Sustainability of the Ballasted Track – A Comprehensive Review on Reducing the Use of Mineral Aggregates and the Role of Sub-ballast as a Protective Layer

Several aspects must be considered when evaluating the sustainability of a railway, such as the use of mineral aggregates in ballasted tracks. To this day, most of the cargo and passenger railways around the world are built on granular layers of rocky material, which is called ballast. It has the structural function of absorbing and dissipating the vertical stresses resulting from train traffic, at the same time as it allows the quick drainage of rain water and facilitates the correction of geometrical defects of the track’s alignment. Nonetheless, ballast grains lose mass due to abrasion, which results in contamination of the layer and differential settlings of the track. Problems also emerge when the railway is constructed without sub-ballast, a blocking and filtering layer located between ballast and subgrade. In developing countries such as Brazil there is still shortage of efficient technology for ballast maintenance, like what is employed in Europe and North America. Likewise, it is common to rebuild or rehabilitate old lines that endure heavy cargo traffic without executing the sub-ballast layer. Considering that ballast is a non-renewable natural material it is compelling that such practices be reviewed and replaced by more sustainable techniques. At the same time, it becomes essential that rocks with adequate properties are used in the construction and maintenance of the railway ballast, so that its life span is the longest possible. The objective of this work is to provoke reflection regarding the rational use of mineral non-renewable resources in the construction and maintenance of rail tracks. A review was made on the desirable properties of rocks to be used as railway ballast. Simultaneously, studies are presented that evaluated the consequences of the absence of sub-ballast in the performance of a railway, as well as different techniques that are used to avoid the interlocking of ballast particles into the subgrade. Finally, recommendations are made to constructors and keepers of rail tracks in developing countries, reinforcing the technical and economic advantages of some already established solutions.