Effective stress (σ') in soil mechanics is defined as:

Effective stress (σ') is a fundamental concept in soil mechanics, critical for understanding soil behavior under load. It is defined as the stress that contributes to the strength of the soil, primarily attributed to intergranular contact between soil particles.

This definition arises from the principle that total stress in a soil mass is composed of two components: the effective stress and pore water pressure. Pore water pressure represents the pressure exerted by water in the soil voids, which does not contribute to the soil's strength.

Thus, effective stress captures the portion of the total stress that is transmitted through the soil skeleton — essentially, it accounts for the forces carried by the soil particles themselves. This is crucial in predicting soil behavior such as settlement, shear strength, and consolidation. Therefore, the correct choice identifies effective stress as being the internal stress supported through grain contact, which influences the ultimate behavior of the soil structure.

In contrast, other options refer to the definitions or situations that do not encapsulate the essence of effective stress: one suggests the sum of pore water pressure and total stress, which misrepresents the concept; another describes total stress without distinguishing effective stress; and the last mentions stress due solely to pore water, which is not representative of effective stress