Why is effective stress, rather than total stress, considered the fundamental parameter controlling a soil's strength and deformation?

Effective stress, rather than total stress, is the fundamental parameter controlling a soil's strength and deformation because these properties arise directly from the forces transmitted through the solid skeleton of the soil, which are governed by effective stress. Total stress (σ) is the total force per unit area acting on a soil mass, encompassing the weight of soil solids, water, and any external applied loads. Pore water pressure (u) is the pressure exerted by the water filling the voids, or pores, within the soil, acting equally in all directions. Effective stress (σ') is the stress carried by the soil solids alone, representing the actual contact forces between individual soil particles. It is defined by Terzaghi's principle as the difference between total stress and pore water pressure: σ' = σ - u. The strength and deformation characteristics of soil originate from the friction, interlocking, and sometimes cohesion between its solid particles. When external loads are applied to a soil mass, these loads are primarily supported by two components: the ....