1 INTRODUCTION 1.1 Railway track components and their functions 1.1.1 Sleepers The main functions of sleepers are to provide a solid, even and flat platform for the rails, and support the rail fastening system. They are laid on the top of a compacted ballast layer. Sleepers receive the rail loads and distribute them over a wider ballast area to decrease the stress to an acceptable level. 1.1.2 Ballast Railway ballast can be defined as granular coarse aggregate. According to Selig and Waters (1994), tradi- tionally, angular, crushed hard stones and rocks, uniformly graded, free of dust and dirt and not prone to cementing action have been considered good ballast materials. Ballast is a granular material with high bearing capacity that is placed above sub-ballast or subgrade to act as a platform, to support the track su- perstructure. Its main function is to spread the high loads of passing axles to the subgrade. 1.1.3 Sub-ballast Sub-ballast is composed of well-graded crushed rock or sand gravel mixtures, and it is positioned be- tween the ballast and the subgrade material. The sub-ballast layer transmits and distributes stress from the ballast layer to the subgrade over a larger area to reduce the magnitude of resultant stress. Another im- portant function is to prevent intermixing between the ballast and subgrade layer. Sub-ballast acts as a fil- ter to stop upward migration of subgrade particles into the ballast and penetration of coarse ballast into subgrade. Geogrid reinforced railway embankment on soft soil – Experiences from 5 years of field monitoring S. Lenart Slovenian National Building and Civil Engineering Institute (ZAG), Ljubljana, Slovenia J. Klompmaker BBG Bauberatung Geokunststoffe GmbH  Co. KG, Espelkamp-Fiestel, Germany ABSTRACT: As a result of increasing railway traffic, speed and axle loads, existing railway tracks, especially those which are built on soft ground, often show signs of distress and settlements. These have a serious influence on the safety and economy of existing railway lines. As an economic measure for the rehabilitation and/or upgrad- ing of existing railway lines, geogrid or geocomposite products for reinforcement, filtration and separation have successfully been installed in many projects and decisive product properties for these materials have been specified by Railway Authorities. The geogrid reinforcement restrains lateral deformations of the ballast/sub-ballast through shear interaction, which reduces permanent horizontal strains and vertical stresses in the long run. The geotextile component, as separation and filtration layer, prevents mixing of the often fine subgrade (clay/silt) with the coarse aggregate, which prevents distortion of the whole track superstructure. The paper presents results from 5 years of field measurement of a rehabilitated railway section (Poljčane – Dolga Gora, Slovenia) with geogrid reinforced sub-ballast layer on soft soil. In addition to this results from 2 years of field measurement of a rehabilitated railway section (Sava, Slovenia) with geogrid reinforced ballast layer on soft soil are presented and compared to an unrein- forced section. Keywords: geogrid, ballast/sub-ballast reinforcement, rehabilitation, aggregate confinement, monitoring