Deformation behaviour of railway embankment ... - Liikennevirasto

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load” is not equal to the failure loads found by static triaxial testing, but describes the
behaviour of the materials under cyclic loading. The failure loads are highly dependent
on the confining pressure. Figure 6.3:3 shows also that more permanent deformations
develop in the first load series than in the beginning of the following series. When the
load approaches the failure line, permanent deformations will start to increase even for the
second, third, and forth load series. This means that the development of permanent
deformations is dependent on both the stress level and the load history.
Even though the effect of stress history on permanent deformation behaviour has been
recognised, very limited research appears to have been done to study this effect. In
laboratory permanent deformation tests, the effect of stress history are normally
eliminated by using a new specimen for each stress path applied (Lekarp 1997 and
Lekarp 1999).
6.4 Number of load applications
Most elasto-plastic material models assume that all the plastic deformations develop
under the first loading and that further loading below this level are purely elastic. This
does not fit well with the behaviour of unbound aggregates, which tend to develop some
additional permanent deformations for each repeated load step.
The number of load cycles is one of the most important factors to consider in the
analysis of the long-term behaviour of granular materials with repeated load triaxial
tests because it is a prerequisite e.g. for the estimation of how much the railway
embankment will deform. The number of load cycles is important also from a practical
testing point of view (time required for completing the tests and hence overall
experimental costs). For instance, 80,000 load cycles are set by the prEN-standard
(prEN 13286-7 2002). Sometimes this number of load repetitions is not adequate. In
addition, the number of load applications is important also because the growth of
permanent deformation in granular materials under loading is a gradual process during
which each load application contributes to the accumulation of strain by a small
increment. If the intensity of the applied loading is not too high, the accumulation of
permanent deformations on a certain stress path is normally assumed to stabilise as the
number of load repetitions increases. The curve representing the accumulated
permanent deformation approaches asymptotically a limiting value, i.e. the permanent
deformation rate per load cycle tends towards zero. Increasing stress ratios lead to a
progressive rise of the accumulating permanent deformations. The significance of the
number of load repetitions has been mentioned many times in the literature.
Some researchers (Morgan 1966, Barksdale 1972, Sweere 1990) have reported
continuously increasing permanent strain under repeated loading. On the other hand,
Brown (1974) and Brown and Hyde (1975), who investigated the behaviour of a wellgraded
crushed granite (5 mm maximum particle size), noted that an equilibrium state
was established after approximately 1000 load applications.
Two types of sands were investigated by Morgan (1966) in a series of repeated load
triaxial tests at both constant and variable confining pressures. Morgan applied up to
2x10 6 load application and noted that the permanent strain was still increasing at the end
of the tests. After an initial period of up to 2x10 5 cycles, the rate of increase of