Cohesion is the component ofshear strength of arock orsoil that is independent of interparticlefriction.

In soils, true cohesion is caused by following:

1. Electrostatic forces in stiffoverconsolidatedclays (which may be lost throughweathering)
2. Cementing byFe₂O₃,CaCO₃,NaCl, etc.

There can also be apparent cohesion. This is caused by:

1. Negativecapillary pressure (which is lost upon wetting)
2. Pore pressure response during undrained loading (which is lost through time)
3. Root cohesion (which may be lost throughlogging orfire of the contributingplants, or throughsolution)

Cohesion (alternatively called thecohesive strength) is typically measured on the basis ofMohr–Coulomb theory. Some values for rocks and some common soils are listed in the table below.

During critical state flow of soil, the undrained cohesion results from effective stress and critical state friction, not chemical bonds between soil particles. All that small clay mineral particles and chemicals do during steady plastic deformation of soft soil is to cause a pore water suction, which can be measured. When we remould soft soil in a classification test, its strength is [(suction) x (friction)], it remains ductile plastic material with constant "apparent cohesion" while it flows at constant volume, because it is at a constant effective stress, and critical state friction is constant.Critical state soil mechanics analyses the bearing capacity of soft clay on the wet side of critical state in terms of a perfectly plastic material with rapid undrained "apparent" cohesion.^[1]

• Collins internet-linked dictionary of Geology

• Mohr–Coulomb failure criterion

• Shear strength
• Soil mechanics