Mohr circles and Bearing capacity

Question 1

What does shear stress equal with cohesion factor?

Answer 1

T = sigma’ * tah(phi’ peak) + C’

Question 2

Why should cohesion factors be treated with caution?

Answer 2

if peak strengths are used the failure envelope isn’t straight but slightly curved. Which can cause a C’ > 0 and a lower phi peak.
Errors in lab/field test can give false C’. for example if the clay isn’t fully drained in a drained triaxial test because of loading too fast leaving false strength.

Question 3

How to find Cu using total stress analysis?

Answer 3

Plot all of the mohr circles for the different cell pressures and draw a horizontal tangent at the tops of the circles and where this crosses the y axis is Cu.