Page 224 - 360.revista de Alta Velocidad - Nº 5

Escobar, Adrián. Zamorano, Clara Isabel. Jiménez, Pablo Lorenzo. Escobar, Jorge.

3. Experimental analysis

3.1 Main ballasted track issues

Although ballast has been widely used for railway infrastructures throughout the world, its use
in High-speed lines will present important limitations that cannot be ignored in the case of
lines with speeds above the 300 km/h, as shown in González-Cancelas et al. (2012). Some of
the main effects that ballast can suffer if placed in lines that have maximum speeds above this
limit are: a higher speed of deterioration that will significantly reduce the useful life of the
ballast and that will modify its elastic and resistant properties; a proliferation of differential
settlements above the normally accepted limits; and, in the third place, second-order effects
such as ballast flight, which may cause significant damage to the rolling stock and imply a risk
to the safety of the circulation.
With respect to the two main effects, the accelerated ageing of the ballast layer and the
appearance of higher differential settlements in the track, it is worth mentioning that both
phenomena are connected and both follow a direct relation with the train operating speed.
Both deterioration mechanisms will follow a law of exponential acceleration with the time due
to the feedback of the mechanisms that generate them.

Firstly, because high speeds produce higher dynamic loads, an important impact will be generated
in the ballast increasing its stiffness and causing the appearance of the first settlements in the
track. Due to these two factors, during the next load cycles that are generated when passing
the trains, the dynamic loads generated by them will be increased, due to the increases of
stiffness and the increase of the imperfections existing in the track due to the settlements, and
there will again be a greater stiffening of the ballast layer and larger settlements in the track.
This mechanism will continue to occur at a higher speed until the track failure occurs.

In conclusion, in two tracks that present the same characteristics and the same number of
load cycles, it is observed that the one in which the trains circulate at a higher speed, bigger
settlements and a greater deterioration of the ballast will be produced, as shown in Nguyen et
al. (2011).

3.2 Analysis of rail superstructure in High-speed lines around the world

For this reason, in spite of the multitude of economic and technical factors that can define
the use of a track typology or another in High-speed lines, it would be necessary to establish
an approximate limit from which a ballasted track can constitute a technical restriction for a
certain line can reach the maximum speeds for which it was designed, without prejudice to
reduce the useful life of the infrastructure.

This fact implies that due to the high proliferation of High-speed lines being built all over the
world, which are continuously seeking speed limits higher than those established as conventional,
such as 300 to 320 km/h, the implementation of slab track systems will be increased, because
they allow to guarantee a high performance and continuous behavior of the superstructure
when the train reach the maximum speeds.

An extract of the main High-speed lines that are in operation and the type of track that they
present will be made, in order to observe the possible trends that are occurring, according to
the data provided by Yokoyama (2010), SSF (2010), Sugrue (2013) and UIC (2017).

222 360.revista de alta velocidad