Alain Robinet

Physical Model for Studying the Migration of Fine Particles in the Railway Substructure

To study the interlayer creation and the mud-pumping phenomena in the conventional French railway substructure, a physical model was developed with a 160-mm-thick ballast layer overlaying a 220-mm-thick artificial silt layer (mixture of crushed sand and kaolin), both layers being compacted in a transparent cylinder of a 550-mm-inner diameter. One positive pore water pressure sensor, three tensiometers, and three time domain reflectometer (TDR) sensors were installed around the ballast/silt interface allowing the evolution of pore water pressure (negative or positive) and volumetric water content to be monitored, respectively. A digital camera was installed allowing direct monitoring of different movements (ballast, ballast/subsoil interface, etc.). The effects of loading (monotonic and cyclic loadings) and degree of saturation of subsoil (w = 16 %, Sr = 55 %, and near-saturation state) were investigated. It was found that the development of pore water pressure in the subsoil is the key factor causing the migration of fine particles, hence, resulting in the creation of interlayer, as well as mud pumping. In particular, the camera exposed the pumping up level of fine particles during the test, showing that the migration of fine particles was not only a result of the interpenetration of ballast particles and subsoil, but also of the pore water pressure that pushed the fine particle upward. The quality of the recorded data showed that the physical model developed worked well and the test protocol adopted for studying the mechanisms of intermixing of ballast and subsoil and mud pumping was appropriate.

Influence of moisture on thermomechanical behaviour of bituminous mixtures used for railway trackbeds

The influence of moisture on the thermomechanical properties of bituminous mixtures used for railway trackbeds is addressed in this paper. The use of asphalt base course materials has been identified as a suitable structural solution for the further development of both high-speed and conventional railways, notably in terms of circulation speed and axle load increases. Moreover, field experiences attest a reduction of the maintenance needs by using asphalt concrete in the railway platform. Nevertheless, the available in-situ feedback is not sufficient for observing the long-term degradation of the bituminous layers in railway trackbeds. The evolution of the thermomechanical properties of bituminous mixtures needs then to be characterized for the specific railway working conditions (loading, frequency and environmental exposure). For this, improved 3D tensioncompression tests were performed on mixtures used in railway trackbed. The tests are performed on laboratory-compacted cylindrical asphalt specimens submitted to sinusoidal loading. A moisture conditioning procedure was defined and used to simulate the exposure of the materials to weather conditions. The thermomechanical performances of the materials are compared for the conditioned and non-conditioned states. A multi-parameter analysis allowed determining the effect of moisture exposure on the materials behaviour. The results allowed establishing selection criteria, based on moisture susceptibility and long-term degradation, for bituminous mixtures to be used in railway trackbeds.

Unsaturated Hydraulic Properties of Fine Particles of Fouled Ballast Layer from an Ancient Railway Track Beds

The ancient railways in France, which was initially built by emplacing ballasts directly on the subgrade soils without a separation layer as that for new railway lines, are facing to various problems that affect the stability of trackbed and thus the proper circulation of trains. One of these problems is related to the loss of efficiency in water drainage of the track. Actually, fouled ballast has been formed progressively along the life time of the track mainly due to the infiltration of fine particles of subgrade layer into the ballast layer. In this study, the hydraulic properties of these fine particles investigated. Soil taken from a fouled ballast layer of a rail track was first sieved at 2 mm and then re-compacted to its initial in-situ dry density. The water retention curve and the unsaturated hydraulic conductivity were finally analyzed.