses before running the test.

 

[YannisChasiotis]

For UU use total stress, for CU use effective...

Considering the way k0 is calculated (it is not, it is estimated and could well be 1.0-1.5 for a heavily overconsolidated clay) I would use:

For lightly overconsolidated soil:
UU: ?3=650
CU: ?'3=400

For heavily overconsolidated soil:
UU: ?3=1250
CU: ?'3=1000


If you are going to run 3-stage tests, arrange your ?3 or ?'3 to include these values...

Do look at the Bishop & Henkel book...fascinating