Key aspects on the behaviour of the ballast and substructure of a modern railway track：research-based practical observations in Finland

This paper presents an overview on the wide-ranging track structure studies at the Tampere University of Technology（TUT）,Finland dealing with the key aspects of track geotechnics related to high-speed passenger traffic on ballasted tracks.Special attention is paid to ballast and sub-ballast,while also considering frost action,embankment stability,track stiffness,track geometry and transition zones.As a result,this paper states that understanding the ballast degradation mechanism and its consequences and assessment of its condition occupy an important role in the construction and maintenance of a smooth high-speed rail line.The choices related to building the sub-ballast also have a dramatic impact on later track deformations and maintenance needs.In cold climate,especially where seasonal frost occurs,understanding and taking into account the frost action mechanism is crucial.Especially in the maintenance and rehabilitation planning of existing tracks,high-class analyses of ground penetrating radar data and its integrated analysis with other data can yield considerable benefits.