Cohesion (soil mechanics)

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Under the cohesion , even adhesion that understands soil mechanics , the cohesive forces in cohesive soils . It is only noticeable in soils that contain very small grains , in which the surface properties of the individual grains outweigh the properties caused by their mass, e.g. B. with sound . Cohesion provides in soil or fine-grained en loose rock to the internal cohesion of the individual particles with each other.

As a special case of shear strength , cohesion is a shear stress and therefore has the physical unit of stress (force per area): N / mm² or MN / m².


Straight line

The cohesion of a soil can be determined in the laboratory in a shear test (at least three individual tests ) with test equipment. The soil sample is z. B. in the shear pressure cylinder , frame shear device or in the triaxial device (also called triaxial device) loaded vertically and horizontally to break.

The two parameters of the shear line of the two-dimensional state of stress are determined with the aid of Mohr's circles of stress . In the stress diagram, the vertical stress is plotted on the horizontal x-axis and the shear stress on the vertical y-axis; the connection of the three measuring points (from the three individual tests) should lie on a straight line (the shear line). This is characterized by its slope (that is the angle of friction ) and the distance at which it intersects the vertical axis (that is the cohesion c).

Coulomb fraction condition

The Coulomb fraction condition says:

  • soil that is in a stress state below the shear line will withstand the load ;
  • above the shear line it fails .

The higher the cohesion and / or the angle of friction, the higher the load-bearing capacity of the floor.


Proof of sliding safety

Cohesion is used together with the angle of friction (angle of internal friction) to calculate the resistance of a building to horizontal displacement (e.g. foundations , (angular) retaining walls , dams ). This proof is called slip safety proof or also shear strength proof .


Cohesion and the angle of friction are responsible for the angle at which an embankment made of soil material can be created without collapsing.

In addition to the angle of friction and the weight of the soil, cohesion is also an important parameter when verifying a slope against slope failure .

Apparent cohesion

Moist sand has an apparent cohesion . It is held together between the grains of sand by capillary forces.

As soon as the sand grains are surface dry, the sand no longer has any cohesive properties, but is only held together by frictional forces. As a result, it can no longer form such a steep slope. (" Sand castle effect")

Individual evidence

  1. Scheffer, F. & Schachtschabel, P. (2010): Textbook of Soil Science. Spektrum Akademischer Verlag, 16th edition, 569 pp. ISBN 978-3-8274-1444-1 , here on pages 184 and 185.
  2. ^ Karl-Eugen Kurrer (2018): The History of the Theory of Structures. Searching for Equilibrium . ISBN 978-3-433-03229-9 , here on page 321