Page 316 - 360.revista de Alta Velocidad - Nº 5

Sol-Sánchez, Miguel. Moreno-Navarro, Fernando. Rubio-Gámez, Mª Carmen.

1. Introduction

Railway over ballasted tracks plays an essential role in most transportation networks, since
it presents a range of benefits in comparison with other alternative systems such as concrete
slabs. However, changes in the geometric parameters, associated with differential settlement
along ballasted tracks, is one of the major problems in this type of infrastructures for railway
transportation. Passing trains generates cyclic movements in ballast particles that cause the
recompaction of the granular layer and its accumulative vertical deformation, and therefore,
to maintain track geometry within adequate safety and ride quality levels, permanent way
maintenance for re-levelling and realignment is necessary. Based on this, automatic tamper
is the machine most used from the 1960s, which consists of lifting and laterally squeezing the
ballast to fill the void space generated beneath the sleeper (Selig and Waters, 1994; Indraratna
et al., 2011). However, this maintenance operation is accompanied by some detrimental effects
such as loosening of compacted ballast layer, particles deterioration and reduced track stability
(Indraratna et al., 2011).
In light of these problems, alternative solutions are being developed for the improvement
of the track quality and the effectiveness of the maintenance tasks. In particular, a
process known as stoneblowing is worthy of note since it allows for the reduction of ballast
breakage during maintenance whilst simultaneously maintaining the vertical strength of
the layer (Hellawell, 1997; Claisse et al., 2003). In addition, using this technique avoids
rapid ballast recompaction after maintenance. This process, developed in the U.K.,
consists of adding small stones (around 14-20 mm) to the existing gap between the
sleeper and the ballast surface once the former has been raised to the desired level.
However, there are some concerns associated with stoneblowing, such as the stiffening of
the granular layer and its retarded capacity to damp loads. Another alternative solution
to reduce ballast maintenance, applied primarily in European transition sections (where
differential settlements are more probable), consists of using elastic Under-Sleeper
pads (USPs) in order to reduce the stress on ballast and the settlement of the granular
layer, among other benefits (Plica, 2007; Dahlberg, 2010). Nonetheless, its widespread
application is limited due to its high costs (which can reach up to 40% of the final price
of each sleeper) and to the need for bonding these elements to the bottom of sleepers.
With a view to considering both alternative measures for reducing ballasted track
maintenance, this paper proposes a new method that consists of replacing part of the
small stones used in the process of stoneblowing with rubber particles obtained from
waste tires, acting as flexible aggregates under the sleeper with capacity to damp loads.
Thus, this proposed solution, refereed as “Stone-Rubber Blowing”, could replace the
incorporation of USPs and would avoid the increase of costs associated with the use of
these elements (since the rubber particles are applied at the time of the small stones
during the stoneblowing process, without the need of fixing the elastic elements to the
bottom of the sleeper) while at the same time an abundant waste material (end-of-life
tires) is reused to improve the effectiveness of stoneblowing process. In addition, no
bonding agents are needed since the rubber particles are used as aggregates, which also
avoids increasing maintenance costs in reference to other solutions (Ho et al., 2013;
Fontserè et al., 2016).
For this purpose, this paper analyses the effect of replacing natural small stones with
different quantities of rubber particles (used as flexible aggregates), comparing the
track section behavior after this process (Stone-Rubber Blowing) to that measured
after conventional stoneblowing technique as well as in comparison to the application
of different USPs (with various stiffness values) used as reference elastic element in
combination with stoneblowing.

314 360.revista de alta velocidad