Deformation behaviour of railway embankment ... - Liikennevirasto

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6 FACTORS AFFECTING PERMANENT DEFORMATION
6.1 Introduction
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.
Most of the research carried out over the years has concentrated on the resilient behaviour
of granular materials. In comparison to the resilient behaviour, less research has been
devoted to the plastic response and permanent deformation development in unbound
granular materials. This is perhaps due to the practical difficulties in studying permanent
deformation behaviour. While resilient tests are fairly quick and each laboratory specimen
can be studied for a great number of stresses, the monitoring of the build-up of
permanent deformations in unbound granular materials and permanent deformation tests
are very time-consuming, and usually separate specimens are required for each set of
stresses. As a result, greater advances have been made in understanding the resilient
response than the long-term performance of granular materials.
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.
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The importance of the applied stress level is strongly emphasized in the literature.
Permanent strain is said to be related directly to the deviator stress and inversely to
the confining pressure (Barksdale 1972). However, several studies have suggested
that permanent strain is principally governed by some form of stress ratio
consisting of both deviatoric and confining stresses (e.g. Brown and Hyde 1975).
A number of limited investigations found in the literature (Chan 1990) indicate that
reorientation of principal stresses due to moving traffic significantly increases the
amount of plastic strains in granular materials.
The growth of permanent strain in granular materials is a gradual process during
which each load application contributes a small increment to the accumulation of
strain. The significance of the number of load repetitions has been recognized
many times in the literature (e.g. Sweere 1990, Paute et al. 1993).
The amount of permanent strain in granular materials is also influenced greatly by
the presence of water. At high levels of saturation, deformation resistance in the
material reduces quite rapidly, probably as positive pore water pressure is
generated. Proper drainage in granular materials is, therefore, a necessity for
improving performances. The reduction in fines content is likely to achieve this but