Deformation behaviour of railway embankment ... - Liikennevirasto

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Past investigations have shown clearly that the resilient response is influenced most by
the level of applied stresses and the moisture content in the material. The influence of
the other factors on the resilient response of granular materials is somewhat unclear.
The literature reveals that researchers do not always agree on the nature or the extent of
the impact of these factors and different, or even completely opposite, conclusions are
often found. The discrepancies found in the literature emphasise the need for more
intensive research into this area in the future.
Most of the research carried out to study the mechanical properties of granular materials
deals with the resilient behaviour of these materials. In comparison, the amount of work
on plastic behaviour is relatively limited. This is probably due to the fact that
monitoring long term behaviour is a very time-consuming and cumbersome process
when very large numbers of load applications need to be employed (10 5 to 10 6 cycles).
Furthermore, each specimen can only be subjected to a single stress path since
permanent deformation behaviour is strongly influenced by stress history.
A railway embankment is exposed to a large number of load applications during its
service life. Although the permanent deformation is normally a fraction of the total
deformation produced by each load repetition, the gradual accumulation of a large
number of these small plastic deformation increments could lead to eventual failure.
Failure and also excessive permanent deformations of the embankment must of course
be prevented. To do so the knowledge of the plastic behaviour of unbound granular
materials is very important.
The permanent strain development in granular materials is affected by several factors:
stress level, stress history, number of load applications, principal stress rotation,
moisture content, density, grading and aggregate type, and physical properties of
aggregate particles.
Modelling is an important necessity for an analytical approach in describing material
performance. Many researchers have outlined several different procedures for predicting
the resilient and permanent strain responses of unbound granular materials. However,
the great number of models available is per se further evidence of the complexities that
overshadow this research area. While many researchers present mathematical
formulations that fit their particular data, greater effort is clearly needed in developing
more general models and procedures that have a sound theoretical basis and wide
applicability.
In general, two approaches are employed for mathematical modelling of the resilient
behaviour of granular materials. In the first approach, the stress-strain relationship is
given by a stress-dependent resilient modulus and a constant or stress-dependent
Poisson's ratio (e.g. K-θ model and Uzan model). In the second approach, the stressstrain
relationship is characterised by decomposing both stresses and strains into
volumetric and shear components. The resilient response of the material is then defined
using bulk and shear moduli instead of resilient modulus and Poisson's ratio (e.g. Boyce
model and contour model). Models of this kind are usually more complex in nature and
the parametric values are more difficult to determine from collected test data.