1) What are the two sources of soil shear strength?
2) Why can't a Mohr circle ever cross the Mohr-Coulomb failure envelope? What happens once a  point on the circle touches the envelope? How does the point of tangency where the Mohr circle touches the failure envelope relates to the orientation of the failure plane in the soil?
3) What are some advantages and disadvantages of the direct shear test?
4) What does it mean for a shear test to be a "drained" shear test? How do you think this would  differ from an "undrained" shear test?