Effect of ballast contamination on the behaviour of track substructure

Ballast contamination by subgrade pumping and coal contamination is understood as major cause of track deterioration in many countries over the world. As contamination clogs the ballast voids, the drainage capacity of the track is compromised. Further, lubrication caused by these fines results in reducing load bearing capacity of the ballast layer. In this study, a series of large scale hydraulic conductivity and triaxial tests were performed to study the influence of contamination of ballast on its drainage and shear strength characteristics. Drainage capacity of the contaminated track under different level of contamination was determined, undertaking a numerical analysis using Seep/W. Shear strength of coal and clay contaminated ballast at different confining pressures were compared. Bearing capacity of contaminated track, using 'foundation under three layers' method was derived and plotted against the degree of contamination. Subsequently, the critical level of contamination by fouling materials such as clay and coal is discussed on the basis of hydraulic conductivity, shear strength and bearing capacity of the ballast.