Behaviour of clay-fouled ballast under drained triaxial testing

Abstract

Contamination or fouling of rail ballast with external fines, including slurried and pumped subgrade material (e.g. clay and silt), is one of the primary reasons for track deterioration. Fouling causes differential settlement of the track, and also decreases the load-bearing capacity, owing to the reduction in the friction angle of the granular assembly. In certain circumstances, fouled ballast needs to be cleaned or replaced to maintain the desired track stiffness, load-bearing capacity and track alignment, all of which influence safety. This paper presents and discusses the results of a series of large-scale triaxial tests conducted on latite basalt, a rail ballast of volcanic origin, commonly used in Australia. Consolidated drained triaxial tests were conducted under three different levels of confining pressure and varying degrees of clay fouling. Stress-strain degradation characteristics are discussed in detail. This paper also describes the non-linear strength envelope and a novel empirical relationship to capture the detrimental effects of clay fouling on the performance of ballasted tracks. Contamination or fouling of rail ballast with external fines, including slurried and pumped subgrade material (e.g. clay and silt), is one of the primary reasons for track deterioration. Fouling causes differential settlement of the track, and also decreases the load-bearing capacity, owing to the reduction in the friction angle of the granular assembly. In certain circumstances, fouled ballast needs to be cleaned or replaced to maintain the desired track stiffness, load-bearing capacity and track alignment, all of which influence safety. This paper presents and discusses the results of a series of large-scale triaxial tests conducted on latite basalt, a rail ballast of volcanic origin, commonly used in Australia. Consolidated drained triaxial tests were conducted under three different levels of confining pressure and varying degrees of clay fouling. Stress-strain degradation characteristics are discussed in detail. This paper also describes the non-linear strength envelope and a novel empirical relationship to capture the detrimental effects of clay fouling on the performance of ballasted tracks.

Micromechanics-Based Investigation of Fouled Ballast Using Large-Scale Triaxial Tests and Discrete Element Modeling

“…Figure 4 shows a DEM model of a large-scale triaxial test (600 mm high by 300 mm in diameter). These dimensions are identical to those carried out in the laboratory by Indraratna et al (2013a). A top wall platen was forced to move downwards to apply a required deviatoric stress while the vertical walls were moved inwards or outwards by a servo to maintain the required confining pressure (Itasca 2014).…”

“…It is noted that a clump acts as a rigid body in which the internal contacts are ignored and the overlapping mass in clumps, which can be affected on particle rotations, is dealt with in this analysis by determining a corrected volume for each clump. The simulated ballast particles were placed at random locations without any overlapping within the specified cylindrical boundary and were then compacted to a bulk unit weight of 14.3 kN/m 3 to represent the actual experimental and field conditions (Indraratna et al 2013a). The initial specimen of simulated ballast was isotopically compressed to three different confining pressures ( 10, 30, and 60 .…”

“…Ballast commonly consists of medium to coarse-sized aggregates whose main purposes are to: (i) distribute the train load to the underlying layers at a reduced stress level; (ii) provide sufficient lateral confinement to the track, and (iii) provide a free draining condition. Under train operations the ballast progressively deteriorates and becomes fouled, which decreases its shear strength (Lackenby et al 2007;Huang and Tutumluer 2011;Indraratna et al 2013a). …”

“…Large-scale triaxial tests were carried out at various levels of fouling, ranging from VCI=10% to VCI=50%, and subjected to three confining pressures of 10 kPa, 30 kPa, and 60 kPa. The confining pressure selected for the current study was determined using field measurements of typical Australian tracks by Indraratna et al (2010) Indraratna et al (2013a) and they highlighted that the peak deviatoric stress ( and the corresponding initial deformation modulus increased with an increase in the confining pressure at a given level of fouling. Mobilised friction angles (ϕ) of the fresh and fouled ballast under different confining pressures measured in laboratory are presented in Figure 2a.…”

“…The fouling materials inhibit the full mobilisation of the inter-particle friction as they coat the aggregate surfaces (lubrication effect), thereby reducing the shear strength of the ballast assembly. Indraratna et al (2013a) observed that at a very high confining pressure (i.e. …”