How do cohesion and friction differ in soil mechanics?

Cohesion and friction play crucial roles in the behavior of soils, and understanding their differences is essential in soil mechanics. Cohesion refers to the intermolecular forces between soil particles that contribute to their ability to stick together. This attraction is particularly significant in fine-grained soils like clay, where particles can bond due to electrostatic forces and other chemical interactions. As a result, cohesive soils retain their structure better under shear stress compared to non-cohesive soils.

Friction, on the other hand, is the resistance that surfaces encounter when sliding against one another. In soil mechanics, this resistance is influenced by factors such as the roughness of the particles and the angle at which they approach one another. Frictional resistance comes into play when there is potential sliding between soil layers or particles.

The correct answer emphasizes that cohesion provides an attraction between soil particles, contributing to their stability, while friction resists the sliding motion between those particles. This duality is fundamental in assessing soil stability, especially in analyses involving slopes, foundations, and earth pressures.