The concept of effective stress proposed by Terzaghi in 1922 is a cornerstone concept in soil mechanics

and its history has been a hundred years so far. On the occasion of the centenary of the birth of effective stress

it is worth summarizing and analyzing the origin

the physical meaning

the expression

and the mechanical effect of effective stress. The summary and analysis show that Terzaghi’s effective stress equation for saturated soil has been widely recognized

whereas the specific expressions for the Bishop-type effective stress equation for unsaturated soil have not been unified. The physical meanings of the stress variables in the effective stress equations for saturated and unsaturated soils have still been explained by different ways. Through further analysis

we find that the stresses existing on the soil skeleton can be divided into three types: the first one is the stress transmitted through the soil skeleton caused by the total stress

i.e.

the effective stress; the second one is the stresses acting on the cross-section of soil particles and on the contact between soil particles induced by the pore fluid pressure

which are not transmitted through the soil skeleton; and the third one is the local stresses acting on the contact between soil particles induced by the van der Waals forces

diffuse double-layer forces

surface tension of water

and cementation forces

which are also not transmitted through the soil skeleton. Therefore

the contributions of these three types of stresses to the shear strength and volumetric strain of soils should be separately quantified.