Which of the following statements is true about effective stress in saturated soils?

Effective stress is indeed defined as the total stress acting on a soil mass minus the pore water pressure within that mass. This fundamental concept, introduced by Karl Terzaghi, helps to explain how effective stress determines the strength and behavior of saturated soils. In saturated conditions, the water within the pores exerts an upward pressure that counteracts some of the external loads applied to the soil.

Therefore, as you apply loads to a saturated soil, the effective stress, which directly influences soil strength, is computed by taking the total stress (the weight of the soil and any overlying materials) and subtracting the pore water pressure. An increase in pore water pressure (such as might occur during heavy rainfall or due to increased loading) reduces the effective stress, which can result in decreased soil stability and increased likelihood of failure.

The other options do not accurately reflect the principles of effective stress. For instance, while cohesion and internal friction contribute to the overall strength of a soil, they are components of soil mechanics rather than definitions of effective stress. An increase in pore water pressure actually decreases effective stress, and stating that effective stress is always zero in saturated soils misrepresents how effective stress functions as it can be positive depending on the specific conditions of loading and soil type