Deformation behaviour of railway embankment ... - Liikennevirasto

97
using the ultimate shear strength of the material. In this approach, the static failure line
is considered as a boundary for permanent strain under repeated loading. This has been
questioned by Lekarp and Dawson (1998), who argue that failure in granular materials
under repeated loading is a gradual process and not a sudden collapse as in static failure
tests. Therefore, the ultimate shear strength and stress levels that cause sudden failure
are of no great interest for the analysis of the material behaviour when the increase in
permanent strain is incremental.
Pappin (1979) conducted repeated load triaxial tests on a well-graded limestone using
cyclic deviator and confining stresses. He concluded that the permanent axial shear
strain can be expressed as a function of the length of the stress path in the p-q space and
the stress ratio (q/p) max , where q and p are the applied deviator stress and the mean
normal stress respectively. His observation further revealed that the resistance to
permanent deformation decreases when the applied stress approaches static failure, i.e.
the accumulated permanent strains increase as the deviator stress increases. This is
illustrated in Figure 6.2:2, in which the accumulation of permanent shear strain in
repeated load triaxial tests is given for two different stress paths A and B.
Figure 6.2:2 Permanent strain and stresses (Brown 1985).
The importance of the static failure stress has also been noted by others. Barret and
Smith (1976) investigated the behaviour of a crushed dolerite (5 mm maximum particle
size) with a 10% clay binder under simulated traffic loadings using cyclic triaxial tests
at constant stress ratio. They found that the static Mohr-Coulomb envelope could be
considered a natural boundary to both axial and horizontal permanent strain. Raymond
and Williams (1978) studied the behaviour of ballast and suggested that at the same
density and confining pressure there is a relationship between the plastic strain response
and the static failure of the material. They also showed that the permanent shear strain
can be estimated using the stress ratio q max /q failure , in which q max is the maximum
deviator stress and q failure the deviator stress at failure. The same stress ratio was used
later by Dawson and Kolisoja (2004) and Kolisoja et al. (2004).
Dawson and Kolisoja (2004) and Kolisoja et al. (2004) used an equation of the form ε p
= a · N b to model the development of plastic deformation. Here, the total plastic