Cohesion is the component of
shear strength of a
rock or
soil
Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...
that is independent of interparticle
friction.
In soils, true cohesion is caused by following:
#
Electrostatic forces in stiff
overconsolidated clay
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...
s (which may be lost through
weathering)
#
Cementing by
Fe2 O3,
Ca CO
3,
Na Cl, 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
In vascular plants, the roots are the plant organ, organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often bel ...
cohesion (which may be lost through
logging
Logging is the process of cutting, processing, and moving trees to a location for transport. It may include skidder, skidding, on-site processing, and loading of trees or trunk (botany), logs onto logging truck, trucks[fire
Fire is the rapid oxidation of a fuel in the exothermic chemical process of combustion, releasing heat, light, and various reaction Product (chemistry), products.
Flames, the most visible portion of the fire, are produced in the combustion re ...]
of the contributing
plant
Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...
s, 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.
Apparent cohesion of soil
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.
K. H Roscoe, Andrew Schofield, C. P Wroth, 1958, On The Yielding of Soils, Géotechnique 8(1), 22-53
/ref>
References
Collins internet-linked dictionary of Geology
See also
* Mohr–Coulomb failure criterion
{{Geotechnical engineering
Shear strength
Soil mechanics