Cohesion is the component of shear strength of a rock or soil that is independent of interparticle friction.
In soils, true cohesion is caused by following:
- Electrostatic forces in stiff overconsolidated clays (which may be lost through weathering)
- Cementing by Fe2O3, CaCO3, NaCl, etc.
There can also be apparent cohesion. This is caused by:
- Negative capillary pressure (which is lost upon wetting)
- Pore pressure response during undrained loading (which is lost through time)
- Root cohesion (which may be lost through logging or fire of the contributing plants, or through solution)