For a CD triaxial compression test the specimen was consolidated under an effective confining stress ?3' = 100 kPa. The stress-strain response reached the critical state when deviatoric stress became equal to 200 kPa. Calculate the critical-state friction angle ?c for the soil. What would be the angle that the failure plane is expected to make with the horizontal?
What are the values of normal and shear stress on the failure plane?