How is effective stress defined in soil mechanics?

Effective stress is a fundamental concept in soil mechanics, originally formulated by Karl Terzaghi. It is defined as the stress carried by the soil skeleton, which is crucial for understanding how soil behaves under load. The effective stress reflects the load that contributes to the structural integrity and strength of the soil, as it accounts for the forces transmitted through the soil grains themselves while excluding the contribution of pore water pressure.

When a soil mass is subjected to external loads, the total stress is the sum of the stresses transmitted through the solid particles and the stresses from the pore water within the voids. The relationship can be expressed through the equation:

Effective Stress = Total Stress - Pore Water Pressure

This relationship illustrates that effective stress is responsible for the strength and failure behavior of soil. When pore water pressure increases, for example during rainfall or irrigation, the effective stress decreases, which can lead to instability, as the soil's ability to support loads diminishes.

Other options describe related concepts but do not capture the essence of effective stress:

• The total stress experienced by the soil includes both the stress carried by the soil skeleton and the pore water pressure, so it cannot accurately define effective stress.

• The pressure exerted by pore water is significant in understanding the behavior of saturated