Mohr's Circle Undrained Triaxial Test

[jpeel73]

Hi, could somebody tell me why, when you test a sample with three cell pressures of 50, 100 and 200 using a undrained triaxial test, then plot them onto the Mohr's Circle you end up with the first circle smaller than the others, I know it is something to do with the cell pressures, but thats about it. I have tried to research it in text books but have came up against a wall.
Thanks John

 

[iandig]

The basic principal of the test is that where a soil has an internal angle of friction, with increasing confining pressures the stress required to cause the soil to fail will also increase.
The difficulty in understanding the results of laboratory analysis is that the technician has to interpret where the soil begins to fail, and when this begins he/she then has to increase the confining pressure of the test. The real flaw in the test is that the conception of a fully cohesive soil is that with any incease in confining stress, there is no increase in shear strength, as a fully cohesive soil in undrained conditions has no angle of internal friction. The failure in this test method is that where a lab undertake a multi-stage test, they will almost always increase the confining pressue before the soil has fully failed and thereby sugest an angle of internal friction which is unrepresentative of the soil.
Solutions to this are to either do a series of samples from the same depth with different confining pressures (i.e. 38mm diameter samples taken from a 100mm diameter sample) or multiple 100mm diameter samples from adjacent boreholes. Or, understand the limitations of the test and use single stage triaxials for undraned shear strength.

 

[BigH]

I'd suggest the OP indicate as to whether he is doing the test with pwp measurements or not. This would help. Soils consolidated to different cell pressures (full consolidation - or near) will show a different "Su" value for quick triaxials - like why undrained shear strength increases with depth . . .

 

[gongxu1492]

If you draw different stages of UU tests using box and stress, if can find that at low confining pressure, the sample is not saturated, so the pore pressure B is less than 1. I think that's the problem.

 

[YannisChasiotis]

Look better, it's in all good textbooks!!!

Anyway, I agree with gongxu1492 for UU and obviously with BigH for CU...Didn't quite understand iandig...