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Cell Pressure in Triaxial
4

Cell Pressure in Triaxial

Cell Pressure in Triaxial

(OP)
I am a naive in triaxial test and looking for your assistance. How do we select the cell pressure (or σ3) for a UU triaxial test? For example, for a clay sample at 10 feet deep (γsat = 125 pcf) and groundwater table at 6 feet:

σv = 10 * 125 = 1,250 psf
σv' = 1,250 – 4 * 62.4 = 1,000 psf
σh' = 1,000 * 0.4 (k0) = 400 psf
σh = 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.
 

RE: Cell Pressure in Triaxial

2
Use the in situ confining stress as the confining pressure.

RE: Cell Pressure in Triaxial

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

RE: Cell Pressure in Triaxial

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.

RE: Cell Pressure in Triaxial

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: σ3=650
CU: σ'3=400

For heavily overconsolidated soil:
UU: σ3=1250
CU: σ'3=1000


If you are going to run 3-stage tests, arrange your σ3 or σ'3 to include these values...

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

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