Introduction to geotechnical engineering - The Mechanics of Soils and Foundations - page 5

Figure 1.3 Branches of mechanics used in geotechnical engineering.

is a mechanism. Otherwise materials may compress (or swell) or distort, as shown in
Fig. 1.4. Figure 1.4(a) shows a block of material subjected to a confining pressure and

Fig. 1.4(c) shows a relationship between the pressure and the change of volume; the
gradient is the bulk modulus K . The stress can be raised more or less indefinitely and
the material continues to compress in a stable manner and does not fail; K continues
to increase with stress and strain.
Figure 1.4(b) shows a block of material subjected to shearing stresses

σ

so that it
distorts in shear. Notice that compression in Fig. 1.4(a) involves a change of size while
shear distortion involves a change of shape; in a general loading, compression and
distortion occur simultaneously. Figure 1.4(d) shows a simple relationship between
shear stress and shear strain; the gradient is the shear modulus G and this reduces with
stress and strain. The material fails when no more shear stress can be added and then it
continues to strain at constant shear stress

τ

τ f ; this is the shear strength of the material.
Figure 1.4 illustrates the two most important aspects of material behaviour: stiffness
and strength. Stiffness relates changes of stress and changes of strain by

d σ d τ

K = G = (1.1)

d ε v d γ