Cohesion (geology)
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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)
Typical values of cohesion[edit]
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.
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 |