Cohesion (geology)


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:
  1. Electrostatic forces in stiff overconsolidated clays
  2. Cementing by Fe2O3, CaCO3, NaCl, etc.
There can also be apparent cohesion. This is caused by:
  1. Negative capillary pressure
  2. Pore pressure response during undrained loading
  3. Root cohesion

    Typical values of cohesion

Cohesion is typically measured on the basis of Mohr–Coulomb theory. Some values for rocks and some common soils are listed in the table below.
MaterialCohesive strength in kPaCohesive strength in psi
Rock
Silt
Clay to to
Very soft clay to to
Soft clay to to
Medium clay to to
Stiff clay to to
Very stiff clay to to
Hard clay> >