Effect of confining pressure on the degradation of ballast under cyclic loading

Traditional railway foundations or substructures have become increasingly overloaded in recent years due to the introduction of faster and heavier trains. A lack of substructure re-engineering has resulted in maintenance cycles becoming more frequent and increasingly expensive. Two significant problems arising from increasing axle loads are differential track settlement and ballast degradation. One potential method of enhancing the substructure is by manipulating the level of ballast confinement. To investigate this possibility, a series of high frequency cyclic triaxial tests have been conducted to examine the effects of confining pressure and deviator stress magnitude on ballast deformation (permanent and resilient) and degradation. Experimental results indicate that for each deviator stress considered, an optimum range of confining pressures exists such that degradation is minimised. This range was found to vary from 15 – 65 kPa for a maximum deviator stress of 230 kPa, to 50 – 140 kPa when deviatoric stresses increase to 750 kPa. Ballast specimens tested at low confining pressures indicative of current in-situ conditions were characterised by excessive axial deformations, volumetric dilation and an unacceptable degree of degradation associated mainly with angular corner breakage. The results suggest that in-situ lateral pressures should be increased to counteract the axle loads of heavier trains, and practical methods of achieving increased confinement are suggested.