Cohesion is the component of
shear strength
In engineering, shear strength is the strength of a material or component against the type of yield or structural failure when the material or component fails in shear. A shear load is a force that tends to produce a sliding failure on a materi ...
of a
rock or
soil
Soil, also commonly referred to as earth or dirt
Dirt is an unclean matter, especially when in contact with a person's clothes, skin, or possessions. In such cases, they are said to become dirty.
Common types of dirt include:
* Debri ...
that is independent of interparticle
friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:
*Dry friction is a force that opposes the relative lateral motion of ...
.
In soils, true cohesion is caused by following:
#
Electrostatic force
Coulomb's inverse-square law, or simply Coulomb's law, is an experimental law of physics that quantifies the amount of force between two stationary, electrically charged particles. The electric force between charged bodies at rest is convention ...
s in stiff
overconsolidated clay
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4).
Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
s (which may be lost through
weathering
Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement) ...
)
#
Cement
A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mi ...
ing 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 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 below the su ...
cohesion (which may be lost through
logging or
fire
Fire is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction products.
At a certain point in the combustion reaction, called the ignition point, flames a ...
of the contributing
plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclu ...
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
Critical state soil mechanics is the area of soil mechanics that encompasses the conceptual models that represent the mechanical behavior of saturated remolded soils based on the ''Critical State'' concept.
Formulation
The Critical State concept ...
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
Soil mechanics