Which factor is crucial in determining the effective stress in soil mechanics?

The crucial factor in determining the effective stress in soil mechanics is pore pressure in the soil. Effective stress is defined as the stress that contributes to the strength of the soil and is calculated by subtracting the pore water pressure from the total stress acting on the soil. This relationship is articulated in Terzaghi's principle of effective stress, which states that the strength and stability of soil are influenced not only by the weight of the soil above it but also by the pressure exerted by water within its pores.

When pore pressure increases, such as during heavy rainfall or groundwater rise, the effective stress decreases, which can lead to reduced soil strength and potential failure in slopes or foundations. Conversely, if pore pressure is lowered, for example through drainage, effective stress increases, enhancing the soil’s load-bearing capacity. Understanding the role of pore pressure is essential for designing foundations, earth structures, and in assessing the stability of slopes, making it a fundamental concept in soil mechanics.

While grain size, moisture content, and density play important roles in soil behavior, they do not directly capture the relationship between total stress, pore pressure, and effective stress as explicitly as pore pressure does. Therefore, pore pressure is identified as the most critical factor in this context.