This lecture introduces the triaxial shear test. The consolidated-drained (CD) and consolidated-undrained (CU) tests are explained and demonstrated. The drained and undrained Mohr-Coulomb failure envelopes are introduced.

Critical Questions to Consider:

1) Why would we want to use a triaxial shear test instead of a direct shear test?

2) What is the difference between contraction and dilation during shear? What does contraction do with the fluid in the void space? What about dilation? What types of soils tend to exhibit contractive behavior? Dilative behavior?

3) What are the differences between the CD and the CU triaxial tests?

4) What is the difference between the drained and the undrained failure envelope?