What does shear strength in soil refer to?

Shear strength in soil pertains to the ability of a soil mass to resist shear stress, which is the force exerted parallel to the surface of the soil. This property is critical when assessing how well the soil can withstand forces that try to cause it to slide or deform, particularly those resulting from gravity, such as downhill forces acting on slopes. Understanding shear strength is fundamental in geotechnical engineering and slope stability analysis, where preventing landslides or structural failures is paramount.

The other options present concepts that do not directly relate to shear strength. Resistance to water flow connects to permeability, while compression under weight pertains to the soil's ability to bear load, and elasticity in changing conditions deals with how soil responds to stress and strain over time. Each of these concepts is important in soil physics and civil engineering but does not encapsulate the specific definition of shear strength.