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

• Collins internet-linked dictionary of Geology

See also

• Mohr–Coulomb failure criterion