If the deviator stress in a soil sample is increased, what will happen to the shear stress?

When the deviator stress in a soil sample is increased, the shear stress behaves in a specific manner due to the fundamental principles of soil mechanics. The deviator stress is essentially the difference between the major and minor principal stresses acting on a soil sample.

As deviator stress increases, it creates a greater differential loading on the soil, which directly influences the distribution and magnitude of shear stress within the material. In a triaxial test scenario, for instance, increasing the deviator stress typically enhances the soil's ability to resist shear loads, thereby increasing the shear stress until reaching the peak strength of the soil.

In terms of the specific planes involved, the increase in shear stress is primarily observed in the vertical plane due to the application of deviator loading, inducing additional stress that acts to distort the soil structure. This response aligns with the concept that as the total stress state changes, particularly when axial loading is applied, the effective stresses and, thus, shear stresses will increase due to the resulting strains generated in the soil matrix.

Therefore, in a scenario where deviator stress is increased, one can reasonably conclude that there will be an increase in shear stress within the vertical plane of the soil sample.