Performance monitoring — case studies of tracks stabilised by geosynthetic grids and prefabricated vertical drains

Abstract

This paper focusses on assessing the performance of rail track through field trials conducted in the towns of Bulli, Singleton and Sandgate in the state of New South Wales (NSW), Australia. In Bulli and Singleton, different types of geosynthetics (geogrid, geotextile and geocomposite) were installed to investigate their relative advantages in relation to various aperture sizes and types of subgrade. A key objective of this study was to evaluate the effect of these artificial inclusions on the vertical and lateral track deformation as the largescale laboratory tests provided evidence that grids reduce particle breakage by restraining the movement of aggregates and by increasing the effective confining pressure such that excessive dilation is curtailed. The geogrids were more affective at controlling displacement when the track was constructed on a soft subgrade such as estuarine soil. With the tracks constructed directly onto estuarine planes consisting of the inevitable deep and saturated soft clay and silt deposits at Sandgate, prefabricated vertical drains (PVDs) were installed to a depth of less than 8 m to swiftly consolidate the soft upper clay stratum thereby helping resist long term track settlement while increasing the bearing capacity and shear strength. A sophisticated finite element program was developed to capture the behaviour of ballast, subballast and subgrade under cyclic loading via a coupled flow-deformation analysis capable of predicting the vertical and lateral displacement of the tracks, as well as the build-up of excess porewater pressure in the soft estuarine clay. The field data provided by the track owner (one year after the initial design and FEM predictions) proved that PVDs improved track stability by reducing the lateral movements of the subgrade while increasing its load bearing capacity.