Nicolas Calon

Development of a large-scale infiltration column for studying the hydraulic conductivity of unsaturated fouled ballast

To study the hydraulic behavior of fouled ballast, an infiltration column 600 mm high and 300 mm in diameter was developed. Five time domain reflectometer (TDR) sensors and five tensiometers were installed at various levels, allowing the measurement of volumetric water content and matric suction, respectively. The material studied was fouled ballast that was formed in the railway track-bed by penetration of fine-grained soil into the ballast. This material is characterized by a high contrast of size between the largest and the smallest particles. During the test, three stages were followed: saturation, drainage, and evaporation. Based on the test results, the water-retention curve and the unsaturated hydraulic conductivity were determined. The quality of the results shows the capacity of this large-scale infiltration column in studying the unsaturated hydraulic properties of such fouled ballast.

Improvement of existing railway subgrade by deep mixing

Abstract Many old railway lines in Europe need to be reinforced as they no longer meet the requirements of modern traffic. This paper presents the results of field tests carried out, as part of the French RUFEX research project (Reinforcement of railway tracks), to study the feasibility of the installation of 600 mm diameter vertical soil-cement columns under existing railway lines without removing the tracks or cementing the ballast. These columns were installed under an execution procedure is described and the quality/homogeneity of the mixed material is studied with excavated columns after curing periods of 28 and 180 days and several samples were taken for laboratory testing. The tests included unconfined compressive strength tests, splitting tensile strength tests, porosity and ultrasonic wave velocity measurements. The results show an increase in strength of 30% between 28 and 180 days. A linear correlation between compressive and tensile strengths is proposed. Besides, data obtained from ultrasonic wave velocity measurements suggests that this test can be used for the estimation of index parameters such as density and porosity but also for preliminary assessments of static stiffness and strength by correlation with the dynamic elastic modulus.

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 Conditioning on Linear Viscoelastic Behaviour of Bituminous Mixtures Used For Railway Trackbeds

The influence of moisture on the linear-viscoelastic properties of bituminous mixtures with and without polymer modified bitumen (PMB) to be used for railway trackbeds was addressed in this paper. Nowadays, the use of bituminous mixtures in railway trackbeds, particularly for new high-speed lines, is a common practice. The variation of the thermomechanical properties of bituminous mixtures needs to be characterized considering railway loading, circulation speed, and environmental exposure conditions. For this purpose, improved 3D complex modulus tests were performed on base-course type mixtures. A tri-dimensional constitutive model, called 2S2P1D (2 Springs, 2 Parabolic creep elements and 1 Dashpot), was used to model the viscoelastic behavior of the materials. A moisture conditioning procedure was used to induce moisture damage to the mixtures. The thermomechanical properties of the materials are compared for the conditioned and non-conditioned states. Low variation of the linear viscoelastic behavior of the studied materials after moisture conditioning was observed. The results allowed assessing the interest of using polymer modified bitumen for bituminous mixtures to be used in railway trackbeds.

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.