Cohesion (geology)

Under cohesion, the bonding force the Soil Mechanics understands the cohesive forces in cohesive soils. It is noticeable only in soils that contain very small grains, ie in which the surface properties of the individual grains of the properties that are caused by their mass, predominate, such as clay. Cohesion ensures the soil or the fine-grained loose rock to the internal cohesion of the individual particles themselves.

Cohesion is usually the sign and the physical unit of a voltage; it is a shear stress. It is a special case of the shear strength.

• 2.1 Gleitsicherheitsnachweis
• 2.2 embankments
• 2.3 Apparent Cohesion

Basics

Just shear

The cohesion of a soil can in the laboratory shear test (at least three individual experiments ) are determined with test equipment. The soil sample (also called triaxial ), for example, in the direct shear device or Dreiaxialgerät vertically and horizontally loaded to failure. It can be found from the Mohr stress circles the parameters of the shear line of the two-dimensional stress state. In the stress diagram, the vertical stress is applied to the horizontal x -axis and the shear stress on the vertical y-axis. The connection of the three measurement points should lie on a straight line ( the shear line). These shear line is characterized by its pitch (which is the angle of friction ), and. By the distance in which it intersects the vertical axis This distance is the cohesion c.

Coulomb failure criterion

A bottom which is in a state of tension below the shear line, the load withstands. Above the shear line it fails. The higher the cohesion and / or friction angle, the higher is the strength.

Applications

Gleitsicherheitsnachweis

The cohesion, together with the friction angle used ( angle of internal friction ) to calculate the resistance of buildings against horizontal displacement (eg foundations, (angular) retaining walls, dams ). This proof is called Gleitsicherheitsnachweis or shear strength test.

Embankments

Cohesion and friction angle are responsible for the angle at which an embankment of earth material may be applied without collapsing.

Also the detection of an embankment against slope failure cohesion in addition to the friction angle and the weight of the soil is an important parameter.

Apparent cohesion

Wet sand has an apparent cohesion. It is held together by capillary forces between the sand grains. Once the sand grains are dry on the surface, the sand does not have cohesive properties anymore, but is only held together by frictional forces. It can thus form no longer so steep embankment. ( " Sand castle " effect )