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 direct contact forces between the solid particles (effective stress) and the pressure of the water within the pores (pore water pressure). The pore water, being essentially incompressible in a short timeframe, bears a portion of the total stress by pushing the soil particles apart. This means that the pore water pressure effectively counteracts the total stress, reducing the direct contact forces between the solid particles.

For example, if you push two blocks together, the force holding them is direct. If you introduce water between them and it pushes back, the net force pressing them together is reduced. Similarly, in soil, higher pore water pressure reduces the force with which soil particles press against each other, thus reducing the effective stress. This reduction in inter-particle forces directly impacts the soil's ability to resist shearing and its tendency to deform.

Regarding strength, specifically shear strength, it is the soil's resistance to sliding or failure along a plane. Shear strength is primarily derived from the frictional resistance and interlocking between soil particles, which are directly proportional to the normal forces at the particle contacts. Since effective stress represents these normal contact forces, a higher effective stress means greater friction and interlocking, leading to greater shear strength. Conversely, an increase in pore water pressure reduces the effective stress, decreasing the inter-particle contact forces and consequently reducing the soil's shear strength, making it more prone to failure.

Regarding deformation, such as compression or settlement, it occurs as soil particles rearrange, move closer together, and often expel water from their voids. This rearrangement and volume change are driven by changes in the forces acting *betweenthe particles. When effective stress increases, the particles are forced into closer and more stable contact, leading to a reduction in void volume and observable settlement. Pore water pressure, by keeping particles separated and resisting their closer arrangement, reduces the effective stress that drives this compaction. Therefore, changes in effective stress directly control the extent and rate of soil deformation. Any change in total stress or pore water pressure that alters the effective stress will consequently alter the soil's strength and deformation behavior, making effective stress the true indicator of the stress state of the soil skeleton and thus its mechanical response.