Civil Engineering Reference
In-Depth Information
Cam clay
12.1 Introduction
Figure 11.7 shows a simple state boundary surface for soil; to develop a simple theo-
retical model for the stress-strain behaviour of soil this could be taken to be a yield
surface. Yield curves are the lines of intersection of elastic walls with the yield surface
as shown in Fig. 11.9 and these could be taken to be plastic potentials. We could then
use the ideas of yielding, hardening and normality set out in Chapter 3 to derive a set
of constitutive equations for soil. All that is required is a mathematical expression for
the shape of the boundary surface.
Suitable equations for the state boundary surface could be obtained by fitting expres-
sions to laboratory test data, by purely theoretical consideration of the mechanics of
granular materials or by a combination of these. A very simple and neat theoretical
equation was obtained by research workers in the University of Cambridge during
the 1960s and this will be described here. Over the years many others have tried to
improve on the original Cambridge equation and while some have succeeded in obtain-
ing better agreement with experimental observations the simplicity and elegance of the
original is inevitably lost. What I am going to do in this chapter is to describe the
original simple theoretical model to get across the basic techniques involved in con-
structing constitutive equations for soil. Anyone seriously interested in applying these
techniques in practice will need to study the more complex, and more realistic, soil
models.
12.2 Basic features of the Cam clay models
The Cambridge theories are known under the umbrella term of Cam clay. The first
model described by Schofield and Wroth (1968) is known as original Cam clay and
a second model described by Roscoe and Burland (1968) is known as modified Cam
clay. All the theories within the Cam clay family are basically similar. Soil is taken to
be frictional with logarithmic compression. The state boundary surface is taken as a
yield surface and as a plastic potential surface, and hardening is related to the plastic
volumetric strains. The principle differences between the various members of the Cam
clay family are in the precise equations used to describe the yield curves. For example,
in original Cam clay they are logarithmic spirals while in modified Cam clay they are
ellipses.
