This lecture introduces the basic concepts of shear strength of soil. Soil friction is described, and the friction angle is derived. The Mohr-Coulomb failure criteria are introduced, which state that soil shear strength is a combination of cohesion and friction. The direct shear test is introduced and explained. An an example calculation with the direct shear test is performed to develop a Mohr-Coulomb failure envelope.

Learning Objectives:
1) Derive and/or explain what the the friction angle is in the soil.
2) Derive the Mohr-Coulomb equation for shear stress in the soil, and explain when failure occurs in the soil.
3) Explain the operation of the direct shear test and list some of its advantages and/or disadvantages.
4) Using direct shear test results, compute the friction angle of a sand.

Critical Questions to Consider:

1) What are the two sources to 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 relate 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, which we will talk about in the next lecture or two?