Stiffness of soil - The Mechanics of Soils and Foundations - page 190

Figure 13.3 Characteristic stiffness-strain curves for soil.

data and secant moduli are often used. The progress of the test can be described by the
change of deviator stress q for the shear modulus G and the change of mean effective
stress p for bulk modulus K . Alternatively the progress of the test can be described
by the shear strain ε s for shear modulus or the volumetric strain ε v for bulk modulus.

If strains are used they are usually plotted to a logarithmic scale.
Figure 13.3 shows the general shapes of stiffness-strain curves for a typical soil.
(Surprisingly, the general shape applies for normally consolidated soils as well as for
lightly and heavily overconsolidated soil and the consequences of this will be considered
later.) The curves for shear and bulk modulus are basically similar at strains less than
1% or so. The tangent shear modulus becomes zero at the critical state while at large
strains the tangent bulk modulus increases as the specific volume decreases. If the soil
has a peak the tangent shear modulus is zero at the peak and is negative as the soil
weakens from the peak towards the critical state.
This stress-strain behaviour is significantly different from that given by the simple
Cam clay theory described in Chapter 12. Figure 13.4 illustrates characteristic stress-
strain behaviour observed in laboratory tests and given by Cam clay. For the drained
constant p A in Fig. 13.4(a), the state reaches the state

boundary surface at Y and travels on the boundary surface along Y

loading path O → Y →

→

A. For Cam clay

Y and, since p and v remain constant for
the particular loading path considered, the shear modulus G =

the behaviour is taken to be elastic along O →

vp / g remains constant,

Figure 13.4 Characteristic stress-strain behaviour for soil observed in laboratory tests and given
by the Cam clay theories.