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 (which may be lost through weathering)
  2. Cementing by Fe2O3, CaCO3, NaCl, etc.

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

  1. Negative capillary 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 through logging or fire of the contributing plants, or through solution)

Typical values of cohesion

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

Cohesive strength (c) for some materials
Material Cohesive strength in kPa Cohesive strength in psi
Rock 10000 1450
Silt 75 10
Clay 10 to 20 1.5 to 3
Very soft clay 0 to 48 0 to 7
Soft clay 48 to 96 7 to 14
Medium clay 96 to 192 14 to 28
Stiff clay 192 to 384 28 to 56
Very stiff clay 384 to 766 28 to 110
Hard clay > 766 > 110

References

See also
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