In which plane can additional stress (deviator stress) affect the shear stress?

The correct answer is the horizontal plane. In the context of soil mechanics and geotechnical engineering, additional stress, or deviator stress, is often applied through vertical loading on soil or earth structures. This loading causes an increase in shear stress in the horizontal plane due to the redistribution of stress in the soil particles.

When a vertical load is applied, the resulting stress field can create horizontal shear within the soil. This phenomenon is related to the concept of effective stress, which states that the effective stress acting on the soil skeleton is influenced not just by the vertical loads but also by how these loads are transmitted laterally.

Understanding how deviator stress affects shear stress in the horizontal plane is crucial for analyzing slope stability, bearing capacity, and overall soil behavior under loading conditions. This relationship is a foundational principle in soil mechanics that shapes design decisions related to foundations, embankments, and retaining structures.