For granular soils, the relationship between shear stress and normal stress is expressed by what?

The relationship between shear stress and normal stress in granular soils is best expressed by the soil frictional angle. This concept is grounded in the theory of soil mechanics, specifically in the Mohr-Coulomb failure criterion, which defines how materials fail under shear stress.

In granular soils, the resistance to shear stress is largely due to the interlocking of particles and the friction that arises between them. The soil frictional angle represents the internal frictional resistance of the granular material as a ratio of shear stress to normal stress. The higher the frictional angle, the greater the shear stress can be resisted at a given normal stress, indicating stronger resistance to movement.

Granular soils, such as sands and gravels, demonstrate this angle more prominently due to their lack of cohesion as seen in cohesive soils, where other factors like cohesion play a significant role. Thus, the soil frictional angle is a key parameter in understanding and predicting the behavior of granular soils under loading conditions, making it the correct answer to the question posed.