True triaxial testing of geogrid for high speed railways

This work describes a series of novel experimental tests to determine the potential of geogrids to confine granular layers within ballasted railway lines 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 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 settlement under high relative train speeds, ballasted railway track samples were subject to combined vertical-horizontal cyclic loading. Three areas were explored: (1) the performance benefit from placing geogrid at the ballast-subballast interface, (2) the performance benefit from placing geogrid at the subballast-subgrade interface, (3) the effect of subgrade stiffness on geogrid performance at the subballast-subgrade interface. Testing was performed using a unique large-scale true triaxial apparatus which had the ability to vary stress levels in three Cartesian directions. Compared to the control conditions, the geogrid offered a settlement improvement of approximately 35% when placed at the ballast-subballast interface, and 10–15% when placed at the subballast-subgrade interface. Regarding subgrade CBR, it was found that the geogrid offered the greatest performance benefits when the subgrade was soft. Therefore it was concluded that for the ballasted rail structures under test, when subject to elevated levels of horizontal stress, geogrids reduced settlements compared to non-geogrid solutions.