Strategies for improving railway track-bed settlement
The operational cost of ballasted railway lines can be reduced by improving track-bed longevity. To aid this, this thesis develops new understandings into ballast behaviour and investigates novel improvement strategies. Three distinct novel contributions are made: 1) A large-scale, true-triaxial testing apparatus (GeoTT) is modified for the large particle test. Then railway ballast is tested under three different confining stresses to determine the Poisson’s ratio and modulus in three dimensions. This is novel because true-triaxial testing of railway ballast has rarely been investigated before. Anisotropic behaviour is clearly evident, with horizontal directions showing a significantly lower modulus compared to the vertical direction. It is also found that confining stress has an important effect on both Poisson’s ratio and modulus. 2) Geogrids are tested to investigate their ability to confine granular ballasted track layers when operating at speeds close to critical velocity. This is important because at low train speeds, vertical stresses are dominant, but when approaching critical velocity conditions, dynamic horizontal stress levels are greatly magnified. Therefore the majority of previous geogrid investigations have been performed assuming constant horizontal stress levels, thus making them more relevant for lower speed lines. To investigate the track-bed settlement under high relative train speeds, ballast railway track samples are subjected to combined vertical-horizontal cyclic loading. It is found that geogrids offer a settlement improvement of approximately 35% when placed at the ballast-subballast, and 10-15% when placed at the subballast-subgrade interface. Regarding the subgrade stiffness, it is found that geogrids offer the greatest performance benefits when the subgrade is soft. 3) Asphaltic layers are investigated to determine whether they can improve the bending stiffness and thus longevity of ballasted tracks. Large-scale, phased cyclic loading and static compression laboratory tests are performed on a large-scale hybrid ballast-asphalt track, supported by subgrade with varying stiffness. This makes it the first large scale laboratory test of hybrid asphalt tracks in the presence of varying subgrade. In the presence of localised poor ground conditions, it is found that an asphaltic layer acts as a bridge to shield the localised area from high stresses. It is also found that the asphalt layer reduces overall track settlement, and is particularly effective when the subgrade stiffness is low.