Laboratory Modelling and Assessment of Internal Instability Potential of Subballast Filter under Cyclic Loading

Abstract

Internal instability occurs when the filtrates under the influence of seepage forces washout the finer fraction from granular soils, resulting in the deterioration of the mechanical and geotechnical properties, e.g. change in soil gradation, large volumetric strains and increased permeability etc. In railway substructures, filters are installed as drainage or subballast layers to arrest the eroding subgrade fines into superstructure that could otherwise endanger the stability of tracks due to ballast fouling and clay pumping. Notably, the design and assessments for internal instability potential for subballast are currently emphasized through existing filtration criteria which neglect the effects of cyclic loading. This paper reports an investigation of the internal stability assessment of saturated subballast filters subjected to cyclic loading. Experimental results conducted on selected soil gradations in specially designed cyclic loading permeameter revealed that repeated loading promotes substantial and premature suffusion failure, the intensity of which increases with the loading frequency. An existing criterion for assessing the internal instability potential is modified to capture the effects of cyclic loading that is subsequently validated through extensive laboratory data. Comparisons between static and cyclic response of test samples revealed that the suffusion could occur at unique hydro-mechanical boundaries, which would have implications in practical design of internally stable subballast filters under unfavorable hydro-mechanical and cyclic loading conditions.