1305
Model tests on settlement behaviour of ballasts subjected to sand intrusion and tie
tamping application
Tests de modélisation sur le comportement en tassement des ballasts sujets à l’intrusion de sable
et au bourrage
Kumara J., Hayano K.
Yokohama National University
ABSTRACT: Effects of sand intrusion into ballast (i.e., ballast fouling) and tie tamping application on settlement characteristics of
ballasted trail track were investigated by series of model tests with cyclic loading. Model tests were conducted on 1/5
th
scale of the
actual size of railway track. Ballast fouling was simulated by sand-gravel mixtures (i.e., gap graded particle size distribution). Tie
tamping application was physically simulated in the model tests using a simple tool. The relationship of number of loading cycles and
settlement was obtained and results were discussed with degree of ballast fouling (i.e., amount of sand in sand-gravel mixtures). The
results indicated that initial settlement process and rate of residual settlement alter after 30% sand. Initial settlement period is higher
for fouled ballast with 30% or more sand after tie tamping application. Rate of residual settlement is higher for fouled ballast with
more than 30% sand after tie tamping application. That’s to say, tie tamping application is effective for fouled ballast up to 30% fines.
RÉSUMÉ : Les effets de l’intrusion de sable dans le ballast et du compactage par bourrage du ballast sur le comportement en
tassement des voies ferrées ballastées ont été étudiés en effectuant une série d'essais cycliques de chargement sur des échantillons
tests. Les tests ont été réalisés sur modèle à une échelle d'un cinquième, et l'exécution du bourrage a été physiquement simulée dans
les essais à l’aide d’un outil simple. La relation entre le nombre de cycles de chargement et le tassement a été obtenue et les résultats
en fonction du degré d’intrusion de sable et du compactage ont été étudiés. Les résultats indiquent que le processus de tassement
initial et le taux de tassement résiduel sont modifiés au delà de 30% d’intrusion de sable. Le taux de tassement résiduel est plus élevé
pour le ballast bourré et compacté comportant plus de 30% de sable, ce qui revient à dire que le compactage pour le ballast bourré est
effectif jusqu’à 30% de fines.
KEYWORDS: Ballast fouling, ballasted railway track, model test, residual settlement, tie tamping application.
1 INTRODUCTION
In railway tracks, ballast fouling occurs when fine materials mix
with ballast due to heavy repeated train loads. Generally, fine
materials come mainly from underneath layers, and to a lesser
extent, due to particle crushing too (Indraratna et al., 2004).
Sand intrusion alters the original particle size distribution (PSD)
of ballast, resulting different settlement characteristics than that
of fresh ballast. Once the settlement reaches the allowable limit,
a maintenance method should be implemented to bring the
railway track into the original position. Usually, tie tamping
application is used worldwide as the main maintenance method.
However, effects of ballast fouling on settlement characteristics
and tie tamping application itself haven’t been well understood
in the past.
In this study, effects of degree of ballast fouling and tie
tamping application on settlement characteristics of ballasted
tracks were investigated. A series of cyclic loading tests were
conducted on a model sleeper of 1/5
th
scale of the actual rail
track as shown in Figure 1. In the model tests, tie tamping
application was physically simulated by inserting a small tool
into the ballasts.
2 MODEL GROUNDS AND CYCLIC LOADING
Figure 1 shows the model test apparatus used in this research.
Model grounds at a scale of 1/5
th
were constructed in a sand box
with interior dimensions of 800mm wide, 304mm deep, and
300mm high. A duralumin footing with a width of 48mm was
used to model the sleeper. Gravel approximately 1/5
th
of the size
of actual ballasts were selected to model the ballasts. Medium
size sand (M sand) was used as sand. PSDs of gravel and M
sand are shown in Figure 2.
The model tests were conducted on fouled ballast (i.e., 5 cases)
and fresh ballast as given in Table 1. Ballast thickness was
made as 50mm (i.e., 1/5 of 250mm of actual ballast layer) in
each case and 100 loading cycles were applied before tie
tamping application. Cyclic loadings were applied to the model
grounds through the sleeper at a constant displacement rate of
0.05mm/s. The amplitude of the cyclic stress applied was
120kN/m
2
(i.e., approximately 70% of maximum stress M sand
can withstand). All the specimens were prepared with 80% of
relative density,
D
r
. How void ratios,
e
max
and
e
min
of fresh
ballast (i.e., gravel) change with amount of sand mixed can be
seen in Figure 3.
Tie tamping application was simulated with the tool shown in
Figure 3. First, the sleeper was lifted to the initial position after
100 loading cycles were applied. Next, a small spoon was
inserted (e.g., about 8-10mm) into the model ground by sides of
the sleeper. After the spoon reached the fixed ground depth (i.e.,
8-10mm), it was tilted several times to permit the particles to
move laterally. This procedure was followed at several locations
until the voids between the sleeper and the ground surface were
completely filled by the particles. Finally, additional gravels
(except in case 6 where M sand was used) were introduced to
the ground surface near the sleeper to produce a flat ground
surface. After this tie tamping application, 100 loading cycles
were applied again. Axial displacement was measured using
two displacement transducers, placed at the front and back of
the sleeper.
