Effective Stress

Effective stress (σ′) is the portion of the total stress in a saturated soil that is actually carried by the soil grain skeleton, given by Terzaghi's principle σ′ = σ − u, where σ is the total stress and u the pore water pressure. It is the single most important concept in soil mechanics because soil strength, stiffness and volume change are governed by effective stress, not total stress — shear strength, consolidation, bearing capacity and slope stability are all effective-stress phenomena.

The practical consequences are everywhere: raising the water table or generating excess pore pressure (rapid loading, pile driving, earthquakes) reduces effective stress and hence strength — the mechanism behind liquefaction, excavation base heave/'boiling', and the difference between drained and undrained behaviour. Dewatering, surcharge/preloading and consolidation all work by changing effective stress. IS 2720 soil testing and IS 1893/IS geotechnical practice are framed so that strength and settlement are assessed on the correct effective-stress basis (drained vs. undrained, short vs. long term).