How is effective stress calculated in geotechnical engineering?

Effective stress in geotechnical engineering is a fundamental concept that reflects the stress carried by the soil skeleton, which is crucial for understanding soil behavior. It is defined as the difference between total stress and pore water pressure. The reason for this relationship is grounded in the fact that total stress includes both the stress due to the weight of the soil and any external loads, while pore water pressure represents the pressure exerted by water in the voids of the soil.

When pore pressure exists within the soil, it reduces the effective stress experienced by the soil particles. Thus, to determine how much of the applied load is truly effective in contributing to the soil's shear strength and stability, pore pressure must be subtracted from total stress. This correct formulation aligns with Terzaghi's principle of effective stress, which is vital in assessing soil properties like strength and consolidation.

Understanding this concept is essential for engineers involved in foundation design, slope stability analysis, and other areas in geotechnics, as it helps ensure that structures are built on adequately supported soil.