This lecture introduces the principal of effective stress, which is arguably the most important principle in geotechnical engineering. Effective stress is derived, and a few examples showing the differences between total stress and effective stress are performed. The effect of groundwater seepage on effective stress is discussed.

Critical Questions to Consider:
1) How is effective stress different than total stress?
2) What is the critical assumption made regarding the area of the particle-to-particle contacts in deriving effective stress?
3) Why doesn't effective stress change when the elevation of surface water is adjusted? What would happen to effective stress if the elevation of groundwater increased? Decreased?
4) How does upward seepage affect effective stress? Why? What about downward seepage?