Civil Engineering Reference
In-Depth Information
Figure 1.2
Principles of mechanics.
pedalling a bicycle turns the wheels. The basic feature of any system of forces and
displacements and stresses and strains are illustrated in Fig. 1.2. Forces give rise to
stresses and these must be in equilibrium or the body will accelerate. Displacements
give rise to strains which must be compatible so the material does not tear or overlap.
(Relationships between forces and stresses and between displacements and strains are
given in Chapter 2.) These two separate requirements (of equilibrium and compat-
ibility) are quite simple and they apply universally to everything. The relationships
between stresses and strains (or between forces and displacements) are governed by
the characteristics of the material.
There are a number of branches or subdivisions of mechanics which depend on the
material, the type of problem and any assumptions made. Obviously soil mechanics is
the mechanics of structures made of soils and there are also rock mechanics for rocks
and fluid mechanics for fluids. Some important branches of mechanics are illustrated
in Fig. 1.3; all of these are used in soil mechanics and appear later in this topic.
Rigid body mechanics deals with mechanisms, such as car engines, in which all
the moving parts are assumed to be rigid and do not deform. Structural mechanics
is for framed structures where deformations arise largely from bending of beams and
columns. Fluid mechanics is concerned with the flow of fluids through pipes and chan-
nels or past wings, and there are various branches depending on whether the fluid is
compressible or not. Continuum mechanics deals with stresses and strains throughout
a deforming body made up of material that is continuous (i.e. it does not have any
cracks or joints or identifiable features), while particulate mechanics synthesizes the
overall behaviour of a particulate material from the response of the individual grains.
You might think that particulate mechanics would be relevant to soils but most of
current soil mechanics and geotechnical engineering is continuum mechanics or rigid
body mechanics.
1.4 Material behaviour
The link between stresses and strains is governed by the properties of the material.
If the material is rigid then strains are zero and movements can only occur if there
