Civil Engineering Reference
In-Depth Information
Figure 13.5
Characteristic ranges of soil stiffness.
as shown in Fig. 13.4(b). When the state reaches the state boundary surface at Y, yield
occurs and the stiffness drops sharply to the value given by the full Cam clay expression
in Eq. (12.16). Figure 13.4(b) indicates that after yield the behaviour observed in
laboratory tests will be very like that given by the Cam clay theories (with suitable
values for the soil parameters), but before yield the observed stiffness-strain behaviour
is very different in character from that given by Cam clay.
The principal features of soil stiffness are illustrated in Fig. 13.5. There are three
regions, as indicated, where the behaviour is different. For very small strains, smaller
than some value corresponding to the first yield (usually of the order of 0.001%), the
stiffness is approximately constant and the stress-strain behaviour is linear. For large
strains, where the state has reached the state boundary surface (usually greater than
about 1%), the behaviour is elasto-plastic and the Cam clay theories are quite good.
In the intermediate, small strain, range the stiffness changes rapidly with strain and
the behaviour is highly non-linear.
13.4 Strains in the ground
In most geotechnical structures that are designed to restrict ground movements, such
as foundations and retaining walls, the strains in the ground are usually quite small.
Figure 13.6 illustrates a stiff retaining wall and a foundation. The outward movement
Figure 13.6
Strains in the ground near typical geotechnical structures.
