# Soil mechanics

Soil mechanics is the study of the physical properties of the soil and its behavior as a building ground (in contrast to rock: rock mechanics ). It examines the movements and forces in loose rock or earth materials, in particular the processes in structures in the ground ( foundation ) and from the earth ( earthwork ). It also deals with how the forces from the statics of a building can be transferred into the subsoil without damage.

## history

Charles Augustin de Coulomb (1773) presented the first mathematical theory of ground movements and forces . The first comprehensive textbook on soil mechanics was written by Karl von Terzaghi (1925). Terzaghi is regarded as the founder of soil mechanics as an independent science.

## Soil mechanics as engineering science

Soil mechanics has developed as a transition area between geotechnics , continuum mechanics and soil science . It is the theoretical basis of earth statics , which provides calculation methods for the behavior of the soil, with which the load on structures is determined in the field of structural analysis , and with the help of which their foundations are measured .

The main difference between soil mechanics and rock mechanics is that the soil can be treated as a continuum in stability calculations, but the rock often only as a discontinuum due to its layer and crevice structure. This requires different approaches.

The necessity of soil mechanical calculations arises from the fact that the subsoil has to safely transfer the loads of the structures in order, for example, to safely prevent ground breaks and thus tilting of the structure. The subsoil found in the immediate vicinity of the structure is therefore mechanically a part of the structure whose failure must be safely avoided. On the other hand, in many cases the influence of the soil on the structure has to be determined, for example the magnitude of the earth pressure . Soil mechanical calculations play an important role in determining the stability of earthworks such as embankments or natural slopes.

## Sub-areas

An important sub-area of ​​soil mechanics (as a sub-area of ​​soil science) is soil classification , as well as research into the mechanical properties of soils and the development of corresponding test methods in order to be able to classify the mechanical properties of soils and model them realistically:

Another area of ​​soil mechanics is the development of material laws for describing the behavior of soils. The simplest material laws with which the behavior of soils is described are Hooke's law (linear elastic behavior) and the Mohr-Coulomb breaking condition (rigid plastic behavior). With these, however, the mechanical behavior of soils can only be roughly approximated; their use is therefore limited to specific issues. For a more realistic description of the material behavior, more and more demanding material laws are applied, whereby this leads to increased effort.

Another important aspect when describing the mechanical behavior of soils is their multiphase nature . Soil consists of different phases: the grain structure (solid), in whose pores there are water (liquid) and air (gaseous). Describing the interactions between these three phases is a task of soil mechanics. Soil mechanical phenomena caused by interactions are e.g. B. the static buoyancy , the capillarity and the permeability, which is usually described with the Darcy law . Due to a change in the phase proportions, the soil can change its state from solid to plastic to liquid (see Casagrande ).